[--MAIN HOME--] [--ALL HABYHINTS--] WINTER PRECIPITATION INDICES PAGE 1000 - 500 mb Thickness (elevations less than 1000 feet) 5,340 gpm Greater than 50% 5,400 gpm 50% of time snow occurs 5,460 gpm Less than 50% 1000 - 700 mb Thickness (elevations less than 1000 feet) 2,820 gpm Snow very likely 2,840 gpm Snow likely 2,860 gpm Snow not likely NOTES: (1) For elevations closer to 3,500 feet add 60 gpm to 1000 - 500 mb Thickness and 20 gpm to the 1000 - 700 mb Thickness for each category. (2) For elevations greater than 5,000 feet add 90 gpm to 1000 - 500 mb Thickness and 40 gpm to the 1000 - 700 mb Thickness for each category. (3) The % chances and likeliness values given for each category are climatological. To assess the threat for a particular precipitation event, forecast soundings need to be analyzed. Thickness criteria do not replace analyzing individual forecast soundings. (4) Snow can occur at higher thickness values when a deep isothermal layer just below freezing exists (temperature between 1000 and 700 millibars just below freezing). (5) The temperatures between the surface and 700 millibars are much more important in determining the precipitation type than the temperatures between 700 and 500 millibars. Because of this, the 1000 - 700 mb Thickness is superior in assessing snow threat. 850 - 700 mb Thickness (elevations less than 1000 feet) 1,525 gpm Subfreezing layer 1,540 gpm All precip types possible 1,555 gpm Complete melting in layer 1000 - 850 mb Thickness (elevations less than 1000 feet) 1,290 gpm Layer below freezing 1,300 gpm All precip types possible 1,310 gpm Rain is favored NOTES: (1) The 850 to 700 mb Thickness provides clues as to whether there is warm air aloft. (2) If complete melting of precipitation occurs (850 - 700 mb thickness greater than 1,555 gpm), precipitation will fall as rain if surface temperature is above freezing or freezing rain if the surface is freezing. Surface temperature is the discriminator of rain versus freezing rain (0 C threshold). (3) Sleet will not occur in a situation in which complete melting of precipitation occurs in the EWL unless the cold layer below the EWL is less than -10 C (rare). Supercooled raindrops will fall in a situation where complete melting of precipitation occurs aloft and the rain falls into a sub-freezing PBL (1000 to 850 Thickness of 1,290 gpm or less). FREEZING RAIN REQUIREMENTS CASE 1: Freezing surface temperature and 850 - 700 Thickness 1,555 gpm or higher CASE 2: Freezing surface temperature and cloud min temperature warmer than -10 C SLEET REQUIREMENTS EWL less than 4 C, EWL less than 400 m thick, 1000 - 850 Thickness less than 1,295 gpm, and cloud min temperature less than -10 C SNOW REQUIREMENTS NO significant EWL, cloud min temperature less than -10 C, 1000 - 850 Thickness less than 1,295 gpm RAIN REQUIREMENTS 1000 - 850 Thickness of at least 1,310 gpm, and surface temperature above freezing NOTES: (1) In clouds with temperatures between 0 C and -10 C, precipitation tends to be in the form of supercooled rain drops. Freezing drizzle / rain can occur when temperatures are below freezing within entire sounding. (2) Complete melting of precipitation occurs when the maximum temperature in the EWL (Elevated Warm Layer) exceeds 3 C or the thickness of the EWL is 400 meters or greater. (3) If maximum temperature in the EWL is 1 C or less, only partial melting of snow will occur. Snow will refreeze once it drops into subfreezing air (the snow will be grainy / pellety). (4) An EWL temperature of between 1 and 3 C results in partial melting of snowflakes which will refreeze into sleet (or a mixture of sleet and freezing rain, depending on the depth of the warm layer). (5) Evaporational cooling will reduce thickness values.