306
FXUS21 KWNC 242057
PMDTHR
US HAZARDS ASSESSMENT
NWS CLIMATE PREDICTION CENTER CAMP SPRINGS MD
300 PM EST NOVEMBER 24 2009
SYNOPSIS: AN INTENSIFYING STORM IS FORECAST TO TRACK NORTHWARD ALONG THE
NORTHEAST COAST DURING FRIDAY AND SATURDAY, BRINGING WIDESPREAD RAIN, STRONG
WINDS, AND MOUNTAIN AND LAKE-EFFECT SNOWS. ANOTHER STORM IS FORECAST TO MOVE
NORTHWARD FROM THE GULF OF MEXICO EARLY NEXT WEEK, SPREADING RAIN ACROSS A
LARGE PORTION OF THE EASTERN HALF OF THE COUNTRY. IN THE NORTHEAST PACIFIC, A
SERIES OF INTENSE LOW PRESSURE SYSTEMS IS EXPECTED TO CONTINUE TO BRING WIND,
RAIN, AND SNOW TO ALASKA.
HAZARDS
HIGH WINDS AND HIGH WAVES ACROSS THE SOUTHERN COAST OF ALASKA NOVEMBER 28-29.
HIGH WINDS FROM PENNSYLVANIA INTO NEW ENGLAND NOVEMBER 27-28.
ONGOING RIVER FLOODING IN ILLINOIS, LOUISIANA, AND SOUTHERN TEXAS.
HEAVY RAINS AND FLOODING OVER THE LOWER MISSISSIPPI VALLEY NOVEMBER 29-30.
SEVERE DROUGHT IN PARTS OF WISCONSIN, TEXAS, AND AREAS IN THE WESTERN U.S. SOME
RELIEF IS EXPECTED ACROSS MOST OF THESE AREAS, EXCLUDING AREAS IN ARIZONA,
SOUTHERN CALIFORNIA, AND NEVADA.
DETAILED SUMMARY
FOR FRIDAY NOVEMBER 27 - SUNDAY NOVEMBER 29: AN UPPER-LEVEL CUTOFF LOW IS
FORECAST TO SLOWLY MOVE EASTWARD FROM THE MID-ATLANTIC REGION TO OFF THE NEW
ENGLAND COAST DURING FRIDAY AND SATURDAY. AS A RESULT, A SURFACE LOW SOUTHEAST
OF CAPE COD IS LIKELY TO INTENSIFY AND TRACK NORTHWARD TO THE NOVA SCOTIA AREA
ON SATURDAY. THE STRONG PRESSURE GRADIENT ASSOCIATED WITH THE LOW AS THE SYSTEM
INTENSIFIES IS FORECAST TO BRING POTENTIALLY DAMAGING NORTHWESTERLY WINDS
ACROSS THE EASTERN GREAT LAKES INTO NEW ENGLAND BY FRIDAY. COLD AIR RUSHING IN
BEHIND THE LOW IS EXPECTED TO CHANGE THE RAIN TO SNOW BY SATURDAY DOWNWIND OF
LAKES ERIE AND ONTARIO, AND OVER THE HIGHER TERRAIN FROM WEST VIRGINIA THROUGH
PENNSYLVANIA INTO NORTHERN NEW ENGLAND. THE LATEST MODEL GUIDANCE INDICATES THE
LIKELIHOOD THAT ANY HEAVY SNOW WILL BE LOCALIZED AND MAINLY LIMITED TO THE
HIGHER TERRAIN OF THE NORTHERN APPALACHIANS, NORTHERN PARTS OF NORTHERN NEW
ENGLAND, AS WELL AS SPOTTY AREAS DOWNWIND OF LAKES ERIE AND ONTARIO. DUE TO THE
LOCALIZED NATURE OF THE HEAVY SNOW, THE SNOW HAZARD AREA HAS BEEN REMOVED FROM
THE MAP.
ON THE WEST COAST, MODELS FORECAST A SHORTWAVE AND A SURFACE LOW TO MOVE
INLAND, SPREADING RAIN AND SNOW EASTWARD FROM WASHINGTON AND OREGON ON FRIDAY
TO MINNESOTA AND THE DAKOTAS ON SUNDAY. FORECAST MAPS FOR SUNDAY INDICATE
SURFACE LOW PRESSURE NEAR THE WESTERN GREAT LAKES REGION, ALTHOUGH THERE IS
CONSIDERABLE SPREAD AMONG FORECAST MODELS.
MORE IMPORTANTLY, A DEEPENING UPPER-LEVEL TROUGH DIGGING SOUTHWARD ACROSS THE
GREAT PLAINS IS FORECAST TO TRIGGER SURFACE LOW DEVELOPMENT IN THE SOUTHERN
PLAINS OR SOUTHERN MISSISSIPPI VALLEY BY SUNDAY. THIS IS LIKELY TO CAUSE HEAVY
RAIN TO EXTEND NORTHWARD FROM THE WESTERN GULF OF MEXICO INTO LOUISIANA AND
ARKANSAS. COLD AIR SPILLING SOUTHWARD BEHIND THE LOW PRESSURE SYSTEM COULD
CAUSE RAIN TO TURN TO SNOW OVER PARTS OF NEW MEXICO, OKLAHOMA AND NORTHERN
TEXAS, BUT THE LATEST FORECAST MODELS DO NOT POINT TO A SIGNIFICANT SNOW EVENT.
