Quantitative Precipitation Forecast
Issued by NWS

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FOUS11 KWBC 272001
QPFHSD

PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
300 PM EST SUN NOV 27 2016

VALID 00Z MON NOV 28 2016 - 00Z THU DEC 01 2016

DAY 1 TO 3...

...WESTERN U.S...

THE DAY 1 HEAVIEST SNOW IS FORECAST TO BE IN THE OR AND WA
CASCADES WITH A FOOT OR MORE OF SNOW POSSIBLE IN THE HIGHER
ELEVATIONS.  CONFLUENT FLOW CONCENTRATES MOISTURE AND ASCENT LATE
THIS AFTERNOON AND EVENING WITH A 700 MB VERTICAL VELOCITY MAXIMA
CENTERED ON NORTHWEST OR/SOUTHWEST WA PROVIDING A PERIOD OF
MODERATE TO HEAVY SNOW AT HIGHER ELEVATIONS AS THE UPPER
DIVERGENCE MAXIMA CROSSES THE AREA ON THE NOSE OF A 150 KT 300 MB
JET MAXIMA FORECAST BY THE ECMWF.  LEFT EXIT JET REGION UPPER
DIVERGENCE ALSO EXTENDS INLAND INTO THE BLUE MNTNS...WHERE A
SECONDARY MAXIMA OF SNOW IS EXPECTED...EXTENDING INTO RANGES ON
THE ID SIDE OF THE OR BORDER.  LIKEWISE...THE JET EXTENDS
DOWNSTREAM ACROSS NV TO NEAR THE AZ/UT BORDER...WITH LEFT EXIT
REGION UPPER DIVERGENCE MAXIMA CROSSING THE UT WASATCH...WHERE
SEVERAL INCHES OF SNOW ARE POSSIBLE.

ON DAY 2 A SHARP SHORT WAVE RIDGING BUILDS INTO THE WEST COAST.
DRYING ALOFT REDUCES SNOW AMOUNTS AND COVERAGE IN THE PACIFIC
NORTHWEST.  THE CONFLUENT FLOW AND BANDS OF ENHANCED MOISTURE IS
INLAND FROM WESTERN MT AND WY SOUTH INTO UT AND WESTERN CO.

ON DAY 3 THE NEXT IN A SERIES OF 850-700 MB WAVES IS FORECAST TO
MOVE ONSHORE FORM THE NORTHEAST PACIFIC INTO THE PACIFIC
NORTHWEST. THE COMBINATION OF UPPER DIVERGENCE/LOWER CONVERGENCE
MAXIMA AND POOLING OF MOISTURE SHOULD LEAD TO SATURATING THE
INITIALLY DRY AIR MASS AND THEN A PERIOD OF SNOW IN THE OR/WA
CASCADES.  THE 0Z ECMWF HAS A HIGHER AMPLITUDE WAVE BUT THE 12Z
RUN HAS DEAMPLIFIED THE WAVE AND MOVED IT FASTER...WHICH REDUCES
THE THREAT OF HEAVY SNOW.

...NORTHERN PLAINS/UPPER MIDWEST...

A SNOWSTORM WITH A FOOT OF SNOW IS EXPECTED ACROSS THE NRN PLAINS
ON MON INTO TUES...WITH HEAVIEST AMOUNTS IN CENTRAL ND.  A DYNAMIC
SHORT WAVE WILL INDUCE A SURFACE LOW NEAR CENTRAL NE SUN EVENING
BEFORE DEEPENING ACROSS THE EASTERN DAKOTAS. THE MOISTURE PLUME
WILL WRAP AROUND THE CIRCULATION...ALLOWING SNOW TO DEVELOP SUN
EVENING FROM CENTRAL SD INTO CENTRAL ND. THIS WILL BE A RESULTANT
OF STRONG DYNAMIC COOLING ALOFT WITHIN A DEVELOPING AND EXPANDING
DEFORMATION ZONE. THIS FRONTOGENESIS ZONE WILL ENHANCE ON MON AND
PERSIST AS THE LOW PRESSURE STALLS. EXPECT HEAVY SNOW WITH WINDY
CONDITIONS IN WRN ND SPREADING INTO ERN MT AND ADJACENT PORTIONS
OF SD.  ON TUES... THE UPPER VORTEX WILL DRIFT EASTWARD INTO MN...
BEGIN TO SLIGHTLY FILL AND THE MODELS FORECAST TO VERTICAL
VELOCITY MAXIMA TO DECREASE IN COVERAGE AND INTENSITY...LIKELY
LEADING TO TAPERING SNOWFALL RATES AND RESULTANT ACCUMULATIONS
ACROSS THE DAKOTAS. A BLEND OF THE LATEST ECMWF/UKMET/09Z SREF
MEAN/12Z NAM12Z GEFS MEAN ON THE QPF/PRECIP TYPE/SNOW AMOUNTS WAS
USED NOW THAT CLUSTERING AMONG THE MODEL SOLUTIONS HAS IMPROVED.


...NORTHERN NEW ENGLAND...

A DYNAMIC SHORT WAVE WILL MOVE TOWARDS THE NORTHEAST ON TUES..WITH
THE WAVE MOVING OFF THE COAST EARLY WED.  DEEP MOISTURE AND WARMER
MID LEVEL AIR WILL SURGE INTO THE REGION... WHILE A SURFACE RIDGE
TO THE NORTH MAINTAINS SURFACE COLD AIR.  NORTH OF THE SFC LOW
TRACK WILL BE A LOW LEVEL FRONTOGENESIS AND CONVERGENCE ZONE. THIS
WILL TRIGGER ASCENT THAT RESULTS IN A VARIETY OF PRECIP TYPES FROM
SOME ICING IN VT/NH/PARTS OF SOUTHERN ME WITH SNOW FURTHER NORTH
IN NORTHERN NH AND NORTHERN MAINE.  DIFFERENCES RELATE TO THE
STRENGTH OF THE LOW LEVEL CYCLONE AND NORTH-SOUTH DIFFERENCES IN
TRACK (ECMWF FURTHER NORTH THAN THE GFS)...WHICH WILL GET RESOLVED
IN LATER RUNS. TIMING DIFFERENCES IMPACT THE DURATION AS THE 12Z
RUN OF THE ECMWF HAS MATCHED THE NAM FASTER MOTION WHILE THE
GFS/GEFS MEAN HAVE BECOMES A FEW HOURS SLOWER IN FORWARD MOTION
WITH THE LOW.

THE PROBABILITY OF SIGNIFICANT (QUARTER INCH OR MORE) ICING IS
LESS THAN 10 PERCENT.

PETERSEN

$$





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