Forecasting Snow
A selected chronology
 George (1960)
 Goree and Younkin (1966)
 Younkin and Browne (1970)
 Cook (1980)
 Beckman (1987)
 Auer (1987)
 Morrison (1989)
 Chaston (1989)
o Chaston, P. R., 1989: The Magic Chart for forecasting snow amounts. Nat. Wea. Dig.,
14, 20-22.
 Garcia (1994)
 Johnston (1995)
 Wetzle and Martin (2001)
Locating the Area of Maximum Snowfall
taken largely from George (1960)
The 0ºC isotherm at 850 mb is used as the basic defining line for the snow area. This isotherm
should be carefully checked, using all data at 850 mb (either in real-time or as a check on model
performance). It should then be checked against the surface map, keeping in mind the following
points:
1. In areas of precipitation, stations reporting snow should be on the cold side of the 0ºC
isotherm at 850 mb; stations reporting mixed types will be very close to this isotherm.
2. In areas of no precipitation, the 0ºC isotherm at 850 mb will roughly parallel the 32ºF
isotherm at the surface. In cloudy areas, the separation will be small, and in clear areas larger.
At the 850 mb level the 0ºC wet bulb temperature should be sketched in, particularly in the area
where precipitation may be anticipated in the next 12 to 24 hours. This line will also serve as the first
approximation of the future position of the 0ºC isotherm at 850 mb. Here, the reasoning holds that,
if the atmosphere is not saturated, snow falling into it from above will be subject to evaporation and
lower the wet bulb temperature to 0ºC.
At the 850 mb level the -5ºC dewpoint line, and at 700 mb level the -10ºC dewpoint line are used as
the basic defining lines for the area of maximum snowfall. The area at 850 mb that lies within
the overlap of the 0ºC isotherm and the -5ºC dewpoint line is the first approximation of the
maximum snowfall area. All stations within this area have temperatures less than 0ºC and
temperature dewpoint spreads of 5ºC or less. This area is then further refined by superimposing the
sketched -10ºC dewpoint line at 700 mb upon the area. Now the final area is defined by the 0ºC
isotherm and the overlapping minimum dewpoint lines from both levels. This final area
becomes the one where moderate or heavy snow will be reported.
The area of maximum snowfall is in motion and must be forecast. The first basic rule for moving
the area is that it maintains the same relative position to the other synoptic features of the 850 mb
level and surface chart. While numerical weather prediction models may not do such a good job
with forecasting the location, extent, or amount of snow, they are comparatively quite good at
predicting thermodynamic values such as geopotential height and temperatures. Although there are
many older methods for forecasting the motion and future location of the 850 mb isotherm, model
output is generally quite reliable for this task.
Using the above information, you may draw a map using the listed criteria to show where the area of
maximum snowfall will occur. Make your maps neat and concise when drawing in surface pressure
systems, dewpoint lines, wet bulb isotherms. Cross hatch the area where you think the area of
maximum snowfall will occur.
Reference
George, J.J., 1960: Weather Forecasting for Aeronautics, Academic Press, New York, pp. 405-406.
The Cook Method
modified from Wetzle web page
The Cook Method for forecasting snowfall amounts relates the strength of the warm air advection at
200mb to the amount of snowfall in a region. Although there is a relationship between the two
under the right circumstances, warm advection so far aloft does NOT cause the snowfall.
The Cook Method can be used to forecast:
1. Location of snow

Where lower-troposphere temperatures support snowfall, the maximum snowfall between t=0
and t=24 occurs near the coldest 200mb temperature downstream of the warmest temperature
at t=0.

Snow bands are almost always parallel to the 200mb geopotential height contours at t=0 or cross
at a small angle from higher to lower values. The width of the snow band is frequently about the
same as the distance between adjacent height contours (120gpm) on the 200mb chart.

The southwest end of the heavy snowband is just downstream from “maximum warm
advection” (warmest 200mb air) and the northeast end should be near the eastern side of the
area of coldest 200mb temperature, or limited to 14 degrees latitude downstream from warmest
200mb air.
2. Amount of snow

Average snow in inches from t=0 (present) to t=24 is 1/2 of the maximum “indicated warm
advection” (in degrees C) at 200mb. Warm advection is defined as the difference between
the warmest temperature at 200mb and the coldest temperature along the height
contours within a distance of 15 degrees latitude (840nm) upstream of the forecast area.
o
If warm advection extends less than 6 degrees latitude (360nm) upstream from the
forecast area, the precipitation is of short duration.
o
If cold advection is observed within 8 degrees latitude upstream of the forecast area at
700mb, divide the temperature difference by 4 instead of 2 to get the snowfall.
Reference
Cook, B.J.,1980: A snow index using 200 mb warm advection. Nat. Wea. Dig., 5, 29-40.