Prognostic Meteorological Discussion
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FXUS21 KWNC 182009
PMDTHR
US HAZARDS OUTLOOK
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD
300 PM EST DECEMBER 18 2014

SYNOPSIS: AT THE BEGINNING OF THIS OUTLOOK PERIOD, THE FOCUS OF UNSETTLED
WEATHER IS EXPECTED TO BE THE PACIFIC NORTHWEST AND NORTHERN ROCKIES. A WEAK
AREA OF LOW PRESSURE MOVING ACROSS THE CAROLINAS IS FORECAST TO HEAD OUT TO
SEA. A MORE SIGNIFICANT STORM IS PREDICTED TO DEVELOP ACROSS THE EAST-CENTRAL
STATES JUST BEFORE CHRISTMAS, AFFECTING PRIMARILY THE GREAT LAKES REGION AND
THE ATLANTIC COAST STATES. A STRONG STORM SYSTEM IS ANTICIPATED TO MOVE ACROSS
THE CENTRAL BERING SEA BETWEEN DECEMBER 23-24. AT THIS TIME, IT DOES NOT LOOK
LIKE HAZARDOUS CONDITIONS ASSOCIATED WITH THIS STORM WILL REACH THE BERING
SEACOAST OF ALASKA.

HAZARDS

HEAVY PRECIPITATION (LOW ELEVATION RAIN AND MOUNTAIN SNOW) IN WESTERN PORTIONS
OF WASHINGTON AND OREGON, SUN-MON, DEC 21-22.

HEAVY PRECIPITATION (LOW ELEVATION RAIN AND MOUNTAIN SNOW) IN THE NORTHERN AND
CENTRAL ROCKIES, SUN-MON, DEC 21-22.

PERIODS OF HEAVY RAIN FOR NORTHERN AND CENTRAL FLORIDA, SUN-WED, DEC 21-24.

HEAVY RAIN FROM THE SOUTHERNMOST APPALACHIAN MOUNTAINS NORTHEAST ACROSS THE
MID-ATLANTIC REGION AND NEW ENGLAND, TUE-WED, DEC 23-24.

HEAVY SNOW DOWNWIND OF THE GREAT LAKES, WED-FRI, DEC 24-26.

HIGH WINDS OVER MUCH OF THE EASTERN CONUS, AS FAR WEST AS THE NORTHERN AND
CENTRAL GREAT PLAINS, AND AS FAR SOUTH AS NORTHERN GEORGIA, WED-THU, DEC 24-25.

POSSIBLE FLOODING IN SOUTHWEST WASHINGTON AND NORTHWEST OREGON, SUN-MON, DEC
21-22.

SLIGHT RISK OF MUCH BELOW-NORMAL TEMPERATURES FROM EASTERN MONTANA TO THE
MID-ATLANTIC REGION, FRI-MON, DEC 26-29.

MODERATE RISK OF MUCH BELOW-NORMAL TEMPERATURES FOR THE UPPER MISSISSIPPI
VALLEY, FRI-MON, DEC 26-29.

SEVERE DROUGHT FOR THE CENTRAL AND SOUTHERN GREAT PLAINS, SOUTHWEST, SOUTHEAST,
LOWER MISSISSIPPI VALLEY, PACIFIC NORTHWEST, GREAT BASIN, AND CALIFORNIA.

DETAILED SUMMARY

FOR SUNDAY DECEMBER 21 - THURSDAY DECEMBER 25: A 500-HPA RIDGE IS FORECAST TO
AMPLIFY NEAR THE WEST COAST, WHILE A DOWNSTREAM TROUGH DIGS ACROSS THE
EAST-CENTRAL CONUS. THE 00Z DETERMINISTIC ECMWF AND THE 12Z DETERMINISTIC GFS
MODEL SOLUTIONS VARY IN THE PREDICTED PATTERN OF PRECIPITATION DURING THIS
PERIOD. SEVERAL WEAKER DISTURBANCES ARE EXPECTED TO DEVELOP ACROSS THE
SOUTHEAST IN ADVANCE OF THE FORMATION OF A MORE CONSOLIDATED, MAJOR STORM
SYSTEM. THE PRIMARY LOW PRESSURE CENTER ASSOCIATED WITH THIS MAJOR SYSTEM IS
FORECAST TO MOVE INTO THE GREAT LAKES/SOUTHERN ONTARIO REGION, DEEPEN RAPIDLY
TO ABOUT 976-HPA, AND THEN SLOWLY MEANDER IN THIS AREA. A LARGE WIND FIELD OF
25-35 MPH IS ANTICIPATED ACROSS THE EAST IN ASSOCIATION WITH THIS STORM. BY
CHRISTMAS EVE, LAKE-ENHANCED SNOW SQUALLS ARE PREDICTED TO SET UP DOWNWIND OF
THE GREAT LAKES, SOME OF WHICH COULD HAVE LOCALLY HEAVY SNOWFALL. THOUGH THE
FORECAST TEMPERATURE DIFFERENCE BETWEEN THE BOUNDARY LAYER AND SURFACE WATERS
OF THE GREAT LAKES IS NOT AS LARGE AS A MONTH AGO WITH THE EXTREME SNOW AMOUNTS
THAT WERE OBSERVED SOUTH OF BUFFALO, NY, IT IS STILL ENOUGH TO GENERATE POTENT
LAKE EFFECT SNOW SQUALLS.



