Quantitative Precipitation Forecast
Issued by NWS

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FOUS11 KWBC 220842
QPFHSD

PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
329 AM EST SUN JAN 22 2017

VALID 12Z SUN JAN 22 2017 - 12Z WED JAN 25 2017

DAYS 1-3...

...CA/GREAT BASIN/SOUTHWEST/CENTRAL TO SOUTHERN ROCKIES...

A PERSISTENT UPPER LEVEL CIRCULATION OFF THE NORTHWEST COAST
COMBINES WITH A POWERFUL 150-170 KT JET AROUND 300 MB COMING
ASHORE IN CENTRAL CA THIS MORNING TO PROMOTE WELL DEFINED UPPER
DIVERGENCE/LOWER CONVERGENCE MAXIMA PLUS STRONG INTEGRATED VAPOR
TRANSPORT ACROSS SOUTHERN CA. THE COMBINED STRONG WATER VAPOR
TRANSPORT AND ASCENT LEAD TO HEAVIER PRECIPITATION IN THE
MOUNTAINS OF CENTRAL AND SOUTHERN CA ON DAY 1. THE MODELS SHOW 3-5
INCHES LIQUID EQUIVALENT IN THE CENTRAL TO SOUTHERN CA SIERRA
NEVADA RANGE. THE GUIDANCE SUGGESTS 3 TO 4 FEET OF SNOW FOR THE
CENTRAL TO SOUTHERN SIERRA NEVADA RANGE. SEVERAL INCHES OF SNOW
ARE EXPECTED IN THE SAN GABRIEL/SAN BERNADINO MOUNTAINS AND IN
FAVORED TERRAIN IN NV...WHERE DEEP LAYER MOISTURE AND MID LEVEL
ASCENT SUPPORT SNOW IN WINDWARD TERRAIN.

AS THE JET AND DEEP LAYER MOISTURE PENETRATE INLAND SUN...HEAVY
SNOW DEVELOPS IN THE UT WASATCH AND CONTINUES INTO EARLY MON AND
SPREADS DOWN IN TO THE AZ MOGOLLON RIM AND THEN INTO THE RANGES OF
SOUTHWEST CO MON NIGHT AS THE PASSAGE OF THE UPPER JET PLACES
THESE AREAS IN FAVORED UPPER DIVERGENCE MAXIMA REGIONS OF THE JET
AND THEN LEFT EXIT REGION OF THE UPPER LEVEL JET. SNOW OCCURS AS
THE 700 MB WAVE PROGRESSES ACROSS THE RANGES OF WY.
ON TUE THE JET DEPARTS FOR THE PLAINS...SO THE UPPER JET AND
ASSOCIATED UPPER DIVERGENCE WEAKENS AND SNOW TAPERS ACROSS THE
RANGES OF CO/NM.  MORE ACCUMULATING SNOW IS POSSIBLE IN FAVORED
TERRAIN...JUST NOT AS MUCH AS ON MON DUE TO THE SHORTER DURATION.
THE FORECASTS USED THE 12-00Z ECMWF AND 00Z UKMET QPF AND
TEMPERATURE PROFILES.

...NORTHERN PLAINS TO UPPER MS VALLEY...

