Figure 4-24.-Graph for delineating the form of precipitation.
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.
Locating the Area of Maximum Snowfall
The various parameters and characteristics that may
be of benefit in locating areas of maximum snowfall are
discussed in the following text.
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