Quantitative Precipitation Forecast
Issued by NWS

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000
FOUS11 KWBC 310922
QPFHSD

PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
421 AM EST SAT JAN 31 2015

VALID 12Z SAT JAN 31 2015 - 12Z TUE FEB 03 2015


DAY 1...

...NORTHEAST...

WRAPAROUND PRECIPITATION ON THE BACKSIDE OF A NEGATIVELY TILTED
UPPER TROUGH/CLOSED LOW IS EXPECTED TO PROLONG THE HEAVY SNOW
THREAT ACROSS EASTERN MAINE THROUGH EARLY SAT...WHERE A
MODERATE-HIGH RISK FOR 4 TO 8 INCHES AND A SLIGHT RISK FOR
8-INCHES OR MORE IS INDICATED.  WITH THE MODELS IN GOOD
AGREEMENT...A MULTI MODEL BLEND WAS USED IN GENERATING THE
DETERMINISTIC PORTION OF THE SNOWFALL.

DAYS 1 THROUGH 3...

...CENTRAL PLAINS/UPPER MISSISSIPPI VALLEY/OHIO VALLEY/LOWER GREAT
LAKES/NORTHERN MID ATLANTIC/NORTHEAST...

AS ENERGY EMANATING FROM A SOUTHERN STREAM CLOSED LOW DROPPING
THROUGH THE SOUTHWEST PHASES WITH A NORTHERN STREAM TROUGH DIGGING
FROM THE NORTHWEST INTO THE CENTRAL PLAINS...A WIDESPREAD
POTENTIALLY HIGH IMPACT SNOW EVENT APPEARS TO BE SHAPING UP ACROSS
A LARGE SWATH OF THE U.S....EXTENDING FROM PORTIONS OF THE CENTRAL
PLAINS-THE UPPER MISSISSIPPI VALLEY ACROSS THE LOWER LAKES/UPPER
OHIO VALLEY...NORTHERN MID ATLANTIC AND ACROSS SOUTHERN NEW
ENGLAND.

OVER THE PAST COUPLE OF DAYS...THE SYSTEM HAS TRENDED FROM A
SHEARED LIGHT QPF SYSTEM TO ONE THAT THREATENS SNOW FARTHER NORTH
THAN PREVIOUS FORECASTS INDICATED.  THIS TREND CONTINUED WITH MANY
OF THE 00Z GUIDANCE MEMBERS...SPREADING PRECIPITATION FURTHER TO
THE NORTH THAN THEIR PREVIOUS RUNS.  UNCERTAINTY REMAINS WHETHER
THIS TREND WILL CONTINUE WITH A MORE AMPLIFIED UPPER
TROUGH...RESULTING IN A MORE NORTHERLY SURFACE LOW
TRACK/PRECIPITATION SHIELD OR IF THE GUIDANCE WILL SHIFT BACK
TOWARD A LESS AMPLIFIED SOLUTION AS THE SYSTEM EVOLVES OVER THE
NEXT COUPLE OF DAYS.

WITH ALL THE UNCERTAINTY REGARDING WHERE THOSE CRUCIAL SNOW/NO
SNOW GRADIENTS SET UP AND WHERE THE CHANGEOVER REGION
OCCURS...THIS SYSTEM APPEARS TO HAVE THE POTENTIAL FOR A SWATH OF
6 TO 12 INCHES OF SNOW SOMEWHERE ALONG AN WEST-EAST CORRIDOR
STRETCHING FROM EASTERN NEBRASKA/IOWA TO SOUTHERN NEW
ENGLAND...BUT WITH PRESENTLY ONLY A SMALL PROBABILITY OF GREATER
THAN A FOOT OF SNOW.  WITH SEVERAL FORECAST CYCLES COMING
UP...THESE NUMBERS AND LOCATIONS WILL LIKELY CHANGE.

