Quantitative Precipitation Forecast
Issued by NWS

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FOUS11 KWBC 210816
QPFHSD

PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
315 AM EST TUE FEB 21 2017

VALID 12Z TUE FEB 21 2017 - 12Z FRI FEB 24 2017

DAYS 1 TO 3...

...RANGES OF THE PACIFIC NORTHWEST/CALIFORNIA/GREAT
BASIN/ROCKIES...

THE MODELS INDICATE THE HEAVY SNOW EVENT TONIGHT IN NORTHERN CA IS
EXPECTED TO CONTINUE THROUGH  TUE AS THE UPPER JET MAXIMA ACROSS
THE REGION IS SLOW TO MOVE AND CONTINUING ANOMALOUS LOW LEVEL
MOISTURE FLUXES AND UPPER LEVEL DIVERGENCE COMBINE WITH
TOPOGRAPHIC LIFT TO PRODUCE PROLONGED SNOW ACROSS THE NORTHERN AND
CENTRAL CA SIERRA NEVADA RANGE. ADDITIONAL SNOW TOTALS ARE LIKELY
TO EXCEED 1 FOOT AND MAY REACH 2 FEET OR MORE ACROSS SOME OF THE
HIGHER PEAKS OF THE NORTHERN AND CENTRAL SIERRA NEVADA.  LEFT-EXIT
REGION UPPER JET DYNAMICS ARE EXPECTED TO EXTEND THE HEAVY SNOW
THREAT FURTHER TO THE NORTH AS WELL INTO THE SOUTHERN OR
CASCADES...CONTINUING INLAND IN THE LOW-MID LEVEL FRONTAL ZONE
ACROSS THE RANGES OF SOUTHERN IS INTO WESTERN WY.

ON WED THE SNOW THREAT DIMINISHES IN CA AS THE UPPER JET MOVES
FURTHER FURTHER SOUTH AND INLAND AS WELL. THE 700 MB WAVE DEPARTS
CA AND CROSS NV AND THEN UT AND INTO NORTHWEST CO..WHERE THE GFS
AND  ECMWF SHOW A CLOSED LOW DEVELOPING.  700 MB CONVERGENCE AND
ASCENT ALONG THE WAVE/DEVELOPING LOW TRACK SHOULD BRING SEVERAL
INCHES OF SNOW TO THE UT WASATCH AND THEN RANGES OF NORTHWEST CO
TO ADJACENT SOUTHEAST WY.  THE SNOW IN SOUTHEAST WY AND FRONT
RANGE OF CO PERSISTS INTO EARLY WED AND THEN TAPERS WITH THE
DEPARTURE OF THE 700 MB LOW ACROSS NEBRASKA.  THE STRENGTHENING
CIRCULATION THOUGH IS EXPECTED TO RESULT IN HEAVY SNOW IN THE
RANGES ACROSS SOUTHEAST WY AND NORTHERN CO...WITH 1-2 FEET LIKELY.


...CENTRAL PLAINS TO UPPER MS VALLEY THU...

THE MODELS/ENSEMBLES SHOW A DEVELOPING 700 MB CLOSED LOW NEAR THE
CO/WY BORDER 12Z THU THAT MOVES EAST
ACROSS NEBRASKA TO NEAR THE NE/IA BORDER NEAR 12Z FRI.  TO THE
LEFT OF THIS TRACK...A LOW-MID LEVEL FRONTOGENESIS MAXIMA PRODUCES
LIFT WITH A BROAD AREA OF SNOW EXPECTED ACROSS EASTERN WY EAST
THROUGH CENTRAL AND NORTHERN NE AND SOUTHERN SD TO AREA ALONG THE
IA/MN BORDER.   THE FRONTOGENESIS MAXIMA SUPPORTS LOCALLY HEAVIER
BANDS EMBEDDED WITHIN THE AREA OF SNOWFALL. THE MODELS SHOW A
LARGE SPREAD IN THE AMOUNTS OF QPF IN THE COLD SECTOR...WITH THE
GFS THE HEAVIEST AMONG THE MODELS. THE 00Z GEFS MEAN QPF IN
NORTHERN IOWA WAS HALF OF THE OPERATIONAL GFS AMOUNTS...SO THE
ENSEMBLES ARE NOT REPLICATING THE HIGH TOTALS OF THE OPERATIONAL
RUN.  THE LATEST 00Z RUN SHIFTED THE SNOW POTENTIAL NORTH IN
SD/MN/WI ACROSS MULTIPLE MODELS/GFS ENSEMBLE MEAN.

...NORTHERN PLAINS TO LAKE SUPERIOR WED...

AS THE LEADING SHORTWAVE EMANATING FROM THE WESTERN U.S. TROUGH
EJECTS NORTHEASTWARD INTO THE NORTHERN PLAINS AND UPPER
MIDWEST...A STRIPE OF LIGHT SNOWS CAN BE EXPECTED NORTH OF THE
ASSOCIATED SURFACE LOW AS IT TRACKS FROM THE NORTHERN PLAINS WED
MORNING TO THE UPPER GREAT LAKES BY EARLY THU.  THE NAM FLIPPED TO
A DRIER SOLUTION SO CONFIDENCE IS LOW IN ITS SOLUTION.  GREATER
WEIGHTING WAS GIVEN TO THE   ECMWF AND GFS SOLUTIONS AND THEIR
RESPECTIVE MEANS...ALONG WITH THE 00Z UKMET.

THE GFS AND ECMWF INDICATE POTENTIAL FOR A BAND OF SNOW TO DEVELOP
WITHIN LOW LEVEL CONFLUENT FLOW IN NORTH CENTRAL MT WITH 850-700
MB CONVERGENCE MAXIMA PERSISTING WED-WED EVENING. THE 12 AND 0Z
ECMWF MOVE THE BAND SLOWLY IN NORTH CENTRAL MT...LEADING TO THE
POSSIBILITY OF LOCALLY HEAVY SNOW TOTALS...AS ASCENT OCCURS WHERE
LAYER SFC-500 MB MEAN RELATIVE HUMIDITY IS 90 PERCENT.

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

PETERSEN



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