The rainfall rate at a particular point is proportional to the magnitude of the vertical moisture flux. This means rising air should have substantial water vapor content and the ascent rate should be large. The atmosphere must be conditionally unstable and there must be a mechanism by which the warm, moist air will rise to its level of free convection.
Skew-T Parameters
- Instability:
- Temperature profile:
- At or very close to moist adiabatic, especially in the lower half of the troposphere
- Low-level moisture and precipitation efficiency:
- High values of K index (usually greater than 35)
- Deep warm cloud layer (from LCL to melting level greater than 4 km)
- Precipitable Water exceeding 150% of average (for most of the continental U.S., greater than 1.75 inches represents a water loaded sounding while less than 0.75 inches represents a fairly dry sounding)
- Wind profile:
- Low-level jet: winds at 850 hPa exceeding 25 kt and at 700 hPa exceeding 30 kt
- Weak wind shear in the mid-troposphere (and sometimes light and variable winds)