12-hour Forecast valid 12 UTC Day 2 (Hour 36)

Hypothetical skewt plot of a sounding for precipitation type forecasting

Task 3: Analyze this model forecast sounding to forecast what type of precipitation will be occurring 12 hours later (12 UTC on Day 2).

Which of the following factors will be important in forecasting the type of precipitation for 12 UTC on Day 2?
(Choose all that apply, then click Done.)
a) Depth of the cold layer at mid levels
b) Depth of the warm layer (T>0°C) aloft
c) Depth of subfreezing layer near the surface
d) Temperature at the surface

The correct answers are b) Depth of the warm air (T>0°C) aloft, c) Depth of subfreezing layer near the surface, and d) temperature at the surface. As before, the depth of the warm, above-freezing layer near the surface will determine how easily snowflakes will melt as they fall through this layer. The depth of the subfreezing layer near the surface will determine how easily the precipitation, if melted, can refreeze before reaching the ground. Whether the surface temperature is above or below 0°C is also an important factor in determining the type of precipitation observed at the surface.

Given this forecast sounding, what type of precipitation do you expect to be observed at the surface at 12 UTC on Day 2?
(Choose the best answer, then click Done.)
a) Rain
b) Snow
c) Ice pellets (sleet)
d) Freezing rain
e) Hail
The correct answer is d) Freezing rain. The model forecast sounding shows a relatively deep layer of warm air (T>0°C) extending from 950 to 765 hPa, with temperatures peaking at 5°C at 850 hPa. This is a depth of 185 hPa or about 6,000 feet, sufficient to allow snowflakes to completely melt as they fall through the layer. The resulting raindrops then fall through a relatively shallow 50 hPa deep layer near the surface. This cold surface layer is only about 1500 feet deep, which is too shallow for the drops to refreeze before hitting the surface. However, the surface temperature of -0.5°C is indication that the supercooled water droplets will freeze on contact with the surface, hence the precipitation falls as freezing rain.

Note that the 1000-500 hPa thickness is 5437 m, which is above the critical rain/snow threshold of 5400 m. Again this points to the utility of detailed sounding analysis. While the thickness value indicates raindrops should be falling, the sounding indicates they are supercooled as they fall through the shallow subfreezing layer near the ground, and they will therefore freeze on contact with the surface.
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