Quantitative Precipitation Forecast
Issued by NWS

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000
FOUS11 KWBC 202117
QPFHSD

PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
417 PM EST SAT JAN 20 2018

VALID 00Z SUN JAN 21 2018 - 00Z WED JAN 24 2018


...WY/CO/NM ROCKIES THROUGH CENTRAL PLAINS/UPPER MS VALLEY/UPPER
GREAT LAKES/NORTHERN NEW YORK AND ENGLAND...

THE LOW OVER WESTERN CO TONIGHT GRADUALLY DECAYS WITH THE LEE LOW
DEVELOPMENT IN SOUTHEAST CO TO NEAR THE KS/OK BORDER SUN ALLOWS
COLD AIR TO DRAIN SOUTH FROM WY ACROSS THE PLAINS OF CO.
700 MB CONVERGENCE/300 MB DIVERGENCE MAXIMA DRIVE SNOW ACROSS
EASTERN WY AND CENTRAL TO EASTERN CO TONIGHT AND THEN FURTHER EAST
ON THE PLAINS TO WESTERN NE TO THE SD BORDER SUN.
HEAVY SNOW ARE EXPECTED ACROSS THE RANGES OF WESTERN TO CENTRAL CO
AND SOUTHEAST WY TO WESTERN NE...
WHERE AREAS OF 8 TO 14 INCHES ARE EXPECTED. MORE MODERATE AMOUNTS
FURTHER SOUTH ACROSS EASTERN CO...SIGHTLY LOWER THE FURTHER SOUTH
YOU GO. THE UNCERTAINTY EXTENDS TO THE NORTHERN EDGE OF THE PRECIP
SHIELD IN WY TO SOUTHERN SD...WITH A TIGHT QPF AND SNOWFALL
GRADIENT.

ON DAY 2....THE NORTHEAST TRACK OF THE LOW-MID LEVEL CIRCULATION
PLACES PEAK 700 MB THETA-E ADVECTION AND CONVERGENCE ALONG A SWATH
FROM NORTHEAST NEBRASKA ACROSS SOUTHEAST SD...SOUTHERN
MN...NORTHWEST IA AND NORTHERN WI TO THE ADJACENT UP OF MI. THE
12Z NAM DROPPED THE AXIS/ORIENTATION OF THIS SWATH SOUTH...WITH
ANOTHER ADJUSTMENT ON THE 18Z RUN.  THE 06-12Z GEFS AND 09-15Z
SREF MEAN HAVE HELD STEADIER WITH THE AXIS/ORIENTATION OF HEAVIER
AMOUNTS. THERE IS POTENTIAL FOR A BAND OF A FOOT OF SNOW WHERE
PEAK LOW-MID LEVEL FRONTOGENESIS AND ASCENT PERSISTS.

PRECIP TYPE UNCERTAINTY DEVELOPS WITH POTENTIAL FOR A TRANSITION
FROM SNOW TO SLEET/FREEZING RAIN ACROSS SOUTHEAST NEBRASKA TO
NORTHERN IOWA...SOUTHERN WI AND NORTHERN LOWER MI.
THE NAM AND 09Z SREF MEAN SHOWS POTENTIAL FOR AREAS OF A QUARTER
INCH OF ICING IN SOUTHEAST MN TO SOUTHERN WI AND NORTHERN LOWER
MI.

ON DAY 3...THE CIRCULATION CROSSES THE UPPER GREAT LAKES AND INTO
CANADA. SNOW PERSISTS THE FIRST SEVERAL HOURS IN THE UP OF MI AND
NORTHERN WI BEFORE TRANSITIONING TO LAKE ENHANCED SNOWS. THE
STRONG LOW-MID LEVEL DOWNSTREAM WARM ADVECTION ACROSS THE LOWER
LAKES INTO NEW YORK AND NEW ENGLAND MEANS THE LIKELIHOOD IS SNOW
QUICKLY TRANSITIONS TO A MIXTURE OF PRECIP TYPES...CUTTING DOWN ON
SNOW AMOUNTS.  THE MODEL CONSENSUS INDICATES POTENTIAL FOR AREAS
OF A QUARTER INCH OF ICING IN THE NY ADIRONDACKS/GREEN MOUNTAINS
OF VT/WHITE MOUNTAINS OF NH/MOUNTAINS OF WESTERN ME.  THE PRIMARY
SNOW THREAT EXTENDS ACROSS NORTHERN MAINE...WITH SEVERAL HOURS OF
SNOW POSSIBLE BEFORE A MIX AND CHANGE OVER TO MIXED SLEET/FREEZING
RAIN.

