MODULE 4.2F
SHORT RANGE FORECASTING OF
CLOUD, PRECIPITATION AND
RESTRICTIONS TO VISIBILITY
Notes on
Quantitative Precipitation Forecasting
Table of Contents
INTRODUCTION .......................................................................................................................................................1
SNOWFALL AMOUNTS RELATED TO OBSERVED VISIBILITIES ..............................................................1
FAVOURABLE LOCATIONS FOR MODERATE TO HEAVY SNOW .............................................................4
A) SURFACE ..............................................................................................................................................................4
B) 850 MB ..................................................................................................................................................................4
C) 700 MB ..................................................................................................................................................................4
D) 500 MB .................................................................................................................................................................4
E) 300 MB ..................................................................................................................................................................5
F) 200 MB ..................................................................................................................................................................5
HEAVY RAINFALL FORECASTING TECHNIQUES .........................................................................................5
NOTES: HEAVY RAINFALL FORECASTING ..................................................................................................................5
ELEVATED THUNDERSTORMS ASSOCIATED WITH HEAVY RAINFALL IN THE MIDWEST .............................................6
LEMO TECHNIQUE .................................................................................................................................................7
TECHNIQUE................................................................................................................................................................7
FORECASTING LIMITATIONS ......................................................................................................................................8
MAGIC CHART .........................................................................................................................................................8
TECHNIQUE................................................................................................................................................................8
FORECASTING LIMITATIONS ......................................................................................................................................8
MIXING RATIO TO SNOWFALL RELATIONSHIP ...........................................................................................8
RATE OF PRECIPITATION ASSOCIATED WITH DIVERGENCE .................................................................9
FORECAST LIMITATIONS ..........................................................................................................................................11
RATE OF PRECIPITATION ASSOCIATED WITH DEW POINTS.................................................................12
FOR A 0 TO 12 HOUR FORECAST WHEN THE 850 MB TROUGH IS WEST OF THE 100TH MERIDIAN: ..............................12
FOR A 12 TO 24 HOUR FORECAST WHEN THE 850 MB TROUGH IS WEST OF THE 100TH MERIDIAN: ............................13
FOR A 0 TO 12 HOUR FORECAST WHEN THE 850 MB TROUGH IS EAST OF THE 100TH MERIDIAN: ...............................14
FOR A 12 TO 24 HOUR FORECAST WHEN THE 850 MB TROUGH IS EAST OF THE 100TH MERIDIAN: .............................15
FORECAST LIMITATIONS ..........................................................................................................................................16
REFERENCES ..........................................................................................................................................................16
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INTRODUCTION
Since the introduction of numerical weather prediction models in the 1950’s, models and model
output statistics have been the primary guidance tools used by meteorologists to predict the
timing and intensity of future precipitation events. Significant advances in NWP over the last 50
years have resulted in improved quantitative precipitation forecasts (QPF; Olson et al. 1995),
but, unfortunately, these improving QPF skills have lagged behind the skill developed in
forecasting mass fields. Mass field forecasts have improved at a greater rate because these fields
can be derived from first principles, and have benefited from advances in model resolution,
analysis, intialization techniques and physical parameterizations. In comparison, QPF fields are
parameterized from mass field forecasts, and have had more difficulties incorporating
modulating diabatic effects, latent and sensible heat fluxes, and smaller-scale atmospheric
circulations. It is for this reason that over the years many studies have been performed in an
attempt to correlate precipitation timing, location and intensity with other meteorological fields.
The following material is a comprehensive list of techniques which may be used in support of, or
in lieu of model output.
Snowfall Amounts Related to Observed Visibilities
The hourly rate of accumulation of snowfall amount is important in short range forecasts of total
accumulation. Two important factors in diagnosing accumulation rate are the horizontal
visibility and the wet bulb potential temperature of the air mass. The following two figures
illustrate the relationship of these factors with the accumulation rate. A handy rule of thumb to
remember, in case these graphs are not nearby or the θw is not known, is that the hourly
snowfall accumulation in centimetres is roughly equal to the inverse of the visibility in
miles. For example, one half mile visibility will give an approximate accumulation of 2
centimetres an hour.
