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WEATHER AND FORECASTING
VOLUME 22
Antecedent Upstream Air Trajectories Associated with Northwest Flow Snowfall in the
Southern Appalachians
L. BAKER PERRY
Department of Geography and Planning, Appalachian State University, Boone, and Department of Geography,
University of North Carolina at Chapel Hill, Chapel Hill, North Carolina
CHARLES E. KONRAD
Department of Geography, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina
THOMAS W. SCHMIDLIN
Department of Geography, Kent State University, Kent, Ohio
(Manuscript received 28 November 2005, in final form 15 May 2006)
ABSTRACT
Northwest flow snow (NWFS) events are common occurrences at higher elevations and on windward
slopes in the southern Appalachians. Low temperatures and considerable blowing and drifting of snow,
coupled with significant spatial variability of snowfall, substantially increase societal impacts. This paper
develops a synoptic classification of NWFS events in the southern Appalachians using 72-h antecedent
upstream (backward) air-trajectory analyses. Hourly observations from first-order stations and daily snowfall data from cooperative observer stations are used to define snowfall events. NCEP–NCAR reanalysis
data are utilized to identify NWFS events on the basis of 850-hPa northwest flow (270°–360°) at the event
maturation hour. The NOAA Hybrid Single-Particle Lagrangian Integrated Trajectory tool is used to
calculate 72-h backward air trajectories at the event maturation hour and composite trajectories are mapped
in a geographic information systems format. Analyses of vertical soundings are coupled with NCEP–NCAR
reanalysis data to determine the synoptic characteristics associated with each trajectory class. Significant
variability of trajectories and synoptic patterns is evident from the analyses, resulting in four distinct
backward air-trajectory classes. Trajectories with a Great Lakes connection result in higher composite mean
and maximum snowfall totals along portions of the higher-elevation windward slopes when compared with
other northwest trajectories, but little effect of the Great Lakes is noted at lower elevations and on leeward
slopes.
1. Introduction
Northwest flow snow (NWFS) events are responsible
for nearly 50% of average annual snowfall totals along
the windward slopes and higher elevations of the southern Appalachian Mountains (Perry 2006). The low
temperatures and considerable blowing and drifting
of snow, coupled with the significant spatial variability
of snowfall, substantially increase the societal impacts. In some cases, snowfall can be quite heavy, as
Corresponding author address: L. Baker Perry, Dept. of Geography and Planning, Appalachian State University, Box 32066,
Boone, NC 28608.
E-mail: [email protected]
DOI: 10.1175/WAF978.1
© 2007 American Meteorological Society
WAF978
evidenced by the widespread totals of 51–76 cm (20–30
in.) along the windward slopes and higher elevations of
eastern Tennessee and western North Carolina during
an event on 18–20 December 2003. Significant lateseason NWFS events in association with 500-hPa cutoff
lows occurred during 2–6 April 1987 [up to 152 cm (60
in.)] and 6–9 May 1992 [up to 102 cm (40 in.)] (NWS
1987; Sabones and Keeter 1989; Fishel and Businger
1993). More commonly, however, NWFS results in
lighter accumulations and occurs in association with
midlevel synoptic-scale subsidence and moisture limited to below 700 hPa, with topography and convection
providing the necessary forcing. In many cases, trajectories extend downwind from the Great Lakes into the
study area (Fig. 1). The general synoptic environment,
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PERRY ET AL.
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FIG. 1. Location of study area and typical synoptic pattern for NWFS.
therefore, displays certain characteristics that are associated with lake-effect snowfall (LES) in the Great
Lakes region, for example, a shallow moist layer that is
present beneath a capping inversion (e.g., Niziol et al.
1995; Lackmann 2001). One important difference is the
role of topography in controlling the spatial patterns of
NWFS, maximizing snowfall on the northwest (windward) slopes and leading to pronounced shadowing on
southeast (leeward) slopes (Fig. 2) (Perry 2006).
Low-level moisture increases as a result of latent heat
fluxes that develop when cold air flows across the relatively warm waters of the Great Lakes in the late autumn and winter months (Niziol et al. 1995). This lowlevel moisture, along with the destabilization of the
lower troposphere through sensible heat fluxes, generates LES on the leeward shores and immediately downwind of the Great Lakes. Even modest topographic
rises of 100 m downwind of the lakes can generate additional lifting through orographic forcing, resulting in
dramatic increases in snowfall (Hjelmfelt 1992; Niziol
et al. 1995). Under favorable synoptic patterns, LES
may extend considerable distances downwind, even as
far away as the northern mountains of West Virginia
(Kocin and Uccellini 1990; Schmidlin 1992). It is quite
plausible therefore, under the right conditions, for the
relatively higher terrain of the southern Appalachians
to extract low-level moisture from the Great Lakes via
orographic processes to produce accumulating snowfall.
It is apparent that air trajectories associated with
NWFS in the southern Appalachians frequently extend
downwind from the Great Lakes, suggesting a possible
Great Lakes influence. Previous work by Schmidlin
(1992) in the northern mountains of West Virginia supports this possibility. He concluded that moisture was
partly of Great Lakes origin for a sample of northwest
flow snow events not tied to synoptic-scale precipitation processes at Snowshoe, West Virginia. Schmidlin
also concluded that at least 25%–30% of the average
annual snowfall at Snowshoe during the 12-yr period of
analysis was tied to periods of LES along the Lake Erie
snowbelt. Johnson (1987) has also suggested that much
of the snow that accumulates in periods of northwest
flow in the southern Appalachians, particularly farther
north in the mountains of West Virginia where accumulations are typically greatest, is LES due to the
modification of the upstream air masses through sensible and latent heat fluxes. Sousounis and Mann
(2000), through simulated model experiments, showed
that the combined effects of the Great Lakes (e.g., the
lake-aggregate effect) play an important role in warming and moistening the lower troposphere at the synoptic scale and thereby enhancing LES in some instances.
Although significant ice cover typically develops over
portions of the Great Lakes by midwinter, large areas
of open water often remain, particularly over southern
portions of Lake Michigan (Assel 2003). Only in the
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FIG. 2. Locations of windward and leeward slopes during periods of northwesterly flow.
coldest winters, such as that of 1976/77, does nearly
continuous ice cover develop across Lake Michigan.
