The various parameters and characteristics that may be of benefit in locating areas of maximum snowfall are discussed in the following text.

TEMPERATURE. The 0C (-3C east coast) isotherm at the 850-hPa level is used as a basis for snow-rain areas, This isotherm should be carefully analyzed by using all data at 850 hpa. It should then be checked against the surface chart. Keep in mind the following two points:

1. In areas of precipitation, locations reporting snow should lie on the cold side of the 0C (3C east coast) isotherm; for locations reporting mixed types of precipitation (e.g., rain and snow, sleet and snow), the 0C (3C) isotherm will lie very close to or through the location.

2. In areas of no precipitation, the 0C (3C east coast) isotherm will roughly parallel the 32F isotherm at the surface. In cloudy areas, the separation will be small, and in clear areas, the separation will be larger. At the 850-hPa level, the 0C wet-bulb temperature should be sketched in, particularly in the area where precipitation may be anticipated within the next 12 to 24 hours. This line will serve as the first approximation of the future position of the 0C isotherm.

MOISTURE. At the 850-hPa level, the 5C dewpoint line is used as the basic defining line; at the 700-hPa level, the 10C dewpoint line is used as the basic defining line. The area at 850 hpa that lies within the overlap of the 0C isotherm and the 5C dewpoint line is the first approximation of the maximum snowfall area. All locations within this area have temperatures less than 0C and spreads of 5C or less. This area is further refined by superimposing the sketched 10C dewpoint line at 700 hPa upon the area. Now the final area is defined by the 0C isotherm and the overlapped minimum dewpoint lines from both levels. This final area becomes the area where moderate or heavy snow will be reported, depending upon the particular synoptic situation. See figure 4-25,

MOVEMENT. The first basic rule for moving the area of maximum snowfall is that it maintains the same relative position to the other synoptic features of the 850-hPa level and surface charts. However, in order to forecast the expansion or contraction of the area, it is necessary to forecast the lines that define it. The 0C isotherm should be forecast according to the roles set forth in the section treating this particular phase. The moisture lines may be advected with the winds. The 0C isotherm should also be moved with rules stated previously in this chapter.

The area of maximum snowfall can be forecast for 12 hours with considerable accuracy, and for 24 hours with fair accuracy, provided a reasonable amount of care is exercised according to rules and subjective ideas mentioned previously.

LEARNING OBJECTIVES: Analyze synoptic features in determining temperature forecasts. Temperature ranks among the most important forecast elements. Temperatures are not only important in the planning and execution of operational exercises, but also are of keen interest to all of us in everyday life.

Many factors are involved in the forecasting of temperatures. These factors include air mass characteristics, frontal positions and movement, amount and type of cloudiness, season, nature and position of pressure systems, and local conditions.

Temperature, which is subject to marked changes from day to night, is not considered a conservative property of an air mass. Too, it does not always have a uniform lapse rate from the surface up through the atmosphere. This means that the surface air temperature will not be representative because of the existence of inversions, which may be a condition particularly prevalent at night. Usually, the noonday surface air temperature is fairly representative, Lets look at factors that cause temperature variations. These factors include insolation and terrestrial radiation, lapse rate, advection, vertical heat transport, and evaporation and condensation.

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