4. Forecast Applications

Convection » Heavy Precip./Flash Floods Example 3: Front Range of the Rocky Mountains in the U.S.

Flash floods also occur where cells regenerate and track along the mountains. The skew-T parameters do not need to exceed the thresholds used for flash floods at lower elevation because orographic lifting becomes very important. When forecasting in areas with significant terrain, examine the lower-tropospheric wind direction relative to the terrain to determine if moisture convergence and lifting will be focused in a small area.

Some skew-T parameters may not be applicable in mountains because the pressure levels at which they are calculated are below ground. For some parameters, calculations can be made at different pressure levels. For example, K index can be calculated using the temperature at 800 hPa instead of 850 hPa.

Sounding taken at Denver, CO, 0000 UTC, 29 Jul 1997

Skew-T Parameters:

  • Instability:
    • Very weak CAPE (<1000 J/kg)
  • Temperature profile:
  • Low-level moisture and precipitation efficiency:
    • K index = 34 (calculated from 800 hPa)
    • Deep warm cloud layer (> 4 km)
    • Precipitable water = 1.3 in (34 mm)
  • Wind profileWind profile:
    • Low-level jet: 600 - 500 hPa winds are > 20 kt, approximately 10 kt greater than the layers above and below
    • Weak wind shear between 500 and 350 hPa