Significant ice cover limits the sensible and latent heat
fluxes and, thus, reduces LES activity in areas immediately downwind of the lakes. It would also follow that
significant ice cover across the entirety of the Great
Lakes would reduce the lake-aggregate effect and result in a noticeable reduction of lower-tropospheric
moisture and humidity considerable distances downwind. We hypothesize that the lake-aggregate effect in
the absence of significant ice cover is important in enhancing snowfall in the southern Appalachians when
trajectories exhibit a Great Lakes connection (GLC).
Additionally, analyses of recent NWFS events have led
us to hypothesize that particular air trajectories downstream of individual lakes may lead to further localized
enhancement.
Recent work (Perry 2006) showed that cold season
northwest flow is tied to the occurrence of upslope
snowfall along the windward slopes of the southern Appalachians, but it is unclear whether or not the air trajectories typically exhibit a GLC. Previous work (Perry
and Konrad 2004) also found that heavy NWFS events
are associated with higher values of lower-tropospheric
relative humidity and lower 1000- and 500-hPa heights,
although the extent to which the heavy events are tied
to trajectories with a GLC is also unknown. Other than
this small body of research, little is known about the
general climatology or antecedent upstream air trajectories associated with NWFS in the southern Appalachians. Therefore, this paper classifies NWFS events on
the basis of backward air parcel trajectories through an
analysis of 191 NWFS events during the period 1975–
2000. We also develop a trajectory classification scheme
and compare composite snowfall totals, analyzed
soundings, and synoptic fields among the different trajectory classes, with particular reference to those trajectories with a GLC.
2. Data and methodology
a. Snowfall data and event definition
Daily snowfall records for the period 1975–2000
served as the source of the daily snowfall data. We
extracted these data from the National Climatic Data
Center’s Cooperative Summary of the Day CD-ROM
(NCDC 2002) for 121 cooperative observer (coop) stations in the southern Appalachian Mountains (Fig. 3).
We defined a snowfall event as having occurred if at
least one coop station in the region reported snow accumulation on a given date. To improve the temporal
resolution of each event, we referenced hourly surface
observation summaries from five nearby first-order stations (Fig. 3). From interpretation of these data, we
were able to approximate the onset, maturation, and
ending times as well as the duration of reported snow-
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337
FIG. 3. Coop and first-order stations in the southern Appalachians.
fall across the region. Assessment of the maturation
time involved determining the hour in which the spatial
extent of snowfall was greatest across the network of
first-order stations. An event remained active if precipitation was reported during a 6-h period. When precipitation was no longer reported at any of the five firstorder stations for more than 6 h, we defined the event
as having ended at the hour precipitation was last reported.
Because of the large areal extent and significant topographic diversity within the southern Appalachians,
the study area was divided into 14 snow regions to facilitate intraregional comparisons. Coop stations were
grouped together into snow regions based on similarities in snowfall patterns, elevation, and topography
(Fig. 4). In most cases these regions correspond to zone
groupings that the National Weather Service (NWS)
uses for forecast products. However, it is important to
note that Snow Region 14 is not contiguous, but rather
consists of all locations in the study area with elevations
greater than 1220 m (4000 ft). These areas are largely
limited to the mountains of eastern Tennessee and
western North Carolina. Recent work (e.g., Perry 2006)
has shown that higher-elevation stations (e.g., Mount
Mitchell, North Carolina) average as much as 75 cm
(1700%) more NWFS on an annual basis than nearby
valley stations (e.g., Asheville, North Carolina), suggesting dramatic differences in snowfall climatologies.
Pronounced differences are also evident between
northwest and southeast slopes and between southern
and northern areas, thereby necessitating a high degree
of complexity in the number and delineation of snow
regions. Mean and maximum event snowfall totals for
those stations reporting snow were calculated for each
snow region. Although useful for analyzing the general
spatial patterns across the southern Appalachians, it is
important to recognize that considerable variability of
snowfall still occurs within snow regions.
b. Identification of NWFS events
We used gridded (2.5° ⫻ 2.5° latitude–longitude
mesh), twice-daily synoptic fields that were extracted
from CDs containing the National Centers for Environmental Prediction–National Center for Atmospheric
Research (NCEP–NCAR) reanalysis dataset (Kalnay
et al. 1996) to obtain the u and ␷ components of 850-hPa
wind direction and vertical velocity ␻. These fields were
spatially interpolated to the center of each snowfall
event. Using the 0000 and 1200 UTC gridded synoptic
fields, we undertook a temporal interpolation to estimate field values during the event maturation time. We
employed an inverse distance technique to carry out all
spatial and temporal interpolations. Using these spatially and temporally interpolated data, we identified
432 NWFS events during this period on the basis of
northwesterly (270°–360°) 850-hPa wind direction at
maturation hour. The 850-hPa level, found at approximately 1450 m, is a good indication of the mean wind
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FIG. 4. Snow regions used in the study.
direction between 1000 and 2000 m, where much of the
orographic enhancement in association with NWFS
likely occurs.
We further narrowed this sample of 432 NWFS
events by identifying only those events with 700-hPa synoptic-scale sinking motions (e.g., vertical velocity ⬎ 0)
with durations between 2 and 3 days. Events with rising
motions were eliminated because a portion of the precipitation may have been tied to synoptic features (e.g.,
approaching troughs), thus confounding the evaluation
of the importance of air trajectories on snowfall totals.
Exceptionally short and long events were also eliminated from the sample in order to control for the influences of event duration (i.e., exceptionally heavy snowfall totals resulting from an extended period of snowfall). We also excluded all events in the months of April
and May because of the complicating effects of diurnal
instability, which can result in locally intense snowshowers. The remaining 191 NWFS events served as the
sample for the backward air-trajectory analysis.
c. Trajectory analysis
The National Oceanic and Atmospheric Administration’s Hybrid Single-Particle Lagrangian Integrated
Trajectory (HYSPLIT) tool (information available online at http://www.arl.noaa.gov/ready/hysplit4.html)
was utilized to calculate 72-h backward air trajectories
for air parcels at 1450 m (4757 ft), or approximately the
850-hPa level. The HYSPLIT tool uses u and ␷ components of the horizontal wind, temperature, height, and
pressure at different levels of the atmosphere to calculate the backward trajectories (Draxler and Hess 1998).
