Quantitative Precipitation Forecast
Issued by NWS

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
000
FOUS11 KWBC 260824
QPFHSD

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

VALID 12Z FRI DEC 26 2014 - 12Z MON DEC 29 2014


DAYS 1 THROUGH 3...


...CENTRAL PLAINS/UPPER MS VALLEY/GREAT LAKES/NORTHERN NEW
ENGLAND...

A BAND OF MID LEVEL FRONTOGENETIC FORCING STRETCHING FROM THE
CENTRAL PLAINS INTO THE GREAT LAKES RAISES THE SPECTER FOR LOCALLY
HEAVY SNOWFALL OVER PORTIONS OF THIS REGION DURING DAY
1...STRETCHING FROM THE GREAT LAKES INTO NORTHERN NEW ENGLAND ON
DAY 2. THERE WAS FAIRLY GOOD MODEL AGREEMENT CONCERNING THE TRACK
OF THE SHORT WAVE AND BANDING THAT OCCURS...SO THE QPF PORTION OF
THE FORECAST WAS BASED MAINLY ON THE MOST RECENT WPC QPF. THE
THERMAL PORTION OF THE FORECAST WAS BASED ON A FAIRLY EVEN BLEND
OF THE 00Z NAM/GFS/21Z SREF MEAN.

DAY 1...

A CLOSED MID LEVEL SYSTEM OVER THE FOUR CORNERS REGION EARLY ON
DAY 1 OPENS UP AND CROSSES THE CENTRAL PLAINS AND THE MIDWEST BY
THE END OF THE PERIOD. THE MID LEVEL SYSTEM SPINS UP A WEAK
SURFACE LOW ON A LOW LEVEL BOUNDARY STRETCHING FROM THE SOUTHERN
PLAINS TO THE UPPER GREAT LAKES. THE MID LEVEL SYSTEM ALSO FOCUSES
MOISTURE NORTHWEST OF THE BOUNDARY IN AN AREA OF FRONTOGENETIC
FORCING FROM SOUTHWEST NE INTO NORTHWEST WI...MAINLY DURING THE
SECOND HALF OF THE PERIOD. THE MID LEVEL LIFT REACHES INTO THE
DENDRITIC GROWTH ZONE ON THIS AXIS AS WELL. THE BEST SNOWFALL
RATES OCCURS BETWEEN 26/18Z AND 27/06Z ACROSS CENTRAL NE...AND
ACROSS EAST CENTRAL MN INTO NORTHWEST WI. IN THESE AREAS...4 TO 6
INCHES OF SNOWFALL IS EXPECTED. THESE VALUES ARE WELL SUPPORTED BY
THE LATEST ECMWF ENSEMBLE OUTPUT (WHICH SHOWS THE POTENTIAL FOR 4+
INCHES ON THIS AXIS)...AS WELL AS SREF PLUMES FROM THE 25/21Z
SREF...WHICH SHOWED MEAN VALUES NEAR 4 INCHES. A MUCH LARGER AREA
OF 2 TO 4 INCHES OF SNOW WAS PLACED OVER MUCH OF WESTERN CO...THE
CENTRAL PLAINS TO SOUTHERN MN AND MUCH OF WI...WHERE THE SYNOPTIC
SCALE LIFT IS MUCH BROADER IN SCOPE.

DAY 2...

THE BEST MID LEVEL FRONTOGENETIC FORCING QUICKLY CROSSES THE UPPER
GREAT LAKES ON DAY 2...LEAVING A SWATH OF 2 TO 4 INCHES OF
SNOWFALL ACROSS NORTHEAST WI INTO THE UP OF MI. THE LIFT BECOMES
MORE DIFFUSE DURING THE SECOND HALF OF DAY 2 ACROSS NORTHERN NEW
ENGLAND...WHERE ISENTROPIC LIFT IN THE STRONG WARM AIR ADVECTION
BECOMES THE PRIMARY DRIVER OF PRECIPITATION PRODUCTION. THE COLUMN
IS INITIALLY COLD ENOUGH FOR SNOW ACROSS THE ROOFTOP OF ME...AS
EVAPORATIVE COOLING RESULTS IN AN ISOTHERMAL PROFILE. A THIN
RIBBON OF 2 TO 4 INCHES OF SNOWFALL IS POSSIBLE HERE. AS THE BEST
LIFT EXITS NORTHERN ME...THE COLUMN DRIES FROM THE TOP
DOWN...WHICH ROBS THE COLUMN  OF ITS ABILITY TO MAKE SNOWFLAKES.
THE SNOW SHOULD END AS A PERIOD OF FREEZING RAIN/FREEZING
DRIZZLE...BUT ICE AMOUNTS SHOULD AVERAGE LESS THAN 0.10 INCHES.


