Prognostic Meteorological Discussion
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FXUS21 KWNC 172105
PMDTHR
US HAZARDS OUTLOOK
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD
300 PM EST DECEMBER 17 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 GULF COAST STATES IS FORECAST TO TRACK
TOWARDS THE CAROLINAS BEFORE HEADING OUT TO SEA. A MORE SIGNIFICANT STORM IS
PREDICTED TO DEVELOP ACROSS THE EAST-CENTRAL STATES TWO DAYS BEFORE CHRISTMAS,
GRADUALLY WORKING ITS WAY TOWARDS THE ATLANTIC COAST AND GREAT LAKES REGION.
SEVERAL INTENSE STORM SYSTEMS ARE ANTICIPATED TO BRING INCLEMENT WEATHER
CONDITIONS TO ALASKA DURING MOST OF THIS OUTLOOK PERIOD.

HAZARDS

HEAVY PRECIPITATION (LOW ELEVATION RAIN AND MOUNTAIN SNOW) IN WESTERN PORTIONS
OF WASHINGTON AND OREGON, AND FAR NORTHWEST CALIFORNIA, SAT-TUE, DEC 20-23.

HEAVY PRECIPITATION (LOW ELEVATION RAIN AND MOUNTAIN SNOW) IN THE NORTHERN
ROCKIES, SAT-TUE, DEC 20-23.

HEAVY RAIN ACROSS NORTHERN FLORIDA AND FAR SOUTHERN GEORGIA, MON-TUE, DEC 22-23.

HIGH WINDS FROM THE NORTHERN PLAINS SOUTHEAST TO THE MIDDLE MISSISSIPPI VALLEY,
MON-TUE, DEC 22-23.

HIGH WINDS FOR THE SOUTH-CENTRAL AND SOUTHEAST COASTS OF ALASKA (INCLUDING THE
PANHANDLE), SAT-SUN, DEC 20-21.

HIGH SIGNIFICANT WAVE HEIGHTS FOR ALL AREAS EAST OF PRINCE WILLIAM SOUND, SAT,
DEC 20.

HIGH SIGNIFICANT WAVE HEIGHTS FOR THE ALASKA PANHANDLE, SUN, DEC 21.

SLIGHT RISK OF MUCH BELOW-NORMAL TEMPERATURES FROM EASTERN MONTANA TO THE OHIO
VALLEY, THU-SUN, DEC 25-28.

MODERATE RISK OF MUCH BELOW-NORMAL TEMPERATURES FOR PORTIONS OF THE DAKOTAS AND
MINNESOTA, THU-SUN, DEC 25-28.

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

DETAILED SUMMARY

FOR SATURDAY DECEMBER 20 - WEDNESDAY DECEMBER 24: A WEAK LOW PRESSURE SYSTEM IS
FORECAST TO MOVE FROM THE CENTRAL GULF COAST REGION TO THE CAROLINAS. HIGH
PRESSURE OVER QUEBEC COULD PREVENT THE DISTURBANCE FROM TRACKING FARTHER NORTH,
INSTEAD DIVERTING IT OUT TO SEA. HOWEVER, SOME LIGHT PRECIPITATION MAY
OVERSPREAD THE SOUTHERN MID-ATLANTIC REGION. TEMPERATURES APPEAR TO BE
MARGINALLY COLD ENOUGH TO SUPPORT A PERIOD OF LIGHT SNOW IN THE INTERIOR, AND
LIGHT RAIN FOR COASTAL AREAS. THE TRAILING COLD FRONT IS FORECAST TO BRING
HEAVY RAIN (1.0-1.5 INCHES) TO FAR SOUTHERN GEORGIA AND MUCH OF NORTHERN
FLORIDA. 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 MAINTAIN STORMY CONDITIONS ACROSS THE REGION. THE EXPECTED PASSAGE
OF SEVERAL STORM SYSTEMS DURING THIS PERIOD MAY RESULT IN PRECIPITATION TOTALS
OF 5-10 INCHES (LIQUID EQUIVALENT) IN THE CASCADES OF WASHINGTON AND OREGON,
AND 2-4 INCHES IN THE NORTHERN ROCKIES. ONCE THIS ENERGY PASSES THE NORTHERN
ROCKIES, A DEEPENING 500-HPA TROUGH ACROSS THE CENTRAL CONUS IS ANTICIPATED TO
STEER THE FLOW SOUTHEASTWARD FROM THE NORTHERN GREAT PLAINS TOWARDS THE MIDDLE
MISSISSIPPI VALLEY. WINDS OF 25-35 MPH ARE PREDICTED ACROSS THIS AREA, AS THE
TROUGH DEEPENS. A SIGNIFICANT STORM SYSTEM IS THEN EXPECTED TO DEVELOP ACROSS
THE MIDWEST AND OHIO VALLEY TOWARDS THE END OF THIS PERIOD.