We used the weighted spatial mean or centroid of each
event and the event maturation time as the starting
point and time for the backward air-trajectory analysis.
Although finer-scaled data could be assessed using the
HYSPLIT tool, NCEP–NCAR reanalysis data (2.5° ⫻
2.5 latitude–longitude) were used because of their
availability over the entire study period. As shown in
Fig. 5, the trajectories computed from these data compared favorably to those calculated from higherresolution final analysis (FNL; 191 km) and Eta Data
Assimilation System (EDAS; 80 km) data.
A visual analysis of the computed trajectory allowed
us to classify each 6-h segment by counting the number
of segments contained within each quadrangle of our
trajectory classification grid (Fig. 6). If the plotted air
trajectory associated with each snowfall event resided
in a respective quadrangle for the specified minimum
number of hours (Table 2), we classified the trajectory
according to the quadrangle number. The Great Lakes
(grid 3) are roughly divided into the western lakes of
Superior and Michigan and the eastern lakes of Huron
and Erie in order to facilitate intraregional comparisons. The Lake Ontario region is not explicitly delin-
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FIG. 5. Comparative air trajectories using NCEP–NCAR reanalysis (solid line), FNL (dashed), and EDAS 80 (dotted) data for
(left) 6 Dec 1997 and (right) 22 Feb 1999 NWFS events.
eated in this study because of the relatively infrequent
occurrence of north-northeasterly flow from this region. Because the western and eastern Great Lakes
subregions (grids 3.1 and 3.2) are the same spatial dimensions, it is possible to directly compare the frequency, snowfall totals, and synoptic characteristics of
air trajectories. The relatively course spatial resolution
of the NCEP–NCAR reanalysis data used to compute
the backward air trajectories also supports using a more
generalized classification grid, and not focusing on the
precise land–water boundary.
The hourly latitude, longitude, and height (hPa) coordinates associated with each event backward air trajectory were used to develop composite trajectories for
each region in the trajectory classification scheme. Additional composite trajectories were developed for T ⫺
12 (12 h prior to event maturation) and T ⫹ 12 (12 h
following event maturation). For each event with a
FIG. 6. Trajectory classification grid and location of Huntington, WV (HTS).
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WEATHER AND FORECASTING
GLC (ⱖ6 h of the trajectory in trajectory grids 3.1 or
3.2), we consulted the Great Lakes Ice Atlas DVD (Assel 2003) and noted the percent ice cover for the western and eastern Great Lakes. Composite mean and
maximum snowfall totals for GLC events in the top
quartile of percent ice cover (ⱖ25%) were compared
with those events in the bottom three quartiles (⬍25%)
to evaluate the importance of ice cover. Last, we used
NCEP–NCAR reanalysis data (Kalnay et al. 1996) to
develop composite plots of the 1000-hPa height for
each trajectory class. For each group we calculated
composite mean and maximum snowfall totals, composite backward air trajectories for each region, and
also values of various synoptic fields. The purpose of
this analysis was to determine if part of the enhanced
snowfall could be associated with antecedent air trajectories across the lakes. For those events with a GLC, we
compared snowfall totals between the North Carolina
high country (region 8) and the Appalachian Plateau of
West Virginia (region 13) to identify events in which
snowfall may have been enhanced in one of these regions due to a GLC.
d. Sounding and synoptic fields
To provide details on vertical thermal and moisture
profiles, an analysis of vertical soundings for Huntington, West Virginia (HTS) (Fig. 6), was undertaken. Unfortunately, upper-air data were only available through
1995 at this site, thereby limiting the sample to 155 of
the original 191 events. These soundings provided
much greater detail regarding the vertical profiles of
temperature and moisture in association with NWFS
events. We chose HTS because it was the closest sounding station upwind of the study area and therefore provides the best assessment of the vertical profiles during
periods of northwest flow. Table 1 summarizes the
sounding variables that were calculated. We defined
the moist layer as that portion of the lower troposphere
with mean relative humidity greater than 80%. The
mean temperature of the moist layer was calculated by
averaging the temperatures at the lower and upper
heights of the moist layer. The presence or absence of
moderately low temperatures (⫺14° to ⫺17°C) in the
moist layer was also noted as ice crystal growth is maximized in the dendritic range of ⫺14° to ⫺17°C (Ryan et
al. 1976; Pruppacher and Klett 1997; Fukuta and Takahashi 1999); furthermore, heavy snowfall is often associated with moist-layer temperatures within the dendritic temperature range (Auer and White 1982). Additionally, forecasters at the Greer, South Carolina,
NWS Office have found that heavy NWFS events are
more likely to occur in the North Carolina mountains
VOLUME 22
TABLE 1. Sounding and synoptic variables calculated in
this study.
Sounding variables
Thickness of moist layer
Mean temperature of moist layer
Top of moist layer
Dendritic temperature range
Lapse rate above moist layer
Synoptic variables
1000-hPa height
500-hPa height
850-hPa mixing ratio
850-hPa relative humidity
850-hPa wind direction
850-hPa wind speed
850-hPa temperature
850-hPa thermal advection
1000–500-hPa mean relative humidity
700-hPa vertical velocity
850–500-hPa lapse rate
Precipitable water
500-hPa relative humidity
850-hPa theta E
500-hPa relative vorticity
Units
hPa
°C
hPa
% of events
°C (100 hPa)⫺1
Units
m
m
g kg⫺1
%
°
m s⫺1
°C
°C (12 h)⫺1
%
hPa h⫺1
°C
in.
%
°C
1 ⫻ 10⫺5 s⫺1
when moist-layer temperatures coincide with the dendritic temperature range (Lee 2005). Values for relative
humidity, temperature, and height were taken from the
University of Wyoming archived text soundings (online
at http://weather.uwyo.edu/upperair/sounding.html).
