598  
FXUS21 KWNC 151925  
PMDTHR  
US HAZARDS OUTLOOK  
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD  
300 PM EST NOVEMBER 15 2017  
 
SYNOPSIS: PERSISTENT MID-LEVEL HIGH PRESSURE CONTINUES TO RESIDE OVER THE  
ALEUTIANS, IMPACTING THE DOWNSTREAM CIRCULATION OVER NORTH AMERICA. MID-LEVEL  
LOW PRESSURE IS FORECAST TO LINGER OVER THE NORTHEAST PACIFIC THROUGHOUT THE  
OUTLOOK, FAVORING PERSISTENT FLOW OF PACIFIC MOISTURE INTO THE WEST COAST.  
ANOTHER AREA OF MID-LEVEL LOW PRESSURE IS ANTICIPATED TO PROGRESS FROM THE  
GREAT PLAINS THROUGH THE CANADIAN MARITIMES OVER THE COURSE OF WEEK-1. BEHIND  
THE AFOREMENTIONED FEATURE, MID-LEVEL HIGH PRESSURE IS FORECAST TO BUILD ACROSS  
THE ROCKIES AND GREAT PLAINS. ALASKA IS FORECAST TO EXPERIENCE A DEEP LOW  
PRESSURE SYSTEM IN THE GULF OF ALASKA DURING WEEK-1, THAT IS FORECAST TO HELP  
BRING COLD AIR OUT OF THE YUKON TO COVER MUCH OF THE STATE.  
 
HAZARDS  
 
HEAVY PRECIPITATION ACROSS PORTIONS OF CALIFORNIA, THE GREAT BASIN, AND THE  
PACIFIC NORTHWEST, SUN-WED, NOV 19-22.  
 
FLOODING LIKELY ACROSS PORTIONS OF WASHINGTON STATE.  
 
MUCH BELOW-NORMAL TEMPERATURES ACROSS MUCH OF MAINLAND ALASKA, SUN-WED, NOV  
19-22.  
 
HIGH WINDS ACROSS PORTIONS OF THE ALASKA PANHANDLE, SOUTHERN MAINLAND ALASKA,  
AND THE ALEUTIANS, SAT-WED, NOV 18-22.  
 
MODERATE RISK OF MUCH BELOW-NORMAL TEMPERATURES FOR PORTIONS OF EASTERN  
MAINLAND ALASKA, THU-SAT, NOV 23-25.  
 
SLIGHT RISK OF MUCH BELOW-NORMAL TEMPERATURES FOR PORTIONS OF THE ALASKA  
PANHANDLE AND MAINLAND ALASKA, THU-MON, NOV 23-27.  
 
SEVERE DROUGHT ACROSS PARTS OF THE THE LOWER AND MIDDLE MISSISSIPPI VALLEY, THE  
NORTHERN AND SOUTHERN PLAINS, THE NORTHERN ROCKIES, ARIZONA, AND HAWAII.  
 
DETAILED SUMMARY  
 
FOR SATURDAY NOVEMBER 18 - WEDNESDAY NOVEMBER 22: BLOCKING AT THE HIGH  
LATITUDES CONTINUES TO HELP DRIVE HAZARDOUS WEATHER IMPACTS OVER NORTH AMERICA  
DURING THE OUTLOOK PERIOD. LARGE POSITIVE HEIGHT ANOMALIES CONTINUE TO BE  
FAVORED IN THE VICINITY OF THE ALEUTIANS, WHILE A LESS INTENSE BLOCK IS ALSO  
FORECAST TO DEVELOP OVER THE DAVIS STRAIT DURING WEEK-1. THE BLOCK OVER THE  
ALEUTIANS IS LIKELY TO CONTINUE TO BE RESPONSIBLE FOR ANOMALOUS TROUGHING OVER  
THE NORTHEAST PACIFIC. THIS PATTERN FOR THE NORTHEAST PACIFIC FAVORS PERSISTENT  
ONSHORE FLOW OF PACIFIC MOISTURE, ENHANCED PERIODICALLY BY SHORTWAVES ROTATING  
AROUND THE LONGWAVE TROUGH DURING WEEK-1. AS SUCH, PERIODIC HEAVY PRECIPITATION  
(LIQUID EQUIVALENT AMOUNTS OF 2 INCHES OR GREATER IN 24 HOURS, WITH SNOW  
CONFINED TO THE CASCADES AND HIGHEST ELEVATIONS IN THE SIERRA NEVADAS) IS  
FORECAST FROM THE 19TH THROUGH 22ND. THE SOUTHERLY COMPONENT TO THE FLOW  
ANTICIPATED IN THIS SCENARIO SUGGESTS RISING SNOW LEVELS DURING THE EVENT, SUCH  
THAT PRECIPITATION IS LIKELY TO BE RAIN CLOSER TO THE END OF THE EVENT, OUTSIDE  
OF THE NORTHERN CASCADES. WITH THIS SURGE OF PACIFIC MOISTURE, RIVER FLOODING  
APPEARS LIKELY ALONG THE CHEHALIS AND SATSOP RIVERS OF WASHINGTON STATE.  
 
