Quantitative Precipitation Forecast
Issued by NWS

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QPFHSD

PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
324 AM EST FRI DEC 09 2016

VALID 12Z FRI DEC 09 2016 - 12Z MON DEC 12 2016


DAYS 1-3...

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

AN ACTIVE PATTERN WILL CONTINUE WITH AREAS OF HEAVY SNOW IN THE
TERRAIN FROM THE PACIFIC NORTHWEST AND NORTHERN/CENTRAL CALIFORNIA
INLAND TO THE NORTHERN/CENTRAL ROCKIES.  THE INITIAL FOCUS WILL BE
WITH SNOW AND PATCHES OF FREEZING DRIZZLE DEVELOPING WITHIN WARM
ADVECTION PRECIPITATION LIFTING  THROUGH THE NORTHWEST...BUT
ATTENTION SHOULD SHIFT TO PERIODS OF HEAVY SNOW OVER THE FAVORED
TERRAIN AS A CLOSED LOW WEAKENS OFF THE NORTHWEST COAST AND
MOISTURE STREAMS ONSHORE AND GETS LIFTED THROUGH UPPER DIVERGENCE
MAXIMA.  AS THE FIRST LOW WEAKENS AS DAY 2 PROGRESSES ANOTHER LOW
FORMS OFF THE COAST NEAR VANCOUVER ISLAND...WHICH PRODUCES AN
ONSHORE FLOW MAXIMUM COMING INTO WESTERN OR..TARGETING CONTINUING
HEAVY SNOW IN THE MOUNTAINS OF WESTERN OR.

ON DAY 2 THE HEAVIER SNOW ARE TARGETED FOR THE CENTRAL CA
SIERRA...AS THE INTENSIFYING 300 MB JET MAX FORECAST TO BE NEAR
130 KT COMES INTO NORTHERN CA.  ENHANCED UPPER DIVERGENCE AND
LOW-MID LEVEL TRANSPORT GETS MAXIMIZED SOUTH OF THE UPPER JET IN
CENTRAL CA AND MULTIPLE MODELS ARE FORECASTING 2 TO 4 INCHES
LIQUID EQUIVALENT...SO HEAVY SNOW IS POSSIBLE.  FORECAST SOUNDINGS
MAINTAIN HIGHER SNOW ELEVATIONS IN CENTRAL CA..SO SOME OF THE
HEAVIEST PRECIP FALLS AS RAIN OR A MIXTURE OF SNOW AND RAIN. ON
DAY 3 THE JET ARCS BACK UP INTO OR TO CONTINUE THE HEAVY SNOW
THREAT IN THE OR CASCADES.

THE WASHINGTON AND OREGON CASCADES...SAWTOOTH MOUNTAINS OF
IDAHO...TETONS OF WYOMING....WASATCH/UINTA MOUNTAINS OF UTAH...AND
COLORADO ROCKIES SHOULD ALL SEE AREAS OF HEAVY SNOW.


...NORTHERN PLAINS AND UPPER MISSISSIPPI VALLEY TO GREAT LAKES AND
OHIO VALLEY/NEW YORK/WESTERN NEW ENGLAND...

A SWATH OF ACCUMULATING SNOW IS EXPECTED WITH 850-500 MB SHORTWAVE
ENERGY MOVING ACROSS THE NORTHERN PLAINS...UPPER MS VALLEY...GREAT
LAKES AND NORTHERN OHIO VALLEY THIS WEEKEND.  AN AREA OF MODERATE
TO LOCALLY HEAVY SNOW IS EXPECTED TO DEVELOP WITHIN AN AXIS OF
DEEP-LAYER FRONTOGENESIS.  ON DAY 2 THE BAND OF SNOWFALL
ACCUMULATIONS ARE FORECAST TO EXTEND FROM EASTERN SOUTH DAKOTA AND
SOUTHERN MINNESOTA TO SOUTHERN WI/ADJACENT IL.