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Figure 1. Hourly Snow Accumulation. From T L. Richards: An Approach to Forecasting
Snowfall Amount. TEC 177, 19 January, 1954.
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Figure 2. θw curves should be representative of the airmass aloft ahead of the warm front.
If airmass snow (circulation flurries) use the θw of the airmass. Assume no melting of the
snow. Visibility restriction is due entirely to snow. If fog or haze is present, a reduction of
as much as 20% would be required. This graph was modified by D. Day, Maritimes
Weather Centre. Original unknown, but probably from Richards’ work (see previous
caption).
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FAVOURABLE LOCATIONS FOR MODERATE TO HEAVY SNOW
A) Surface
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Around 2 to 2.5 degrees latitude to the left of the track of the low (150-240 nm).
Approximately 5 degrees latitude ahead of the low.
As long as the low is deepening, heavy snow will occur.
When the low begins the fill, the heavy snow usually ends.
When the cold surface anticyclone is the north or northwest, snowfall amounts enhanced by
confluent mid-level flow.
Optimum surface temperature -3 to 0°C.
B) 850 mb
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Around 1.5 degrees latitude to the left of the track of the low (60-240 nm).
When cooling occurs in the rear quadrant of the low during early stages of development,
heavy snow occurs.
Heavy snows occur more frequently with lows that move towards the northeast.
Heavy snows less rare with lows that move towards the southeast.
Heavy snow lies north of the 0°C isotherm (-3°C for the east coast).
-2°C to -8°C for moderate snow.
> 5°C of warm air advection moving into the area of interest.
C) 700 mb
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Heavy snow band between -6°C and -8°C temperature, and south of the -10°C dew point
line.
Heavy snow favours an area under the 1000-700 mb thickness ridge between 2830 and 2855
metre thickness lines.
Heavy snow band along path and just to the left of the low.
North of the 700 mb closed contour.
D) 500 mb
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Favoured location is about 6 to 7 degrees latitude downstream of the vorticity maximum.
2 to 3 degrees left of the track of the closed low/strong vorticity maximum.
Slightly downstream from where the curvature changes from cyclonic to anticyclonic.
Heavy snow favours an area under the 1000-500 mb thickness ridge between 3830 and 2855
meter thickness lines.
When the low or trough deepens, look for significant height falls greater than 90 m.
When the average lowest 500 mb temperature within 3 degrees latitude of the vorticity
maximum is near -30°C.
Heavy snow band between -20 and -25°C (-23°C is the best)
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If warming occurs at 500 mb, heavy snow band occurs left of 500 mb low. Otherwise heavy
snow band tends to occur left of surface low track.
Heavy snow begins at 500 mb ridge line. Ends at either the trough or the inflection point
between the trough and ridge.
E) 300 mb
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Heavy snow along right entrance and left exit regions of a jet maximum.
Heavy snow in area between coupled jets.
Look for strongest Q-vector convergence.
Heavy snow occurs with a deep or deepening long wave.
F) 200 mb
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Look for stratospheric warming.
Heavy snow occurs just to the north of the 164 height line.
Heavy Rainfall Forecasting Techniques
Notes: Heavy Rainfall Forecasting
Theodore W. Funk
Central Region Tech. Attachment (1993)
-No one method can be utilized by itself without consideration of all other parameters.
-Parameters/Techniques - Examples of pattern recognition; Synoptic, Mesohigh, cyclonic
circulation, and SHARS - Subtle heavy Rainfall Signatures.
-Moisture availability - high ambient and/or inflow moisture must be present and maintained.
