There are four distinct types of synoptic patterns with associated maximum snow area.

BLIZZARD TYPE. The synoptic situation features an occluding low. In the majority of cases, the "wrapped around" high pressure and ridges are present. The track of the low is north of 40N, and its speed, which initially may be average or about 25 knots, decreases into the slow category during the occluding process. In practically all cases, a cold closed low at the 500-hPa level is present and captures the surface low in 24 to 36 hours.

The area of maximum snowfall lies to the left of the track. At any particular position, the area is located from due north to west of the low-pressure center. When this type occurs on the east coast with its large temperature contrast and high moisture availability, heavy snowfall may occur. The western edge of the maximum area is limited by the 700-hPa level trough, or low center, and the end of all snow occurs with the passage of the 500-hPa level trough or low center.

MAJOR STORM AND NONOCCLUDING LOWS. The synoptic situation consists of a nonoccluding wave-type low. The track of the low or wave is south of 40 latitude, and its speed is at least the average of 25 knots, often falling into the fast-moving category. The upper-air picture is one of fast-moving troughs, generally open, but on occasion could have a

Figure 4-23.-Graphs for computing wet-bulb temperatures. (A) Computation of 1,000-hPa level wet-bulb temperature; (B) computation of 850-hPa level wet-buib temperature.

minor closed center for one or two maps in the bottom of the trough. The area of maximum snowfaIl lies in the cold air to the left of the track and usually describes a narrow belt oriented east-west or northeast-southwest about 100 to 200 miles wide. At any position, the area is parallel to the warm front and north of the low-pressure center. Within the maximum area, the rate of snowfall is variable from one case to another, depending upon available moisture, amount of vertical shear, etc. However, it is not uncommon for heavy snow (1 inch

per hour or greater) to occur. You must remember that even though heavy snow occurs, the duration is short, This means that a location could lie in the maximum snow area only 4 to 8 hours; whereas, in the case of the blizzard type, it usually remains in the area in excess of 10 hours.

WARM ADVECTION TYPE. This type was separated from the other types because of the absence of an active low in the vicinity of the maximum snow area. A blocking high or ridge is located ahead of a sharp warm front. The overrunning warm air is a steady current from the south to southwest. The area of maximum snowfall is a narrow band parallel to the warm front and moves north or northeast. A rate of fall of moderate to heavy for a 6- to 12-hour duration may occur. The usual history is a transition to freezing rain, and then rain.

POST-COLD FRONTAL TYPE. The synoptic situation consists of a sharp cold front oriented nearly north-south in a deep trough. A minor wave may form on the front and rapidly travel to the north or northeast, Strong cold advection from the surface to the 850-hPa level is present west of the front. The troughs at the 700-hPa and 500-hPa levels are sharp and displaced to the west of the surface trough 200 to 300 miles. Ample moisture is available at the 850-hPa and 700-hPa levels. This type of heavy snow may occur once or twice per season.

The area of maximum snowfall is located between the 85MPa and 700-hPa troughs, where moisture at both levels is available, The rate of fall is moderate, although for a brief period of an hour or less, it may be heavy. The duration is short, on the order of 2 to 4 hours at anyone location. The area as a whole generates and dissipates in a 12- to 18-hour period. The normal history is one of a general area of light snow within the first 200 miles of a strong outbreak of cold air. After the cold air moves far enough south and the cold front becomes oriented more north to south and begins moving steadily eastward, the troughs aloft and moisture distribution reach an ideal state, and a maximum snow area appears. After 12 to 18 hours, the advection of dry air at the 700-hPa level decreases the rate or fall in the area, and soon thereafter, the area, as a whole, dissipates.

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