IN THE NORTHEAST PACIFIC, THE PATTERN CONTINUES TO FAVOR A SERIES OF DEEP LOW
PRESSURE SYSTEMS AFFECTING ALASKA. A VERY LARGE AND POWERFUL LOW PRESSURE
SYSTEM IS FORECAST TO TRACK EASTWARD ACROSS THE GULF OF ALASKA INTO THE ALASKA
PANHANDLE FROM SATURDAY TO SUNDAY, BRINGING HIGH WINDS AND HIGH WAVES TO
COASTAL ALASKA.
ONGOING FLOODING FROM EARLIER RAINFALL EPISODES IS FORECAST FOR THE ILLINOIS
RIVER IN ILLINOIS AND SEVERAL RIVERS IN NORTHERN LOUISIANA AND SOUTHERN TEXAS.
RAIN OR SNOW IS EXPECTED TO BENEFIT DROUGHT AREAS IN WISCONSIN, WASHINGTON, AND
NORTHERN CALIFORNIA. LITTLE CHANGE IS SEEN FOR DROUGHTS IN ARIZONA, NEVADA, AND
SOUTHERN CALIFORNIA.
A TIGHT PRESSURE GRADIENT BETWEEN A SURFACE TROUGH OVER COASTAL CALIFORNIA AND
HIGH PRESSURE OVER THE GREAT BASIN MAY CAUSE STRONG WINDS TO DEVELOP OVER
CALIFORNIA ON SATURDAY AND SUNDAY. THERE IS TOO MUCH UNCERTAINTY TO POST A
HAZARD AREA AT THIS TIME.
FOR MONDAY NOVEMBER 30 - FRIDAY DECEMBER 04: BIG DIFFERENCES EXIST BETWEEN
MODELS FOR THIS PERIOD, MAKING DELINEATION OF THE TRACK AND INTENSITY OF THE
DEVELOPING SURFACE LOW OVER THE SOUTH QUITE DIFFICULT. TODAY'S 12Z GFS MODEL
RUN INDICATES A NEGATIVELY-TILTED UPPER-LEVEL TROUGH EXTENDING SOUTHEASTWARD ON
TUESDAY FROM THE GREAT LAKES, WITH A DEEP SURFACE LOW OVER THE MID-ATLANTIC
STATES. THE 12Z ECMWF MODEL HAS A POSITIVELY TILTED TROUGH EXTENDING
SOUTHWESTWARD FROM THE GREAT LAKES INTO TEXAS AND A WEAK SURFACE LOW NEAR THE
FLORIDA PANHANDLE. REGARDLESS OF THE DETAILS, THERE IS A LIKELIHOOD THAT HEAVY
RAINS WILL SPREAD EASTWARD FROM THE LOUISIANA-ARKANSAS AREA INTO MISSISSIPPI ON
MONDAY, RESULTING IN INCREASED ODDS FOR FLOODING IN AREAS THAT HAVE RECENTLY
SEEN MUCH RAIN. WIDESPREAD RAINS ARE LIKELY TO CONTINUE NORTHWARD AND EASTWARD,
BUT IT IS TOO EARLY TO INDICATE ADDITIONAL HAZARDS AT THIS TIME GIVEN MODEL
UNCERTAINTIES. THERE IS ALSO AN ELEVATED CHANCE FOR SEVERE WEATHER OVER THE
SOUTHEAST WITH THIS STORM SYSTEM, BUT MODEL UNCERTAINTIES PRECLUDE DELINEATION
OF AN AREA AT THIS TIME.
ANOTHER UPPER-LEVEL TROUGH IS FORECAST TO DEEPEN OVER THE PLAINS BY FRIDAY,
ALLOWING COLD CANADIAN AIR TO DROP SOUTHWARD INTO THE ROCKIES AND PLAINS.
OVER THE PACIFIC, ANOTHER IN THE SERIES OF STORMS IS FORECAST TO AFFECT WESTERN
ALASKA ON TUSDAY AND WEDNESDAY.
FOR SATURDAY DECEMBER 05 - TUESDAY DECEMBER 08: RECENT GFS ENSEMBLES CONTINUE
TO SHOW A POSITIVE PNA PATTERN THIS PERIOD, FEATURING WEAK RIDGING NEAR THE
WEST COAST AND TROUGHING OVER THE CENTRAL OR EASTERN STATES. THE PATTERN
SUGGESTS COLD WEATHER OVER MUCH OF THE COUNTRY AWAY FROM THE WEST COAST, WITH
THE POTENTIAL FOR ADDITIONAL WET WEATHER ACROSS THE SOUTH AND EAST.
FORECASTER: DOUG LECOMTE
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