Quantitative Precipitation Forecast
Issued by NWS

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FOUS11 KWBC 262148
QPFHSD

PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
448 PM EST FRI DEC 26 2014

VALID 00Z SAT DEC 27 2014 - 00Z TUE DEC 30 2014


DAYS 1 THROUGH 3...


...CENTRAL PLAINS/UPPER MS VALLEY/GREAT LAKES...

A BAND OF MID LEVEL FRONTOGENESIS STRETCHING SOUTHWEST TO
NORTHEAST ACROSS THE CENTRAL PLAINS/UPPER MIDWEST TO THE GREAT
LAKES SHOULD RESULT IN BANDS OF HEAVY SNOW WITHIN AN AREA OF
GENERALLY LIGHT SNOW TONIGHT INTO SATURDAY.  BROADSCALE SYNOPTIC
ASCENT AIDED BY INCREASING JET LEVEL WINDS...A MID LEVEL VORT
CENTER EJECTING NORTHEAST FROM THE WESTERN HI PLAINS TONIGHT AND A
CENTER OF LOW PRESSURE MOVING NORTHEAST ALONG A COLD FRONT WILL
EVENTUALLY BE AIDED BY A PERIOD OF ENHANCED MID LEVEL LIFT FROM
MESOSCALE PROCESSES.  INITIALLY THE BEST CHANCES OF 4 OR MORE
INCHES OF SNOW WILL BE OVER THE PLAINS THIS EVENING AS UPPER LEVEL
FLOW BACKS AND BECOMES HIGHLY DIFLUENT AHEAD OF THE MID LEVEL VORT
CENTER.  THE ENVIRONMENT WILL INITIALLY BE LESS FAVORABLE FOR
HEAVY SNOW WHEN COMPARED TO THE AREAS OVER THE PLAINS...BUT
INCREASING JET LEVEL WINDS ALOFT AND 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.

...NEW ENGLAND...

THE COLD FRONT MENTIONED ABOVE WILL BE TRANSLATING EASTWARD
SATURDAY NIGHT INTO SUNDAY WHILE AREAS OF LOW PRESSURE CONTINUE TO
MOVE NORTHEAST ALONG THE FRONT.  WHILE THE THREAT OF 4 INCHES OF
SNOW OR MORE STAYS WEST OF THE AREA...THERE APPEARS TO BE A WINDOW
OF OPPORTUNITY FOR SOME LIGHT FREEZING PCPN IN ME WHEN THE SURFACE
LOW IS STILL UP IN CANADA AND AN ASSOCIATED WARM FRONT IS TO THE
SOUTH. GIVEN THE SHORT AMOUNT OF TIME IN WHICH TEMPS ALOFT HAVE
WARMED ABOVE FREEZING WHILE TEMPERATURES AT THE SURFACE WERE
SUBFREEZING...THE AMOUNT OF ICING LOOKS MINIMAL AND NOT LIKELY TO
EXCEED A QUARTER INCH OR GREATER.


...PACIFIC NORTHWEST/NORTHERN ROCKIES...

A STRONG SHORT WAVE TRACKING SOUTHEAST OUT OF BRITISH COLUMBIA
INTO THE NORTHERN ROCKIES ON DAY 2 WHICH THEN DROPS INTO A LONG
WAVE TROUGH POSITION FORMING OVER THE WESTERN UNITED STATES ON DAY
3. FOR THE MOST PART...THE 12Z MODEL GUIDANCE HANDLED THE SYSTEM
COMPARABLY...WITH ONLY THE NAM LOOKING TO BE A STRONG/SLOW
OUTLIER.

THE LOW LEVEL FLOW FOCUSES THE MOISTURE ON THE HIGHER
TERRAIN...WHERE SNOW LEVELS START THE DAY AS LOW AS 4500 FEET
ACROSS THE CASCADES OF WA/OR...THE BLUE MOUNTAINS IN OR...AND THE
BITTERROOT AND SAWTOOTH RANGES OF ID. THE BEST SYNOPTIC SCALE LIFT
OCCURS DURING THE SECOND HALF OF THE PERIOD...AND THE COMBINATION
OF LIFT AND MOISTURE SUPPORTS AN AXIS OF 8 INCHES OR MORE OF
SNOWFALL ACROSS THE WA CASCADES...WITH LESSER AMOUNTS (4 TO 8
INCHES) OVER THE OR CASCADES. DOWNSLOPING SCOURS SOME OF THE
PACIFIC MOISTURE FROM THE SYSTEM...BUT THE COMBINATION OF STRONG
SYNOPTIC SCALE LIFT AND UPSLOPE FLOW OVER THE BITTERROOTS AND
SAWTOOTH RANGES SHOULD MAKE THE MOST OF THE MOISTURE REMAINING IN
THE COLUMN.

A CLOSED MID LEVEL SYSTEM CROSSES THE PACIFIC NORTHWEST DURING THE
SECOND HALF OF DAY 3...WHICH USHERS ARCTIC AIR IN FROM WESTERN
CANADA INTO THE NORTHERN ROCKIES...WITH A BROAD AREA OF UPSLOPE
FLOW DEVELOPING ACROSS THE NORTHERN ROCKIES BEHIND THE ARCTIC
FRONT. MOISTURE AVAILABILITY INCREASINGLY BECOMES A LIMITING
FACTOR FOR WINTRY PRECIPITATION...ESPECIALLY ACROSS PORTIONS OF
ID/WESTERN MT/NORTHWEST WY WITH TIME. THE COMBINATION OF
STEEPENING MID LEVEL LAPSE RATES AND INCREASING UPSLOPE FLOW
SHOULD RESULT IN AREAS OF 4 TO 6 INCHES OF SNOW ACROSS THE HIGHER
TERRAIN OF THE BITTERROOTS AND SAWTOOTH RANGES IN ID...THE ROCKY
MOUNTAIN FRONT RANGE IN MT...AS WELL AS THE GRAND TETONS IN WY.

BANN

$$





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