Table 1 lists 15 synoptic fields extracted from the
NCEP–NCAR reanalysis dataset that will be incorporated into the analysis. Recent work (Perry and Konrad
2004) suggests that many of these synoptic fields are
tied to heavy NWFS in the southern Appalachians and,
thus, may be tied to certain air trajectories. In particular, high values of relative humidity in the lower troposphere are critical, as are lower heights from 1000 to 500
hPa. Modest, as opposed to strong, cold advection also
appears to be tied to heavy NWFS. The substantial
pressure rises and subsidence associated with strong
cold advection may work to counteract available lowlevel lift and decrease snowfall totals. Upper-level divergence has also been linked to heavier NWFS (Perry
and Konrad 2004). Differential cyclonic vorticity advection below 500 hPa is known to help raise the height of
the capping inversion in situations of lake-effect snow
in the Great Lakes (Niziol 1989; Lackmann 2001), and
a vorticity maximum also may play a role in creating an
environment more conducive for heavy NWFS in the
southern Appalachians. The sounding and synoptic
fields together provide a comprehensive assessment of
the vertical and synoptic environments associated with
different air trajectories.
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PERRY ET AL.
TABLE 2. Summary of backward air trajectory analysis.
Class
Percent
No.
Description
1
2
3.1
3.2
3.3
4
7.9
26.7
30.9
5.2
11.0
8.4
15
51
59
10
21
16
9.9
19
191
ⱖ36 h in region 1, no GLC
ⱖ36 h in region 2, no GLC
ⱖ6 h in W Great Lakes
ⱖ6 h in E Great Lakes
ⱖ6 h in both W and E Great Lakes
ⱖ36 h in region 4, no GLC, ⬍36 h
in either regions 1 or 2
Remainder
Tot
5
3. Results and discussion
a. Trajectory classification
Table 2 summarizes the results of the trajectory classification. The most common air trajectory associated
with NWFS in the southern Appalachians is trajectory
class 3.1 (30.9%), followed closely by class 2 (26.7%).
Interestingly, almost half (47.1%) of the 191 NWFS
events analyzed in this study exhibit a GLC (trajectory
class 3) and approximately three-quarters (73.8%) of
the events display a northwesterly trajectory (trajectory
classes 2 or 3). Besides subclasses 3.2 and 3.3 of trajectory class 3, the most infrequent trajectories are trajectory classes 1 and 4, which are characterized by a more
westerly trajectory, as opposed to northwesterly, at
event maturation. The remaining events did not meet
any of the trajectory classification requirements and
were therefore grouped together in trajectory class 5.
The temporal pattern of trajectory classes by month
(Fig. 7) indicates significant variability within and
among the classes. Most of the trajectory class 1 events
were observed during the coldest months of the winter
(i.e., December, January, and February), when westerly
trajectories are associated with air sufficiently cold for
snowfall to occur. Trajectory class 3.1, on the other
hand, displayed a relatively equal monthly distribution,
as the other trajectory classes were largely limited to
the months of December, January, February, and
March. Interestingly, the overwhelming majority of
events in the month of November were associated with
GLC trajectories. The lake–air temperature contrast,
which is greatest in the late autumn and early winter
when the sensible and latent heat fluxes are maximized
(Niziol et al. 1995), may contribute somewhat to the
observed monthly patterns for trajectory class 3.1. The
very low number of February events for trajectory class
2 is noteworthy, as is the spike of events for trajectory
class 3.3 in the same month. Both of these anomalies
likely result from small sample sizes rather than
monthly variations in the synoptic environment.
341
b. Composite trajectory plots
Northwesterly trajectories, or trajectory classes 2 and
3, will be discussed first, followed by the more westerly
trajectories of trajectory classes 1 and 4. Composite trajectory plots (Fig. 8) for trajectory class 2 show a backward air trajectory that extends to the northwest, but
well to the west of the western Great Lakes. Subsidence
is noted in the vicinity of the Ohio River Valley, with
the air parcels descending from 800 hPa to slightly below 850 hPa, before ascending slightly as a result of
orographic forcing. Trajectory class 3.1 is very similar to
class 2, but in this case part of the trajectory at event
maturation (T0) and T ⫹ 12 crosses the extreme southern portion of Lake Michigan. Subsidence is also evident from 800 to 850 hPa over the 72-h period, although
it is not quite as abrupt in the vicinity of the Ohio
Valley as with class 2. Air parcels in association with
trajectory class 3.2 spend a considerable amount of time
in the vicinity of the Great Lakes, with the backward air
trajectory centered over the eastern Great Lakes at T0.
Air parcels are also near the surface for an extended
period of time, with modest lift occurring as a result of
the topography. A veering of the trajectory to a more
northerly component is evident between T ⫺ 12 and T0
for all three composite trajectories, with less consistency noted between T0 and T ⫹ 12, particularly in
trajectory class 3.2, where backing is noted. The position of the composite cyclone is related to the orientation of the air trajectories (Fig. 8). Cyclones positioned
farther south (e.g., off the mid-Atlantic coast) favor a
more northerly low-level airflow with a GLC while cyclones positioned farther north off the New England
coast favor a more westerly trajectory that is positioned
west of the Great Lakes.
Trajectory class 1 (Fig. 9) extends west from the
study area in association with strong subsidence and
backing of winds as air parcels descend from 650 to 850
hPa over the 72-h period. The composite 1000-hPa
height field reveals a strong anticyclone located to the
west, with a departing surface cyclone to the east. Several of the 72-h backward air trajectories extend as far
west as the Pacific Ocean, suggesting that moisture for
many of these events may be of Pacific origin. A veering of the flow from southwest to northwest is evident
from the temporal trajectory analysis, consistent with
the temporal pattern for trajectory class 2. Trajectory
class 4 (Fig. 9) occurs in situations of weak southwest
flow at T ⫺ 12 that becomes northwest near event
maturation, and then increases and becomes a more
northwesterly trajectory as the event approaches T ⫹
12. The vertical trajectory plot clearly shows that air
parcels are lifted from the surface in this case, which
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FIG. 7. Trajectory class by month.
stands in contrast to the pronounced subsidence evident in trajectory class 2. The composite 1000-hPa
height pattern indicates a cyclone to the east and southeast off the North Carolina–South Carolina coast,
which is evidently far enough away to preclude synoptic-scale lift, but close enough to result in northwest
flow at 850 hPa. It is important to recognize that the
more westerly trajectories characterized by classes 1
and 4 are infrequent, together representing just 16.3%
of the sample.
c. Composite snowfall totals
Snowfall totals exhibit substantial variability among
the different trajectory classes and across the study area
(Fig. 10). The high country (region 8), central and
northern plateau (regions 12 and 13, respectively), and
highest elevations (region 14) received the greatest
amounts of snowfall across the different trajectories.
