Quantitative Precipitation Forecast
Issued by NWS

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FOUS11 KWBC 272021
QPFHSD

PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
321 PM EST MON FEB 27 2017

VALID 00Z TUE FEB 28 2017 - 00Z FRI MAR 03 2017

DAYS 1 THROUGH 3...

IN THE SOUTHWEST ON DAY ONE...HEAVY SNOWFALL IS LIKELY TONIGHT
INTO EARLY TUE ACROSS THE HIGHER TERRAIN OF THE MOGOLLON RIM OF AZ
NORTHEAST ACROSS THE SAN JUAN MNTNS OF SOUTHWEST CO AND NORTHERN
NM.  STRONG LOW-MID LEVEL CONVERGENCE IN ADVANCE OF THE 700 MB
TROUGH WILL INTERACT WITH A SUBTROPICAL PLUME OF MOISTURE MOVING
IN FROM THE PACIFIC TO PRODUCE THE HEAVY SNOWFALL AMOUNTS.
LOCALIZED AMOUNTS OF 1-2 FEET ARE POSSIBLE ACROSS THE HIGHEST
TERRAIN OF CENTRAL AZ AND PORTIONS OF THE SAN JUAN MOUNTAINS NEAR
THE CO/NM BORDER. OVERALL THERE IS GOOD MODEL AGREEMENT WITH THE
EVOLUTION OF THIS SYSTEM...THUS HIGH CONFIDENCE IN THE
FORECAST...WITH A MULTI MODEL BLEND GENERALLY FOLLOWED. SNOWFALL
WILL COME TO AN END BY TUESDAY NIGHT ACROSS THIS REGION AS THE
TROUGH MOVES OFF TO THE EAST AND UPPER LEVEL RIDGING BEGINS TO
BUILD INTO THE AREA.

IN THE NORTHWEST DAYS 1-3...PROLONGED SNOWFALL IS EXPECTED IN THE
WA CASCADES.  A WELL DEFINED SHORTWAVE MOVING ONSHORE AND INLAND
WITH ENHANCED 700 MB RELATIVE HUMIDITY NEAR 95 PERCENT AND ASCENT
WILL BE RESPONSIBLE FOR SNOWFALL TONIGHT ACROSS THE WA/OR CASCADES
AND THE HIGHER TERRAIN OF NORTHERN ID AND WESTERN MT AND
EVENTUALLY NORTHWEST WY.  A LULL IN THE SNOWFALL IS EXPECTED
TUESDAY.

A SURGE IN SNOWFALL COVERAGE AND INTENSITY IS EXPECTED TUESDAY
NIGHT INTO WEDNESDAY AS ADDITIONAL SHORTWAVE ENERGY AND INCREASED
UPSLOPE FLOW MOVES ACROSS THE WA CASCADES...WITH LESS COVERAGE
WITH TIME IN THE OR CASCADES AS DRIER AIR ALOFT MOVES INTO THE
REGION. THE GFS HAS A STRONG 50 KT 700 MB JET...WHICH FOCUSES
STRONG CONVERGENCE AND MOISTURE ADVECTION COMING INTO THE
WA CASCADES.

ON WED NIGHT-THU...THE NET IN THE SERIES OF WAVES MOVES ONSHORE
FROM THE NORTHEAST PACIFIC ACROSS WA AND EVENTUAL ACROSS NORTHERN
ID AND NORTHWEST MT...SO ANOTHER ROUND OF MODERATE TO HEAVY SNOW
IS EXPECTED ACROSS THE WA CASCADES.  NOT REALLY EXPECTING SNOWFALL
RATES TO BE THAT INTENSE...HOWEVER THE LONG DURATION OF THE
SNOWFALL WILL RESULT IN TOTALS OF 1-2 FEET ACROSS THE WA CASCADES
DOWNSTREAM INTO THE NORTHERN ROCKIES...WITH LOCALLY HIGHER TOTALS
IN THE NORTHERN CASCADES APPROACHING 3 FEET POSSIBLE.  AMOUNTS
NEAR A FOOT ARE EXPECTED IN THE OR CASCADES.

...UPPER MS VALLEY/UPPER GREAT LAKES...

ON DAY ONE...
THE APPROACH OF A SHORTWAVE AND LOW/MID LEVEL FRONTOGENESIS NORTH
OF THE DEVELOPING SURFACE LOW SHOULD RESULT IN SOME SNOWFALL
TONIGHT INTO EARLY TUESDAY ACROSS PORTIONS OF THE ARROWHEAD OF MN
INTO THE UP OF MI.
THE LATEST TRENDS IN THE 12Z GUIDANCE ARE FOR A FASTER SHORTWAVE
PROGRESSION.  THE RESULT WAS LIGHTER AMOUNTS THAN PRIOR
FORECASTS...PARTICULARLY IN THE 12Z ECMWF...WHICH WAS PREVIOUSLY A
SLOWER AND HEAVIER SOLUTION. A CONSENSUS OF 2-4 INCHES IS SHARED
AMONG THE 12Z GFS/NAM/ECMWF AND GEFS MEAN.

ON DAY TWO...
THE NEXT WAVE OF LOW PRESSURE MOVES A BIT FURTHER SOUTH THAN THE
PRIOR LOW...PROGRESSING ACROSS NORTHERN INDIANA AND SOUTHEAST
LOWER MI. THIS ALLOWS COLDER AIR IN THE UP OF MI TO FILTER FURTHER
SOUTH ACROSS NORTHERN MI AND CHANGE RAIN OVER TO SNOW.  THE 12Z
GFS/GEFS MEAN/12Z NAM/00Z ECMWF ENSEMBLE MEAN ARE ALL CLUSTERED
PRETTY CLOSE TOGETHER...WITH A CONSENSUS OF THESE MODELS
PREFERRED.  THE ECMWF IS SLOWER THAN THE MAJORITY TIMING WISE.
DIFFERENCES REMAIN REGARDING DURATION OF SNOW AND HOW FAR
SOUTHEAST THE PRECIP TYPE TRANSITION ZONE GETS. THE MULTI-MODEL
CLUSTERING AROUND A MOST LIKELY SNOWFALL OF 3-5" ACROSS PORTIONS
OF NORTH CENTRAL TO NORTHERN LOWER MI.

...THE NORTHEAST ON DAY 3...
THE WAVE WHICH CROSSES THE UPPER LAKES ON DAY 2 MOVES NORTHEAST
ACROSS ONTARIO AND QUEBEC...WITH THE SOUTHERN EXTENT CLIPPING
WESTERN AND NORTHERN NY AND NORTHERN NEW ENGLAND.  IN MOST AREAS
THE INITIAL TEMPERATURES ARE TOO WARM FOR SNOW.  THE LOW LEVEL
FRONTAL PASSAGE LEADS TO COLD ADVECTION AND EVENTUAL CHANGEOVER
FROM RAIN TO SNOW. THE LOW LEVEL NORTHWEST FLOW FAVORS
ACCUMULATING SNOW IN WINDWARD TERRAIN ACROSS NORTHERN NY/NORTHERN
VT AND NH AND WESTERN AND NORTHERN MAINE.

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

PETERSEN



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