Quantitative Precipitation Forecast
Issued by NWS

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FOUS11 KWBC 270811
QPFHSD

PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
311 AM EST SAT DEC 27 2014

VALID 12Z SAT DEC 27 2014 - 12Z TUE DEC 30 2014


DAYS 1 THROUGH 3...

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

THE MODELS SHOW A LONGWAVE TROUGH GRADUALLY DRIFTING SOUTH FROM
BRITISH COLUMBIA WITH THE 300 MB JET AXIS ON DAY 1 EXTENDING
ACROSS WA/OR INLAND INTO ID.  UPPER DIVERGENCE MAXIMA IN THE
PROXIMITY OF THE JET HELP PRODUCE ASCENT AND COMBINE WITH HIGH
AVAILABLE MOISTURE TO MODERATE TO HEAVY SNOW ACROSS
THE HIGHER TERRAIN ACROSS THE CASCADES OF WA/OR...THE BLUE
MOUNTAINS IN OR...AND THE BITTERROOT AND SAWTOOTH RANGES OF ID.
THE COMBINATION OF LIFT AND MOISTURE SUPPORTS AN AXIS OF 8 INCHES
OR MORE OF SNOWFALL ACROSS THE WA CASCADES...CONTINUING INLAND TO
THE RANGES OF NORTHERN ID.  LESS COVERAGE OF HEAVIER AMOUNTS IS
EXPECTED OVER THE OR CASCADES TO THE BLUE MNTNS AND RANGES OF
SOUTHERN ID.

ON DAY 2...THE CONTINUED AMPLIFICATION OF THE WESTERN TROUGH
RESULTS IN A STEADY SOUTH MOVEMENT OF THE JET ACROSS ACROSS THE
NORTHERN TO CENTRAL GREAT BASIN...WITH SNOW DEVELOPING AS FAR
SOUTH AS UT AND CO AS THE 300 MB JET DROPS INTO THESE STATES
SUNDAY NIGHT. THE PERSISTENCE OF THE JET OVER OR LEADS TO A
CONTINUING THREAT OF SNOWS IN THE OR CASCADES...WITH THE LONGER
DURATION HERE LEADING TO HIGHER AMOUNTS THAN OTHER AREAS.  STEEP
LAPSE RATES AND HIGH RELATIVE HUMIDITY LEAD TO FAVORABLE
CONDITIONS FOR SNOW SHOWERS IN THE RANGES OF ID INTO WESTERN MT
WITH HIGH SNOW RATIOS RESULTING FROM THE 7-8 DEG C/KM LAPSES FROM
700-500 MB.  A BROAD AREA OF UPSLOPE FLOW DEVELOPS ACROSS THE
NORTHERN ROCKIES BEHIND THE ARCTIC FRONT.

THE ECMWF SHOWS ANOTHER SHORTWAVE ROTATING THROUGH THE BASE OF THE
TROUGH ACROSS OR AND ID ON MONDAY...WITH DIFLUENT FLOW IN THE
UPPER TROUGH SUPPORTING SPORADIC UPPER DIVERGENCE MAXIMA AND MORE
SNOW SHOWERS IN THE OR CASCADES INTO THE RANGES OF ID.  THE JET AT
THE BASE OF THE TROUGH SUPPORTS MORE SNOW SHOWS ALONG THE JET
STREAM CROSSING THE HIGHER ELEVATIONS OF NV AND THEN UT AND CO. AS
A LOW AMPLITUDE UPPER TROUGH EXITS FOR THE CENTRAL PLAINS...THE
WEST TO EAST JET PROMOTES SNOW SHOWERS EXTENDING EAST ON TO THE
PLAINS OF CO AND NORTHWEST KS AND ADJACENT SOUTHWEST NEBRASKA.
THE MODELS HAVE COME INTO BETTER AGREEMENT WITH THE
GFS/ECMWF/UKMET CLUSTERING WELL WITH THE SOUTH DRIFT TO THE
LOW-MID LEVEL FRONT AND CIRCULATION.

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

A BAND OF MID LEVEL FRONTOGENESIS STRETCHING SOUTHWEST TO
NORTHEAST ACROSS THE UPPER MIDWEST TO THE UPPER GREAT LAKES SHOULD
RESULT IN AREAS OF SNOW OVER SOUTHERN MN OVERNIGHT TO CONTINUE
MOVING NORTHEAST ACROSS NORTHERN WI AND THE UP OF MI.  BROAD SCALE
SYNOPTIC ASCENT AIDED BY UPPER DIVERGENCE MAXIMA IN THE LEFT EXIT
REGION OF THE 300 MB JET AND A CENTER OF LOW PRESSURE MOVING
NORTHEAST ALONG A COLD FRONT WILL EVENTUALLY BE AIDED BY A PERIOD
OF ENHANCED MID LEVEL LIFT.  POCKETS OF NEGATIVE EQUIVALENT
SATURATED POTENTIAL VORTICITY WITHIN A REGION OF FRONTOGENESIS
SHOULD LEAD TO SOME LOCALLY HEAVIER SNOWFALL AMOUNTS ACROSS THE
UPPER MIDWEST INTO THE WESTERN GREAT LAKES BEFORE THE SYSTEM MOVES
NORTH OF THE BORDER AND FRONTOGENESIS WEAKENS SAT NIGHT.  THE
MODELS HAVE AVERAGE AGREEMENT ON THE AXIS OF SNOW AND AMOUNTS AND
A MULTI-MODEL/21Z SREF MEAN CONSENSUS BASED APPROACH WAS TAKEN
ALONG WITH CONTINUITY FOR AMOUNTS.

THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT.

PETERSEN


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