Quantitative Precipitation Forecast
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FOUS11 KWBC 100827
QPFHSD

PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
327 AM EST SAT DEC 10 2016

VALID 12Z SAT DEC 10 2016 - 12Z TUE DEC 13 2016

DAYS 1-3...

...PACIFIC NORTHWEST AND NORTHERN CALIFORNIA TO THE NORTHERN AND
CENTRAL ROCKIES...

MULTIPLE ROUNDS OF HEAVY SNOW OVER THE HIGHER TERRAIN FROM THE
PACIFIC NORTHWEST AND NORTHERN CALIFORNIA TO THE NORTHERN AND
CENTRAL ROCKIES.  THE WASHINGTON AND OREGON CASCADES...SAWTOOTH
MOUNTAINS OF IDAHO...TETONS OF WYOMING...WASATCH/UINTA MOUNTAINS
OF UTAH...AND COLORADO ROCKIES SHOULD ALL HAVE AREAS OF FORECAST
HEAVY SNOW ACCUMULATIONS.
A SERIES OF WEAK PACIFIC SYSTEMS MOVE ONSHORE AND INLAND ALONG A
FRONTAL BNDRY DRAPED ACROSS THE NORTHERN ROCKIES.  MULTIPLE 300 MB
JET MAXIMA ARE FORECAST WITH EACH CONCENTRATING FOCUSED ASCENT AND
RESULTANT PRECIP AND SNOW IN HIGHER ELEVATIONS.  THE NORTHERN
STREAM BRANCH CROSSING WA AND OR ON DAY 1 BRINGS AN UPPER
DIVERGENCE MAXIMA THAT TRIGGERS LIFT AND COMBINES WITH OROGRAPHIC
LIFT TO INDUCE HEAVY SNOW IN THE WA/OR CASCADES.

FURTHER SOUTH...A SECOND BRANCH OF THE JET CROSSES OR AND ID/WY
AND DRIFTS SOUTH INTO NORTHERN CA/NORTHERN NV/NORTHERN UT.  THIS
BRINGS A SWATCH OF SNOW TO HIGHER ELEVATIONS OF NORTHERN
NV/NORTHERN UT AND WESTERN WY/NORTHWEST CO.

ON DAY 2 THE ECMWF PRODUCES CONFLUENCE OF THE 2 JET BRANCHES WITH
ONE PRIMARY MAXIMA AT 300 MB WHICH STREAMS ONSHORE INTO OR AND ID.
 HIGHER MOISTURE CONTENT STREAMING INTO WESTERN OR AND FLOW
PERPENDICULAR TO THE OR CASCADES LEADS TO A STRONG UPSLOPE
RESPONSE WITH HEAVY SNOW EXPECTED TO PERSIST FOR A LONG DURATION
IN THE OR CASCADES. MULTI-DAY TOTALS NEAR 3 FEET ARE POSSIBLE IN
THE OR CASCADES.  LIKEWISE...FAVORABLE UPPER DIFLUENT FLOW AND
LIFT FROM THE FRONTAL ZONE PRODUCE HEAVY SNOW IN NORTHERN ID AND
NORTHWEST MT.

ON DAY 3...THE MODELS INDICATE LOW PRESSURE DEVELOPS IN THE
EASTERN PACIFIC WITH A NORTH-SOUTH SPREAD OF WHERE THE LOW
PRESSURE DEVELOPS AND ITS TIMING.  THE 12Z ECMWF WAS ON ITS OWN
WITH DEVELOPMENT FURTHER NORTH WHOSE PRECIP SPREAD ONSHORE FASTER
THAN THE OTHER SOLUTIONS AND ITS MEAN INTO NORTHERN CA.  MORE
WEIGHTING WAS GIVEN TO THE LIGHTER AMOUNTS OF THE OTHER SOLUTIONS
LIKE THE GFS/GEFS MEAN AND UKMET.