FOR ALASKA, THE GFS HAS COME AROUND TO THE ECMWF'S FORECAST REGARDING A  
DEEPENING LOW PRESSURE CENTER DESCENDING FROM THE BERING SEA INTO THE GULF OF  
ALASKA. THE GFS REMAINS FURTHER SOUTH AND WEST WITH THE TRACK RELATIVE TO THE  
ECMWF, BUT THE INTENSITY OF THE LOW IS NOW MUCH CLOSER BETWEEN THE TWO MODELS.  
THE TRACK OF THIS LOW FAVORS HIGH WINDS BEGINNING ACROSS THE ALEUTIANS AND  
SHIFTING EASTWARD INTO COASTAL PORTIONS OF THE GULF OF ALASKA OVER THE COURSE  
OF THE 18TH THROUGH 22ND. ONCE OVER THE GULF OF ALASKA, EASTERLY FLOW WELL TO  
THE NORTH OF THIS LOW IS FORECAST TO HELP DRIVE BITTERLY COLD AIR OUT OF THE  
YUKON AND ACROSS MUCH OF ALASKA. MUCH BELOW-NORMAL TEMPERATURES ARE FORECAST  
ACROSS MOST OF THE STATE FROM THE 19TH THROUGH 22ND, AS WIDESPREAD MINIMUM  
TEMPERATURE ANOMALIES OF -20 DEGREES F OR GREATER ARE FORECAST. THE COLD AIR  
POOLING ACROSS THE ALASKA RANGE AND OTHER MOUNTAINOUS AREAS IN SOUTHERN PARTS  
OF THE STATE IS LIKELY TO GIVE RISE TO THE POTENTIAL FOR LOCALLY HIGH WINDS IN  
ASSOCIATION WITH KATABATIC FLOW SPILLS DOWN THE TOPOGRAPHY. A GAP WIND EVENT IS  
ALSO LIKELY IN THE VICINITY OF YAKUTAT, WHERE WINDS COULD GUST TO 100 MPH OR  
GREATER AS THE ARCTIC AIR SPILLS SOUTHWARD OFF OF THE HUBBARD GLACIER.  
 
FOR THURSDAY NOVEMBER 23 - WEDNESDAY NOVEMBER 29: MIXED SIGNALS IN THE MODEL  
TREATMENT OF ANOMALOUS TROUGHING DURING WEEK-2 IN THE EASTERN U.S. PERSIST FROM  
YESTERDAY. THE GEFS HAS TRENDED SLIGHTLY MORE PROGRESSIVE WITH THE TROUGH  
RELATIVE TO YESTERDAY'S RUNS, SOMEWHAT MORE IN LINE WITH THE ECMWF FORECASTS.  
THE MODELS ARE TRENDING TOWARDS A WARMER SOLUTION IN THE EAST, WITH A STABLE  
ECMWF FORECAST CONFIRMING THE DECISION YESTERDAY TO DROP THE SLIGHT RISK OF  
MUCH BELOW-NORMAL TEMPERATURES FOR THIS REGION. TODAY'S 12Z DETERMINISTIC GFS  
APPEARS TO BE AN OUTLIER FOR WEEK-2, WITH ANOMALOUS TROUGHING OVER MOST OF THE  
WEST AND A RIDGE SITUATED EAST OF THE APPALACHIANS.  
 
COLD-RELATED HAZARDS LOOK TO CONTINUE FROM WEEK-1 ACROSS ALASKA. WEEK-2 MEAN  
SEA LEVEL PRESSURE FIELDS CONTINUE TO PLACE HIGH PRESSURE NEAR THE BORDER WITH  
THE YUKON, IMPLYING THE COLDEST AIR ACROSS EASTERN PARTS OF THE STATE. A  
MODERATE RISK OF MUCH BELOW-NORMAL TEMPERATURES IS FORECAST FOR THE EASTERN  
PORTION OF MAINLAND ALASKA ON THE 23RD THROUGH 25TH. THIS PERIOD AND REGION IS  
HIGHLIGHTED BY THE GEFS PROBABILISTIC EXTREMES TOOL GIVING A BETTER THAN 40%  
CHANCE OF MINIMUM TEMPERATURES BEING BELOW THE 15TH CLIMATOLOGICAL PERCENTILE.  
A BROADER SLIGHT RISK OF MUCH BELOW-NORMAL TEMPERATURES IS GIVEN FOR  
APPROXIMATELY THE SOUTHERN TWO-THIRDS OF MAINLAND ALASKA FROM THE 23RD THROUGH  
27TH. THE SLIGHT RISK IS ASSOCIATED WITH A 20% RISK OF MINIMUM TEMPERATURES  
BEING BELOW THE 15TH CLIMATOLOGICAL PERCENTILE BY THE GEFS PROBABILISTIC  
EXTREMES TOOL, WHILE CORRESPONDING TO THE AREA FAVORING BELOW-NORMAL  
TEMPERATURES IN THE WEEK-2 ECMWF REFORECAST GUIDANCE.  
 
THE U.S. DROUGHT MONITOR, VALID ON NOV 7, INDICATES THAT SEVERE TO EXCEPTIONAL  
(D2-D4) DROUGHT COVERS 2.90 PERCENT OF THE CONTINENTAL U.S., WHICH IS A SLIGHT  
INCREASE FROM 2.67 PERCENT LAST WEEK. THE MOST WIDESPREAD IMPROVEMENTS MADE TO  
THE DROUGHT DEPICTION THIS WEEK WERE OVER THE NORTHWESTERN CONUS. AREAS ACROSS  
THE SOUTHERN AND CENTRAL PLAINS AND THE LOWER AND MIDDLE MISSISSIPPI VALLEY HAD  
THE MOST WIDESPREAD DROUGHT CLASS DEGRADATION.  
 
FORECASTER: CPC FORECASTER  
 

 
 
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