The lower-elevation regions, and particularly those on
the leeward slopes [southern, central, and northern
foothills (regions 3, 9, and 11, respectively)], received
no measurable snowfall across most of the trajectories.
These results are consistent with the climatology of orographic precipitation and those reported by Perry and
Konrad (2006), in which the greatest snowfall totals for
NWFS events occur along the windward slopes and at
higher elevations. The lowest composite mean snowfall
totals are found in association with trajectory classes 1
and 2 across all snow regions, whereas the highest
snowfall totals generally are found with trajectory
classes 3.2, 4, and, in some cases, 5. The low composite
snowfall totals in association with trajectory class 1, in
particular, may be tied to the strong subsidence along a
westerly trajectory. Trajectory class 4 stands out as producing the highest snowfall totals across all but one of
the snow regions, suggesting the presence of more
abundant moisture and/or higher relative humidity.
The southern, central, and northern foothills (regions
3, 9, and 11, respectively) exhibit the lowest composite
mean snowfall totals across most of the trajectory
classes, although the totals are substantially higher for
trajectory classes 4 and 5. These results should be
viewed with caution, however, as synoptic-scale lift in
association with a coastal low and broad upper trough
apparently extended just far enough to the west to
bring significant snowfall to these areas in a small number of events (3 out of the 16 in the sample). Interestingly, some of the lowest composite mean snowfall totals across the different trajectory classes are also found
in the New River Valley of southwestern Virginia (region 10). Even though the mean elevation is relatively
high (686 m), this snow region is heavily shadowed in
periods of northwest flow by the higher terrain of the
northern plateau (region 13) to the west and northwest.
Therefore, it is rare for accumulating snowfall to occur
in the absence of synoptic-scale ascent in periods of
low-level northwest flow. A general latitudinal gradient
in the composite mean snowfall, with the exception of
the New River Valley (region 10), is evident across the
different trajectory classes as northern snow regions at
the same elevation and exposure display higher totals
[e.g., southern plateau (region 6) versus central plateau
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FIG. 8. (left) Composite trajectories at T ⫺ 12, T0, and T ⫹ 12 and (right) 1000-hPa height pattern at T0 for trajectory classes (top)
2, (middle) 3.1, and (bottom) 3.2.
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FIG. 9. (left) Composite trajectories at T ⫺ 12, T0, and T ⫹ 12 and (right) 1000-hPa height pattern at T0 for trajectory classes (top)
2 and (bottom) 4.
(region 12)]. This latitudinal gradient likely continues
northward to the central Appalachian Mountains of
northern West Virginia, western Maryland, and Pennsylvania.
d. Composite sounding and synoptic fields
Results from the sounding analysis and the NCEP–
NCAR reanalysis data provide some additional insight
into the observed snowfall patterns by trajectory class
(Table 3). The lower snowfall totals observed in trajectory classes 1 and 2 in particular are associated with a
“thinner” moist layer, lower values of relative humidity
and moisture at 850 hPa, and higher values of cold
advection. Previous work (e.g., Auer and White 1982)
has demonstrated the importance of a deep moist layer
in producing heavy orographic snowfall in the western
United States. Perry and Konrad (2004) also found that
the heavy NWFS events in the southern Appalachians
were tied to higher values of lower-tropospheric relative humidity, so the results of this study are consistent
with previous work. The higher values of 850-hPa cold
advection for trajectory classes 1 and 2 also imply stronger subsidence and, therefore, a less favorable synoptic
environment for snowfall in the lower troposphere. The
500-hPa vorticity, though still positive (cyclonic), is
lower in trajectory classes 1 and 2, while the remaining
trajectory classes are tied to higher absolute values of
500-hPa cyclonic vorticity.
Several of the sounding and synoptic variables in association with the higher snowfall totals of trajectory
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PERRY ET AL.
FIG. 10. Composite mean snowfall totals by trajectory class and snow region.
class 3.2 are quite distinctive from the other trajectory
classes. Temperatures were in the dendritic temperature range for only approximately one-third (38%) of
the events, and the mean temperature of the moist layer
was also considerably greater than the other trajectory
classes. In addition, the 850-hPa mixing ratio was ex-
ceptionally high (2.66 g kg⫺1), as were 500-hPa relative
humidity, precipitable water, 1000–500-hPa mean relative humidity, and 850-hPa ␪e. The warmth of the moist
layer is particularly striking, since previous work has
demonstrated that ice crystal growth is most rapid and
efficient in the dendritic temperature range (Ryan et al.
TABLE 3. Composite sounding and synoptic values for each trajectory class.
Field
Top of moist layer
Dendritic temperature range
Depth of moist layer
Avg temp of moist layer
Lapse rate above moist layer
1000-hPa height
500-hPa height
850-hPa mixing ratio
850-hPa relative humidity
850-hPa wind direction
850-hPa wind speed
850-hPa temperature
850-hPa thermal advection
1000–500-hPa mean relative
humidity
700-hPa vertical velocity
850–500-hPa temperature change
Precipitable water
500-hPa relative humidity
850-hPa ␪e
500-hPa vorticity
Trajectory 1 Trajectory 2 Trajectory 3.1 Trajectory 3.2 Trajectory 3.3 Trajectory 4 Trajectory 5
n ⫽ 13
n ⫽ 40
n ⫽ 48
n⫽8
n ⫽ 17
n ⫽ 11
n ⫽ 18
703
0.54
203
⫺9.3
⫺4.0
142
5453
1.86
70.4
306
13.2
⫺8.1
⫺8.9
56.1
688
0.65
140
⫺11.5
⫺3.7
146
5392
1.72
73.1
300
14.2
⫺9.8
⫺7.0
54.4
689
0.69
183
⫺11.7
⫺4.5
135
5373
1.75
77.9
306
13.8
⫺9.9
⫺6.2
55.7
604
0.38
238
⫺6.6
⫺5.4
114
5459
2.66
82.5
309
14.3
⫺5.5
⫺2.7
64.8
634
0.65
237
⫺8.6
⫺6.2
110
5405
2.28
81.5
301
14.5
⫺6.9
⫺5.2
60.7
590
0.64
272
⫺8.5
⫺5.8
122
5437
2.45
82.0
309
8.7
⫺6.3
⫺2.9
62.7
584
0.5
188
⫺8.3
⫺4.6
117
5413
2.38
82.8
296
12.7
⫺7.4
⫺3.6
66.0
11.7
14.9
0.28
27.3
9.4
2.6
20.0
17.4
0.22
29.4
7.3
4.4
23.6
17.9
0.22
28.7
7.2
5.2
16.4
17.3
0.37
37.4
14.3
4.5
16.3
19.2
0.30
34.2
11.8
6.0
17.4
19.3
0.33
30.8
12.9
4.4
13.9
18.3
0.33
35.8
11.6
4.1
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WEATHER AND FORECASTING
VOLUME 22
FIG. 11. Composite mean and maximum snowfall totals between trajectory class 2 (no GLC)
and trajectory class 3 (GLC) by snow region. The star symbol denotes significance at p ⬍ 0.05.
