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
l 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 0°C isotherm will move eastward
with respect to the closed low as cold air is advected
around the low.
MOVEMENT OF THE 850-hPa 0°C
temperature can be forecast by the above procedures and
rules. Remember that the wet-bulb temperature is
dependent upon dewpoint as well as the temperature.
The dewpoint will be advected with the winds at nearly
the full velocity, whereas the temperature under
nonsaturated conditions moves slower. The following
observations with respect to the 0°C wet-bulb isotherm
after saturation is reached may help:
. The 0°C wet-bulb isotherm does not move far
offshore in the Gulf and the Atlantic because of upward
vertical motion in the cold air over the warmer water.
. If the 0°C wet-bulb isotherm lies in a ribbon of
closely packed isotherms, movement is slow.
. Extrapolation works well on troughs and ridges.
After the forecast of the surface and 850-hPa level
temperature and dewpoint values are made, you are
ready to convert these values to their respective
wet-bulb temperature. The following procedures are
l Use figure 4-23, views (A) and (B), to compute
the wet-bulb temperatures for the 1,000- and 850-hPa
levels, respectively. (The surface chart may be used for
the 1,000-hPa level.) Admittedly, the wet-bulb
temperatures at just these two levels do not give a
complete picture of the actual distribution of moisture
and temperature, and error is introduced when values
are changing rapidly, but these are values the forecaster
can work with and predict with reasonable accuracy.
. Refer to figure 4-24. From the surface wet-bulb
temperature at the bottom, go up vertically until you
intersect the computed 850-hPa level wet-bulb
temperature to the left. This intersection indicates the
form of precipitation that can be expected. A necessary
assumption for use of this graph is that the wet-bulb
temperatures at these two levels can be predicted with
Known factors affecting the
wet-bulb temperature at any particular station should be
carefully considered before entering the graph. Some
of the known factors are elevation, proximity to warm
bodies of water, known layers of warm air above or
below the 850-hPa level, etc. Area A on the graph
calls for a rain forecast, area B for a freezing rain
forecast, and area C for a snow forecast. Area D is
not so clear cut because it is an overlap portion of the
graph; however, wet snow or rain and snow mixed
predominate in this area. Sleet occurring by itself for
more than 1 or 2 hours is rare, and should be forecast
FORECASTING THE AREA OF
The intent of this section is to introduce the patterns
associated with maximum snowfall and to present
techniques for predicting the areas where snowstorms
are likely to appear.
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 40°N, 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