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Winter Weather Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 0842Z Nov 26, 2014)
 
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Abbreviations and acronyms used in this product
 
Geographic Boundaries -  Map 1: Color  Black/White       Map 2: Color  Black/White


PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
342 AM EST WED NOV 26 2014

VALID 12Z WED NOV 26 2014 - 12Z SAT NOV 29 2014

DAY 1...

...APPALACHIANS/MID ATLANTIC/NORTHEAST/NEW ENGLAND...

THE WELL ADVERTISED COASTAL STORM IS STILL FORECAST TO MOVE
NORTHEASTWARD ALONG THE EAST COAST ON WEDNESDAY WITH HEAVY SNOW
STILL FORECAST FOR MOSTLY INTERIOR PORTIONS OF THE MIDDLE ATLANTIC
STATES AND NEW ENGLAND. THERE WILL BE A BAND OF HEAVY
SNOWFALL...WHICH IS PROVING NOT TOO DIFFICULT TO FORECAST. 
HOWEVER...THERE IS A NARROW TRANSITION ZONE WHERE RAIN CHANGES TO
SNOW THAT IS AND HAS BEEN THE MOST DIFFICULT ASPECT OF THE
FORECAST. THIS TRANSITION ZONE IS ESPECIALLY IMPORTANT SINCE IT
APPEARS TO OCCUR ACROSS THE MOST HEAVILY POPULATED OF THE I-95
CORRIDOR BETWEEN WASHINGTON AND NEW YORK.

AS AN EXAMPLE OF HOW CRUCIAL THIS TRANSITION ZONE IS TO THE
FORECAST...THE AVERAGE DISTANCE BETWEEN LOW AND HIGH FORECAST
PROBABILITIES FOR GREATER THAN 4 INCHES OF SNOW ALONG THE I95
CORRIDOR IS BETWEEN 15 MILES AND 30 MILES. THEREFORE...SLIGHT
SHIFTS IN THE TRACK HAVE HAD IMPORTANT IMPLICATIONS FOR
ACCUMULATIONS. 

A COMPARISON OF THE SURFACE LOW POSITIONS FROM THE OPERATIONAL
NAM/GFS AND EUROPEAN MODELS OVER THE PREVIOUS 24 HOURS SHOWS A
SLIGHT BUT IMPORTANT TREND IN THE MODELS EVOLUTION OF THE SURFACE
LOW. IN GENERAL...ALL 3 MODELS FORECAST OF THE SURFACE LOWS ARE
SLIGHTLY SLOWER...HAVE SLIGHTLY LOWER PRESSURES AND ARE SLIGHTLY
CLOSER TO THE COAST. THIS TREND HAS HELPED ADJUST OUR
DETERMINISTIC SNOWFALL FORECAST...AN IMPORTANT PART OF OUR
GENERATION OF THE PROBABILISTIC FORECAST...SLIGHTLY
WESTWARD...ALLOWING SNOWFALL AMOUNTS WITHIN THE CITIES THEMSELVES
TO BE ON THE LOW END OF POTENTIAL ACCUMULATIONS. IT ALSO MAY
EXPLAIN A SLIGHT TREND FOR HEAVIER SNOW AMOUNTS TO CREEP FARTHER
WEST AND NORTH AS THE STORM MOVES TOWARD NEW ENGLAND. IN THIS
INSTANCE...WITH A 15 TO 30 MILE WIDE TRANSITION ZONE BETWEEN
MOSTLY SNOW AND MOSTLY RAIN...SUCH SLIGHT ADJUSTMENTS CAN HAVE
LARGE IMPLICATIONS.

IN GENERAL...THE PROBABILISTIC FORECAST OF SNOWFALL AMOUNTS SHOW
HIGH PROBABILITIES OF GREATER THAN 4 TO 12 INCHES OF SNOW FROM THE
MOUNTAINS OF WESTERN VIRGINIA/EASTERN WEST VIRGINIA ALL THE WAY
THROUGH NEW ENGLAND BUT AWAY FROM THE COAST UNTIL NEW
HAMPSHIRE/MAINE. OVER THE PAST SEVERAL DAYS...THERE HAS ONLY BEEN
A LOW TO MODERATE PROBABILITY FOR GREATER THAN 12 INCHES OF SNOW
WITH THIS STORM.  THE LATEST OPERATIONAL ECMWF RUN HAS LARGER QPF
AMOUNTS THAN THE 12Z RUN AND FARTHER WEST. IN GENERAL...EXPECT
SOME ACCUMULATIONS TO EXCEED 12 INCHES ESPECIALLY ACROSS HIGHER
TERRAIN FROM NORTHEAST PENNSYLVANIA TO NEW ENGLAND.

