Quantitative Precipitation Forecast
Issued by NWS

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FOUS11 KWBC 210829
QPFHSD

PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
328 AM EST SUN DEC 21 2014

VALID 12Z SUN DEC 21 2014 - 12Z WED DEC 24 2014


DAYS 1-2...

...PACIFIC NORTHWEST TO THE NORTHERN...CENTRAL AND SOUTHERN
ROCKIES...

A POWERFUL PACIFIC JET WILL DRIVE MOISTURE INLAND ACROSS THE
PACIFIC NORTHWEST AND NORTHERN AND CENTRAL ROCKIES AS IT ENTERS A
HIGHLY DIFFLUENT REGION DOWNWIND OF THE JET.  THE JET WILL
CONTINUE SOUTH AND EASTWARD AND BEGIN TO CARVE OUT A DEEPENING
UPPER TROUGH OVER THE CENTER OF THE NATION AS HEIGHTS RISE OVER
THE EASTERN PACIFIC OCEAN.  SURGES OF SYNOPTICALLY DRIVEN ASCENT
AIDED BY TOPOGRAPHY WILL CONTINUE TO PRODUCE SNOW ACROSS THE
HIGHER ELEVATIONS OF THE WEST WITH SNOWFALL TOTALS EXCEEDING 1 TO
2 FEET ACROSS PORTIONS OF THE WASHINGTON CASCADES ACROSS THE HIGH
ELEVATIONS OF CENTRAL IDAHO AND WESTERN MONTANA ACROSS THE
MOUNTAINS OF WYOMING/UTAH AND COLORADO.

ON DAY 1/SUNDAY MORNING TO MONDAY MORNING...THE 150+ KNOT JET NEAR
250 MB WILL DEEPEN THE TROUGH OVER THE CENTRAL US AS THE JET DROPS
SOUTHEASTWARD ACROSS THE ROCKIES.  THE LOWER EXTENSION OF THE JET
WILL CROSS MUCH OF THE WESTERN MOUNTAINS...YIELDING HEAVY UPSLOPE
SNOW AND STRONG GUSTY WINDS ACROSS THE WASHINGTON CASCADES/THE
SAWTOOTH/SALMON RIVER AND BITTERROOTS OF IDAHO/MONTANA...ALL THE
MAJOR RANGES IN WYOMING...THE WASATCH/UINTA IN UTAH AND ACROSS
MUCH OF THE HIGH PEAKS OF THE ROCKIES.

ON DAY 2/MONDAY MORNING TO TUESDAY MORNING...THE UPPER TROUGH OVER
THE CENTER OF THE COUNTRY CONTINUES TO AMPLIFY AS AN UPPER LEVEL
LOW FORMS OVER THE NORTHERN PLAINS TO THE UPPER MISSISSIPPI
VALLEY.  WHILE THE DEVELOPMENT OF THE UPPER LOW IS ASSOCIATED WITH
SNOW OVER THE NORTHERN PLAINS AND UPPER MISSISSIPPI
VALLEY...DISCUSSED IN THE NEXT SECTION...THE STRONG JET CONTINUES
TO PLUNGE SOUTHWARD OVER THE ROCKIES WITH THE JET CORE OVER THE
CENTRAL ROCKIES.  SNOW WILL TAPER OFF FROM NORTHWEST TO SOUTHEAST
ACROSS THE WASHINGTON CASCADES/THE MOUNTAINS OF IDAHO/MONTANA AND
WYOMING WHILE HEAVY SNOW WILL CONTINUE TO FALL OVER THE
WASATCH/UINTA IN UTAH AND ESPECIALLY OVER THE COLORADO ROCKIES AND
MOVE INTO THE HIGHER ELEVATIONS OF NORTHEASTERN NEW MEXICO.  THE
HIGHEST PROBABILITIES OF GREATER THAN A FOOT OF SNOW IS MOST
LIKELY OVER THE COLORADO ROCKIES.  IN GENERAL...THE 00Z PARALLEL
RUN OF THE GFS AND THE 00Z ECMWF APPEAR TO BE A GOOD MATCH FOR A
SYNOPTIC SITUATION WITH RELATIVELY HIGH UNCERTAINTY AS THE UPPER
LEVEL EVOLUTION OF THE TROUGH OVER THE CENTER OF THE COUNTRY IS
TRENDING FROM A MAJOR CYCLONE TOWARD A MORE COMPLEX AND POSSIBLY
WEAKER SOLUTION.


