084  
FXUS21 KWNC 221851  
PMDTHR  
US HAZARDS OUTLOOK  
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD  
300 PM EST NOVEMBER 22 2017  
 
SYNOPSIS: A COLD FRONT IS FORECAST TO PROGRESS EAST FROM THE GREAT LAKES TO OFF  
THE NORTHEAST COAST THIS WEEKEND. ONSHORE FLOW IS EXPECTED TO CONTINUE TO  
AFFECT THE PACIFIC NORTHWEST, NORTHERN CALIFORNIA, AND THE NORTHERN ROCKIES  
THROUGH NOV 26 AND THEN SURFACE HIGH PRESSURE IS FORECAST TO BECOME CENTERED  
OVER THE GREAT BASIN. MULTIPLE SURFACE LOW PRESSURE SYSTEMS ARE EXPECTED TO  
TRACK ACROSS THE ALEUTIANS NEXT WEEK. AN AREA OF UPPER-LEVEL HIGH PRESSURE IS  
FORECAST TO BUILD OVER THE NORTHEAST PACIFIC AND WESTERN NORTH AMERICA BY THE  
END OF NOVEMBER.  
 
HAZARDS  
 
HEAVY SNOW FOR THE SOUTHERN CASCADES AND NORTHERN SIERRAS, SUN, NOV 26.  
 
HEAVY SNOW FOR PARTS OF THE NORTHERN AND CENTRAL ROCKIES, MON, NOV 27.  
 
HIGH WINDS FOR PARTS OF THE WESTERN U.S., MON, NOV 27.  
 
HIGH WINDS FOR SOUTHERN CALIFORNIA, TUE, NOV 28.  
 
MUCH BELOW-NORMAL TEMPERATURES ACROSS PARTS OF EASTERN MAINLAND ALASKA, SAT,  
NOV 25.  
 
PERIODS OF HIGH WINDS AND HIGH WAVES FOR THE ALEUTIANS AND ALASKA PENINSULA,  
SUN-WED, NOV 26-29.  
 
FLOODING OCCURRING OR IMMINENT ALONG MULTIPLE RIVES ACROSS WEST-CENTRAL  
WASHINGTON.  
 
SEVERE DROUGHT ACROSS PARTS OF THE LOWER AND MIDDLE MISSISSIPPI VALLEY, GREAT  
PLAINS, NORTHERN ROCKIES, ARIZONA, AND HAWAII.  
 
DETAILED SUMMARY  
 
FOR SATURDAY NOVEMBER 25 - WEDNESDAY NOVEMBER 29: 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  
WEST-CENTRAL WASHINGTON. FLOODING IS OCCURRING OR IMMINENT ALONG THE FOLLOWING  
RIVERS: COWLITZ, DUNGENESS, NISQUALLY, NOOKSACK, SKAGIT, SKOKOMISH, AND  
SNOQUALMIE. ADDITIONAL RAINFALL ALSO LEADS TO AN INCREASING RISK OF LANDSLIDES.  
AS THE UPPER-LEVEL TROUGH SHIFTS INLAND FROM THE EAST PACIFIC ON NOV 26, SNOW  
LEVELS ARE EXPECTED TO DECREASE BELOW 4,000 FEET (NEAR HIGHWAY PASS LEVELS)  
ACROSS THE SOUTHERN CASCADES AND NORTHERN SIERRAS WHERE HEAVY SNOW (6 INCHES OR  
MORE PER 24 HOURS) IS FORECAST.  
 
