[--MAIN HOME--] [--ALL HABYHINTS--] [--FACEBOOK PAGE--] THE SOIL MOISTURE'S IMPACT ON WEATHER PREDICTION METEOROLOGIST JEFF HABY Soil moisture is important to forecasting. It effects both the temperature forecast and precipitation forecast. Let's first start with the temperature forecast. High soil moisture will produce high evaporation, especially if temperatures warm significantly during the day. This evaporation will produce evaporative cooling. Although the temperature warms during the day, the evaporation does prevent the temperature from getting as warm as it otherwise would have. A good example is comparing Mississippi to Arizona in the summer time (both states are on about the same latitude). Mississippi generally has a high soil moisture content and evapotranspiration while Arizona generally has dry soils and low values of evapotranspiration. High temperatures in southern Arizona average over 100 degrees in the summer while temperatures generally do not rise above 100 in Mississippi (unless a drought reduces soil moisture). The humidity does make it feel more uncomfortable outside though. It is hotter in Arizona, but the humidity in Mississippi can make it feel just as hot in the summer. High soil moisture values will tend to increase the dewpoint. This has a major consequence on forecasted lows. The overnight low under uniform weather conditions will not drop by more than a couple degrees below the evening dewpoint, especially if the dewpoint is above 60 F. Condensation (a warming process) occurs when the temperature tries to drop below the dewpoint at night. Therefore, high dewpoints limit the amount of overnight cooling. If dewpoint are low, such as when a continental high pressure is in place or a location is located in a dry climate (or dry weather pattern), the overnight low will be much cooler than the afternoon high. Since the dewpoint is low, the temperature can continue falling at night without condensation warming the air and limiting the cooling. Rule of thumb: If the dewpoint depression is large during the afternoon, there will be a large temperature range between the high and low temperature. Soil moisture is also important to precipitation forecasts. High soil moisture increases the likelihood of moisture convergence. A trigger mechanism such as a front or low pressure will not produce precipitation unless there is moisture in place to lift. Moist air rising has a much better chance of producing precipitation than dry air that is rising. High soil moisture continuously evaporates moisture into the air, which helps to supply low level moisture. The best combination is to have moist soils along with moisture being advected from a moisture source such as the Gulf of Mexico into a trigger mechanism. Droughts and Floods can produce a positive feedback loop that can continue the drought or flood. When the soils dry out, there is less moisture for fronts and other trigger mechanism to lift and therefore there is a continuation of less rainfall. When floods occur, the supply of evaporating moisture to the atmosphere is continuous and there is always moisture in place for a trigger mechanism to lift. It takes a dramatic shift in the weather pattern sometimes to end a drought or flood because of this positive feedback loop. There are several ways to infer the soil moisture across a forecast region. One way is the study the 24-hour precipitation charts each day. From these you can determine which locations have wet or dry soils. Some states have mesonets that measure soil moisture directly. Keeping track of the soil moisture can make you both a better temperature and precipitation forecaster.