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 25 26 27 28 29 30 31
000
FOUS11 KWBC 212056
QPFHSD

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

VALID 00Z MON DEC 22 2014 - 00Z THU DEC 25 2014


...NORTHERN AND CENTRAL AND SOUTHERN ROCKIES DAY 1 AND PACIFIC
NORTHWEST TO NORTHERN ROCKIES DAYS 2-3...

THE MODELS INDICATE DEEP LAYER WEST NORTHWEST FLOW TONIGHT ACROSS
THE UT WASATCH/UINTAS AND CONTINUING INTO CO.  DEEP LAYERED
MOISTURE EXISTS EARLY AND COUPLETS OF LOWER LEVEL
CONVERGENCE/UPPER DIVERGENCE COMBINE WITH UPSLOPE FLOW IN FAVORED
TERRAIN TO PRODUCE AREAS OF HEAVY SNOW.  SNOW TAPERS IN UT MONDAY
AS DRIER AIR ALOFT AND SINKING MOTION DEVELOPS DUE TO THE UPSTREAM
MID-UPPER RIDGE.
FASTER DRYING ALOFT LEADS TO LOWER AMOUNTS IN THE RANGES OF ID AND
THE PACIFIC NORTHWEST.

ON DAY 2...
STRONG LOW LEVEL SOUTHWEST FLOW DEVELOPS IN THE PACIFIC OCEAN AND
THEN MOVES ONSHORE AND INLAND. THIS FLOW WILL DRIVE MOISTURE
INLAND ACROSS THE PACIFIC NORTHWEST AND NORTHERN ROCKIES.  THE
BUILDING HEIGHTS ALOFT AREA ASSOCIATED STRONG WARM ADVECTION SO
SNOW ELEVATIONS WILL RISE.  WHILE PRECIP WILL BE WIDESPREAD ACROSS
THE WA CASCADES RISING SNOW LEVELS MAKE THE DURATION OF SNOW
UNCERTAIN.

ON DAY 3...
FALLING HEIGHTS WITH THE APPROACH OF THE MID-UPPER TROUGH LEADS TO
A GRADUAL DECREASE IN SNOW LEVEL ACROSS THE NORTHWEST.  AREAS OF
CONFLUENT FLOW AND ASSOCIATED 700 MB CONVERGENCE STREAK ACROSS THE
PACIFIC NORTHWEST AND NORTHERN ROCKIES.  THE GFS FCST IS DRIER
THAN THE ECMWF/NAM/SREF MEAN AND ITS WON GEFS MEAN SO THE GFS WAS
GIVEN LESS WEIGHT THAN THE BETTER CLUSTERED SOLUTIONS.

...NORTHERN PLAINS TO UPPER MISSISSIPPI VALLEY/MIDWEST DAYS 1-3...

THE DEEPENING TROUGH OVER THE CENTRAL US WILL DRIVE A LOW LEVEL
CIRCULATION MOVING SOUTHEAST ACROSS THE NORTHERN PLAINS.  THE MORE
SOUTHERN TRACK OF THE 850-700 MB LOW TODAY VS YESTERDAY HAS LED TO
SNOW POTENTIAL DECREASING IN NORTHEAST ND TO NORTHWEST MN WHILE
INCREASING FURTHER SOUTH IN SOUTH DAKOTA AND SOUTHERN MN.  THE
UNCERTAINTY DAY ONE IS THE AIR MASS IS INITIALLY TOO WARM FOR SNOW
AHEAD OF THE CIRCULATION WITH COOLING AS THE LOW LEVEL CIRCULATION
CENTER PASSES.  ONE MAXIMA IS IN FAVORED UPSLOPE AREAS OF THE
BLACK HILLS OF SD.

ON DAY 2...THE MODELS AGREE ON THE LOW LEVEL CIRCULATION MOVING
NORTHEAST ACROSS THE UPPER MS VALLEY TOWARDS LK SUPERIOR.  ALONG
THE TRACK OF THE 850 MB LOW WILL BE A PRECIP TYPE TRANSITION ZONE
ACROSS SOUTHERN MN AND CENTRAL WI/CENTRAL UP OF MI.
MODERATE SNOW IS POSSIBLE FROM CENTRAL MN TO NORTHERN WI AND THE
FAR WESTERN UP OF MI TO THE MN ARROWHEAD WITHIN THE MID LEVEL
DEFORMATION OF THE 850-700 MB CIRCULATION.   THE 12Z NAM BECOMES
FASTER THAN THE 00-12Z GFS AND ECMWF SO LESS WEIGHTING WAS GIVEN
TO THE 12Z NAM AND MORE TO THE ECMWF/GFS/09Z SREF MEAN.

DAY 3 THE DEPARTING LOW-MID LEVEL CIRCULATION LEADS TO SNOW
COVERAGE/INTENSITY QUICKLY TAPERING WITH NO SIGNIFICANT SNOWFALL
SHOWN IN THE GFS/ECMWF 09Z SREF MEAN/12Z NAM.  THE 12Z GFS
PARALLEL WAS AN OUTLIER SOLUTION WITH A CLOSED LOW DEVELOPING ON
THE WAVY FRONT IN THE MID MS VALLEY...WITH THE 06Z RUN BETTER
MATCHING UP WITH THE MAJORITY OF MODELS.

THE PROBABILITY OF SIGNIFICANT ICING (QUARTER OF AN INCH) IS LESS
THAN 10 PERCENT DAYS 2-3.  A LOW RISK APPEARED ON DAY 1 IN
RESPONSE TO SOME WETTER 09Z SREF MEMBERS NEAR THE WV/VA BORDER.
MANUAL PROGS GAVE MORE WEIGHTING TO THE DRIER GFS/NAM/ECMWF
FORECASTS...MINIMIZING THE THREAT OF SIGNIFICANT ICING.

PETERSEN

$$





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