Prognostic Meteorological Discussion
Issued by NWS

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FXUS21 KWNC 271847
PMDTHR
US HAZARDS OUTLOOK
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD
300 PM EST NOVEMBER 27 2014

SYNOPSIS: AN AREA OF UPPER-LEVEL LOW PRESSURE IS EXPECTED TO SLOWLY PROGRESS
INLAND FROM THE EAST PACIFIC NEXT WEEK. A STRONG SURFACE HIGH IS FORECAST TO
SHIFT SOUTHEAST FROM THE PACIFIC NORTHWEST AND NORTHERN ROCKIES ON SUNDAY TO
THE MIDWEST ON MONDAY. MEANWHILE, AN UPPER-LEVEL RIDGE OF HIGH PRESSURE IS
FORECAST TO BUILD ACROSS THE SOUTHEASTERN U.S. IN THE BEGINNING OF DECEMBER. A
SURFACE HIGH IS EXPECTED TO PERSIST ACROSS NORTHEAST ALASKA DURING THE NEXT
WEEK, WHILE A LOW PRESSURE SYSTEM STRENGTHENS AS IT MOVES INTO THE BERING SEA
THIS WEEKEND.

HAZARDS

MUCH BELOW NORMAL TEMPERATURES EXPANDING EASTWARD ACROSS THE PACIFIC NORTHWEST,
NORTHERN ROCKIES, NORTHERN/CENTRAL GREAT PLAINS, AND UPPER MISSISSIPPI VALLEY,
SUN-MON, NOV 30-DEC 1.

PERIODS OF HEAVY SNOW FOR THE SIERRAS OF CALIFORNIA, SUN-TUE, NOV 30-DEC 2.

PERIODS OF HEAVY RAIN FOR PARTS OF NORTHERN/CENTRAL CALIFORNIA, SUN-TUE, NOV
30-DEC 2.

HEAVY RAIN FOR SOUTHERN CALIFORNIA, TUE-WED, DEC 2-3.

SIGNIFICANT RIVER FLOODING POSSIBLE, LIKELY, OR OCCURRING FOR PARTS OF
WASHINGTON.

HIGH WINDS FOR THE COLUMBIA RIVER GORGE IN THE PACIFIC NORTHWEST, SUN, NOV 30.

HIGH WINDS FOR THE ALEUTIANS, SUN, NOV 30.

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

DETAILED SUMMARY

FOR SUNDAY NOVEMBER 30 - THURSDAY DECEMBER 04: AN UPPER-LEVEL TROUGH IS
FORECAST TO SEPARATE FROM WESTERLY FLOW ACROSS THE NORTHEAST PACIFIC THIS
WEEKEND. THE EVOLUTION OF THIS TROUGH REMAINS UNCERTAIN AS MODEL DIFFERENCES
INCREASE NEXT WEEK. THE HIGH-RESOLUTION GFS MODEL CONTINUES TO BE MUCH MORE
PROGRESSIVE WITH THIS UPPER-LEVEL TROUGH AND IS ALSO A WETTER SOLUTION COMPARED
TO THE HIGH-RESOLUTION ECMWF MODEL. DUE TO LARGE DIFFERENCES AMONG THE
HIGH-RESOLUTION MODELS, THE LOCATION AND TIMING OF HEAVY PRECIPITATION HAZARDS
FOR CALIFORNIA ARE BASED PRIMARILY ON THE 0Z AND 6Z GFS ENSEMBLE MEANS. SEVERAL
INCHES OF PRECIPITATION (LIQUID EQUIVALENT) ARE EXPECTED ACROSS THE COASTAL
RANGES OF NORTHERN CALIFORNIA AND THE SIERRAS FROM SUNDAY THROUGH AT LEAST
TUESDAY. SNOW LEVELS AROUND 7,000 FEET ARE EXPECTED FOR THE SIERRAS. ONSHORE
FLOW AND THE POTENTIAL FOR TROPICAL MOISTURE TO BECOME ENTRAINED INTO THE
UPPER-LEVEL TROUGH INCREASE PROSPECTS FOR LOCALLY HEAVY RAIN (MORE THAN 1 INCH)
ACROSS SOUTHERN CALIFORNIA ON DECEMBER 2 AND/OR 3.



