Quantitative Precipitation Forecast
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FOUS11 KWBC 212115
QPFHSD

PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
414 PM EST FRI NOV 21 2014

VALID 00Z SAT NOV 22 2014 - 00Z TUE NOV 25 2014


PACIFIC NORTHWEST/GREAT BASIN/NORTHERN AND CENTRAL ROCKIES...

MODELS ARE IN GENERAL AGREEMENT THAT PERIODIC MODERATE AND EVEN
HEAVY SNOWS WILL OVERSPREAD THE MOUNTAINS OF THE PACIFIC NORTHWEST
INTO THE GREAT BASIN AND ROCKIES THE NEXT 3 DAYS...WITH THE
HIGHEST RATES OCCURRING DURING THE PASSAGE OF 2 SHORTWAVE TROUGHS.
 THE FIRST TROUGH CROSSES FROM THE PACIFIC NORTHWEST ACROSS THE
NORTHERN ROCKIES AND GREAT BASIN TONIGHT INTO SAT. SPORADIC
COUPLETS OF 300 MB DIVERGENCE AND 700 MB CONVERGENCE COMBINE WITH
OROGRAPHIC LIFT TO PRODUCE PERIODS OF SNOW ACROSS THE HIGHER
ELEVATIONS OF WA/OR/ID/WESTERN MT/WY/UT.

THE SECOND WAVE ARRIVES OVER THE PACIFIC NORTHWEST EARLY
SUNDAY...WITH WELL DEFINED CONVERGENCE ON THE NOSE OF THE 700 MB
JET FCST TO BE 50-60 KT BY THE GFS COMING INTO THE WA CASCADES.
THE CONVERGENCE MAXIMA STREAMS INTO THE RANGES OF NORTHEAST OR AND
ID/WESTERN MT LATER ON SUNDAY AS THE 700 MB JET CONTINUES ONSHORE
AND INLAND. THE BROAD NORTHWEST TO SOUTHEAST 300 MB JET CROSSING
NORTHERN CA/NORTHERN NV/SOUTHERN UT SUPPORTS DIFLUENT FLOW ALOFT
AND EMBEDDED DIVERGENCE MAXIMA CROSSING THE RANGES OF UT INTO
WESTERN CO TO HELP SNOW SHOWERS CONTINUE THERE.

ON SUN NIGHT...MON...CONTINUED MOIST WEST NORTHWEST DEEP LAYER
FLOW GRADUALLY BECOMES NORTHWEST WITH VEERING ACROSS THE NORTHWEST
IN RESPONSE TO A BUILDING UPSTREAM MID-UPPER LEVEL RIDGE.  INITIAL
DEEP MOISTURE LEADS TO CONTINUING SNOW SHOWERS ACROSS THE NORTHERN
TO CENTRAL ROCKIES BUT THE UPSTREAM RIDGE STARTS ADVECTING DRIER
AIR ALOFT ACROSS THE REGION AND INDUCES SINKING MOTION ALOFT.
CONSEQUENTLY...COVERAGE AND INTENSITY ACROSS THE RANGES OF
ID/WESTERN MT SHOULD DECREASE ON MON.

GIVEN THE INFLUENCE OF OROGRAPHIC ASCENT WITH THESE SNOW
EVENTS...HAVE CONTINUED TO RELY UPON THE HIGHER RES GUIDANCE FOR
BETTER TARGETING OF PREFERRED OROGRAPHIC SNOW FALL AREAS AND
GRADIENTS.

...UPPER GREAT LAKES...

ON DAY 1...THE MODELS/ENSEMBLES INDICATE INITIAL SUB FREEZING
CONDITIONS EXIST AT THE GROUND.  STRONG LOW-MID LEVEL WARM
ADVECTION LEADS TO WARMING ABV FREEZING FROM 900-800 MB.  A BURST
OF 700-500 MB VERTICAL MOTIONS OCCURS AND CAUSES PRECIP FROM THE
UPPER MS VALLEY TO THE GREAT LAKES TONIGHT...RESULTING IN A LIKELY
FREEZING RAIN EVENT ACROSS THE REGION.  THE SHORT DURATION OF
PRECIP LEADS TO MOSTLY LIGHT AMOUNTS AS PRECIP TAPERS WITH RAPID
DRYING ABOVE 850 MB SAT. A MODEL/SHORT RANGE ENSEMBLE CONSENSUS
WAS USED FOR QPF/TEMP PROFILES.

ON DAY 3...THE MODELS INDICATE A DECAYING OCCLUDED LOW MOVING EAST
ACROSS NORTHERN MN...GIVING WAY TO A SOUTHERN STREAM LOW
DEVELOPING IN THE MID MS VALLEY AND MOVING INTO THE UPPER GREAT
LAKES. CONVERGENCE MAXIMA ALONG THE 700 MB TROUGH AXIS AS IT
ROTATES EAST FROM NORTHERN MN ACROSS THE UP OF MI AND ADJACENT
NORTHERN WI SHOULD LEAD TO LIGHT SNOW MON AFTERNOON. INITIAL
TEMPERATURES SUN NIGHT ARE TOO WARM FOR SNOW WITH A COLD FRONTAL
PASSAGE BRINGING STRONG LOW LEVEL COLD ADVECTION ON
MONDAY...LEADING TO A CHANGE TO SNOW. NORTHWEST BNDRY LAYER WINDS
ACROSS LK SUPERIOR LEAD TO LEE SIDE CONVERGENCE TO AID IN
PRODUCING ASCENT IN THE UP OF MI TO ADJACENT WI MON DURING THE
DAY.  THE MODEL TIMING/CYCLONE INTENSITY LEAD TO A SPREAD OF
SEVERAL INCHES IN SNOW ACCUMULATION POTENTIAL.

THE PROBABILITY OF SIGNIFICANT ICING (QUARTER INCH) IS LESS THAN
10 PERCENT ALL 3 DAYS.

PETERSEN




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