Precipitable water (PW) values greater than one inch and at or above normal. K index values
(ambient or inflow) 30 to 40 or more. 850 mb and surface dew points (ambient or inflow) near or
especially above 12oC and 17oC (60 oF) respectively (warm season)
-Low-level Inflow and Convergence - Moderate to strong moist surface to 850 mb inflow (10 kts
or more at the surface, 25 kts or more at 850 mb). Such persistent southerly inflow converging
toward a quasi-stationary low-level frontal or outflow boundary can signify the potential for
rainfall amounts approaching or exceeding 5 inches in a 24 hour period.
-Jet stream structure - favored locations:1)right entrance, 2) left exit region, 3) exit region of a jet
streak approaching the top of a ridge axis, 4) area of upper level divergence, 5) anticyclonic
shear axis to the right of a jet core. coupled upper-level jet streaks (convection within the right
entrance of the polar jet and left exit of the subtropical jet simultaneously). Upper-level/lowerlevel jet coupling through direct and indirect circulations.
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-Warm air advection - Associated heavy precipitation occurs most often at night and in the early
morning. Associated with the exit region of the low-level wind maximum. Heavy precipitation
potential exists if model forecast thickness or 850 mb temperatures hold steady or sink
southward in the face of southerly warm air advection, since the warm air is being lifted instead
of actually warming the air at a particular level.
-Theta-E - low level (especially 850mb) ridge axis coincident with upward motion and unstable
air is a prime location for convective development. In warm air advection/overrunning situations,
a region of positive advection of theta-e by the low-level wind is where overrunning convection
will be found. Tight gradients of theta-e (baroclinic situations) are favored for heavy
precipitation.
-Thickness difluence - Implies low-level convergence or upper-level divergence, but most likely
a combination of the two, and, therefore, is an area conducive for convective development.
-Thickness saturation - When the ambient or inflow PW's represent at least 70 percent saturation
of the ambient 1000-500 mb thickness. Sufficient moisture must be present to cause saturation in
a diffluent thickness region, or else convection likely will form farther north (downwind) within
tighter thickness packing, as in overrunning situations.
-Preferred Thickness- Useful tool for determining the location of initial convection when forcing
mechanisms are weak.
-Rules of Thumb - Large volume convective rainfall tends to occur farther south or southeast
with time over the central U.S. if outflow boundaries from current or previous convection can
intercept moist southerly flow.
Heavy rainfall producing convection often develops within or along the upstream edge of a
vorticity minimum ridge axis at 500 mb. Watch for convection behind a weak short wave if
moist unstable inflow persists into a low-level boundary. The convection then is maintained by
low-level forcing. If a well-defined middle and high level tropical moisture connection exists in
water vapor imagery, rain potential is typically higher than normal. Inverted isobars signal the
possibility of heavy rainfall. Models are subject to “convective feedback” short waves, which
models induce through deep convection, strong vertical velocities, and latent heat release.
Models then generate subsequent precipitation (often bull’s eyes).
Elevated Thunderstorms Associated with Heavy Rainfall in The Midwest
J.T. Moore, S.M. Rochette, F.H. Glass, D.L. Ferry, & P.S. Market
Preprints 18th Conf. of Severe Local Storms, 1996 San Francisco, CA Amer. Meteor. Soc.
-Introduction - Colman (1990a,b) notes that elevated thunderstorms are isolated from surface
diabatic effects and occur above frontal surfaces. Elevated thunderstorms with attendant heavy
rainfall often fit the synoptic patterns described by Maddox et al. (1979) as the "frontal" or
"mesohigh" type flash flood scenarios.
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-Methodology - Augustine and Caracena (1994) showed that large, long-lived MCSs tend to
form overnight in the region where the LLJ encounters a frontogenetic region at 850 mb, north of
the surface boundary. Small MCSs were associated with a weaker front, if present, which was
not subject to frontogenetical forcing. Frontogenetical forcing typically enhances the direct
thermal circulation pattern and thus the subsequent convective activity. Glass et al. (1995) found
the favorable region of heavy convective rainfall as being north of a west-east surface boundary,
and south of the positive 850 mb theta-e advection maximum that is coupled with a southerly
LLJ. This region is especially favored if the surface boundary is quasi-stationary and the 850300 mb thickness field is diffluent. In this study, each of the seven cases formed in a region of
elevated convective instability (i.e., a region above the stable boundary layer where theta-e
decreases with height).