THE 12Z GFS MODEL INDICATES LIGHT PRECIPITATION SPREADING NORTH INTO THE
PACIFIC NORTHWEST ON SUNDAY AND MONDAY AHEAD OF THE EAST PACIFIC TROUGH. IF
PRECIPITATION OCCURS, SURFACE TEMPERATURES ARE EXPECTED TO SUPPORT FREEZING
RAIN ACROSS THE WILLAMETTE VALLEY AND COLUMBIA RIVER GORGE. LARGE MODEL
DIFFERENCES ON PRECIPITATION AMOUNTS PRECLUDE DESIGNATION OF A FREEZING RAIN
HAZARD AT THIS TIME.



FORECAST CONFIDENCE FOR HEAVY PRECIPITATION ACROSS ARIZONA IS LOWER TODAY DUE
TO LARGE MODEL DIFFERENCES UPSTREAM OVER THE EAST PACIFIC. THE PAST FEW RUNS OF
THE HIGH-RESOLUTION GFS MODEL INDICATE LESS PRECIPITATION FOR ARIZONA WITH THE
SHORTWAVE TROUGH BECOMING LESS AMPLIFIED AS IT MOVES INLAND.



MEANWHILE, A STRONG SURFACE HIGH BUILDING SOUTH FROM WESTERN CANADA IS EXPECTED
TO ENHANCE EASTERLY WINDS THROUGH THE COLUMBIA RIVER GORGE WHERE A HIGH WIND
HAZARD IS DEPICTED. HEAVY RAINFALL THIS WEEK MAY TRIGGER FLOODING ALONG THE
MOST FLOOD-PRONE RIVERS ALONG THE WESTERN SLOPES OF THE NORTHERN WASHINGTON
CASCADES.



A 1036-HA SURFACE HIGH BUILDING SOUTH FROM CANADA IS LIKELY TO RESULT IN MUCH
BELOW-NORMAL TEMPERATURES ACROSS PARTS OF THE PACIFIC NORTHWEST, NORTHERN
ROCKIES, AND NORTHERN/CENTRAL GREAT PLAINS ON SUNDAY. THE MUCH-BELOW NORMAL
TEMPERATURES ARE THEN EXPECTED TO EXPAND EAST INTO THE UPPER MISSISSIPPI VALLEY
ON MONDAY. SUBZERO MINIMUM TEMPERATURES COULD EXTEND AS FAR SOUTH AS NEBRASKA
AND IOWA BY MONDAY MORNING.



A LOW PRESSURE SYSTEM IS FORECAST TO STRENGTHEN AS IT TRACKS INTO THE BERING
SEA DURING THE WEEKEND. THE HIGH RESOLUTION 0/6Z GFS AND 0Z ECMWF MODELS
INDICATE A 968-HPA LOW ACROSS THE BERING SEA LATER IN THE WEEKEND. ON SUNDAY,
HIGH WINDS ACROSS THE ALEUTIANS ARE EXPECTED TO ACCOMPANY THIS LOW PRESSURE
SYSTEM.

FOR FRIDAY DECEMBER 05 - THURSDAY DECEMBER 11: SINCE MODEL DIFFERENCES ARE
LARGE AS EARLY AS DAY 5, NO SPECIFIC HAZARDS CAN BE DEFINED DURING WEEK-2
EXCEPT FOR THE AREAS OF ONGOING SEVERE TO EXCEPTIONAL DROUGHT.



THE MOST RECENT U.S. DROUGHT MONITOR, RELEASED ON NOVEMBER 27, INDICATES A
SLIGHT DECREASE IN THE AREAL COVERAGE OF SEVERE TO EXCEPTIONAL DROUGHT (D2 TO
D4) FROM 17.13 PERCENT TO 16.81 PERCENT ACROSS THE CONTINENTAL U.S. THIS THIS
THE LOWEST COVERAGE OF SEVERE TO EXCEPTIONAL DROUGHT SINCE DECEMBER 2013.

FORECASTER: BRAD PUGH

$$



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