Forecast Exercise:

To answer this question and others that follow, you first need to open the Interactive Skew-T with this sounding.

1. Modify the sounding, based on forecast 18 UTC surface conditions of T=29°C and Td= 17°C.

2. Based on the modified sounding, determine the following parameters listed below, then click Done.

CAPE: J/kg      Correct: about 300 J/kg

K index (KI):       Correct: 18

Convective Temperature (Tc): °C      Correct: 29°C

LCL: hPa      Correct: 820Pa

Melting Level: hPa      Correct: 588 hPa

Precipitable Water: in      Correct: 1.3 in.

Mid-tropospheric Wind Shear 700-300 hPa: kt      Correct: 20 kt

Compare your answers with the correct answers above.

3. Based on the above parameters and your overall examination of the sounding, evaluate the flash flood potential, then click Done.

Feedback:
There is a moderate potential for flash flooding with this modified sounding. The convective temperature (29°C) has already been reached, and the CAPE value (250-300 J/kg) is relatively small as is typical for flash flood soundings that are nearly moist adiabatic. The K index of 18 is below typical flash flood threshold, but this is reflective of a drier layer at 700 hPa, whereas the sounding is moist above and below that level. The precipitable water of 1.3 in is above the climatological norm and there is a moderately deep warm layer (melting level to LCL) of about 3 km depth. Midtropospheric wind shear is also relatively weak. While not all parameters exceed typical flash flood thresholds, other factors (e.g. the synoptic/mesoscale situation with a stationary front over the threat region) would indicate a moderate threat for flash flood producing thunderstorms.

What Happened:
Thunderstorms initiated along a stationary front over Central PA on the afternoon of 17 June, producing 3-5 inches of rain in a two-hour period in and around State College.
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