GIVEN THE RELATIVELY SIMILAR FORECASTS FROM THE OPERATIONAL MODELS
AND THEIR ENSEMBLE MEANS...USED A MULTI BLEND APPROACH.  THE
FORECAST DID NOT TAKE INTO ACCOUNT THE 00Z ECMWF...SINCE IT WAS
ISSUED AFTER DEADLINE. IT IS ALSO NOT NECESSARILY A GOOD IDEA TO
INCORPORATE A MODEL SHOWING A CHANGE FROM PREVIOUS GUIDANCE.

...UPPER MIDWEST/MID TO UPPER MISSISSIPPI VALLEY/OHIO VALLEY...

A DIGGING SHORTWAVE TROUGH MOVING FROM THE MISSISSIPPI VALLEY
ACROSS THE TENNESSEE VALLEY DOES NOT HAVE MUCH MOISTURE TO WORK
WITH SINCE IT FOLLOWS ON THE HEELS OF THE WET EAST COAST SYSTEM. A
WEAKENING SURFACE LOW IS ASSOCIATED WITH THIS SYSTEM DESPITE A
STRONG UPPER JET AND DIFFLUENT EXIT REGION.  IT IS STILL EXPECTED
TO GENERATE GENERAL 1 TO 3 INCHES OF SNOW ACROSS
MINNESOTA/IOWA/EASTERN MISSOURI/SOUTHERN ILLINOIS AND WESTERN
KENTUCKY WITH LESS THAN A 10 PERCENT OR LESS PROBABILITY OF MORE
THAN 4 INCHES OF SNOW.

DAYS 1 TO 3...

...PACIFIC NORTHWEST/NORTHERN ROCKIES/NORTHERN PLAINS/UPPER
MIDWEST...

A SERIES OF WEAK SHORTWAVE SYSTEMS DAMPING DOWN THE UPPER RIDGE
OVER THE WEST COAST WILL CROSS NEAR THE US CANADIAN BORDER ON
WEDNESDAY THRU EARLY FRIDAY. A WARM FRONT WILL MOVE NORTHWARD
ACROSS THE NORTHERN ROCKIES ON WEDNESDAY AS A BAND OF SNOW WILL
MOVE ACROSS MONTANA WITH LOW TO MODERATE PROBABILITIES OF GREATER
THAN 4 TO 8 INCHES OF SNOW ACROSS PORTIONS OF THE STATE.  THIS
BAND OF SNOWFALL WILL MOVE EASTWARD INTO NORTH DAKOTA/MINNESOTA
AND INTO THE MIDWEST THROUGH EARLY FRIDAY AS WARM ADVECTION SPANS
MUCH OF THE MIDDLE OF THE COUNTRY AS THE FLOW BECOME RELATIVELY
ZONAL.  MOST OF THE SNOWFALL IS EXPECTED TO BE LESS THAN 4 INCHES.

AS A SHORTWAVE STARTS MOVING ACROSS THE NORTHWEST ON DAY 3...MUCH
COLDER AIR WILL BEGIN TO POUR INTO MONTANA AS PACIFIC MOISTURE IS
AGAIN ENTRAINED EASTWARD. SIGNIFICANT SNOW IS EXPECTED ACROSS THE
MOUNTAINS OF WESTERN MONTANA AS WELL AS ACROSS THE BITTERROOTS AND
THE SALMON RIVER/SAWTOOTH RANGES WITH HIGH PROBABILITIES OF
GREATER THAN 4 TO 8 INCHES AND A LOW TO MODERATE CHANCE OF A FOOT
OR MORE.

THE PROBABILITY FOR SIGNIFICANT ICING IS LESS THAN 10 PERCENT.

KOCIN