A VIGOROUS SHORTWAVE TROUGH IS EXPECTED TO TRACK ACROSS THE NORTHERN ROCKIES ON  
NOV 27 WITH SNOW AND INCREASED WINDS. SINCE THERE HAS BEEN A MODEL TREND  
TOWARDS A STRONGER SHORTWAVE WITH HIGHER PRECIPITATION AMOUNTS, A HEAVY SNOW  
HAZARD IS POSTED FOR PARTS OF THE NORTHERN AND CENTRAL ROCKIES WHERE SNOWFALL  
AMOUNTS IN EXCESS OF 6 INCHES ARE MOST LIKELY. HIGH WINDS (SPEEDS OF NEAR 40  
MPH) ARE ALSO EXPECTED TO ACCOMPANY THE VIGOROUS SHORTWAVE TROUGH WHICH COULD  
LEAD TO BLIZZARD CONDITIONS IN AREAS OF SNOWFALL.  
 
DETERMINISTIC MODEL SOLUTIONS ARE IN GOOD AGREEMENT THAT SURFACE HIGH PRESSURE  
(AROUND 1036-HPA) BECOMES CENTERED OVER THE GREAT BASIN EARLY NEXT WEEK. THIS  
SURFACE PATTERN IS EXPECTED TO PROMOTE A SANTA ANA WIND EVENT (WIND GUSTS ABOVE  
50 MPH) FOR SOUTHERN CALIFORNIA ON NOV 28.  
 
A COLD FRONT IS FORECAST TO USHER IN A BRIEF PERIOD OF BELOW-NORMAL  
TEMPERATURES TO THE GREAT LAKES AND NORTHEAST THIS WEEKEND. THE HEAVY,  
LAKE-EFFECT SNOW HAZARD POSTED DOWNWIND OF LAKES ERIE AND ONTARIO ON THE  
PREVIOUS OUTLOOK HAS BEEN REMOVED TODAY SINCE SNOWFALL AMOUNTS ARE EXPECTED TO  
BE LESS GIVEN A SHORTER DURATION OF COLD AIR ADVECTION.  
 
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. AS THE HIGH AMPLITUDE RIDGE UPSTREAM OVER THE  
BERING SEA WEAKENS, ANOMALOUS OFFSHORE FLOW IS EXPECTED TO END WHICH WILL LEAD  
TO A RAPID INCREASE IN TEMPERATURES ACROSS MAINLAND ALASKA. A COUPLE OF INTENSE  
LOW PRESSURE SYSTEMS ARE FORECAST TO BRING HIGH WINDS (SPEEDS ABOVE 50 MPH) AND  
HIGH WAVES (PEAK HEIGHTS OF MORE THAN 36 FEET) TO THE ALEUTIANS AND ALASKA  
PENINSULA FROM NOV 26 TO 29.  
 
FOR THURSDAY NOVEMBER 30 - WEDNESDAY DECEMBER 06: THE ENSEMBLE MEANS DURING  
WEEK-2 FEATURE AN AMPLIFIED UPPER-LEVEL RIDGE EXTENDING FROM THE NORTHEAST  
PACIFIC TO ALASKA WITH ITS AXIS CENTERED AT 140 TO 150 DEGREES WEST LONGITUDE.  
THIS UPPER-LEVEL RIDGE IS LIKELY TO RESULT IN A MUCH DRIER PATTERN FOR THE  
PACIFIC NORTHWEST AT THE START OF DECEMBER. THERE IS AN INCREASING RISK OF  
COLDER TEMPERATURES RETURNING TO THE NORTHWESTERN U.S. DURING EARLY DECEMBER AS  
THE UPPER-LEVEL RIDGE BUILDS UPSTREAM OVER ALASKA.  
 
THE U.S. DROUGHT MONITOR, VALID ON NOV 21, INDICATES THAT SEVERE TO EXCEPTIONAL  
(D2-D4) DROUGHT COVERS 4.14 PERCENT OF THE CONTINENTAL U.S., WHICH IS A SLIGHT  
INCREASE FROM 3.52 PERCENT LAST WEEK. SEVERE DROUGHT HAS EXPANDED THIS MONTH  
ACROSS ARKANSAS AND MISSOURI.  
 
FORECASTER: BRAD PUGH  
 

 
 
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