1976; Pruppacher and Klett 1997; Fukuta and Takahashi 1999); furthermore, Auer and White (1982) have
tied heavy snowfall to moist-layer temperatures within
the dendritic growth range. These results indicate that
heavier NWFS can occur—albeit infrequently—with
moist-layer temperatures outside of the dendritic
growth range. However, a deep moist layer with exceptionally high values of lower-tropospheric moisture appears to be needed to make up for the reduced precipitation efficiency.
Trajectory class 4 is also associated with a considerably thicker moist layer, much higher values of humidity and moisture at 850 and 500 hPa, and lower values
of cold advection when compared with the remaining
trajectory classes. Interestingly, however, the 850-hPa
wind speed for trajectory class 4 is more than 4 m s⫺1
below the speeds for the other classes. This reduction in
850-hPa wind speed is associated with a less pronounced pressure gradient, apparently due to a weaker
surface anticyclone to the west (Fig. 9). The weak flow
is evident in the composite trajectory plots (Fig. 8), with
the 72-h backward trajectories extending only slightly
west of the Mississippi River. In this trajectory class,
which once again occurs relatively infrequently (n ⫽ 16,
or 8.4% of the sample) compared with the others, abundant and deep low-level moisture is lifted orographically, resulting in heavier snowfall. Therefore, significant NWFS can also result across the region when high
values of relative humidity and moisture throughout
the lower troposphere are lifted, albeit weakly compared with other NWFS events.
e. Influence of the Great Lakes
Comparison of the northwesterly trajectories of class
2 (no GLC) with classes 3.1, 3.2, and 3.3 (GLC) reveals
that trajectories with a GLC are associated with higher
composite mean and maximum snowfall totals across
windward slopes and higher elevations of the study
area, with the influence the greatest in the high country
and northern plateau (regions 8 and 13, respectively)
(Fig. 11). In both of these regions, the composite mean
and maximum snowfall totals are significantly greater
( p ⬍ 0.05) for events with a GLC, with the composite
maximum snowfall 1.8–2.1 cm greater (150%–135%).
Modest increases are noted in several of the other snow
regions, particularly those with portions along the
windward slopes. At the lower elevations and along the
leeward slopes, very little if any snowfall occurs with
either trajectory, highlighting the importance of topographic controls on the spatial patterns of snowfall for
events with a northwesterly trajectory.
The increase in snowfall totals in portions of the
study area is tied to higher values of moisture and humidity, particularly in the low levels (Table 4). The difference is most pronounced and statistically significant
( p ⬍ 0.05) with the sounding variables of thickness of
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PERRY ET AL.
TABLE 4. Composite sounding and synoptic field values for trajectory class 2 vs class 3.
Variable
Trajectory 2
Trajectory 3
Absolute diff
T score
Thickness of moist layer
Precipitable water
500-hPa vorticity
850-hPa relative humidity
1000-hPa height
1000–500-hPa mean relative humidity
850-hPa mixing ratio
850-hPa thermal advection
Lapse rate above moist layer
850-hPa wind direction
850-hPa ␪e
850-hPa temperature
Avg temp of moist layer
700-hPa vertical velocity
Top of moist layer
850-hPa wind speed
500-hPa height
Dendritic temperature range
140
0.22
4.44
73.1
146
54.4
1.7
⫺7.0
⫺3.7
300.0
7.4
⫺9.8
⫺11.5
20.0
688
14.2
5392
65
202
0.26
5.35
79.3
127
57.9
2.0
⫺5.6
⫺5.0
305.0
8.9
⫺8.8
⫺10.4
21.1
667
14.0
5390
64
61.61
0.04
0.89
6.14
⫺19.59
3.43
0.26
1.46
⫺1.34
5.02
1.54
0.99
1.14
1.11
⫺21
⫺0.18
⫺2
⫺1
2.61
2.34
2.34
2.32
⫺2.06
1.96
1.87
1.8
⫺1.76
1.45
1.23
1.22
1.04
0.55
⫺0.53
⫺0.26
⫺0.13
⫺0.07
the moist layer and lapse rate above the moist layer.
The synoptic variables of precipitable water, 850-hPa
relative humidity, 1000–500-hPa mean relative humidity, and 850-hPa mixing ratio also display significant ( p
⬍ 0.05) differences, suggesting a more moist and saturated lower troposphere for events with a GLC. Significant differences in 500-hPa relative vorticity are also
evident. Events without a GLC typically are accompanied by lower values of cyclonic vorticity, whereas
events with a GLC display higher values of cyclonic
vorticity. In GLC events, a vorticity maximum at 500
hPa rotating through the base of the 500-hPa trough
may help to raise the height of the low-level capping
inversion, as suggested by Lackmann (2001) and Niziol
(1989). Our sample, however, was restricted to events
in which subsidence was observed at 700 hPa (i.e., no
approaching troughs to provide synoptic-scale ascent).
Interestingly, no significant differences in 700-hPa vertical velocity, 850-hPa temperature, or percent of
events in the dendritic temperature range were noted.