-Results - Moisture transport vectors, computed as the product of the vector wind and the mixing
ratio, clearly demonstrate that heavy rainfall producing MCSs are favored about 400 km
downstream and to the east of the maximum horizontal transport of moisture. The 500 mb
composite height and vorticity analysis reveal a weak short wave trough upstream from the MCS
initiation point with weak PVA.
-Conclusions - MCSs with elevated thunderstorms associated with heavy rainfall intiated...
1) About 200 km downstream from the LLJ maxima
2) In a region characterized by maximum strong theta-e advection and moisture convergence at
850 mb
3) About 400 km downstream and to the east of the maximum moisture transport vectors at 850
mb
4) In a region of anticyclonic curvature and downstream of a weak 500 mb short wave trough
5) In the right entrance region of the 200 mb ULJ, near the divergence maxima.
6) In a region characterized by a stable LI, a slightly unstable SI and having a relatively high
value of the KI and elevated convective instability between 500 and 850 mb
7) In a region of >60% mean surface to 500 mb relative humidity and a precipitable water >1.2
in.
LEMO TECHNIQUE
Maximum snowfall in inches = (Va - 10) x (30/S)
where Va is the absolute vorticity interpreted from the progs, and S is the estimated speed of the
vorticity maximum in knots.
Technique
1. Va is found by extending a straight line from the centre point of the forecast area of
maximum snowfall until it intersects the estimated path of the 500 mb vorticity max. This
point is then interpolated from your model of choice.
2. Estimate the speed of the vorticity maximum in knots then plug in the values.
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Forecasting Limitations
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Systems which move from the northwest to southeast overestimate snowfall by
Increase LEMO estimate for thundersnow situations.
Technique is limited by the accuracy of the model output.
25%.
Note: this procedure is best applicable when dealing with an upper-level feature as opposed to a
system which has considerable development on the surface.
MAGIC CHART
The Magic Chart uses 700 mb Net Vertical Displacement (NVD) and NGM 850 mb temperature
to predict inches of snowfall in a 12-hour period. This technique was developed for usage in
the United States and can be used by weather offices who have access to PC grids.
Technique
1. Calculate the NVD in mb for air that will arrive at the 700 mb level in 24 hours over the area
of interest.
2. Using the 12 or 24 h NGM 850 mb forecast fields, identify regions between -3 and -5°C.
The heaviest snowfall occurs where the greatest NVD coincides within the aforementioned
temperature range.
3. Snowfall amounts may be computed from the table below. The snowfall given is a forecast
for the 12-hour period between 12 and 24 hours after the initial forecast time. Note that the
snowfall amount in inches is the millibars of vertical displacement divided by 10.
NVD
20 to 40 mb
40 mb
60 mb
80 mb
100 mb
12-hr Snowfall
2 to 4 in (5 to 10 cm)
4 in (10 cm)
6 in (15 cm)
8 in (20 cm)
10 in (25 cm)
Forecasting Limitations
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Does not apply to mesoscale snowstorms, including topographic and lake effect.
Adequate moisture should be forecast or available.
Technique does not work well with very cold systems.
Technique only as good as model data.
Note: this procedure works best in scenarios where the forecaster is certain of a heavy snow
event.
MIXING RATIO TO SNOWFALL RELATIONSHIP
The following procedure is a condensed version modified from Garcia (1994). This table below
may be useful tool when trying to calculate convective snowfall totals.
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1. Identify the geographic area of concern for the expected snowfall.
2. Determine what isentropic surface (K) best intersects the 700-750 layer over the area of
concern.
3. The isentropic surface should be analyzed for pressure (every 50 mb) and for mixing ratios
(every g kg-1).
4. Calculated an average mixing ratio value over the area of concern along the chosen
isentropic surface. From the isentropic wind field approximate the mixing ratio to be
advected in during the next 12 hours. Average these two values to calculate an average
mixing ratio for the 12 hour period.