...NORTHERN PLAINS AND UPPER MISSISSIPPI VALLEY TO THE GREAT
LAKES...OHIO VALLY...CENTRAL APPALACHIANS...AND NORTHEAST...

ON DAY 1..SATURDAY...SNOW WILL INITIALLY FOCUS IN AN AXIS OF
DEEP-LAYER FRONTOGENESIS SETTING UP OVER THE NORTHERN PLAINS AND
UPPER MISSISSIPPI VALLEY.  HIGHER TOTALS ARE EXPECTED SOMEWHERE IN
THE VICINITY OF THE IA/MN BORDER WHERE A BAND OF 5-7 INCHES IS
EXPECTED...WITH A TIGHT NORTH-SOUTH GRADIENT OF AMOUNTS.  THE
ECMWF AND ITS ENSEMBLE MEAN AND THE UKMET ARE A SHADE NORTH OF THE
WRF NMMB/ARW/CANADIAN REGIONAL GEM/NCAR 3 KM ENSEMBLE MEAN.

ON DAY 2...SUNDAY...INCREASING MOISTURE/WARM AIR ADVECTION AHEAD
OF LOW-MID LEVEL WAVE ALLOW SNOW TO BECOME MORE WIDESPREAD AS IT
SPREADS DOWNSTREAM OVER THE GREAT LAKES...AND NORTHEAST SUNDAY
INTO MONDAY.  ALSO...SNOW WILL CHANGE TO SLEET AND RAIN OVER THE
OH VALLEY AND PA/SOUTHEAST NEW YORK/SOUTHERN NEW ENGLAND AS THE
WARM MOIST AIR OVERRUNS AN ARCTIC AIRMASS CURRENTLY IN PLACE.
MODEL FOCUS ON ENHANCED POTENTIAL FOR LIGHT FREEZING RAIN IN
PORTIONS OF CENTRAL PA SUNDAY DURING THE PRECIP TYPE TRANSITION
PERIOD.

ON MONDAY THE MODELS FORECAST A TRIPLE POINT LOW TO DEVELOP IN THE
VICINITY OF THE NEW ENGLAND COAST.  THIS LOW WOULD FUNCTION TO
KEEP THE COLD AIR IN PLACE IN INTERIOR MAINE AND SHUT OFF THE WARM
ADVECTION. THOSE SOLUTIONS WHICH DELAY THE LOW DEVELOPMENT ALLOW
WARMER AIR TO CONTINUE INLAND...LEADING TO LESS SNOW.  FEW
SOLUTIONS BRING HEAVY SNOW TO THE MAINE COAST.


...LAKE EFFECT SNOW SATURDAY...

A COLD AIRMASS BEING ADVECTED OVER RELATIVELY WARMER LAKES WILL
CONTINUE TO SUPPORT LAKE EFFECT SNOW BANDS SATURDAY.  WITH BNDRY
LAYER WINDS BACKING TOWARDS THE WEST...PEAK LOW LEVEL CONVERGENCE
SHIFTS TODAY TO THE LEE SHORE AREAS DOWNSTREAM FROM LK ERIE NEAR
THE PA/NY BORDER INTO SOUTHWEST NY AND ALSO NEAR THE SOUTHEAST
CORNER OF LK ONTARIO.  SNOW COVERAGE/INTENSITY WANES AS WINDS
WEAKEN AND IN RESPONSE LOW LEVEL CONVERGENCE WEAKENS AND DRYING
ALOFT COMMENCES.  THE SNOW WILL QUICKLY RETURN TO THE GREAT LAKES
WITH WARM ADVECTION PRECIPITATION MOVING INTO THE REGION ON SUNDAY
AS DISCUSSED ABOVE.


THE PROBABILITY OF SIGNIFICANT ICING (GREATER THAN 0.25 INCHES) IS
LESS THAN 10 PERCENT DAYS 1-3.

PETERSEN


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