ANOTHER FACET OF THIS ANTICIPATED MAJOR STORM IS THE HEAVY RAIN ACROSS THE EAST
COAST STATES. LIKE YESTERDAY`S 12Z GFS SOLUTION, TODAY`S 12Z RUN PREDICTS ONLY
A WEAKNESS IN THE SURFACE PRESSURE FIELD NEAR THE MID-ATLANTIC COAST, AND NOT A
MAJOR TRANSFER OF ENERGY FROM THE PRIMARY LOW IN THE GREAT LAKES REGION TO A
LOW OFF THE MID-ATLANTIC COAST. IN ANY EVENT, HEAVY RAIN IS ANTICIPATED FROM
FLORIDA AND GEORGIA NORTHEASTWARD ACROSS THE MID-ATLANTIC AND NORTHERN ATLANTIC
REGIONS. THIS STORM IS EXPECTED TO HAVE A SIGNIFICANT IMPACT ON PRE-CHRISTMAS
TRAVEL. MODEL GUIDANCE IS IN GENERAL AGREEMENT THAT WITH THE POSSIBLE EXCEPTION
OF THE HIGHER ELEVATIONS OF THE APPALACHIANS, THIS IS PRIMARILY A RAIN EVENT.
TOWARDS THE END OF THE PRECIPITATION EVENT, TEMPERATURES MAY BE COLD ENOUGH TO
SUPPORT LIGHT SNOW ACROSS THE COASTAL PLAIN AND MAJOR NORTHEAST CITIES.



MEANWHILE, FAST WESTERLY FLOW ACROSS THE PACIFIC NORTHWEST AND NORTHERN ROCKIES
(IN-BETWEEN A LOW PRESSURE CENTER OVER THE NORTHERN GULF OF ALASKA, AND A HIGH
PRESSURE CENTER OFF THE CENTRAL CALIFORNIA COAST) IS EXPECTED TO BRING HEAVY
PRECIPITATION TO THE PACIFIC NORTHWEST AND NORTHERN AND CENTRAL ROCKIES DURING
THE FIRST TWO DAYS OF THIS PERIOD. THE CASCADES IN WASHINGTON AND OREGON COULD
RECEIVE 4-7 INCHES OF PRECIPITATION (LIQUID EQUIVALENT), WHILE 2-4 INCH AMOUNTS
ARE ANTICIPATED FOR THE ROCKIES. LOCALIZED AREAS OF POSSIBLE FLOODING ARE
INDICATED ON THE MAP IN SOUTHWEST WASHINGTON AND NORTHWEST OREGON.



A MAJOR STORM SYSTEM IS FORECAST TO MOVE ACROSS THE BERING SEA DURING TUESDAY
AND WEDNESDAY, DECEMBER 23-24. AT THE STORM`S PEAK, STORM-FORCE WINDS (50
KNOTS) AND 10-11 METER SIGNIFICANT WAVE HEIGHTS ARE PREDICTED BY THE NOAA
WAVEWATCH MODEL FOR THE CENTRAL BERING SEA, BEFORE THE STORM CURVES
NORTHWESTWARD INTO EXTREME EASTERN RUSSIA. THOUGH THIS STORM SYSTEM POSES
SERIOUS HAZARDS FOR COMMERCIAL SHIPPING IN THE BERING SEA, IT APPEARS THAT THE
WEST COAST OF ALASKA WILL BE SPARED THE WORST OF ITS IMPACTS.

FOR FRIDAY DECEMBER 26 - THURSDAY JANUARY 01: MODELS ARE IN AGREEMENT ON A
HIGHLY AMPLIFIED WAVE PATTERN OVER NORTH AMERICA IN THE WEEK-2 RANGE. A HIGHLY
AMPLIFIED RIDGE IS PREDICTED OVER ALASKA AND THE GULF OF ALASKA, WHILE A DEEP
TROUGH IS EXPECTED OVER EAST-CENTRAL NORTH AMERICA. THERE IS A SLIGHT CHANCE OF
MUCH BELOW-NORMAL MINIMUM TEMPERATURES FROM THE DAKOTAS AND NEBRASKA
EAST-SOUTHEASTWARD ACROSS THE GREAT LAKES AND OHIO VALLEY DURING THE FIRST FOUR
DAYS OF THIS PERIOD. A MODERATE CHANCE OF MUCH BELOW-NORMAL MINIMUM
TEMPERATURES IS ANTICIPATED OVER THE UPPER GREAT LAKES AND UPPER MISSISSIPPI
VALLEY.



THE MOST RECENT U.S. DROUGHT MONITOR, RELEASED ON DECEMBER 18, INDICATES A VERY
SLIGHT INCREASE IN THE AREAL COVERAGE OF SEVERE TO EXCEPTIONAL DROUGHT (D2 TO
D4) FROM 17.09 TO 17.27 PERCENT ACROSS THE CONTINENTAL U.S.. PERHAPS THE
LARGEST AND MOST SIGNIFICANT CHANGE FROM LAST WEEK`S DROUGHT MONITOR IS THE
ONE-CLASS IMPROVEMENT (FROM D4 TO D3) ACROSS NORTHERN CALIFORNIA AND THE SAN
FRANCISCO AREA IN RESPONSE TO THE RECENT HEAVY RAINS IN THAT REGION.

FORECASTER: ANTHONY ARTUSA

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