5. Determine if there is sufficient dynamic lift over the area of concern for part of the 12 hour
period.
6. Empirical observation has shown that a 2 to 1 relationship exists between maximum snowfall
amount (in inches) and the average mixing ratio.
Snowscale (12 hour period)
1-2 g kg-1
2-3 g kg-1
3-4 g kg-1
4-5 g kg-1
5-6 g kg-1
6-7 g kg-1
2-4 (5-10)
4-6 (10-15)
6-8 (15-20)
8-10 (20-25)
10-12 (25-31)
12-14 (31-36)
inches (centimetres)
inches (centimetres)
inches (centimetres)
inches (centimetres)
inches (centimetres)
inches (centimetres)
Note: The mixing ratios shown on this scale should be the average mixing ratios between the
700-750 mb layer. The snowfall amounts should be considered to be the maximum possible 12
hour snowfall.
RATE OF PRECIPITATION ASSOCIATED WITH DIVERGENCE
1. For the station you are forecasting, measure the 3 hourly pressure tendency and station
pressure (both in millibars) for four stations, each 150 nautical miles from your station (one
station directly north, south, east and west).
2. Measure the pressure tendency (3 hourly) and pressure at your forecast station.
3. Plug your measurements into the equation below.
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Divergence =
- (dp1 + dp2 + dp3 + dp4 - 4dpo)
(p1 + p2 + p3 + p4 - 4po + f2bd2)
dp1 = 3 hourly pressure change at station directly north of forecast station
dp2 = 3 hourly pressure change at station directly south of forecast station
dp3 = 3 hourly pressure change at station directly east of forecast station
dp4 = 3 hourly pressure change at station directly west of forecast station
p1 = actual pressure (mbs) at station directly north of forecast station
p2 = actual pressure (mbs) at station directly south of forecast station
p3 = actual pressure (mbs) at station directly east of forecast station
p4 = actual pressure (mbs) at station directly west of forecast station
po = actual pressure (mbs) at forecast station
f = coriolis parameter
b = surface air density
d = distance from the four stations to the forecast station
Note: The values for the term f2bd2 are given in the table below. These values are valid when d
= 150 nautical miles. Also note that the negative sign in front of the equation corresponds to
convergence.
Latitude
40°N
45°N
50°N
55°N
60°N
65°N
70°N
Value for f2bd2
8.3
10.1
11.9
13.6
15.2
16.6
17.9
4. Next, determine the 850 mb temperature, or the surface dew point in the moistest air near the
surface warm front.
5. Enter the values obtained from the equation, and from number 4 above, into the graph below.
Note: This graph gives the 6 hourly precipitation rate. To get an idea of the hourly rate divide
the obtained 6 hour rate by six. For precipitation events lasting longer than six hours, one has to
recalculate the parameters again.
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Forecast Limitations
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Do not use this method to forecast thunderstorm rain amounts.
This method is not accurate if terrain plays a major role in precipitation accumulations.
This method is not accurate if the surface low is expected to pass directly over the forecast
station.
For best results, the surface low should be southwest of the forecast station and moving east,
or south of the forecast station and moving northeast.
After precipitation has begun, or after surface pressures have fallen rapidly, it is best to
recalculate the parameters again.
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RATE OF PRECIPITATION ASSOCIATED WITH DEW POINTS
For a 0 to 12 hour forecast when the 850 mb trough is West of the 100th Meridian:
1. Determine the area where one expects precipitation to occur during the next 12 hours.
2. Draw a line indicating the maximum warm air advection through the forecasted precipitation
area.
3. Obtain the 850 mb dew point where the line drawn through the maximum 850 mb warm air
advection first intersects the forecasted precipitation area.
4. Measure the surface dew point directly below where you obtain the 850 mb dew point.
5. Measure the distance (in miles) from the 700 mb trough line to the centre of the forecasted
precipitation area.