WHILE THE SOUTHWESTERN TROUGH HEADS SOUTHWARD INTO MEXICO...ENERGY
AND MOISTURE EJECTING EASTWARD ACROSS THE PLAINS AND MISSISSIPPI
VALLEY WILL COMBINE WITH THE AMPLIFYING TROUGH MOVING
SOUTHEASTWARD ACROSS THE NORTHERN ROCKIES ON SATURDAY NIGHT AND
BECOME A SIGNIFICANT TROUGH OVER THE MISSISSIPPI VALLEY BY SUNDAY.
THE CONSOLIDATION OF THESE SYSTEMS APPEAR TO PRODUCE A HIGH
PROBABILITY OF GREATER THAN 4 INCHES OF SNOW FROM EASTERN
NEBRASKA..IOWA..NORTHERN MISSOURI TO NORTHERN ILLINOIS ON DAY 1...
SATURDAY MORNING INTO SUNDAY MORNING.   THERE IS A LOW PROBABILITY
OF GREATER THAN 12 INCHES OF SNOW ACROSS PORTIONS OF THIS AREA.
AS THIS STORM DEVELOPS...MUCH COLDER AIR WILL BUILD NOT ONLY
BEHIND THE SYSTEM AS A LARGE ARCTIC ANTICYCLONE HEADS SOUTHWARD
INTO THE NORTHERN PLAINS...BUT COLDER AIR WILL ALSO BLEED EASTWARD
AS PRESSURES RISE OVER SOUTHEASTERN CANADA AND NORTHERN NEW
ENGLAND IN ASSOCIATION WITH A PRONOUNCED UPPER CONFLUENCE ZONE.
THE COMBINATION OF THESE TWO SYSTEMS WILL HELP GENERATE A
INCREASINGLY BAROCLINIC ZONE UPON WHICH THE DEVELOPING SURFACE LOW
WILL MOVE ALONG.  ON DAY 2...SUN MORNING INTO MON ...A MODERATE TO
HIGH RISK FOR 4 TO 8 INCHES IS FORECAST TO EXTEND FROM THE MID MS
VALLEY EASTWARD THROUGH THE LOWER LAKES REGION/UPPER OH VALLEY
INTO SOUTHEASTERN NY AND PARTS OF CENTRAL/SOUTHERN NEW ENGLAND.  A
MODERATE TO HIGH RISK FOR 8-INCHES IS FORECAST TO CENTER IN THE
VICINITY OF NORTHERN IND/OH INTO WESTERN PA...WITH A SLIGHT RISK
FOR A FOOT OR MORE CENTERING THERE AS WELL.  BY DAY 3...MON
MORNING INTO TUE MORNING...THE SYSTEM IS FORECAST TO MOVE FROM THE
NORTHERN MID ATLANTIC OFFSHORE...PAINTING A MODERATE TO HIGH RISK
FOR 4-8 INCHES ACROSS THE REMAINDER OF SOUTHERN NY AND
CENTRAL/SOUTHERN NEW ENGLAND INTO DOWN EAST MAINE...WITH A
MODERATE RISK FOR 8-12 INCHES AND A SLIGHT RISK FOR A FOOT OR MORE.

THE WPC FORECAST REFLECTS THE PREVAILING TREND IN THE 00Z MODELS
TOWARD A MORE NORTHERLY TRACK WITH THIS SYSTEM.  HOWEVER FORECAST
CONFIDENCE IS TEMPERED BY THE RUN-TO-RUN INCONSISTENCIES THE
MODELS HAVE SHOWN ALONG THE WITH DISTINCT POSSIBILITY THAT THE
MODELS MAY REVERSE TRENDS IN SUBSEQUENT RUNS.

DAYS 2-3...

...PACIFIC NORTHWEST TO THE NORTHERN ROCKIES...

A SERIES OF LOW AMPLITUDE SHORTWAVES MOVING ACROSS THE TOP OF AN
UPPER RIDGE CENTERED NEAR  THE WEST COAST ARE EXPECTED TO RAISE
THE THREAT FOR HEAVY SNOWS ACROSS PORTIONS OF THE NORTHERN
CASCADES TO THE NORTHERN ROCKIES...WITH COVERAGE INCREASING BY DAY
3.  TOTALS EXCEEDING A FOOT ARE POSSIBLE ACROSS THE HIGHER PEAKS
OF THE NORTHERN CASCADES AND THE BITTERROOTS BY THE END OF THE
PERIOD.

THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT.

PEREIRA

$$




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