The composite plots of 850-hPa relative humidity and
1000–500-hPa mean relative humidity for trajectory
classes 2 and 3 (Fig. 12) further highlight the significant
differences in lower-tropospheric relative humidity between the two trajectory classes. In both classes, the
highest values of relative humidity are generally found
to the northeast, but a distinct ridge of higher values
extends southwest toward the Ohio Valley in association with trajectory class 3. The greatest mean differences between the two fields for both trajectory classes
are located over western North and South Carolina, as
higher values of lower-tropospheric relative humidity in
these areas in particular are tied to trajectories with a
GLC. These composite plots also show that lowertropospheric relative humidity is higher across a large
area downwind of the Great Lakes for trajectories with
a GLC.
Interestingly, although higher values of lowertropospheric moisture and humidity across the southern Appalachians occur in association with trajectory
class 3, significant (at p ⬍ 0.05) increases in composite
mean and maximum snowfall only occur in the high
country and northern plateau (regions 8 and 13). The
mean elevation in these two snow regions is considerably higher than the others, with the exception of the
highest elevations (snow region 14 or areas ⬎1220 m).
Therefore, the increase in snowfall totals appears to be
somewhat elevation dependent, suggesting the important role orographic processes play in extracting lowertropospheric moisture. The greater regional topographic relief along the windward slopes of the high
country and northern plateau translates into a longer
period of orographic lifting. In addition, in a conditionally unstable lower troposphere, the orographic ascent
along the windward higher-elevation slopes may provide sufficient forcing for convection to develop, further extracting lower-tropospheric moisture. Even
though an increase in the composite mean and maximum snowfall totals is evident for events with a GLC in
the highest elevations (region 14), it is not as great as
that seen in the high country and northern plateau (regions 8 and 13, respectively). This may be related to the
more efficient scavenging of low-level moisture at the
highest elevations in trajectory class 2 (e.g., no GLC)
when compared with stations at slightly lower elevations. No significant topographic features are present
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WEATHER AND FORECASTING
VOLUME 22
FIG. 12. Composite plots of (left) 850-hPa relative humidity and (right) 1000–500-hPa mean relative humidity for
trajectory classes (top) 2 and (middle) 3, and (bottom) the mean difference between classes 3 and 2.
APRIL 2007
PERRY ET AL.
349
FIG. 13. Composite trajectories and interpolated snowfall totals for events with a GLC in which (a) NW NC received considerably
more than SE WV and (b) SE WV received more than NW NC.
upwind of the study area in association with northwesterly trajectories, so it is safe to assume that upstream
moisture robbing snow associated with topographic
forcing is not connected to the differences in snowfall
totals.
Additional findings related to the influence of the
Great Lakes indicate that no significant difference ( p ⬍
0.05) exists in the composite mean and maximum snowfall totals between events with percent ice cover in the
top quartile (ⱖ25%) and those in the bottom three
quartiles (⬍25%). This finding may be explained by the
very small number of events with a GLC in which the
percent ice cover was greater than 75% (n ⫽ 3 or 3.3%)
or even 50% (n ⫽ 10 or 11.1%). In these cases, much of
the ice cover existed on Lake Superior and the northern
portions of Lake Michigan, leading to only a small effect on the sensible and latent heat fluxes for trajectories crossing the southern end of Lake Michigan. Although extensive ice cover greater than 75% greatly
limits the sensible and latent heat fluxes, the small
sample size (n ⫽ 3) limits our ability to draw definitive
conclusions.
f. Comparative air trajectories
The spatial patterns of snowfall across the southern
Appalachians in association with different trajectories
are best illustrated by comparing air trajectories for two
samples of events: (a) ones in which the highest totals
occurred in the high country (region 8) and (b) ones in
which the highest totals occurred in the northern plateau (region 13). These regions were chosen for comparative purposes because of the significant increase ( p
⬍ 0.05) in the composite mean and maximum snowfall
totals for events with a northwesterly trajectory and a
GLC. When considerably more snow fell in the high
country (top quartile of the snow region, 8:13 ratio), air
trajectories for this region appeared to coincide approximately with the long axis of Lake Michigan (Fig.
13), whereas farther north, air trajectories were less
favorable for lake interaction (e.g., across lower Michigan). Snowfall was observed along the windward slopes
and higher elevations throughout the study area, with
the greatest amounts centered over the high country.
For events in which the highest totals occurred in the
northern plateau (bottom quartile of the snow region,
8:13 ratio), air trajectories were most favorable for lake
interaction in that respective snow region (Fig. 13). To
the south, only trace amounts of snow fell in the higher
elevations—with no snowfall occurring elsewhere—in
association with air trajectories well west of the western
Great Lakes.
Measures of lower-tropospheric relative humidity
represent some of the most significant differences in
synoptic fields between the bottom quartile and top
quartile of the ratio of mean snowfall between the two
snow regions (Table 5). These same fields also showed
the greatest differences between trajectory class 2 (no
GLC) and trajectory classes 3.1, 3.2, and 3.3 (GLC),
and are consistent with a moistening of the lower troposphere as a result of the latent heat fluxes from the
lake surfaces. Air trajectories, however, certainly are
not the only factor influencing the spatial patterns of
snowfall in these cases, as indicated by the significant
differences in 1000- and 500-hPa heights, 500-hPa vorticity, and 850-hPa thermal advection. In particular,
NWFS events in the bottom quartile (e.g., no snow in
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WEATHER AND FORECASTING
VOLUME 22
TABLE 5. Synoptic field values over the high country (region 8) for events with a GLC in the bottom vs top quartile of the mean
snowfall ratio between the high country and northern plateau (region 13).