6. Enter the values obtained from the above steps into the graph below. This graph will give
one an indication of the expected precipitation accumulation during the next 12 hours.
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For a 12 to 24 hour forecast when the 850 mb trough is West of the 100th
Meridian:
1. Change 12 hours in number 1 above to read 24 hours.
2. Use the same dew point criteria as described above.
3. Same as number 5 above, but this time measure from the 700 mb trough line to the centre of
the 24 hour forecasted precipitation area.
4. Enter the values obtained in the graph below. The graph will give you the expected
precipitation accumulation during the next 24 hours.
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For a 0 to 12 hour forecast when the 850 mb trough is East of the 100th Meridian:
1. Use the same dew point criteria as described earlier.
2. Determine the area where one expects precipitation to occur during the next 12 hours.
3. As before, draw a line indicating the maximum warm air advection at 850 mb through the
forecasted precipitation area.
4. Measure the difference in surface pressure (mbs) between the point where the line (indicating
the maximum 850 mb advection) first intersects the forecasted precipitation area, to the point
where it exits the forecasted precipitation area.
5. Enter the values into the graph below. This graph will give one an indication of the expected
precipitation accumulation during the next 12 hours.
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For a 12 to 24 hour forecast when the 850 mb trough is East of the 100th Meridian:
1. Determine the area where one expects precipitation to occur during the next 12 to 24 hours.
2. Forecast the maximum warm air advection in °C at 850 mb through the forecasted
precipitation area. If precipitation is already occuring, measure the maximum warm air
advection into this region.
3. Measure the maximum 850 mb cold air advection in °C into the precipitation area.
4. Enter the values obtained into the graph below. This graph will give one the maximum
expected precipitation amount for the next 12 to 24 hours.
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Forecast Limitations
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Developed for use in the United States; graphs may not work as well in Canada.
May not accurately capture thunderstorm rain amounts.
References
Augustine, John A., Fernando Caracena, 1994: Lower-Tropospheric Precursors to Nocturnal
MCS Development over the Central United States. Weather and Forecasting: 9, pp. 116–135.
Colman, Bradley.R., 1990a: Thunderstorms above Frontal Surfaces in Environments without
Positive CAPE. Part I: A Climatology. Monthly Weather Review: 118, pp. 1103–1122.
Colman, Bradley R., 1990b: Thunderstorms above Frontal Surfaces in Environments without
Positive CAPE. Part II: Organization and Instability Mechanisms. Monthly Weather Review:
118, pp. 1123–1144.
Funk, Theodore W.: Central Region Tech. Attachment (1993)
Garcia, C. Jr., 1994: Forecasting snowfall using mixing ratios on an isentropic surface. NOAA
Tech. Memo., NWS CR-105, U.S. Dept. Of Commerce/NOAA/NWS, 31 pp.
Glass, F.H., D.L. Ferry, J.T. Moore, and S.M. Nolan, 1995: Characteristics of Heavy Convective
Rainfall Events across the Mid-Mississippi Valley during the Warm Season: Meteorological
Conditions and a Conceptual Model. Preprints, 14th Conf. on Weather Analysis and
Forecasting, Dallas, TX, Amer. Meteor. Soc., 34-41.
Maddox, R. A., C. F. Chappell and L. R. Hoxit, 1979: Synoptic and mesoscale aspects of flash
flood events. Bull. Amer. Meteor. Soc., 60, 115-123.
Moore, J.T., S.M. Rochette, F.H. Glass, D.L. Ferry, & P.S. Market: Elevated Thunderstorms
Associated with Heavy Rainfall in the Midwest. Preprints 18th Conf. of Severe Local Storms,
1996 San Francisco, CA Amer. Meteor. Soc., 772-776.
Olson, David A., Norman W. Junker, Brian Korty, 1995: Evaluation of 33 Years of Quantitative
Precipitation Forecasting at the NMC. Weather and Forecasting: 10, pp. 498–511.
Richards, T. L.: An Approach to Forecasting Snowfall Amount. TEC 177, 19 January, 1954
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