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Weather Analysis

A thorough study of the weather on the surface chart is made as the map is analyzed. It is now time to indicate the intensity and characteristics of the precipitation and other weather phenomena. This is done by carefully checking every plotted report. Shade and/or use the symbols shown in figure 7-4-4. Use an ap-propriate colored pencil and lightly shade areas, as needed. The symbols are done in a darker tone so they can easily be seen within the shading.

Steady precipitation is shaded in green. Shower areas are marked with shower symbols; thunderstorm areas, with thunderstorm symbols; and drizzle areas, with drizzle symbols, accord-ing to figure 7-4-4. The edges of altostratus shields and fog areas may be outlined in blue or yellow pencil respectively.

Figure 7-4-3.—Example of air-mass labeling.

Warm-front rain, or corresponding steady rain area in advance of an occluded front, is usually assumed in the absence of data to extend over a band of approximately 300 miles ahead of the front. This typical model may be, of course, modified by geographical factors or complicated frontal structure. A known boundary of solid altostratus will often indicate that the edge of the precipitation area is near. Also, the presence of "mid 7" clouds frequently shows that a rain area is approaching. The existence of this cloud type, or of an altostratus layer, may be the first indication of a front approaching a coastline. Pressure falls within a precipitation area may indicate warm-front type precipitation, the fall of pressure tending to be greatest where the rain-fall is heaviest.

If the band of precipitation is appreciably wider than 300 miles, a double-front structure may exist, or there may be a shallower-than-normal slope to the warm-front surface. Some-times there is an increase of slope well in advance of the surface position. If considered significant, the line along which the front begins to increase in slope may be shown as a warm front aloft. Where relatively dry air ascends over a warm front, precipitation is often lacking or may occur only some distance ahead of the surface position of the front.

Shower symbols are used to outline regions where showery conditions are occurring or have occurred. In the absence of data, they can be used to show expected shower conditions based on continuity or model.

In cold air masses, showers are prevalent where rapid warming caused by warmer water sur-faces takes place; for example, the Great Lakes in the winter; over mountainous country, such as the western slope of the Appalachians; and in general where the isobars are curved cyclonically and the cold air contains sufficient moisture. Look for drizzle where warm, moist air moves rapidly over a cooler surface, especially in the warm sector of a cyclone. It may occur in cold air, especially under anticyclonic or stagnant conditions if there is sufficient moisture and instability in the lower levels and a stable layer or inversion above the low clouds. The principal cloud or precipitation systems to look for in a typical extratropical cyclone are as follows:

1. Warm-front rain areas, and the pre-warm-front cirrus and altostratus preceding them.

2. Warm sector drizzle, low clouds, or fog.

Figure 7-4-4.—Weather symbols and shading schemes.

3. Warm sector showers; in many cyclones, most showers occur in the warm sector rather than along the cold front.

4. Cold-front (squall line) cumulonimbus and showers.

5. Post-cold-front cumulus and showers (usually only where flow is cyclonic or showers are favored by topographic features).

All systems are subject to modifications by geographical features, moisture content of the air, and other characteristics pertaining to the given cyclone.

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