12-hour Forecast valid 00 UTC Day 2 (Hour 24)

Hypothetical skewt plot of a sounding for precipitation type forecasting

Task 2: Analyze this model forecast sounding to forecast what type of precipitation will be occurring 12 hours later (00 UTC Day 2). (To assist with this question and others that follow, you can open the interactive skew-T with this sounding).

Is any part of the modified sounding above freezing?
(Choose the best answer, then click Done.)
a) Yes
b) No

The correct answer is a) yes. The 800 hPa temperature in the forecast sounding warmed by 2°C to yield a 850 hPa temperature of +1.5°C.

Assuming that the modified sounding corresponds to the model forecasts for 00 UTC on Day 2 for warm advection aloft and that the model forecasts are reasonably correct, which of the following factors are most useful for forecasting precipitation type?
(Choose all that apply, then click Done.)
a) Depth of above freezing (T>0°C) layer aloft
b) Depth of subfreezing (T<0°C) near the surface
c) Depth of the mixed layer
d) Height of the lifting condensation level
e) Height of the convective condensation level
f) Temperatures in the warm (T>0°C) layer aloft

The correct answers are a) Depth of above freezing (T>0°C) layer, b) Depth of the subfreezing (T<0°C) near the surface and f) Temperatures in the warm (T>0°C) layer aloft. The depth of the warm (T>0°C) layer aloft will determine how easily a snowflake can melt as it falls through that layer. Temperatures in the warm layer are also important, since higher temperatures will enable snowflakes to melt more easily. The depth of the subfreezing layer near the surface will determine the precipitation particles' ability to refreeze before reaching the surface.

Based on the modified sounding, what type of precipitation would you expect at the surface at 00 UTC on Day 2?
(Choose the best answer, then click Done.)
a) Rain
b) Snow
c) Ice pellets (sleet)
d) Freezing rain
e) Hail
Animation showing effects of temperature profile on precipitation type The correct answer is c) Ice pellets (sleet). The warm, above freezing layer is about 60 hPa (780-840 hPa) or about 2000 feet deep, and has temperatures that peak at 2°C around 800 hPa. This is sufficient to melt at least part, if not all of the snowflakes. However, the particle then falls through a 160 hPa (840-1000 hPa) deep, subfreezing layer near the surface. This corresponds to a layer depth of approximately 5000 feet, which is sufficiently deep to allow for the precipitation particles to completely refreeze into ice pellets.

Note also that the 1000-500 hPa thickness is 5391 m, just below the critical value of 5400 m, which would seem to indicate that snow should be observed at the surface. This shows that thickness must be used with care and that the detailed vertical sounding profile should always be examined to come up with complete and accurate diagnosis and/or prediction of precipitation type.
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