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March 20th White Rain/Slush Event Discussion and Obs


yoda

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Afternoon LWX AFD:

 

 

 

A COMPLEX WEATHER SCENARIO CONTINUES TO BE ADVERTISED FOR THURSDAY
NIGHT INTO FRIDAY WITH ACCUMULATING SNOW POSSIBLE.
SYNOPTICALLY...HIGH PRESSURE WILL EXTEND OVER SOUTHERN NEW ENGLAND
AND THE NORTHERN MID-ATLANTIC...BUT BE PROGRESSIVE TO THE EAST UNDER
NEAR-ZONAL FLOW ALOFT. A PAIR OF LOW PRESSURE CENTERS WILL APPROACH
THE AREA...ONE FROM THE CENTRAL PLAINS AND ANOTHER FROM THE GULF
COAST. ENERGY FROM THE FORMER WILL TRANSFER TO THE LATTER OFFSHORE
BY FRIDAY EVENING. LIFT WILL BE PROVIDED BY THE LFQ OF UPPER JET AND
ISENTROPIC LIFT BY MODEST SOUTHERLY LLJ. EFFECTIVE COLD FRONT
CROSSES AROUND FRIDAY EVENING...WITH DRYING QUICKLY TAKING PLACE
FROM THE TOP OF THE COLUMN DOWN. THIS FCST SLOWS PRECIP ONSET BY A
BIT AND HASTENS THE EXIT FRIDAY EVENING.

PRECIP WILL SPREAD FROM SW TO NE THURSDAY NIGHT. THE COLUMN MAY TAKE
A FEW HOURS TO SATURATE...BUT THIS WILL ALSO CAUSE WET BULB
PROCESSES TO TAKE PLACE. IN ADDITION THE NRN CWA MAY BE ABLE TO COOL
DIURNALLY WITH MAINLY HIGH LEVEL CLOUDS. THERE IS STILL SPREAD
AMONGST MODEL GUIDANCE IN REGARDS TO TEMPS...WITH THE NAM RUNNING
COLDEST AND STAYING COLD THROUGHOUT THE EVENT. DON/T THINK THIS IS A
BAD IDEA AT LEAST THROUGH EARLY MORNING
...BUT THINK TEMP PROFILES
WILL AT LEAST WARM SOME DURING THE DAY GIVEN THE TIME OF YEAR. FCST
SOUNDING INDICATE THIS WILL MAINLY BE A RA/SN EVENT...WITH ANY PL OR
FZRA BRIEF AND DUE TO MINOR NUANCES WITH THE PROFILE AND ELEVATION.

SO WITH THAT SAID...TEMPS IN THE LOWEST THOUSAND FT OR SO WILL BE
CRITICAL. ELEVATION WILL LIKELY PLAY A ROLE...WITH THE GREATEST
CHANCE FOR SNOW ACCUMS N/W OF DC AND BALTIMORE AND THE NRN SHEN
VALLEY. THE HEAVIEST PRECIP IS EXPECTED WITHIN SEVERAL HOURS EITHER
SIDE OF DAYBREAK...FAVORABLE FOR COLDEST TEMPS. GIVEN THE FCST QPF
AMOUNTS...SNOWFALL TOTALS WILL BE BUMPED UP A LITTLE...BUT NOT TO
THE FULL EXTENT SHOWN BY MODEL DIAGNOSTICS/BUFR OUTPUT DUE TO
LIMITED ACCUMS AFTER THE SUN RISES AND AMBIENT GROUND TEMPS WHICH
WILL BE RELATIVELY WARM. MOST ACCUM WILL BE ON GRASSY SFCS...BUT
HEAVIER RATES BEFORE THE SUN RISES MAY ALLOW PAVEMENT ACCUMS TOO.

TEMPS RISE ABOVE FREEZING IN MOST LOCATIONS BY FRIDAY
AFTERNOON...ENDING THREAT FOR IMPACTS.
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