Top down approach uses soundings to forecast precip type. Four Steps: 1. Is there ice in the cloud? 2. Consider dry layers. 3. Consider warm layers. 4. Consider near surface conditions.
Is There Ice in the Cloud? Start at top of sounding Find first cloud layer Determine temperature at this cloud layer Coldest temperature Minimum TemperatureIs ice present? -4CNo ice -10C60% chance -12C70% chance -15C90% chance -20CThere is ice
Consider the Dry Layers Will the precip particle evaporate? Will temperature cool to the wet bulb temperature? If so, the layer will become saturated. Is there a seeder feeder situation? 2 cloud layers, one with ice Ice does not evaporate and falls into lower cloud Whole cloud layer eventually contains ice
Consider the Warm Layers If there is no warm layer, ice continues to fall If there is, must determine if ice will melt Keep wet bulb temp in mind Layer will cool if precip is falling If ice is falling: Max Temp of Warm LayerResult < -1CIce does not melt 1C to 3CIce partially melts (mix falling now) >3CIce completely melts
Lastly, Look Near the Surface Does precip re-freeze, continue to melt, freeze on contact, etc. If ice is falling: ConditionResult/Precip Type Wet bulb < 0CSnow Wet bulb > 0CRain with some wet snow If mix is falling: ConditionResult/Precip Type Wet bulb < 0CMix refreezes --> Sleet Wet bulb > 0CRain
If super cooled water is falling: ConditionResult/Precip Type Min near surface temp > - 10C & surface wet bulb < 0C Freezing rain Min near surface temp > - 10C & surface wet bulb > 0C Rain Min near surface temp < - 10C & surface wet bulb < 1C Snow/sleet mix Min near surface temp 1C Rain Basically, look at depth and temp of near surface cold layer and surface wet bulb temperature. There are always exceptions!
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