Tropospheric moisture is critical to the forecasting process: (1) No amount of rising air will produce precipitation unless moisture is present. The more moisture that is present, the higher the potential for precipitation if uplift mechanisms are in place. In order for thermodynamic thunderstorms to occur, there must be moisture in the low levels of the troposphere (between the surface and 700 millibars). A dewpoint of 55 degrees in used as a critical value for having enough moisture to sustain a thunderstorm. Thunderstorms generally do not occur when the surface dewpoint is less than 55 degrees F (exception: thunderstorms/snow/sleet due to elevated convection). This is why thunderstorms are not common well behind drylines or cold fronts. Most thunderstorms occur in the warm sector of a mid-latitude cyclone or due to strong low level instability (summer time air mass thunderstorms). Warm air has the capacity to contain more water vapor. Higher concentrations of water vapor release abundant latent heat. (2) Precipitation amounts are shown on the synoptic scale forecast models. Part of the formula for determining precipitation amounts is the quantity of moisture in the air. The quantity of moisture is found by summing the actual mixing ratios from the surface to the upper levels of the troposphere. Keep in mind the synoptic models can not handle mesoscale precipitation well. The synoptic models depict precipitation over a large region but can not predict mesoscale rainfall gradients or precipitation amounts at a point location with a high degree of accuracy especially when precipitation is convective. (3) Cloudiness without precipitation will result when enough uplift is present to saturate the troposphere, but not enough uplift to condense out raindrops that can reach the surface. When a forecast model has positive UVV over the forecast region with a 70% or higher 850-500 average RH, cloudiness becomes likely. If RH is lower, UVV will need to be higher to develop clouds. If UVV is 3 or greater, and RH is 70% or greater, expect synoptic scale cloud development. The PBL moisture is very important. If the PBL is dry, UVV will have to increase for precipitation to reach the surface compared to a moist PBL. (4) The dewpoint can be used to forecast temperatures. In a barotropic troposphere (no fronts around), the low temperature tends to be close to the evening dewpoint (especially when dewpoints are above 60 degrees F) on a night with clear skies and wind less than 15 mph. A high dewpoint can limit afternoon warming due to evaporative cooling from the soil, vegetation, etc. (5) Moisture convergence is the advection of moisture into a fixed region. Moisture convergence results in increasing low level dewpoints, the potential for heavier rain if rain occurs, and a higher value of Theta-E. Moisture advection can cause rainfall totals to be higher than the Precipitable Water value given on a thermodynamic diagram because moisture is continually transported toward a fixed location. Moisture advection is common along cold fronts or any other low-level boundary that causes general convergence. Moisture advection is common when a mid-latitude cyclones transports moisture from a warm water body within the warm sector of the mid-latitude cyclone. (6) The drying power of the air can be estimated using the RH or dewpoint depression. A low RH in warm air or a large dewpoint depression in warm air (they both go hand in hand) will produce a large drying potential. Soil and vegetation will dry quickly in this situation. Precipitation falling into dry air will lose some or all of its mass to evaporation. |