DAYS 1 TO 3...

...NORTHERN PLAINS TO UPPER MISSISSIPPI VALLEY/MIDWEST...

THE DEEPENING TROUGH OVER THE CENTRAL US WILL BE ASSOCIATED WITH A
SURFACE LOW MOVING SOUTH AND EASTWARD ALONG THE LEE OF THE
CANADIAN ROCKIES ACROSS NORTHEAST MONTANA INTO NORTH DAKOTA ON DAY
1.  A CONVERGENT BOUNDARY IS EXPECTED TO BECOME ESTABLISHED FROM
THE LOW CENTER NORTHWESTWARD TOWARD THE NORTH DAKOTA/MONTANA
BORDER SEPARATING NORTHEASTERLY FROM NORTHWESTERLY FLOW AND SHOULD
HELP SET UP AN AREA OF SNOW ESPECIALLY ALONG AND WEST OF THIS
BOUNDARY WITH MOSTLY LIGHT AMOUNTS EXPECTED ON DAY 1/SUNDAY TO
MONDAY MORNING. IN ADDITION...WARM AIR ADVECTION AHEAD OF THE
DEEPENING TROUGH WILL CAUSE MOSTLY LIGHT SNOWS FROM NORTHEAST
IOWA/EASTERN MINNESOTA ACROSS CENTRAL AND NORTHERN WISCONSIN AND
THE UP OF MICHIGAN.  HOWEVER...THERE ARE ONLY VERY LOW ODDS THAT
LOCAL SNOWFALL AMOUNTS WILL APPROACH 4 INCHES.

ON DAY 2/MONDAY MORNING TO TUESDAY MORNING...A MORE WIDESPREAD
AREA OF SNOWFALL DEVELOPS AROUND THE SLOW MOVING LOW PRESSURE
SYSTEM MOVING FROM THE NORTHERN PLAINS TO THE UPPER MISSISSIPPI
VALLEY.  WHILE SNOWFALL WILL  BE WIDESPREAD...IT IS AGAIN LIKELY
TO BE RELATIVELY LIGHT WITH A LOW TO MODERATE PROBABILITY OF 4 TO
8 INCHES OF SNOW ACROSS SOUTH DAKOTA INTO EASTERN NORTH DAKOTA AND
ACROSS NORTHERN MINNESOTA.  AGAIN...A BLEND OF THE 00Z PARALLEL
RUN OF THE GFS AND THE 00Z ECMWF WAS GENERALLY FOLLOWED FOR BOTH
TEMPERATURES AND QPF AMOUNTS.

DAY 3 STILL REMAINS HIGHLY UNCERTAIN WITH THE EVOLUTION OF THE
COMPLEX UPPER TROUGH OVER THE MIDDLE OF THE COUNTRY. BY FOLLOWING
THE 00Z GFS PARALLEL AND ECMWF ALLOWS US TO EMPHASIZE A RATHER
WEAK/BROAD CYCLONIC CIRCULATION EMPHASIZING AGAIN MOSTLY LIGHT
SNOW ACROSS NORTHERN IOWA/MINNESOTA/CENTRAL AND NORTHERN WISCONSIN
AN UPPER MICHIGAN.  SINCE SOME SNOW IS LIKELY ON ALL THREE
DAYS...A THREE DAY TOTAL OF 4 TO 8 INCHES IS HIGHLY PROBABLE
ALTHOUGH THERE IS ONLY A SMALL TO AT BEST MODERATE PROBABILITY OF
GREATER THAN 4 INCHES ON ANY ONE DAY.

THE PROBABILITY OF SIGNIFICANT ICING (QUARTER OF AN INCH) IS LESS
THAN 10 PERCENT DAYS 1-3.

KOCIN

$$




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