The determining factor in the form of precipitation in this study was found to be the distribution of temperature and moisture between the surface and the 700-hPa level at the time of the beginning of precipitation. The median level of 850-hPa was studied in conjunction with the precipitation area and the 32F

Figure 4-21.-Map showing 1,000- to 500-hPa thickness values for which probability of rain or frozen precipitation is equal (after Wagner).

isotherm sketched on the surface chart. This method presents an objective and practical method by which the forecaster can make a decision on whether the precipitation in winter will be rain, snow, freezing rain, sleet, or some combination of these.

The following objective techniques can be applied to the land areas south of 50 north latitude and east of a line drawn through Williston, North Dakota; Rapid City, South Dakota; Goodland, Kansas; and Amarillo, Texas.

The area outlined by the 0C isotherm at 850-hPa and the 32F isotherm on the surface chart, when superimposed upon the precipitation area, generally separates the forms of precipitation. Most of the pure rain was found on the warm side of the 32F isotherm, and most of the pure snow on the cold side of 0C isotherm, with intermediate types falling generally within the enclosed area between these two isotherms. It was also observed that in a large majority of situations, evaporation and condensation was a sizable factor, both at 850-hPa and at the surface level in its affect upon temperature. With this in mind, the wet-bulb temperature was selected for investigation because of its conservative properties with respect to evaporation and condensation, and also because of its ease of computation directly from the temperature and dewpoint. The surface chart is used for computations of the 1,000-hPa level because the surface chart approximates the 1,000-hPa level for most stations during a snow situation; therefore, little error is introduced. IT MUST BE REMEMBERED THAT ALL PREDICTIONS ARE BASED ON FORECAST VALUES.

MOVEMENT OF THE 850-hPa 0C ISOTHERM. A reasonably good approximation for forecasting the 0C isotherm at the 850-hPa level can be made subjectively by use of a combination of extrapolation and advection, considerations of synoptic developments, and the rules listed in the following paragraphs. (See figure 4-22, views (A) and (B), for typical warm and cold air advection patterns at 850-hPa.)

The following rules for the movement of the 24-hour, 850-hPa level temperature change areas have been devised

. Maximum cooling takes place between the 850-hPa contour trough and the 850-hPa isotherm ridge east of the trough.

. Maximum warming takes place between the 850-hPa contour ridge and the 850-hPa isotherm trough east of the contour ridge.

. Changes are slight with ill-defined isotherm/contour patterns.

. Usually, little change occurs when isotherms and contours are in phase at the 850-hPa level.

. The temperature falls at the 850-hPa level tend to replace height falls at the 700-hPa level in an average of 24 hours. Conversely, temperature rises replace height rises.

Figure 4-22.-Typical cold and warm air advection patterns at 850-hPa. (A) Cold; (B) Warm.

. With filling troughs or northeastward moving lows, despite northwest flow behind the trough, 850-hPa level isotherms are seldom displaced southward, but follow the trough toward the east or northeast

. Always predict temperature falls immediately following a trough passage.

. Do not forecast temperature rises of more than 10 to 2F in areas of light or sparse precipitation in the forward areas of the trough. If the area of precipitation is widespread and moderate or heavy, forecast no temperature rise.

. With eastward moving systems under normal winter conditions (trough at the 700-hPa level moving east about 10 per day), a distance of 400 nautical miles to the west is a good point to locate the temperature to be expcted at the forecasting point 24 hours later. A good 850-hPa temperature advection speed seems to be about 75 percent of the 700-hPa trough displacement.

The following is step-by-step procedures for moving the 850-hPa 0C isotherm:

1. Extrapolate the movement of the thermal ridge and troughs for 12 and 24 hours. If poorly defined, this step may be omitted. The amplitude of the thermal wave may be increased or decreased subjectively if, during the past 12 hours, there has been a corresponding increase or decrease in the height of the contours at 500-hPa.

2. The thermal wave patterns will maintain their approximate relative position with the 850-hPa level contour troughs and ridges. Therefore, the 12- and 24-hour prognostic positions of the contour troughs and ridges should be made, and the extrapolated positions of the thermal points checked against this contour prognosis. Adjustments of these thermal points should be made.

3. Select points on the 0C isotherm that lie between the thermal ridge and trough as follows: one or two points in the apparent warm advection area, and one or two points in the apparent cold advection area. Apply the following rules to these selected points.

. Warm advection area. If the points lie in a near saturated or precipitation area, they will remain practically stationary with respect to the contour trough. If the points lie in a nonsaturated area, but one that is expected to become saturated or to lie in precipitation area, then it will remain stationary or move upwind slightly to approximate y the prognostic position of the 0C wet-bulb. If the point does not fall in the above two categories, it will be advected with about 50 percent of the wind component normal to the isotherm. Note in all three cases above, the movement is related to the contour pattern.

. Cold Advection area. Advect the point with approximately 75 to 80 percent of the wind component normal to it.

In the case of a slow moving, closed low at the 850-hPa level, the 0C isotherm will move eastward with respect to the closed low as cold air is advected around the low.

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