The relative humidity prog helps a forecaster determine how close a deep layer of the troposphere is to saturation. The relative humidity given on graphical model progs is the average 850 mb to 500 mb relative humidity. If the air is rising through a deep layer of the troposphere (low levels and middle levels), the relative humidity will increase. If the relative humidity from 850 mb to 500 mb is high, a trigger mechanism such as a front or upper level divergence will be able to condense clouds and produce precipitation. The relative humidity prog is available on UNISYS weather at: http://weather.unisys.com/nam/rh.php The highest relative humidities will tend to be located near strong low-pressure regions and fronts. The lowest relative humidities will occur in regions experiencing downsloping wind, behind strong/deep cold fronts, upper level convergence due to high pressure, and over dry interior regions of the U.S. that currently have no UVV mechanisms. At times you may notice the forecast model NOT producing precipitation over a region where the UVV's are moderately high. Most likely, the cause of the model not producing precipitation is dry air (the lifting in place is not enough to condense water vapor into precipitation). Dry air, near the surface or aloft, can also evaporate precipitation that does occur (producing virga). The relative humidity value says nothing about the amount of moisture in the air or how much precipitation will occur. It can be used for cloud and general precipitation forecasting however. UVV combined with a high relative humidity has a high potential to produce clouds and precipitation (especially those regions in the troposphere with the highest relative humidity). Relative humidity is contoured on the prog in increments of 10%'s. The color code of interpretation is given under the model panel. 80 to 100% RH indicates nearly saturated air, less than 60% indicates air that, when averaged from 850 mb to 500 mb, is fairly far from saturation. However, an average RH may misrepresent layers of dry or moist air. An average can be misleading. Example: the troposphere may be near saturation from the surface to 700 millibars and dry above 700 millibars. The average will yield a low RH that is not indicative of the nearly saturated air in the low levels. Generally, the RH will be high only when lifting is occurring through a deep layer of the troposphere. The 850 to 500 mb RH is much more relevant when used to help forecast dynamic precipitation as opposed to convective precipitation. This is because high relative humidity above 700 millibars is generally not required in surface based convective type situations in order to produce precipitation. |