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CC Hennon ATMS 350 UNC Asheville
Forecasting Winter Precipitation
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CC Hennon ATMS 350 UNC Asheville
Outline
• Precipitation Type– Rain, Snow, Sleet, Freezing Rain
• Snow Amounts– Heavy snow band– Snow’s liquid equivalent
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CC Hennon ATMS 350 UNC Asheville
Precipitation Types
• Rain• Snow
– Passage through any warm layer thicker than ~600 ft. will melt (no return to crystal form)
• Sleet– Rain freezes before contact with surface
• Freezing Rain– Rain freezes upon contact with surface
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CC Hennon ATMS 350 UNC Asheville
Method For Evaluating Precipitation Type
• Determine if there will be any precipitation– Model surface forecasts– MOS QPF
• If yes, then perform vertical analysis of forecast temperature– Skew-T / Stuve diagram
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CC Hennon ATMS 350 UNC Asheville
All Snow (below freezing throughout column
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CC Hennon ATMS 350 UNC Asheville
All Rain (melting layer at least 1200 ft. all the way to the ground)
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CC Hennon ATMS 350 UNC Asheville
Freezing Rain (freezing layer near surface not thick enough (need ~ 800 ft.)
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CC Hennon ATMS 350 UNC Asheville
Possible Sleet (Deep freezing and melting level)
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CC Hennon ATMS 350 UNC Asheville
Rain/Snow Mix
• If there is a warm layer near the surface between 600-1200 ft. thick, some snow crystals will melt while others may not
• Results in a rain/snow mixture
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CC Hennon ATMS 350 UNC Asheville
Another factor to consider
• Beware of dry air near the surface– Precipitation will begin to fall from above and
sublimate/evaporate before hitting the ground• Moistens lower atmosphere• Lowers temperature through evaporative cooling• Can drastically alter the temperature profile, and
thus precipitation type
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CC Hennon ATMS 350 UNC Asheville
Forecasting Snow Amounts
• Check model QPF• Average conversion is 10:1, e.g. every 1”
or liquid water equates to 10” of snow• Must also factor in “wetness” of snow
– Snow that accumulates at temperatures near freezing will have a lower ratio (~5-9:1)
– Snow at very cold temperatures may have ratios as high as 30:1
• Lake effect snow – up to 100:1!!!
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CC Hennon ATMS 350 UNC Asheville
Where Will the Heaviest Snow Fall?
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CC Hennon ATMS 350 UNC Asheville
Rules of Thumb
• 120-150 miles left of the surface low track• Along the track of the 700 mb and/or 500 mb low• Just north of the 164 height line at 200 mb• Between the -2°C and -5°C 850 mb temperature• Between the 534 and 540 1000-500 mb
thickness contours• K Index values of 10-20 (instability measure)• Several others.......