Prognostic Meteorological Discussion
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FXUS21 KWNC 261907
PMDTHR
US HAZARDS OUTLOOK
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD
300 PM EST NOVEMBER 26 2014

SYNOPSIS: ONSHORE FLOW IS FORECAST TO SHIFT SOUTH ALONG THE WEST COAST BY EARLY
NEXT WEEK. A STRONG AREA OF SURFACE HIGH PRESSURE IS EXPECTED TO PROGRESS SOUTH
FROM CANADA INTO THE PACIFIC NORTHWEST, NORTHERN ROCKIES, AND NORTHERN GREAT
PLAINS DURING THE WEEKEND BEFORE IT RETREATS TO THE NORTH. MEANWHILE, AN
UPPER-LEVEL RIDGE OF HIGH PRESSURE IS FORECAST TO BUILD ACROSS THE SOUTHEASTERN
U.S. AT 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 APPROACHES THE ALEUTIANS THIS WEEKEND.

HAZARDS

MUCH BELOW NORMAL TEMPERATURES FOR PARTS OF THE PACIFIC NORTHWEST, NORTHERN
ROCKIES AND NORTHERN GREAT PLAINS, SAT-MON, NOV 29-DEC 1.

HEAVY SNOW FOR PARTS OF THE NORTHERN ROCKIES, SAT, NOV 29.

PERIODS OF HEAVY SNOW FOR THE SIERRAS OF CALIFORNIA, SAT-TUE, NOV 29-DEC 2.

PERIODS OF HEAVY RAIN FOR SOUTHWEST OREGON AND PARTS OF NORTHERN/CENTRAL
CALIFORNIA, SAT-TUE, NOV 29-DEC 2.

HEAVY RAIN FOR SOUTHERN CALIFORNIA, MON-TUE, DEC 1-2.

HEAVY PRECIPITATION FOR MUCH OF ARIZONA, TUE-WED, DEC 2-3.

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

HIGH WINDS FOR THE ALEUTIANS, SUN, NOV 30.

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

DETAILED SUMMARY

FOR SATURDAY NOVEMBER 29 - WEDNESDAY DECEMBER 03: A PATTERN CHANGE IS LIKELY BY
THE BEGINNING OF DECEMBER AS AN UPPER-LEVEL TROUGH DEVELOPS ALONG THE WEST
COAST. THIS PATTERN CHANGE IS EXPECTED TO BRING HEAVY RAIN ALONG COASTAL AREAS
OF CALIFORNIA AND HEAVY SNOW TO THE SIERRAS DURING THIS PERIOD. TOTAL
PRECIPITATION AMOUNTS (LIQUID EQUIVALENT) ARE EXPECTED TO LOCALLY EXCEED 5
INCHES DURING THIS 5-DAY PERIOD ACROSS COASTAL RANGES OF NORTHERN CALIFORNIA
AND ALSO ACROSS THE NORTHERN SIERRAS. EARLY NEXT WEEK, AN INFLUX OF SUBTROPICAL
MOISTURE INTO A SOUTHERN STREAM SHORTWAVE TROUGH INCREASES THE PROSPECTS OF
LOCALLY MORE THAN 1 INCH OF RAIN ACROSS COASTAL AREAS OF SOUTHERN CALIFORNIA
AND UP TO 5 INCHES FOR THE MOUNTAINS OF SOUTHERN CALIFORNIA EARLY NEXT WEEK.



FORECAST CONFIDENCE ON TIMING OF THE HEAVIEST PRECIPITATION FOR CALIFORNIA IS
LOWER TODAY SINCE THE LATEST MODEL RUNS HAVE SLOWED THE PROGRESSION OF A
SHORTWAVE TROUGH PUSHING INLAND FROM THE EAST PACIFIC. ADJUSTMENTS MAY BE
NEEDED IN FUTURE HAZARDS OUTLOOKS ON THE EXACT TIMING OF THE HEAVIEST
PRECIPITATION FOR CALIFORNIA.



PRECIPITATION AMOUNTS AND TIMING ACROSS THE SOUTHWEST ALSO CONTINUE TO VARY
AMONG MODEL SOLUTIONS, BUT HEAVY PRECIPITATION (RAIN AND HIGH-ELEVATION SNOW)
IS EXPECTED FOR ARIZONA ON DECEMBER 2 AND/OR 3.



MEANWHILE, ACROSS THE NORTHERN ROCKIES, HEAVY SNOW IS EXPECTED TO CONTINUE INTO
SATURDAY BEFORE IT SHIFTS SOUTH. HEAVY RAINFALL DURING THE NEXT FEW DAYS MAY
TRIGGER FLOODING ALONG THE MOST FLOOD-PRONE RIVERS ALONG THE WESTERN SLOPES OF
THE NORTHERN WASHINGTON CASCADES.



A STRONG AREA OF HIGH PRESSURE IS EXPECTED TO SHIFT SOUTH FROM WESTERN CANADA
AND BRING MUCH BELOW-NORMAL TEMPERATURES TO PARTS OF THE PACIFIC NORTHWEST,
NORTHERN ROCKIES, AND NORTHERN GREAT PLAINS FROM SATURDAY THROUGH MONDAY
MORNING. MINIMUM TEMPERATURES COULD FALL BELOW -10 DEGREES F ACROSS THE
NORTHERN HIGH PLAINS FROM NOVEMBER 29-DECEMBER 1.



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 964-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 THURSDAY DECEMBER 04 - WEDNESDAY DECEMBER 10: HIGH RESOLUTION MODELS
CONTINUE TO DIFFER ON HOW FAST A SOUTHERN STREAM SHORTWAVE TROUGH DEPARTS THE
SOUTHWEST AT THE BEGINNING OF WEEK-2. HEAVY PRECIPITATION MAY LINGER ACROSS
ARIZONA INTO WEEK-2, BUT MODEL DIFFERENCES PRECLUDE A DESIGNATION OF HEAVY
PRECIPITATION AT THIS TIME



MULTIPLE DAYS OF RETURN FLOW FROM THE GULF OF MEXICO ARE EXPECTED ACROSS THE
EAST-CENTRAL U.S. AT THE BEGINNING OF DECEMBER. AN INCREASE IN LOW-LEVEL
MOISTURE INTERACTING WITH THE UPPER-LEVEL TROUGH MOVING OUT OF THE WESTERN
CONUS RAISES THE RISK OF HEAVY RAINFALL ACROSS THE MIDDLE/LOWER MISSISSIPPI AND
OHIO VALLEYS DURING WEEK-2. HOWEVER, DUE TO UNCERTAINTY ON TIMING AND THE
POTENTIAL FOR PROGRESSIVE FLOW TO LIMIT RAINFALL AMOUNTS, A HEAVY RAIN HAZARD
IS NOT DEFINED AT THIS TIME.



THE MOST RECENT U.S. DROUGHT MONITOR, RELEASED ON NOVEMBER 20, INDICATES A
SLIGHT DECREASE IN THE AREAL COVERAGE OF SEVERE TO EXCEPTIONAL DROUGHT (D2 TO
D4) FROM 17.27 PERCENT TO 17.13 PERCENT ACROSS THE CONTINENTAL U.S.

FORECASTER: BRAD PUGH

$$



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