BY MON EVENING INTO TUES... THE STORM SYSTEM EMERGES FROM THE
CENTRAL ROCKIES AND BEGINS TO EJECT OUT INTO THE CENTRAL PLAINS.
THE CONSENSUS SHOWS A 700 MB WAVE DEVELOPING IN WY AND NORTHERN CO
MON AND MON NIGHT WITH A CLOSED LOW DEVELOPING NEAR 12Z TUE AND
PROGRESSING ROUGHLY ACROSS NEBRASKA AND IOWA.  MID LEVEL
FRONTOGENESIS NORTH OF THE CIRCULATION APPEARS TO BE THE FOCUS FOR
SNOW...SO SEVERAL INCHES ARE EXPECTED IN THIS BAND...WHICH THE
CURRENT CONSENSUS TARGETS TO BE SOUTHERN SD AND SOUTHERN MN AND
ADJACENT WISCONSIN. THERE ARE STILL TRACK AND TIMING DIFFERENCES
TO BE RESOLVED SO THIS COULD SHIFT LATER.  FOR EXAMPLE...THE GFS
SHOWS A FASTER WEST-EAST TRACK...WHICH IS ITS TYPICAL BIAS.  AL;SO
THE 00Z NAM QPF AXIS IS FURTHER SOUTH IN IOWA...SO THESE CONTRASTS
WILL BE WATCHED FOR LATER RESOLUTION.  FOR NOW WE GAVE GREATER
WEIGHTING TO THE MORE CONSISTENT ECMWF AND NOW THE 00Z UKMET.


...INTERIOR NEW YORK AND NEW ENGLAND DAYS 2 AND 3...

A SOUTHERN STREAM SYSTEM IS FORECAST ON MONDAY TO MOVE ACROSS THE
MID ATLANTIC STATES...AND CONNECT WITH A NORTHERN STREAM TROUGH
OVER THE GREAT LAKES.  MOISTURE ADVECTS RAPIDLY INLAND ON THE NOSE
OF THE LOW LEVEL JET...BRINGING A THREAT OF SNOW BACK INTO WESTERN
NY AND THE LAKES. AN IMPRESSIVE RIDGE OF HIGH PRESSURE REMAINS TO
THE NORTH IN EASTERN CANADA...WHICH ACTS TO HOLD IN LOW LEVEL COLD
AIR OVER NORTHERN NEW YORK/NORTHERN NEW ENGLAND.

THE MID LEVEL WARM ADVECTION CAUSES A LIKELY CHANGE OVER FROM SNOW
TO MIXED PRECIP ACROSS SOUTHERN NEW YORK/FAR NORTHERN NJ/NORTHEAST
PA AND SOUTHERN NEW ENGLAND.  THIS TRANSITION TAKES A LITTLE
LONGER THE HIGHER UP AND THE FURTHER NORTH YOU GO.  AS A
CONSEQUENCE...SIGNIFICANT ICE ACCUMULATIONS ARE POSSIBLE IN
MOUNTAIN AREAS OF WESTERN MA/VT/NH INTO INTERIOR MAINE.

SEVERAL INCHES OF SNOW IS POSSIBLE IN AREAS NEAR EASTERN LK
ONTARIO AND INTO THE ST LAWRENCE VALLEY AND NORTHERN MAINE...BUT
AS YOU GO FURTHER SOUTH...THE MORE LIKELY THERE WILL BE A CHANGE
OVER TO MULTIPLE PRECIP TYPES.

A DEVELOPING AREA OF LOW PRESSURE FORMS NEAR OR JUST OFFSHORE FROM
CAPE COD ON TUE.  THIS FOCUSES PRECIP IN EASTERN NEW ENGLAND AND
SHUTS OFF WARM ADVECTION INLAND. THE MODELS EVEN INDICATES
POTENTIAL FOR PRECIP TO END AS A PERIOD OF SNOW IN NORTHEASTERN NY
AND VT/NORTHERN NH AND NORTHERN ME.

CONFIDENCE IS LOWER THAN NORMAL OWING TO THE POTENTIAL FOR
MULTIPLE TYPES OF PRECIPITATION AND UNCERTAIN DURATION OF EACH
TYPE. THE MODELS HAVE TYPICAL SPREAD AS TO WHEN THE TRIPLE POINT
LOW DEVELOPS AND WHERE.  THE FORECAST EQUALLY WEIGHTED THE
NM/12-00Z ECMWF/00Z UKMET/00Z GEFS MEAN/21 SREF MEAN QPF AND
TEMPERATURE PROFILES.

THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT ON
DAY 1.

PETERSEN

$$




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