00 UTC Day 1 (Hour 0)

Hypothetical skewt plot of a sounding for precipitation type forecasting

This sounding shows a relatively warm (T > 0°C) layer extending from the surface at 1000 hPa up to 733 hPa. A nearly isothermal layer exists between 950 and 800 hPa. Winds near the surface are from the NE, veering across the warm frontal zone to ESE at 850 hPa, and to SW above the isothermal layer. The surface temperature/dewpoint is +5.0°C / -10.3°C, yielding a wet-bulb temperature of 0°C and a relative humidity of 32%, and the sounding is unsaturated throughout. The 1000-500 hPa thickness is 5449 m.

Task 1: Predict what type of precipitation will be falling 12 hours from now. (To assist with this question and others that follow, you can open the Interactive Skew-T with this Sounding).

Assuming that the combined effects of temperature advection and vertical motion will have negligible impacts on the sounding, which of the following parameters might be useful in assessing precipitation type? (Choose all that apply, then click Done.)
a) 1000-500 hPa thickness
b) Level of free convection (LFC)
c) Showalter Stability Index
d) Wet-bulb temperature
e) Wet-bulb zero level

The correct answers are a)1000-500 hPa thickness, d) wet-bulb temperature, and e) wet-bulb zero level. Since the 00 UTC sounding is unsaturated, the wet-bulb temperature curve should give you a reasonable approximation of what the sounding will look like when it becomes saturated and precipitation is occurring. Thickness is directly related to the mean virtual temperature of a layer, in this case, the layer from 1000-500 hPa, and can be related empirically to precipitation type. Generally for continental locations near sea level, the critical 1000-500 hPa thickness value is 5400 m, i.e. rain is observed for values above 5400 m, snow at values below 5400 m. The wet-bulb zero level is important for identifying the freezing level assuming the atmosphere becomes saturated in association with the onset of precipitation.

Given the above information, what type of precipitation do you expect at the surface at 12 UTC?
(Choose the best answer, then click Done.)
a) Rain
b) Snow
c) Ice pellets (sleet)
d) Freezing rain
e) Hail
The correct answer is b) snow.
The wet-bulb temperature sounding from the 00 UTC sounding is at or below 0°C throughout the sounding. Therefore, precipitation that initiates as snow at mid levels (e.g. 600 hPa) will not undergo melting and will therefore fall as snow at the surface. In addition, saturating the sounding via evaporative cooling to the wet-bulb values lowers the 1000-500 hPa thickness to 5391 m, which is below the critical value of 5400 m, indicating that snow is more likely to be observed. The wet-bulb zero level is at the surface, so it stands to reason that saturation of the sounding will yield surface temperatures of 0°C.
Previous       Next