...PACIFIC NORTHWEST/NORTHERN ROCKIES...

A STRONG SHORT WAVE TRACKING SOUTHEAST OUT OF BRITISH COLUMBIA
INTO THE NORTHERN ROCKIES ON DAY 2 DROPS INTO A LONG WAVE TROUGH
POSITION FORMING OVER THE WESTERN UNITED STATES ON DAY 3. FOR THE
MOST PART...THE 00Z GFS/12Z ECMWF BLEND HAS HANDLED THE EVOLUTION
OF THE MID LEVEL PATTERN BEST...SO THE QPF PORTION OF THE FORECAST
WAS BASED ON A BLEND OF THE 00Z GFS/12Z ECMWF/LATEST WPC QPF. THE
THERMAL PORTION OF THE FORECAST WAS BASED ON A BLEND OF THE 00Z
GFS/12Z ECMWF.

DAY 2...

A STRONG SHORT WAVE DROPS SOUTHEAST FROM BRITISH COLUMBIA EARLY ON
DAY 2 INTO THE PACIFIC NORTHWEST AND NORTHERN ROCKIES BY THE END
OF THE PERIOD.  AHEAD OF THE SHORT WAVE...A LOW LEVEL WESTERLY
FLOW TRANSPORTS PACIFIC MOISTURE INTO THE REGION. 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 TO 14 INCHES 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. THE UPSLOPE
COMPONENT TO THE FLOW IS STRONGEST OVER THE BITTERROOT
RANGES...WHERE AN AXIS OF 8 TO 12 INCHES OF SNOWFALL IS EXPECTED.
LESSER AMOUNTS (BETWEEN 5 AND 10 INCHES) ARE EXPECTED OVER THE
BLUE MOUNTAINS IN OR AND THE SAWTOOTH RANGE IN ID.

DAY 3...

A CLOSED MID LEVEL SYSTEM CROSSES THE PACIFIC NORTHWEST DURING THE
SECOND HALF OF DAY 3. THE CLOSED LOW USHERS ARCTIC AIR FROM
WESTERN CANADA INTO THE NORTHERN ROCKIES...WITH A BROAD AREA OF
UPSLOPE FLOW DEVELOPING ACROSS THE NORTHERN ROCKIES BEHIND THE
ARCTIC FRONT. STRONG SYNOPTIC SCALE LIFT ASSOCIATED WITH THE
CLOSED MID LEVEL SYSTEM CROSSING THE PACIFIC NORTHWEST IS
AUGMENTED BY STEEP MID LEVEL LAPSE RATES (WHICH COULD RESULT IN
MARGINAL INSTABILITY). THE LOW LEVEL WEST NORTHWEST FLOW CONTINUES
TO FOCUS MOISTURE ON THE CASCADES OF WA/OR. WARM AIR ADVECTION
AHEAD OF THE CLOSED MID LEVEL SYSTEM RAISES SNOW LEVELS ACROSS THE
TERRAIN CLOSE TO 6000 FEET...PRODUCING AN AXIS OF 4 TO 8 INCHES OF
SNOW FALL ACROSS THE WA/OR CASCADES...WITH THE HIGHEST AMOUNTS
OVER NORTH CENTRAL OR.

FURTHER EAST...BOTH SYNOPTIC SCALE ASCENT AND UPSLOPE BECOME
PRIMARY PRECIPITATION DRIVERS ACROSS THE NORTHERN ROCKIES.
MOISTURE 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.


HAYES

$$





USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.