Quantitative Precipitation Forecast
Issued by NWS

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FOUS11 KWBC 220745
QPFHSD

PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
244 AM EST WED FEB 22 2017

VALID 12Z WED FEB 22 2017 - 12Z SAT FEB 25 2017

...HEAVY SNOW EVENT FORTHCOMING FOR PORTIONS OF WYOMING ACROSS THE
ADJACENT PLAINS TO UPPER MS VALLEY...

...WESTERN U.S. TO THE UPPER MIDWEST ...

ON DAY ONE /WED/ A 700 MB WAVE PROGRESSES EAST NEAR THE CA/OR
BORDER EAST TO THE NV/ID AND UT/ID BORDERS UNTIL FINALLY REACHING
NEAR THE CO/WY BORDER AND LIKELY FORMING A CLOSED CIRCULATION.
ENHANCED LOW LEVEL CONVERGENCE ALONG THE TRACK OF THE WAVE AND
DEVELOPING LOW LEADS TO THE EXPECTED AREAS
OF SNOW NEAR THE FRONTAL BNDRY...AND ALSO OVER THE UT WASATCH
WHERE PRE-FRONTAL CONVERGENCE PRODUCES LIFT..AIDED BY WINDWARD
TERRAIN.

FURTHER NORTH IN MT..ONE AREA OF LIGHT SNOW IS STILL EXPECTED
WITHIN A WEAKLY CONFLUENT 700 MB FLOW WITH CONVERGENCE MAXIMA
ALONG THE AXIS OF DILATATION.  ANOTHER ENHANCED AREA IS IN THE
RANGES OF SOUTH CENTRAL MT WHERE TOPOGRAPHIC LIFT AIDS THE MODEST
700 MB CONVERGENCE MAXIMA IN PRODUCING LIFT THERE.

ON DAY 2...THE BIGGER SNOW EVENT IS FORECAST OVER WY DUE TO
PERSISTENT 700 MB CONVERGENCE/300 MB DIVERGENCE IN THE VICINITY OF
THE DEVELOPING LOW...WITH ENHANCEMENTS IN WINDWARD TERRAIN.
SNOW INTENSIFIES ALONG THE WELL DEFINED LOW LEVEL WARM FRONT IN
NEBRASKA. AS THE LOW LEVEL CYCLONE DRIFTS EAST...SNOW DEVELOPS
WITHIN STRONG 700 MB THETA-E ADVECTION AND CONVERGENCE ACROSS
NORTHERN NEBRASKA INTO SD.  BY THU NIGHT-FRI THE ENHANCED LIFT
EXTENDS INTO NORTHERN IOWA AND MN/WI.
THE INTENSIFYING LOW DOES PRESENT THE PROSPECT FOR PRODUCING BANDS
OF HEAVY MESOSCALE SNOWFALL.  FOR EXAMPLE...THE 00Z NAM DOES SHOW
CONDITIONS MEETING THE CRITERIA FOR CONDITIONAL SYMMETRIC
INSTABILITY NEAR THE SOUTHEAST CORNER OF SD INTO SOUTHERN
MN...WHICH IS ALREADY A TARGET AREA FOR HEAVY SNOW ANYWAY.

ON DAY 3/FRI/ THE 700 MB IS FORECAST BY THE GFS/ECMWF TO MOVE
NORTHEAST FROM MN/WI TO THE UPPER GREAT LAKES.  HEAVY SNOW IS
LIKELY TO OCCUR IN THE MID LEVEL DEFORMATION ZONE NEAR THE MID
LEVEL CIRCULATION IN MN AND ADJACENT WI. THE WPC GIVES MORE
WEIGHTING TO THE 18Z GEFS MEAN/18Z GFS AND THE 12 UTC
ECMWF...DISCOUNTING THE NAM WHICH PRODUCED HEAVIER AMOUNTS FURTHER
TO THE NORTH IN SD AND MN...AS IT HAS THE FURTHEST NORTH TRACK OF
THE 700 MB LOW OF THE VARIOUS MODEL SOLUTIONS. LIKEWISE...THE
CANADIAN GLOBAL WAS AN OUTLIER WITH A SLOW CYCLONE TRACK...SO
LITTLE WEIGHTING WAS GIVEN TO ITS FORECAST.  OVERALL...THERE WAS
NOT MUCH OF A MOVEMENT TO RESOLVE MODEL TO MODEL TRACK AND CYCLONE
TIMING DIFFERENCES...WITH SIGNIFICANT DIFFERENCES PERSISTING.

THE PROBABILITY OF ICE ACCUMULATIONS GREATER THAN 0.25 IS LESS
THAN 10 PERCENT DAYS 1-3.

PETERSEN

$$





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