Prognostic Meteorological Discussion
Issued by NWS

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FXUS21 KWNC 211852
PMDTHR
US HAZARDS OUTLOOK
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD
300 PM EST NOVEMBER 21 2017

SYNOPSIS: A COLD FRONT IS FORECAST TO PROGRESS EAST FROM THE GREAT LAKES TO THE
NORTHEAST LATER THIS WEEK. A WEAK SURFACE LOW IS EXPECTED TO CROSS THE FLORIDA
PENINSULA ON NOV 24. ONSHORE FLOW IS EXPECTED TO CONTINUE TO AFFECT THE PACIFIC
NORTHWEST AND NORTHERN CALIFORNIA THROUGH THE WEEKEND. SURFACE HIGH PRESSURE IS
FORECAST TO PREVAIL OVER EASTERN MAINLAND ALASKA THROUGH NOV 25. AN AREA OF
UPPER-LEVEL HIGH PRESSURE IS FORECAST TO BUILD OVER WESTERN NORTH AMERICA BY
THE END OF NOVEMBER.

HAZARDS

HEAVY SNOW DOWNWIND OF LAKES ERIE AND ONTARIO, SUN, NOV 26.

MUCH BELOW-NORMAL TEMPERATURES ACROSS PARTS OF EASTERN MAINLAND ALASKA,
FRI-SAT, NOV 24-25.

FLOODING LIKELY OR POSSIBLE FOR PARTS OF WASHINGTON.

SEVERE DROUGHT ACROSS PARTS OF THE LOWER AND MIDDLE MISSISSIPPI VALLEY, GREAT
PLAINS, NORTHERN ROCKIES, ARIZONA, AND HAWAII.

DETAILED SUMMARY

FOR FRIDAY NOVEMBER 24 - TUESDAY NOVEMBER 28: AN UPPER-LEVEL TROUGH IS EXPECTED
TO REMAIN OFFSHORE OF THE WEST COAST THROUGH THE WEEKEND, MAINTAINING PERIODS
OF RAIN AND HIGH-ELEVATION SNOW ACROSS THE PACIFIC NORTHWEST AND NORTHERN
CALIFORNIA. ALTHOUGH RAINFALL AMOUNTS (GENERALLY 1.5 INCHES OR LESS PER 24
HOURS) ARE NOT EXPECTED TO EXCEED HAZARDS CRITERIA ON NOV 25 AND 26, THE
ADDITIONAL RAINFALL MAY PROLONG OR EXACERBATE RIVER FLOODING, ESPECIALLY IN
NORTHERN WASHINGTON. FLOODING IS LIKELY ALONG PARTS OF THE SKOKOMISH, WHILE
LEVELS ALONG THE FOLLOWING RIVERS MAY NEAR FLOOD STAGE: COWLITZ, DUNGENESS,
NISQUALLY, NOOKSACK, SKAGIT, AND SNOQUALMIE. ADDITIONAL RAINFALL THIS WEEK ALSO
LEADS TO AN INCREASING RISK OF LANDSLIDES. SOUTHWESTERLY FLOW IS FORECAST TO
MAINTAIN RELATIVELY HIGH SNOW LEVELS (ABOVE 6,000 FEET) ACROSS THE CASCADES.



A COLD FRONT IS FORECAST TO USHER IN BELOW-NORMAL TEMPERATURES TO THE GREAT
LAKES, NORTHEAST, AND MID-ATLANTIC THIS WEEKEND. COLD AIR ADVECTION IS FORECAST
TO RESULT IN 850-HPA TEMPERATURES FALLING TO NEAR -15 DEGREES C ACROSS THE
EASTERN GREAT LAKES ON NOV 26. THE MOST LIKELY AREA FOR HEAVY, LAKE-EFFECT SNOW
(6 INCHES OR MORE) IS EXPECTED DOWNWIND OF LAKES ERIE AND ONTARIO DUE TO A
LARGE DIFFERENCE BETWEEN THOSE LAKE SURFACES AND LOW-LEVEL TEMPERATURES.



AN ARCTIC SURFACE HIGH OVER THE YUKON IS FORECAST TO RESULT IN MUCH BELOW
NORMAL TEMPERATURES (NEGATIVE ANOMALIES OF 10 TO 15 DEGREES F) ACROSS SOUTHEAST
MAINLAND ALASKA THROUGH NOV 25. THE GEOGRAPHIC EXTENT OF THIS HAZARD IS BASED
ON GUIDANCE FROM THE 0Z ECMWF MODEL. AS THE HIGH AMPLITUDE RIDGE UPSTREAM OVER
THE BERING SEA WEAKENS, ANOMALOUS OFFSHORE FLOW IS EXPECTED TO END WHICH WILL
LEAD TO A RAPID MODERATION IN TEMPERATURES ACROSS MAINLAND ALASKA. THE HIGH
WIND HAZARD POSTED FOR NORTHWEST ALASKA ON THE PREVIOUS OUTLOOK HAS BEEN
REMOVED SINCE THE SURFACE LOW APPROACHING THAT REGION IS FORECAST TO BE WEAKER
AMONG TODAY`S MODEL SOLUTIONS.

FOR WEDNESDAY NOVEMBER 29 - TUESDAY DECEMBER 05: THE ENSEMBLE MEANS DURING
WEEK-2 INDICATE POSITIVE 500-HPA HEIGHT ANOMALIES AT THE HIGHER LATITUDES OF
THE NORTHERN HEMISPHERE WHICH WOULD TYPICALLY SIGNAL AN INCREASING RISK FOR
ARCTIC AIR OUTBREAKS ACROSS THE MIDDLE LATITUDES OF NORTH AMERICA. HOWEVER, THE
LONGWAVE PATTERN AND VARIOUS TEMPERATURE TOOLS FAVOR NEAR OR ABOVE NORMAL
TEMPERATURES THROUGHOUT THE LOWER 48 AND ALASKA DURING WEEK-2. ANY RENEWED
OUTBREAKS OF ARCTIC AIR ARE EXPECTED TO OCCUR BEYOND THE WEEK-2 PERIOD. THE
BUILDING RIDGE UPSTREAM OVER ALASKA MAY EVENTUALLY ALLOW COLDER TEMPERATURES TO
RETURN TO THE WESTERN AND NORTH-CENTRAL U.S. DURING EARLY DECEMBER. A
STRENGTHENING RIDGE ALOFT OVER THE EAST PACIFIC AND WESTERN NORTH AMERICA LEADS
TO A MUCH DRIER PATTERN FOR THE NORTHWESTERN U.S. FOR THE END OF NOVEMBER AND
START OF DECEMBER.



THE U.S. DROUGHT MONITOR, VALID ON NOV 14, INDICATES THAT SEVERE TO EXCEPTIONAL
(D2-D4) DROUGHT COVERS 3.52 PERCENT OF THE CONTINENTAL U.S., WHICH IS A SLIGHT
INCREASE FROM 2.90 PERCENT LAST WEEK. SEVERE DROUGHT HAS EXPANDED THIS MONTH
ACROSS ARKANSAS AND MISSOURI.

FORECASTER: BRAD PUGH

$$



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