WET-BULB ISOTHERM. The wet-bulb temperature can be forecast by the above procedures and rules. Remember that the wet-bulb temperature is dependent upon dewpoint as well as the temperature. The dewpoint will be advected with the winds at nearly the full velocity, whereas the temperature under nonsaturated conditions moves slower. The following observations with respect to the 0C wet-bulb isotherm after saturation is reached may help:

. The 0C wet-bulb isotherm does not move far offshore in the Gulf and the Atlantic because of upward vertical motion in the cold air over the warmer water.

. If the 0C wet-bulb isotherm lies in a ribbon of closely packed isotherms, movement is slow.

. Extrapolation works well on troughs and ridges. After the forecast of the surface and 850-hPa level temperature and dewpoint values are made, you are ready to convert these values to their respective wet-bulb temperature. The following procedures are recommended

. Use figure 4-23, views (A) and (B), to compute the wet-bulb temperatures for the 1,000- and 850-hPa levels, respectively. (The surface chart may be used for the 1,000-hPa level.) Admittedly, the wet-bulb temperatures at just these two levels do not give a complete picture of the actual distribution of moisture and temperature, and error is introduced when values are changing rapidly, but these are values the forecaster can work with and predict with reasonable accuracy.

. Refer to figure 4-24. From the surface wet-bulb temperature at the bottom, go up vertically until you intersect the computed 850-hPa level wet-bulb temperature to the left. This intersection indicates the form of precipitation that can be expected. A necessary assumption for use of this graph is that the wet-bulb temperatures at these two levels can be predicted with reasonable accuracy. Known factors affecting the wet-bulb temperature at any particular station should be carefully considered before entering the graph. Some of the known factors are elevation, proximity to warm bodies of water, known layers of warm air above or below the 850-hPa level, etc. Area "A" on the graph calls for a rain forecast, area "B" for a freezing rain forecast, and area "C" for a snow forecast. Area "D" is not so clear cut because it is an overlap portion of the graph; however, wet snow or rain and snow mixed predominate in this area. Sleet occurring by itself for more than 1 or 2 hours is rare, and should be forecast with caution.

FORECASTING THE AREA OF MAXIMUM SNOWFALL

The intent of this section is to introduce the patterns associated with maximum snowfall and to present techniques for predicting the areas where snowstorms are likely to appear.

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