1000–500-hPa mean relative humidity
500-hPa height
500-hPa vorticity
850-hPa thermal advection
1000-hPa height
850-hPa relative humidity
850-hPa temperature
850-hPa ␪e
500-hPa relative humidity
700-hPa vertical velocity
850-hPa mixing ratio
Bottom quartile
Top quartile
Absolute diff
T score
45.4
5517
3.03
⫺8.0
176
63.0
⫺5.5
12.3
22.5
27.0
1.92
57.9
5395
5.43
⫺4.6
131
76.0
⫺9.0
8.6
29.8
21.1
1.86
12.5
⫺122
2.41
3.4
⫺45
13.1
⫺3.5
⫺3.8
7.3
⫺5.9
⫺0.06
4.08
⫺3.81
3.4
3.18
⫺2.99
2.89
⫺2.46
⫺1.98
1.93
⫺1.51
⫺0.31
the high country or region 8) of events with a GLC are
tied to higher 500- and 1000-hPa heights, and lower
values of 500-hPa vorticity. In these events, not only is
the lower troposphere much drier, but the 500-hPa
trough is displaced farther to the north. In addition,
cold advection at 850 hPa is significantly greater, suggesting stronger subsidence in the lower troposphere.
Therefore, the characteristics of the air mass being advected into the high country (region 8) are not conducive for snowfall development, and may be partly explained by an air trajectory that is less favorable for
interaction with the southern portion of Lake Michigan.
4. Summary and conclusions
NWFS events occur frequently in the southern Appalachians during the period from late autumn through
spring and contribute substantially to annual snowfall
along windward slopes at higher elevations. Additionally, they are often tied to synoptic-scale subsidence
and a relatively shallow moist layer present beneath a
capping inversion below 700 hPa. Forcing is primarily
of an orographic and convectional nature, thereby presenting a considerable forecast challenge. Previous
work (e.g., Johnson 1987; Kocin and Ucellini 1990;
Schmidlin 1992) has suggested that antecedent upstream air trajectories with a GLC may enhance snowfall in portions of the Appalachian Mountains, particularly immediately downwind of Lake Erie in the northern mountains of West Virginia. Therefore, this paper
developed an antecedent trajectory classification
scheme for a sample of 191 NWFS events during the
period 1975–2000. Results of a detailed sounding analysis were paired with NCEP–NCAR reanalysis data to
further describe the vertical profiles and synoptic characteristics associated with each trajectory class, with
particular interest in the differences between non-GLC
and GLC trajectory classes.
NWFS events in the southern Appalachians display
considerable variability in 72-h backward air trajectories. In fact, over a quarter of the sample of NWFS
events analyzed in this study do not exhibit northwesterly trajectories. The remaining events do exhibit
northwesterly trajectories, with almost half (47.1%) of
all NWFS events in the sample interacting with portions
of the Great Lakes region for a minimum of 6 h during
the analyzed 72-h backward air trajectories. Trajectories passing over the western Great Lakes of Superior
and Michigan are most prevalent, with the southern
section of Lake Michigan seeing the highest frequency.
Trajectories with higher composite mean and maximum
snowfall totals are characterized by air parcels lifted
from near the surface in weak to moderate cold advection, a deep moist layer, and high values of 850-hPa
relative humidity. In contrast, trajectories with lighter
totals are tied more strongly to subsidence, stronger
cold advection, a thinner moist layer, and considerably
lower values of 850-hPa relative humidity.
Northwesterly trajectories with a GLC are also associated with higher values of moisture and humidity in
the lower troposphere and enhanced snowfall totals
along windward slopes and at higher elevations. This is
particularly the case when comparisons are made between northwesterly trajectories, or when trajectory
class 2 is compared with trajectory classes 3.1, 3.2, and
3.3. An increase in the composite mean and maximum
snowfall totals is evident across the southern Appalachians, with significant ( p ⬍ 0.05) differences for the
high country (region 8 or northwestern North Carolina)
and northern plateau (region 13 or southeastern West
Virginia). This increase in snowfall totals is also associated with higher values of lower-tropospheric moisture and relative humidity, suggesting that the Great
APRIL 2007
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PERRY ET AL.
Lakes may act to enhance snowfall totals by destabilizing and moistening the lower troposphere considerable
distances downwind. The exact air trajectories in association with events that display a GLC also appear to
influence the spatial patterns of snowfall across the
southern Appalachians. In particular, the heaviest
snowfall occurs in the high country when trajectories
extend southeastward from the long axis of Lake Michigan into this respective snow region, whereas the heaviest snowfall shifts to the northern plateau when upstream antecedent trajectories with a Great Lakes interaction are limited to extreme northern portions of
the study area.
Although this study has found that air trajectories
with a GLC aid in enhancing snowfall totals under certain circumstances, it remains unclear how important
air trajectory is in differentiating between light and
heavy NWFS. In particular, further investigation is
needed to evaluate the importance of duration of the
air trajectory–lake interaction. The trajectory classification scheme presented in this study evaluated antecedent upstream air trajectories at event maturation,
which is only a snapshot in time. Over the course of a
2–3-day NWFS event, a wide range of trajectories is
evident, signifying that a trajectory without a GLC at
the event maturation may develop one toward the end
of the event (e.g., trajectory class 4 at T ⫹ 12). Additionally, the actual source of the low-level moisture associated with NWFS remains poorly understood. In
some cases, moisture may be partly of Great Lakes
origin. This study has focused solely on those NWFS
events in which synoptic-scale subsidence was observed
at event maturation, and we therefore intentionally excluded those NWFS events with synoptic-scale ascent.
A question remains, therefore, as to the degree of enhancement, if any, that may occur in NWFS events with
synoptic-scale ascent and a GLC. The relatively coarse
(⬃2.5°) spatial resolution of the NCEP–NCAR reanalysis data used in the calculation of the synoptic
fields may also be a limitation in the study, particularly
in resolving moisture and instability plumes that may
extend downwind of the individual Great Lakes and
into the southern Appalachians.
Further research incorporating the higher-resolution
North American Regional Reanalysis dataset might
prove beneficial in resolving some of the mesoscale features associated with NWFS, such as the role of 500-hPa
vorticity maxima in lifting the height of the low-level
capping inversion. Additionally, case study analyses
and high-resolution modeling of selected NWFS events
could also provide valuable insight into the mesoscale
orographic and convectional processes associated with
NWFS events.
Acknowledgments. Peter Robinson, Tom Whitmore,
Walt Martin, and Laurence Lee provided helpful guidance in the development of this study. Blair Holloway,
Gary Lackmann, Kermit Keeter, Steve Keighton, and
Michael Mayfield provided beneficial feedback on the
preliminary results and suggestions for future research.
Last, Jim Young offered valuable cartographic expertise.
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