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.