...PACIFIC NORTHWEST INTO NORTHERN ROCKIES...

A MID-UPPER LEVEL JET WITH 700 MB WINDS FORECAST BY THE ECMWF TO
BE 50-70 KT COMING ONSHORE INTO OR BY SUN MORNING PRODUCES AN
EXTENDED FETCH OF PAC MOISTURE FROM VANCOUVER ISLAND SOUTH ACROSS
WESTERN WA/OR/NORTHWEST CA.  ON DAY 1 THE QPF/LIQUID EQUIVALENT
MAXIMA ARE FCST INTO THE WA OLYMPICS AND CASCADES....ENDING SOUTH
ALONG THE COASTAL RANGES OF WA/OR TO FAR NORTHWEST CA....WHERE
SEVERAL INCHES OF SNOW ARE EXPECTED  INTO THE SISKIYOUS/SOUTHERN
OR CASCADES.
THE IMPRESSIVE AMOUNTS OF MOISTURE LEADS TO THE POSSIBILITY OF
18-24 INCHES OF SNOW IN THE HIGHER ELEVATIONS OF THE WA OLYMPICS
AND NORTHERN CASCADES.

ON DAY 2...SEVERAL ADDITIONAL INCHES ARE EXPECTED IN THE SISKIYOU
AND CRUSADES OF NORTHERN CA/ADJACENT SOUTHEAST OR AS WARM
ADVECTION SOUNDINGS PROFILES SUPPORT CONTINUED LIFT.
THE LOW LEVEL FRONT PROGRESSES INLAND IN WESTERN OR/WA BUT SNOW
ARE EXPECTED TO CONTINUE IN THE WA CASCADES IN THE POST FRONTAL
AIR MASS AS DEEP MOISTURE REMAINS AND DEEP LAYER WESTERLY FLOW
PRODUCES TOPOGRAPHICALLY FORCED ASCENT IN WINDWARD FACING TERRAIN.
 LOCAL HEAVY TOTALS AGAIN ARE POSSIBLE IN THE WA OLYMPICS AND
NORTHERN CASCADES. SNOW DEVELOPS FURTHER INLAND AS THE LOW LEVEL
FRONT CROSSES THE RANGES OF ID AND INTO NORTHWEST MT.

ON DAY 3 THE UPPER TROUGH DEAMPLIFIES WITH WEAKER UPPER
DIVERGENCE/LOWER CONVERGENCE MAXIMA...SO THE TREND IS TOWARDS
LIGHTER AMOUNTS ACROSS THE NORTHWEST AS A MIDDLE LEVEL RIDGE
CROSSES THE REGION. A CYCLONE DEVELOPS OVER THE NORTHEAST PACIFIC
BUT AHEAD OF THE SYSTEM...DEEP SOUTHWEST FLOW SENDS WARMER AIR
INTO THE PACIFIC NORTHWEST...KEEPING SNOW LEVELS IN NORTHERN CA
AND WESTERN OR HIGH. SLIGHTLY HIGHER AMOUNTS ARE EXPECTED IN THE
NORTHERN WA CASCADES AS INITIAL CONDITIONS ARE COOLER.

EXPECT HEAVY SNOWFALL OF 3-4 FEET TO OCCUR FROM THE WA
CASCADES/OLYMPICS...WITH A FOOT POSSIBLE IN  PORTIONS OF THE OR/CA
CASCADES INTO THE NORTHERN CA SIERRA NEVADA RANGE, AND IN THE ID
PANHANDLE.

THE PROBABILITY OF 0.25 INCHES OF ICING IS LESS THAN 10 PERCENT ON
DAY 1.

PETERSEN



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