Skew-T's are handy forecasting tools for predicting winter precipitation type. If temperatures from a 1000 meters above the surface to the top of the troposphere are below freezing, precipitation type is likely to be snow. It may, for a short period, fall as a cold rain or a wintry mix, but through evaporational cooling the precipitation type will change to snow (unless low level warm air advection is occurring or inadequate evaporative cooling occurs in the boundary layer). In some cases, temperatures in the PBL will be below freezing but an inversion just above the PBL will have above freezing temperatures. This inversion could be the top of a shallow cold front or a layer of warm air advection. This situation is conducive to producing sleet. If the below freezing temperatures near the surface are fairly deep are are capped with above freezing temperatures further aloft, precipitation type will likely be sleet. Precipitation type will be freezing rain if the warm air aloft is well above freezing and/or deep. The set up for freezing rain is similar to that of sleet except the below freezing temperatures extend only a short distance above the surface (ranging from below freezing temperatures just at the surface or extending to as high as 500 or so meters above the surface). An inversion of above freezing temperatures will cap the below freezing low level temperatures, but the inversion will be closer to the ground than inversions associated with sleet. Only two balloon soundings are launched each day, therefore soundings can change rapidly in just a few hours. From studying the analysis and forecast panels, gain an insight into thermal and moisture advections that will change the soundings. A slight change in thermal advection can change the precipitation type from one to the other (i.e. warm air advection... snow to rain, freezing rain to rain, sleet to freezing rain;;;; cold air advection. ... rain to snow, freezing rain to sleet, freezing rain or sleet to snow). Sometimes the vertical depth of cold air and the inversion of above freezing temperatures will be near the cusps of a change in precipitation type. This produces a wintry mix, precipitation type changes from one type to another or even sleet, snow, and cold rain all falling at the same time. Evaporative cooling also plays a key role. Monitor the wet bulb temperatures from the surface to the top of the inversion to monitor possible changes in precipitation type (if wet bulb is at or below freezing at all levels, precipitation type will eventually change to all snow, until or unless warm air advection again changes it back to another type. Remember that winter precipitation is "elevated lifting". Parcels of air will begin their ascent from the top of the inversion. Calculate indices using the top of the inversion as a base for the convection or lift. Forecasting winter precipitation using Thickness values: http://www.theweatherprediction.com/winterwx/thicknesscriteria/ |