Quantitative Precipitation Forecast
Issued by NWS

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FOUS11 KWBC 211425
QPFHSD

PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
924 AM EST SAT FEB 21 2015

...CORRECTION FOR TYPO...

VALID 12Z SAT FEB 21 2015 - 12Z TUE FEB 24 2015


...OH VALLEY/CENTRAL TO NORTHERN APPALACHIANS ACROSS
MID-ATLANTIC/NEW ENGLAND...

A LARGE AREA OF MIXED PRECIPITATION IN THE OH AND TN VALLEYS WILL
MOVE NORTHEAST SATURDAY.   AN ANOMALOUS FLUX OF MOISTURE WILL
SURGE NORTHEAST ACROSS THE CENTRAL APPALACHIANS TO THE MID
ATLANTIC ON THE NOSE OF AN 850 MB JET OF 50-60 KT. THE STRONG WARM
AND MOIST ADVECTION ALOFT OCCURRING OVER LOW LEVEL COLD AIR
RESULTS IN A LARGE AREA OF SNOW AND MIXED PRECIPITATION ACROSS THE
REGION.  THE HEAVIEST PRECIPITATION OCCURS WHERE THE STRONGER
850-700 MB MOISTURE FLUX ANOMALIES INTERACT WITH THE TERRAIN OF
THE CENTRAL APPALACHIANS.  THE HEAVIEST SNOW TOTALS ARE
ANTICIPATED IN THE MOUNTAINS OF WV TO THE VA BORDER.  THE
UNCERTAINTIES ARE HOW FAR NORTH HIGHER QPF AMOUNTS EXTEND ACROSS
PA/NY/NEW ENGLAND...AND THE DISTRIBUTION OF PRECIPITATION TYPES
WHERE SNOW CHANGES TO SLEET/FREEZING RAIN OR RAIN FROM SOUTHEAST
NEW ENGLAND TO THE MID ATLANTIC.
THE WARM ADVECTION MAY ALSO RESULT IN THE EVENT WINDING DOWN AS A
PERIOD OF FREEZING DRIZZLE ACROSS WV AND THE MID ATLANTIC TO
SOUTHEAST NY AND PORTIONS OF SOUTHERN NEW ENGLAND.

AS THE WAVE OF LOW PRESSURE LIFTS UP INTO THE ERN LAKES AND CANADA
ON SUN... THE LOW LEVEL CONVERGENCE WEAKENS AND THE THREAT OF
FROZEN PRECIP SHOULD WIND DOWN.
MANUAL PROGS USED THE QPF FROM THE ECMWF/GEFS MEAN/00Z /00Z GFS
WITH MORE WEIGHTING ON
THE NAM TEMPERATURE PROFILES.

...INTER-MOUNTAIN WEST/ROCKY MOUNTAINS INTO CENTRAL/SRN HIGH
PLAINS...

ON DAY 1...LOW LEVEL FRONTOGENESIS AND MOISTURE POOLING ALONG THE
FRONTAL ZONE LEADS TO SEVERAL INCHES OF SNOW IN THE HIGH PLAINS OF
WY AND CO INTO SOUTHWEST KS.
UNCERTAINTY REVOLVES AROUND QPF AMOUNTS AS THE NAM AMOUNTS ARE
HIGHER THAN THE SREF AND GEFS MEANS.  ANOTHER ASPECT OF
UNCERTAINTY IS TEMPERATURES ARE TOO WARM FOR SNOW IN CENTRAL
KS....WITH PRECIP STARTING OFF AS RAIN AND THEN CHANGING TO SNOW
WITH THE FRONTAL PASSAGE AND POST-FRONTAL COLD ADVECTION.  THE
SHORTER DURATION SNOW LEADS TO LOWER PROBABILITIES.  THE HIGHER
AMOUNTS AND PROBABILITIES ARE IN EASTERN CO WHERE THE BEST OVERLAP
OF BOTH HIGHER QPF AND LONGER DURATION OF SNOW.

AN UPPER LEVEL TROUGH WILL DIG INTO CA/NV SUNDAY TO CLOSE OFF A TO
MID TO UPPER LOW AND ALLOW CONFLUENT UPPER-LEVEL FLOW TO DEVELOP
DOWNSTREAM. THIS SHOULD RESULT IN BROAD SWRLY UPPER DIFLUENT FLOW
WITH STRONG FRONTOGENETIC FORCING FOR INTENSE UPSLOPE FLOW AND
HEAVY OROGRAPHIC SNOWFALL.  THE BULK OF THE HEAVY SNOW WILL OCCUR
SUN AND MON FROM THE SRN SIERRA/SRN WASATCH INTO THE CO/SRN
ROCKIES... INCLUDING THE SAN JUANS AND SANGRE DE CRISTO RANGES...
SPREADING OUT INTO ERN CO/NM... WRN KS...OK/TX PANHANDLES.

THE GUIDANCE VARIES ON THE TIMING OF THE UPPER LOW IN THE
SOUTHWEST WITH THE GFS NOW JOINED BY THE 00Z ECMWF WITH FASTER
FORWARD PROGRESSION THAN THE SREF MEAN/NAM/UKMET AND OLD 12Z ECMWF
RUN. WPC GENERALLY FOLLOWED A MULTI-MODEL BLEND FOR HEAVY SNOW
AMOUNTS AND PROBABILITIES TO MITIGATE THE DIFFERENCES...AS THE
MODELS WITH FASTER TIMING OF THE MID LEVEL CIRCULATION HAVE LESS
SNOW EXTENDING NORTH ACROSS UT AND CO.

THE PROBABILITY OF 0.25 INCHES OR GREATER OF FREEZING RAIN IS LESS
THAN 10 PERCENT DAYS 2-3.

PETERSEN

$$





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