Quantitative Precipitation Forecast
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FOUS11 KWBC 222038
QPFHSD

PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
337 PM EST MON DEC 22 2014

VALID 00Z TUE DEC 23 2014 - 00Z FRI DEC 26 2014

DAY 1

...NORTHERN PLAINS TO UPPER MS VALLEY/UPPER LAKES...

A CLOSED 850-700 MB LOW MOVES EAST THEN NORTHEAST ACROSS THE UPPER
MS VALLEY. THE INITIAL BURST OF SNOW OCCURS IN UPSLOPE AREAS OF
THE BLACK HILLS OF SD AND THEN INTO THE MID LEVEL DEFORMATION ZONE
IN SOUTHERN SD...THEN NORTHEAST NEBRASKA TO NORTHWEST IA/SOUTHEAST
MN/NORTHERN WI/CENTRAL UP OF MI AS THE MID LEVEL CIRCULATION
CONTINUES TO SUPPORT A DEFORMATION BAND TO THE LEFT OF ITS TRACK.
PRECIP TYPE UNCERTAINTY CONTINUES SOUTH OF THIS AXIS AS INITIAL
TEMPERATURES ARE TOO WARM FOR SNOW.
QPF UNCERTAINTY EXISTS AS THE NAM QPF IS FURTHER SOUTHEAST IN IOWA
VS THE GFS/ECMWF/SREF MEAN.  THE LIQUID EQUIVALENT VALUES AND
THERMAL PROFILES OF THE 12Z GFS/12Z GEFS MEAN/12Z ECMWF/09Z SREF
MEAN WERE USED FOR SNOW AMOUNTS.

THE PROBABILITY OF ICE GREATER THAN 0.25 INCHES IS LESS THAN 10
PERCENT.

DAYS 2 AND 3

...MID MS VALLEY/MIDWEST/GREAT LAKES...

THE MODELS STILL SHOW CONSIDERABLE SPREAD OF THE CYCLONE
DEVELOPMENT ALONG THE FRONT WITH THE WAVE MOVING NORTH INTO THE OH
VALLEY AND LAKES.  BOTH TIMING AND INTENSITY DIFFERENCES EXIST.
THE SREF MEMBERS AND GEFS MEMBERS SHOW POTENTIAL FOR A FURTHER
WEST LOW DEVELOPMENT THAN THE 00-12Z ECMWF AND UKMET.  THERE IS
LITTLE OVERLAP AMONG WHERE GEFS AND SREF MEMBERS INDICATE WHERE
THERE IS SUFFICIENT COOLING TO RESULT IN A CHANGE FROM RAIN TO
SNOW...WITH THE SREF MEMBERS SOUTH OF WHERE THE GEFS MEMBERS
SUPPORT A RAIN TO SNOW CHANGE OVER.  THE 12Z NAM WAS AN OUTLIER
WITH SLOWER TIMING AND HEAVIER SNOW ON DAY 3 IN MI.   MANUAL PROGS
GAVE MORE WEIGHTING TO THE 00-12Z ECMWF AND 12Z GFS
QPF/TEMPERATURE PROFILES. CONFIDENCE REMAINS BELOW NORMAL DUE TO
THE LARGE SPREAD OF SOLUTIONS.

...PACIFIC NORTHWEST/NORTHERN ROCKIES...

THE 00-12Z ECMWF HAVE SHOWN GOOD RUN TO RUN AGREEMENT ON DEPICTING
A HIGH AMPLITUDE/SHORT WAVELENGTH 700 MB TROUGH STEADILY CROSSING
THE NORTHERN ROCKIES AND GREAT BASIN  NORTHWEST LATE WEDNESDAY
INTO THURSDAY.  THE 12Z GFS SPED UP FROM ITS 00Z RUN AND GETS TO
BE ON THE FASTER END OF GUIDANCE MOVING THE 700 MB WAVE EAST ONTO
THE PLAINS.  CONSEQUENTLY...GREATER WEIGHTING WAS GIVEN TO THE
ECMWF/NAM TROUGH TIMING AND QPF/TEMPERATURE PROFILES.
WELL DEFINED 700 MB CONVERGENCE AND STRONG 700-500 MB ASCENT
OCCURS ON THE NOSE OF A 50 KT JET CROSSING THE RANGES OF NORTHERN
CA AND THEN NORTHERN UT. A COUPLED 300 MB JET REGION DEVELOPS IN
THE NORTHERN ROCKIES AND LEADS TO SECONDARY MAXIMA OF SNOW ACROSS
THE RANGES OF SOUTHERN TO CENTRAL ID AND WESTERN MT...WITH THE
UPPER DIVERGENCE MAXIMA EVENTUALLY ARRIVING DOWNSTREAM IN THE
RANGES OF WY.

THE PROBABILITY OF ICE GREATER THAN 0.25 INCHES IS LESS THAN 10
PERCENT.

PETERSEN

$$




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