Quantitative Precipitation Forecast
Issued by NWS

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FOUS11 KWBC 140849
QPFHSD

PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
349 AM EST THU DEC 14 2017

VALID 12Z THU DEC 14 2017 - 12Z SUN DEC 17 2017

DAYS 1 THROUGH 3...

...UPPER MIDWEST/GREAT LAKES...

WITHIN DEEP-LAYERED CYCLONIC FLOW SPINNING OVER MUCH OF THE
NORTHEASTERN SECTION OF NORTH AMERICA...A SERIES OF MID-LEVEL
PERTURBATIONS SHOULD SWING ACROSS THE GREAT LAKES THROUGH SATURDAY
MORNING BEFORE HEIGHTS MARKEDLY RISE. AT THE START OF THE PERIOD
ON 14/1200Z...A DEEP SYNOPTIC-SCALE LOW WILL BE IN THE PROCESS OF
EXITING OFF THE NJ COAST WITH SOME SCATTERED SNOW SHOWERS IN THE
WAKE ACROSS THE LOWER GREAT LAKES. THERE SHOULD BE A BRIEF LULL IN
THE PATTERN BEFORE THE NEXT SHORTWAVE IN THE CHAIN MOVES THROUGH.
SHARP TEMPERATURE FALLS WILL STEEPEN LOW-LEVEL LAPSE RATES AS
NOTED IN SOME OF THE MODEL SOUNDINGS. STRONG ASCENT WITHIN THIS
COLUMN COUPLED WITH FAVORABLE DEEP-LAYERED WESTERLY FLOW WILL
PROMOTE LAKE EFFECT SNOW ON THE EASTERN EXTENT OF ALL THE MAJOR
GREAT LAKES. IT SHOULD BECOME PARTICULARLY HEAVY DOWNWIND OF LAKES
ERIE/ONTARIO GIVEN ADEQUATE PARCEL TRAJECTORIES. ONE LIMITING
FACTOR TO BETTER SNOWFALL POTENTIAL IS THE LOW MOISTURE CONTENT.
IT APPEARS PRECIPITABLE WATER VALUES CAP OUT AT AROUND 0.20
INCHES. MOVING TOWARD THE EARLY PART OF THE WEEKEND...SOME
SYNOPTIC-SCALE ASCENT ALONG A WEST-EAST FRONTAL ZONE SHOULD KEEP A
THREAT FOR SNOW INTO EARLY/MID-SATURDAY. OVERALL...THE BEST CHANCE
FOR HEAVY SNOW WOULD BE ON DAY 2 ACROSS THE LOWER GREAT LAKES. THE
WINTER WEATHER PROBABILITY GRAPHICS ADVERTISE A MODERATE RISK OF 8
INCHES DOWNWIND OF LAKES ERIE/ONTARIO.


...WA CASCADES/UPPER INTERMOUNTAIN WEST/NORTHERN TO CENTRAL
ROCKIES...

AS A MEAN UPPER RIDGE ALONG THE FAR EASTERN PACIFIC QUICKLY
ERODES...SHARP HEIGHT FALLS WILL PUSH INTO THE PACIFIC NORTHWEST
BY FRIDAY AFTERNOON. VIGOROUS LIFT IN ADVANCE OF THESE HEIGHT
FALLS IN CONJUNCTION WITH AREAS OF UPSLOPE FLOW WILL PRODUCE
MODERATE SNOWS OVER AREAS OF THE WA CASCADES EASTWARD INTO THE
BITTERROOTS AND SAWTOOTH RANGES. GIVEN THE EXTENT OF COLD AIR
SURGING IN WITH THIS LONGWAVE TROUGH...SNOW LEVELS SHOULD FALL
CONSIDERABLY...PERHAPS TO AS LOW WAS 3000 FT. AS THE UPPER TROUGH
CONTINUES ITS MARCH TOWARD THE SOUTH AND EAST...SNOW WILL EXPAND
INTO THE NORTHERN/CENTRAL ROCKIES BY DAY 3. WHILE EXPECTED AMOUNTS
ARE NOT HIGH BY ANY MEASURE...THERE ARE SLIGHT CHANCES FOR 8
INCHES ON DAY 2 OVER WESTERN MT AND ACROSS CENTRAL/EASTERN WY BY
THE FOLLOWING DAY.


THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT DAYS
1-3.



RUBIN-OSTER

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