Quantitative Precipitation Forecast
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000
FOUS11 KWBC 110909
QPFHSD

PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
408 AM EST SUN DEC 11 2016

VALID 12Z SUN DEC 11 2016 - 12Z WED DEC 14 2016

CA/PACIFIC NORTHWEST/GREAT BASIN/ROCKIES...

THE UPPER TROUGH SPLITTING WHILE PROGRESSING ACROSS THE NORTHERN
ROCKIES CREATES FAVORABLE CONDITIONS FOR SNOW ACROSS A LARGE
PORTION OF THE HIGHER TERRAIN. ONCE THE SPLIT JET CONSOLIDATES
FURTHER SOUTH INTO NORTHERN CA/NORTHERN NV/UT/CO...RANGES IN THESE
AREAS ARE GOING TO BE THE FOCUS FOR HEAVIER PRECIPITATION AND
HIGHER ELEVATION SNOW AS WE HEAD TOWARDS MID WEEK.

THE UNCERTAINTY DEVELOPS IN RESPONSE TO DIFFERENT TIMING AND
AMOUNTS OF SPLITTING. PROBABILITIES OF 8 AND 12 INCHES OF SNOW ARE
DEPICTED FOR THE CASCADES/BITTERROOTS/WASATCH/COLORADO ROCKIES ON
DAY 1/SUNDAY.

ON MONDAY NIGHT AND TUE...LOW PRESSURES FORMS IN THE EASTERN
PACIFIC AND MOVES EAST TOWARDS CA. A SURGE OF MOISTURE DEVELOPS
AND MOVES ONSHORE WITH LIFT PRODUCING AN INCREASE IN
PRECIPITATION...INCLUDING HIGHER ELEVATION SNOW. BY TUE THE ARCTIC
FRONT WILL BE SITUATED CLOSE TO THE NV AND UT BORDER WITH ID INTO
SOUTHWEST WY AND CENTRAL CO.  LIFT ALONG THE FRONTAL BNDRY WILL
OCCUR AND AID IN PRODUCING HIGHER ELEVATION SNOWS.
ALSO...THE ECMWF ENSEMBLE MEAN INDICATES A 150 KT JET STREAK AT
300 MB TRAVERSES CO TUE NIGHT...WITH UPPER DIVERGENCE MAXIMA
CROSSING THE CO ROCKIES SUPPORTING A PERIOD OF ENHANCED ASCENT.
THE GFS FORECAST JET IS A LITTLE FURTHER NORTH IN WY/NE BUT STILL
SUPPORTS FORECAST UPPER DIVERGENCE MAXIMA IN THE NORTHERN CO
ROCKIES. WITH THE JET REMAINING IN PLACE ALL THE WAY ACROSS
NORTHERN NV/NORTHERN CA/EASTERN PACIFIC...THE PROBABILITIES FOR
SNOW TUE ARE ENHANCED FOR THE CA SIERRA NEVADA MOUNTAINS...RANGES
OF NORTHERN NV AND UT...AND DOWNSTREAM INTO THE CENTRAL ROCKIES.
SINCE TH 850-700 MB MOISTURE FLUX STANDARDIZED ANOMALIES FORM THE
GFS ARE GREATEST IN CA COMING INTO THE SIERRA NEVADA RANGE...THE
HIGHEST POTENTIAL SNOWFALL TOTALS ON ARE FORECAST HERE.


...GREAT LAKES/NEW YORK/PA/NEW ENGLAND...

SNOW IS EXPECTED TO BE WIDESPREAD ACROSS THE GREAT LAKES ON DAY
1... IN RESPONSE TO AN 850 MB LOW AND SFC REFLECTION THAT DEVELOPS
AND MOVES ACROSS THE UPPER LAKES.  UPPER DIVERGENCE MAXIMA IN
ADVANCE OF THE UPPER TROUGH INDUCES ASCENT. FURTHER DOWNSTREAM
OVER THE LOWER LAKES AND NY/NEW ENGLAND...STRONG MID LEVEL THETA-E
ADVECTION AND CONVERGENCE ON THE NOSE OF A STRONG SOUTHWEST JET
WILL DRIVE ASCENT AND RESULT IN A PERIOD OF SNOW...WITH SNOW
CHANGING OVER TO MIXED PRECIP TYPES IN PA AND SOUTHERN NY/SOUTHERN
NEW ENGLAND AS THE WARMER AIR ALOFT PROGRESSES STEADILY NORTH. ON
DAY 1 THE HIGHEST CHANCE OF 4 AND 8 INCHES OF SNOW IS FORECAST
ACROSS NORTHERN INDIANA INTO SOUTHERN MICHIGAN...WITH SECONDARY
MAXIMA IN FAVORED UPSLOPE AREAS LIKE THE CATSKILLS/TUG HILL
PLATEAU/ADIRONDACKS.

ON DAY 2 THERE IS THE POSSIBILITY OF A TRIPLE POINT LOW DEVELOPING
NEAR COASTAL MAINE OR A BIT FURTHER OFFSHORE...WITH THE CLOSER IN
SOLUTIONS KEEPING SNOW INLAND AND THE FURTHER OFFSHORE SOLUTIONS
ALLOWING SIGNIFICANT SNOW ACCUMULATIONS TO REACH THE COAST. THE
STRONG ONSHORE JET PRODUCES STRONG LOW LEVEL
CONVERGENCE...FAVORING A BURST OF HEAVY SNOW EARLY DAY 2 IN
EASTERN MAINE.  THE DEPARTURE OF THE LOW DURING THE MIDDLE OF DAY
2 LEADS TO SNOW WINDING DOWN IN COVERAGE/INTENSITY.  UNCERTAINTY
PERSISTS AS TO WHETHER A MIXTURE OF PRECIP TYPES OCCURS IN COASTAL
NH AND MAINE OR IF THE COASTAL WATERS LOW DEVELOPMENT ALLOWS THE
COLD AIR TO REMAIN IN PLACE AND SUPPORT HEAVIER ACCUMULATIONS.

THE PROBABILITY OF ICE GREATER THAN 0.25 INCHES IS LESS THAN 10
PERCENT.

PETERSEN

$$





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