ON DAY 3 A STRONGER SURFACE-850 MB LOW IS FORECAST BY A FEW OF THE
MODELS SUCH AS THE 18Z GFS...SO THERE IS POTENTIAL FOR HIGHER
ACCUMULATIONS.  THE SPREAD IN FORECAST SOLUTIONS GROWS WITH THE
SREF AND GEFS MOVING THE LOW LEVEL CYCLONE NORTH-NORTHEAST FASTER
THAN THE NAM/CANADIAN...WHICH IN TURN CAUSES THOSE SOLUTIONS TO
SPREAD SNOW FURTHER NORTH-NORTHEAST ACROSS BOTH THE GREAT LAKES
AND NEW YORK/ADJACENT NEW ENGLAND.  THE STRONG LOW-MID LEVEL WARM
ADVECTION CAUSES THE PRECIP TO CHANGE OVER TO MIXED PRECIP TYPES
IN CENTRAL IL TO CENTRAL IN/CENTRAL OH/PA/SOUTHEAST NY...SO THE
DURATION/AMOUNT OF EACH PRECIP TYPE ADDS TO THE UNCERTAINTY IN
THESE AREAS.


...GREAT LAKES DAY 1 INTO EARLY DAY 2...

A COLD AIRMASS BASING ADVECTED OVER RELATIVELY WARMER LAKES WILL
CONTINUE TO FOSTER LAKE EFFECT SNOW BANDS FRIDAY AND INTO
SATURDAY.  NORTHWEST FLOW ACROSS LK SUPERIOR SUPPORTS A PRIMARY
BAND WHERE LOW LEVEL CONVERGENCE IS MAXIMIZED IN THE EASTERN UP OF
MI WITH A SECONDARY MAX FURTHER WEST IN THE WESTERN UP..WITH  THE
STRENGTH AND ENDURANCE OF LOW LEVEL CONVERGENCE NOT AS GREAT..SO
AMOUNTS NOT QUITE AS HIGH.  NORTHWEST LOWER MI DOWNSTREAM FROM
TRAVERSE BAY IS FAVORED TODAY BUT WINDS BACK ON SAT SO THE BANDS
REVERSE COURSE AND MOVE FURTHER NORTH WITH TIME..FOLLOWED BY
DRYING ALOFT TO END THE THREAT.

NORTHWEST FLOW ACROSS LAKE ERIE BECOMES CONVERGENT WITH NEAR SHORE
WEST-NORTHWEST FLOW SO NEAR THE NORTHWEST PA BORDER WITH NY AND OH
SHOULD SEE THE HEAVIER SNOW SHOWER ACTIVITY TODAY AND TONIGHT.  ON
SAT THE FLOW BACKS WITH FLOW MORE PARALLEL TO THE LAKE SO OH
SHOULD SEE LESS ACTIVITY...PARTICULARLY INLAND FROM THE LAKE. THE
LONGER CROSS LK TRAJECTORY COULD BECOME A SHORT-LIVED THREAT FOR
SOUTHWEST NEW YORK BEFORE THE FLOW CONTINUES BACKING AND THE LOSS
OF LOW LEVEL CONVERGENCE FORCES ACTIVITY TO BECOMES LESS IN
COVERAGE AND INTENSITY.
LIKEWISE...ACTIVITY CONTINUES IN THE LEE SHORE AREAS OFF LK
ONTARIO IN WESTERN NY BUT DECREASES LATE SAT AND SAT NIGHT AS THE
FLOW CHANGES AND THE LOSS OF CROSS LAKE TRAJECTORIES AND WEAKENING
LOW LEVEL CONVERGENCE SPELL LESS SNOW SHOWER ACTIVITY.


THE PROBABILITY OF SIGNIFICANT ICING (GREATER THAN 0.25 INCHES) IS
LESS THAN 10 PERCENT DAYS 1-3.

PETERSEN

$$





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