Models
The Nested Grid Model (NGM) was developed from one of the first numerical
weather models of the late 1970s and the early 1980s, the Limited Fine Mesh (LFM)
model. The NGM model is very outdated, but is still run twice daily at 00UTC and
12UTC.
The four panels in the chart are as follows:
Upper-Left: 500mb Heights and Vorticity
Lower-Left: 700mb Heights and Relative Humidity
Upper-Right: Surface Pressure and 1000-500mb Thickness
Lower-Right: 6hr Precipitation, 700mb Omega, and 850mb 0°C isotherm
Weather Research and Forecasting Model (WRF-NMM), released for
public use in 2000, is an eighty-four (84) hour model that is used for research and
operational forecasting purposes. Another name for this model is the North American
Model (NAM). The WRF Model is issued four times daily at 00UTC, 06UTC, 12UTC,
and 18UTC.
The four panels in the chart above are as follows:
Upper-Left: 500mb Heights and Vorticity
Lower-Left: 700mb Heights and Relative Humidity
Upper-Right: Surface Pressure and 1000-500mb Thickness
Lower-Right: 6hr Precipitation, 700mb Omega, and 850mb 0°C isotherm
What each panel tells you:
500mb Heights and Vorticity Panel
The 500mb Heights and Vorticity Panel can be used to roughly describe the steering flows of the
atmosphere. These steering flows are about 20,000 feet up in the atmosphere (roughly halfway through the
atmosphere) and generally direct major weather systems across the country. The black contours are
isohypse (lines of constant height), and are contoured every 6 decameters (the values on the isohypse are in
decameters (dm), i.e. 570 would actually be 5700m). The height of the 500mb level is how far you would
have to go up in the atmosphere before the pressure drops to 500mb. Lower heights are usually found to
the north while higher heights are usually found off to the south.
The red and purple shading on the map indicate areas of vorticity in the atmosphere. Vorticity is a measure
of rotation throughout the atmosphere. Vorticity can either strengthen or weaken storm systems. Areas of
red shading on the panel indicate positive vorticity, while areas of purple shading indicate negative
vorticity. The darker the shading is, the stronger the vorticity. Areas of positive vorticity can strengthen
areas of low pressure, and are sometimes referred to as “upper-level disturbances” or “upper-level energy”.
The numbers on the map near areas of vorticity indicate the magnitude of the vorticity.
700mb Heights and Relative Humidity Panel
The 700mb Heights and Relative Humidity Panel is a very important panel used by meteorologists. The
black contours are isohypse at the 700mb level (roughly 10,000ft above ground level), and are contoured
every 3dm (the isohypse are in decameters, so 300dm is actually 3000m). The height of the 700mb level is
how far you would have to go up in the atmosphere before the pressure drops to 700mb. The green and
brown shading indicates the amount of relative humidity in the atmosphere at 700mb. (NOTE: This is NOT
the relative humidity found at the surface). The green shading on the map represents areas with 700mb RH
values greater than 70%. The dark green shading represents areas with RH values greater than 90%. Light
brown areas are regions where 700mb RH is less than 30%, with dark brown areas less than 10%. White
areas on the map indicate regions where the RH is between 30% and 70%.
Meteorologists use the 700mb RH panel to predict areas where clouds and precipitation may develop. For
more information on how to use the 700mb RH to forecast, please visit the Forecasting Page here.
Surface Pressure and 1000-500mb Thickness Panel
The Surface Pressure and 1000-500mb Thickness Panel is very useful when predicting where storm systems
may move and the type of precipitation that may fall from them. The black lines on this panel are isobars,
(lines of constant pressure), and are contoured every four millibars (mb). Some of the isobars are labeled
with numbers that usually range from 960mb (a strong low-pressure system), to 1050mb (a strong high
pressure system). Low pressure systems are denoted by a red “L” while high pressure systems are denoted
by a blue “H”.
The red and green dashed lines on the map are 1000-500mb thickness contours. The 1000-500mb thickness
is the average depth of a column of air from 1000mb (roughly the surface) to 500mb (about halfway up
through the atmosphere). The thickness can also be inferred to be the average temperature of a column of
air from 1000-500mb. The red dashed thickness lines are primary thickness contours (contoured every
30dm, the value on the thickness contours is in decameters), while the green dashed lines are secondary
thickness contours (contoured every 6dm). The three primary thickness lines on the panel above are 510dm,
540dm, and 570dm. Thicknesses lower than 510dm usually represent a dry, arctic airmass found in Canada
or along the northern portion of the United States during winter, while thicknesses higher than 570dm
usually represent a warm and moist tropical airmass from the tropics. Thickness values can also be very
useful in forecasting the different types of precipitation associated with major winter storms. More on
forecasting precipitation types using thickness values can be found here.
6hr Precipitation, 700mb Omega, and 850mb 0°C Isotherm Panel
The last panel on the map is basically a forecast produced by the model of how much precipitation will fall
across any given region during a six hour period of time ending at the time at the bottom of the panel. (For
more information on how to read the time at the bottom of each panel, click here.) Precipitation amounts are
measured in hundredths of an inch, so 10 on the scale would actually be 0.10” of precipitation. The black
solid and dashed contours on the panel display the 700mb Omega. Omega is used to describe where there
are rising and sinking motions in the atmosphere. Areas with negative numbers and dashed contours
indicate areas of rising motion in the atmosphere (which may lead to clouds and precipitation), while areas
with positive numbers and soild contours indicate areas of sinking motion in the atmosphere (fair weather).
The red dashed contour across the map is the 850mb 0°C isotherm. The 850mb level is roughly 5000ft up
from the surface, and the 0°C isotherm can be used to determine which areas are below or above freezing at
that level. Areas to the north of the isotherm are below freezing, while areas to the south of the isotherm are
above freezing. The 850 0°C isotherm can also be useful in predicting the various precipitation types
associated with storm systems during the winter season.