BY THIS TIME, UNCERTAINTY INCREASES RAPIDLY REGARDING THE FUTURE EVOLUTION OF
THIS STORM SYSTEM. IF THE LATEST GFS MODEL RUN (INITIALIZED AT 12Z) IS CORRECT,
TWO LOW PRESSURE CENTERS WILL CONSOLIDATE INTO ONE CENTER OVER THE OHIO VALLEY.
THE 12Z GFS RUN PREDICTS THIS CONSOLIDATED LOW WILL GRADUALLY ARC IN A COUNTER-
CLOCKWISE MOTION FIRST NORTHWARD ACROSS FAR WESTERN NEW YORK, AND THEN WESTWARD
INTO THE CENTRAL GREAT LAKES REGION. AT THE SAME TIME, THE CENTRAL PRESSURE OF
THIS LOW IS FORECAST TO FALL RAPIDLY TO A MINIMUM OF 960-965 HPA DURING
CHRISTMAS DAY. IN ADDITION, LOCALLY INTENSE LAKE-ENHANCED SNOW BANDS CAN BE
ANTICIPATED DOWNWIND OF THE GREAT LAKES. IT IS TOO EARLY TO SPECIFY WHICH AREAS
MAY BE HARDEST HIT WITH THESE SQUALLS. IF THE 12Z GFS IS CORRECT, AREAS ALONG
THE SOUTHERN SHORE OF LAKE SUPERIOR, AND AREAS NORTHEAST OF LAKES ERIE AND
ONTARIO WOULD BE PARTICULARLY VULNERABLE. ANOTHER CONSIDERATION WITH THIS
POTENTIAL WINTER STORM IS THE LATEST GFS RUN DOES NOT PREDICT A CLEAR TRANSFER
OF ENERGY FROM THE PRIMARY LOW TO A COASTAL LOW NEAR THE MID-ATLANTIC COAST
WITH THIS STORM SYSTEM, AS IS FREQUENTLY THE CASE. HOWEVER, IF ONE DOES DEVELOP
OFF THE DELMARVA PENINSULA, ENHANCED PRECIPITATION WOULD BE FAVORED ACROSS
EASTERN LONG ISLAND AND EASTERN NEW ENGLAND. INTERESTINGLY, THE 12Z OPERATIONAL
GFS AND THE 00Z OPERATIONAL ECMWF BOTH FORECAST RELATIVELY MILD TEMPERATURES
WITH THIS SYSTEM, SUPPORTING MOSTLY A RAIN EVENT (ESPECIALLY FOR COASTAL
AREAS), WITH SNOW FAVORED ON THE REAR-SIDE OF THE STORM AS IT DEPARTS THE
REGION. FOR THE TIME BEING, THIS POTENTIAL STORM IS BEING HIGHLIGHTED ONLY IN
THE TEXT, AND NOT ON THE MAP, GIVEN ALL THE UNCERTAINTY IN THE DETAILS.



IN ALASKA, A FAIRLY STRONG LOW PRESSURE SYSTEM JUST OFF THE SOUTH-CENTRAL COAST
IS FORECAST TO BRING HIGH WINDS AND WAVES FROM NEAR PRINCE WILLIAM SOUND
EASTWARD AND SOUTHWARD ALONG THE COASTAL AREAS TO KETCHIKAN IN THE SOUTHERN
PANHANDLE.  ANOTHER STRONG CYCLONIC SYSTEM IS EXPECTED TO AFFECT THE BERING SEA
AND WESTERN ALASKA BY THE MIDDLE OF NEXT WEEK. HOWEVER, NO HAZARDOUS AREAS
ASSOCIATED WITH THIS SECOND STORM ARE DESIGNATED ON THE MAP AT THIS TIME, GIVEN
TOO MUCH UNCERTAINTY A WEEK OUT.

FOR THURSDAY DECEMBER 25 - WEDNESDAY DECEMBER 31: 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 EASTERN MONTANA SOUTHEASTWARD INTO
THE OHIO VALLEY DURING THE FIRST FOUR DAYS OF THIS PERIOD. A MODERATE CHANCE OF
MUCH BELOW-NORMAL MINIMUM TEMPERATURES IS ANTICIPATED OVER PORTIONS OF THE
DAKOTAS AND MINNESOTA FOR THE SAME PERIOD.



THE MOST RECENT U.S. DROUGHT MONITOR, RELEASED ON DECEMBER 11, INDICATES A VERY
SLIGHT INCREASE IN THE AREAL COVERAGE OF SEVERE TO EXCEPTIONAL DROUGHT (D2 TO
D4) FROM 16.90 TO 17.09 PERCENT ACROSS THE CONTINENTAL U.S.

FORECASTER: ANTHONY ARTUSA

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