. The 0C 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 0C 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 recommended
. 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 reasonable
accuracy. 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 with
caution.
