Quantitative Precipitation Forecast
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FOUS11 KWBC 202024
QPFHSD

PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
323 PM EST SAT DEC 20 2014

VALID 00Z SUN DEC 21 2014 - 00Z WED DEC 24 2014


DAYS 1-3...

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

A POWERFUL PACIFIC JET WILL DRIVE MOISTURE INLAND ACROSS THE
PACIFIC NORTHWEST AND NORTHERN AND CENTRAL ROCKIES.  THIS COMBINES
WITH SURGES IN SYNOPTICALLY DRIVEN ASCENT AIDED BY TOPOGRAPHY TO
PRODUCE HEAVY SNOW ACROSS HIGHER ELEVATIONS
ACROSS THESE REGIONS. FAVORED MOUNTAINS/RANGES SHOULD RECEIVE
MULTI-DAY TOTALS OF 2-3 FEET OF SNOW.

ON DAY 1/SATURDAY NIGHT TO SUNDAY AFTERNOON...A POWERFUL 300 MB
JET NEAR 150 KT SURGES ONSHORE INTO WA STATE WITH WELL DEFINED
UPPER DIVERGENCE MAXIMA SUPPORTING STRONG ASCENT AND MODERATE TO
HEAVY PRECIPITATION IN THE WA CASCADES DOWNSTREAM INTO THE RANGES
OF SOUTHERN TO CENTRAL ID/NORTHEAST OR/WESTERN MT AND THEN
NORTHWEST WY AND NORTHERN UT.  AS THE 700 MB JET OF 50 KT + SPEEDS
MOVES TOWARDS THE WY/CO BORDER...MOISTURE FLUXES INTO THE RANGES
OF NORTHERN CO COMBINED WITH 700 MB CONVERGENCE LEADS TO SNOW
INCREASE IN COVERAGE/INTENSITY SUN AFTERNOON.

ON DAY 2...SUNDAY EVENING TO MONDAY AFTERNOON...SNOW TAPERS IN THE
WASHINGTON CASCADES THROUGH THE RANGES OF ID/OR/NORTHWEST MT AS
THE BUILDING UPSTREAM MID-UPPER RIDGE ADVECTS DRIER AIR INLAND.
HOWEVER...HEAVY SNOW WILL CONTINUE ACROSS THE MOUNTAINS IN
SOUTHERN WYOMING/THE WASATCH/UINTAS IN UTAH AND THE COLORADO
ROCKIES...AS LAYER 700-500 MB ASCENT CONTINUES MOST OF THIS
FORECAST PERIOD...WITH LOW DIRECTIONAL SHEAR IN THE WEST NORTHWEST
LAYERED FLOW. FAVORED UPSLOPE AREAS SHOULD RECEIVE HEAVY SNOW.

ON DAY 3...THE 700 MB RIDGE IS FCST TO DRIFT EAST ACROSS THE
NORTHWEST WITH ANOTHER 700 MB JET INCREASING TO NEAR 50 KT COMING
INTO VANCOUVER ISLAND...WITH 30-40 KT COMING INTO THE RANGES OF
NORTHERN WA.  THIS INDUCES ANOTHER SURGE OF MOISTURE THAT GETS
ADVECTED ONSHORE AND THEN INLAND INTO THE NORTHERN CASCADES OF
WA...WHERE SEVERAL MORE INCHES OF SNOW ARE INDICATED. SNOW
DECREASES IN THE CO ROCKIES AS THE UPPER TROUGH DEPARTS AND DRIER
AIR ALOFT STARTS MOVING INTO THE REGION.

A MULTI MODEL/SHORT RANGE ENSEMBLE(SREF) BLEND WAS USED ON DAY 1
WHILE A COMBINATION OF 00Z ECMWF/09Z SREF MEAN QPF/TEMPERATURE
PROFILES WAS FAVORED ON DAYS 2/3.


DAYS 2 TO 3...

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

THE DEEPENING TROUGH OVER THE CENTER OF THE COUNTRY ON MONDAY INTO
TUESDAY IS EXPECTED TO PRODUCE THE DEVELOPMENT OF MORE THAN ONE
CENTER OF LOW PRESSURE AS THE BROAD MID-UPPER CIRCULATION MOVES
SLOWLY EAST SOUTHEAST ACROSS THE NORTHERN PLAINS AND UPPER MS
VALLEY.  A BROAD AREA OF MOSTLY LIGHT SNOW IS EXPECTED TO DEVELOP
FROM THE DAKOTAS ACROSS MINNESOTA INTO WISCONSIN...WITH A LOW TO
MODERATE PROBABILITY OF  4 OR MORE INCHES OF SNOW.  THE HIGHER
CONFIDENCE AND AMOUNTS ARE EXPECTED IN THE AREAS OF LAKE
ENHANCED/LK EFFECT SNOWS WITH ONSHORE FLOW RESULTING IN LOW LEVEL
LEE SHORE CONVERGENCE OFF LK SUPERIOR INTO THE ARROWHEAD OF MN.
THIS MAXIMA IS SHOWN IN THE GFS PARALLEL/09Z SREF MEAN/12Z NAM AND
GFS PARALLEL FORECASTS.
LIGHT FREEZING RAIN OR DRIZZLE IS POSSIBLE ACROSS THE NORTHERN
PLAINS TO UPPER MS VALLEY WHERE POCKETS OF DRIER MID LEVEL AIR
WORK AGAINST SNOW GROWTH OCCURRING.
MANUAL PROGS GAVE MORE WEIGHTING TO THE 00Z ECMWF/09Z SREF MEAN
CYCLONE TIMING BUT MODEL TO MODEL AND RUN TO RUN DIFFERENCES STILL
RESULT IN SNOW/FREEZING RAIN LOCATION/TIMING VARIABILITY THAT
REMAIN TO BE RESOLVED.

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

PETERSEN

$$




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