377  
FXUS21 KWNC 201926  
PMDTHR  
US HAZARDS OUTLOOK  
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD  
300 PM EST NOVEMBER 20 2017  
 
SYNOPSIS: A COLD FRONT IS FORECAST TO ADVANCE SOUTH ACROSS THE EASTERN U.S.  
LATER THIS WEEK. A WEAK SURFACE LOW IS EXPECTED TO DEVELOP OVER THE GULF OF  
MEXICO BY THANKSGIVING WITH A SUBSEQUENT TRACK ACROSS THE FLORIDA PENINSULA AND  
NEAR THE OUTER BANKS. ONSHORE FLOW IS EXPECTED TO CONTINUE TO AFFECT THE  
PACIFIC NORTHWEST THROUGH THE WEEKEND. SURFACE HIGH PRESSURE IS FORECAST TO  
PREVAIL OVER EASTERN MAINLAND ALASKA DURING THE NEXT WEEK, WHILE A COUPLE OF  
LOW PRESSURE SYSTEMS TRACK ACROSS THE BERING SEA. THE PERSISTENT AREA OF  
UPPER-LEVEL HIGH PRESSURE OVER THE BERING SEA IS EXPECTED TO BE REPLACED BY  
UPPER-LEVEL LOW PRESSURE BY THE BEGINNING OF WEEK-2.  
 
HAZARDS  
 
HEAVY SNOW DOWNWIND OF LAKE ONTARIO, SUN, NOV 26.  
 
MUCH BELOW-NORMAL TEMPERATURES ACROSS PARTS OF EASTERN MAINLAND ALASKA,  
THU-SAT, NOV 23-25.  
 
HIGH WINDS FOR PARTS OF NORTHWEST ALASKA, SAT-SUN, NOV 25-26.  
 
SLIGHT RISK OF MUCH BELOW-NORMAL TEMPERATURES FOR PARTS OF THE NORTHEAST, TUE,  
NOV 28.  
 
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 THURSDAY NOVEMBER 23 - MONDAY NOVEMBER 27: 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. ALTHOUGH  
RAINFALL AMOUNTS (GENERALLY LESS THAN 1.5 INCHES PER 24 HOURS) ARE NOT EXPECTED  
TO EXCEED HAZARDS CRITERIA, THE FREQUENT RAIN 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  
7,000 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 AROUND -15 DEGREES C ACROSS THE  
GREAT LAKES ON NOV 25 AND 26. THE MOST LIKELY AREA FOR HEAVY, LAKE-EFFECT SNOW  
(6 INCHES OR MORE) IS EXPECTED DOWNWIND OF LAKE ONTARIO DUE TO THE LONGEST  
DURATION OF COLD AIR ADVECTION ON NOV 26.  
 
DETERMINISTIC MODEL SOLUTIONS CONTINUE TO INDICATE THAT A SHORTWAVE TROUGH  
SEPARATES FROM THE WESTERLIES AND INDUCES SURFACE CYCLOGENESIS IN THE GULF OF  
MEXICO BY NOV 23. THIS SURFACE LOW IS THEN FORECAST TO TRACK EAST ACROSS THE  
FLORIDA PENINSULA, BUT RAINFALL AMOUNTS ASSOCIATED WITH THIS LOW PRESSURE  
SYSTEM ARE EXPECTED TO BE AROUND AN INCH OR LESS.  
 
AN ARCTIC SURFACE HIGH OVER THE YUKON IS FORECAST TO RESULT IN MUCH BELOW  
NORMAL TEMPERATURES (NEGATIVE ANOMALIES OF 10 TO 20 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 EASTERN ALASKA. A SURFACE LOW  
IS FORECAST TO STRENGTHEN AS IT TRACKS EAST FROM THE KAMCHATKA PENINSULA AND  
BRING HIGH WINDS (SPEEDS OF 40 TO 50 KNOTS, OR HIGHER) TO PARTS OF NORTHWEST  
ALASKA ON NOV 25 AND 26.  
 
FOR TUESDAY NOVEMBER 28 - MONDAY DECEMBER 04: THE ECMWF AND CANADIAN ENSEMBLE  
MEANS ARE IN GOOD AGREEMENT WITH AN UPPER-LEVEL TROUGH (RIDGE) OVER THE BERING  
SEA (WESTERN NORTH AMERICA). HOWEVER, THE GFS ENSEMBLE MEAN INDICATES AN  
UPPER-LEVEL RIDGE NEAR THE BERING SEA. HIGH UNCERTAINTY EXISTS IN THE EVOLVING  
LONGWAVE PATTERN ACROSS THE MIDDLE AND HIGH LATITUDES OF NORTH AMERICA DUE TO  
THESE DIFFERING MODEL SOLUTIONS. ALSO, SPREAD AMONG ENSEMBLE MEANS IS MODERATE  
TO LARGE. THE ENSEMBLE MEANS AGREE ON A LARGE POSITIVE 500-HPA HEIGHT ANOMALY  
CENTER OVER THE NORTH ATLANTIC, WHICH MAY MAINTAIN NEAR TO BELOW NORMAL  
TEMPERATURES ACROSS THE NORTHEASTERN U.S. INTO THE BEGINNING OF DECEMBER. A  
SLIGHT RISK OF MUCH BELOW NORMAL TEMPERATURES IS POSTED FOR THE NORTHEAST WHERE  
THE GEFS TEMPERATURE TOOL INDICATES THAT MINIMUM TEMPERATURES HAVE CLOSE TO A  
20 PERCENT CHANCE OF FALLING BELOW THE 15TH PERCENTILE ON NOV 28.  
 
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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