347  
FXUS21 KWNC 141936  
PMDTHR  
US HAZARDS OUTLOOK  
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD  
300 PM EST NOVEMBER 14 2017  
 
SYNOPSIS: PERSISTENT MID-LEVEL HIGH PRESSURE NEAR THE ALEUTIANS CONTINUES TO  
DOMINATE DOWNSTREAM IMPACTS OF THE FLOW ON NORTH AMERICA. A GENERAL TENDENCY  
FOR MID-LEVEL LOW PRESSURE EXISTS OVER THE NORTHEAST PACIFIC THROUGHOUT THE  
PERIOD, WITH TRANSIENT SURFACE LOW PRESSURE SYSTEMS EJECTING FROM THIS AREA AND  
DRIFTING EASTWARD ACROSS THE CONTINENT. WHILE TRANSITIONING TOWARDS WEEK-2,  
MID-LEVEL HIGH PRESSURE IS ANTICIPATED TO BUILD OVER THE WEST, WHILE MID-LEVEL  
LOW PRESSURE IS FORECAST TO EXIST IN THE VICINITY OF THE ATLANTIC SEABOARD.  
PERIODIC SURFACE DISTURBANCES ARE FORECAST IN THE BERING SEA AND GULF OF ALASKA  
DURING BOTH WEEKS, BUT MODEL INCONSISTENCY IS RELATIVELY HIGH ON THEIR EXACT  
TIMING AND TRACKS.  
 
HAZARDS  
 
HEAVY PRECIPITATION ACROSS PORTIONS OF CALIFORNIA, THE PACIFIC NORTHWEST, AND  
THE NORTHERN GREAT BASIN, FRI-TUE, NOV 17-21.  
 
HEAVY SNOW ACROSS PORTIONS OF THE CENTRAL ROCKIES, FRI, NOV 17.  
 
HIGH WINDS ACROSS PORTIONS OF THE GREAT PLAINS, ROCKIES, GREAT BASIN, AND THE  
SOUTHWEST, FRI, NOV 17.  
 
HEAVY SNOW ACROSS PORTIONS OF NORTHERN MAINE, SAT, NOV 18.  
 
HEAVY LAKE-EFFECT SNOW ACROSS PORTIONS OF NEW YORK, MON, NOV 20.  
 
HIGH WINDS ACROSS PORTIONS OF SOUTHERN MAINLAND ALASKA, THE ALEUTIANS,AND PARTS  
OF THE ALASKA PANHANDLE, SAT-TUE, NOV 18-21.  
 
MUCH BELOW NORMAL-TEMPERATURES ACROSS PORTIONS OF THE ALASKA PANHANDLE AND MUCH  
OF MAINLAND ALASKA, SUN-TUE, NOV 19-21.  
 
MODERATE RISK OF MUCH BELOW-NORMAL TEMPERATURES FOR PORTIONS OF SOUTH-CENTRAL  
MAINLAND ALASKA, WED-FRI, NOV 22-24.  
 
SLIGHT RISK OF MUCH BELOW-NORMAL TEMPERATURES FOR PORTIONS OF THE ALASKA  
PANHANDLE AND MUCH OF MAINLAND ALASKA, WED-SUN, NOV 22-26.  
 
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 FRIDAY NOVEMBER 17 - TUESDAY NOVEMBER 21: AT THE OUTSET OF WEEK-1, A  
POSITIVELY TILTED 500-HPA TROUGH IS FORECAST TO EXTEND FROM CALIFORNIA THROUGH  
THE CANADIAN PRAIRIES. THIS FEATURE IS ANTICIPATED TO PUSH EASTWARD ACROSS  
NORTH AMERICA AND BECOME NEGATIVELY TILTED OVER THE GREAT LAKES BY MIDWEEK. AS  
THE TROUGH PUSHES THROUGH THE ROCKIES LEE CYCLOGENESIS IS FORECAST OVER KANSAS,  
RESULTING IN POTENTIAL LOCALIZED GUSTY HIGH WINDS (EXCEEDING 35 KNOTS) ON THE  
17TH FOR MUCH OF THE ROCKIES AND WESTERN PARTS OF THE GREAT PLAINS. ACROSS THE  
SOUTHWEST, SOUTHERN ROCKIES, AND CENTRAL AND SOUTHERN PLAINS, THESE DRY,  
DOWNSLOPE WINDS COULD ALSO MARGINALLY ENHANCE FIRE WEATHER CONDITIONS ON THE  
17TH. ELSEWHERE, WESTERLY UPSLOPE FLOW ACROSS COLORADO AND WYOMING COULD  
ENHANCE PACIFIC MOISTURE AND RESULT IN HEAVY SNOW (EXCEEDING 8 INCHES IN 24  
HOURS) ON THE 17TH.  
 
THE AFOREMENTIONED SURFACE LOW PRESSURE SYSTEM DEVELOPING IN THE LEE OF THE  
ROCKIES IS ANTICIPATED TO INTENSIFY AND LIFT TOWARDS QUEBEC OVER THE COURSE OF  
THE WEEK. MODELS VARY WITH HOW DEEP THIS SYSTEM BECOMES (THE 0Z GFS BELOW  
980-HPA, WHILE THE ECMWF IS CLOSER TO 990-HPA), BUT THE MORE INTENSE SOLUTIONS  
WOULD SUGGEST HIGH WIND POTENTIAL FOR THE GREAT LAKES DUE TO ISALLOBARIC  
EFFECTS. THE 12Z GFS GUIDANCE HAS TRENDED CLOSER TO THE WEAKER ECMWF FORECASTS.  
REGARDLESS OF HOW DEEP THE LOW GETS, THERE DOES APPEAR TO BE POTENTIAL FOR  
HEAVY SNOW (EXCEEDING 6 INCHES IN 24 HOURS) ACROSS NORTHERN MAINE ON THE 19TH  
WITH THE SYSTEM'S PASSAGE. YESTERDAY'S OUTLOOK ANTICIPATED THE POTENTIAL FOR  
HEAVY LAKE-EFFECT SNOW ON THE BACK SIDE OF THIS SYSTEM, BUT TODAY MODEL  
GUIDANCE HAS A MORE NORTHERLY COMPONENT TO THE LOW-LEVEL FLOW, WHICH WOULD  
LIMIT THE POTENTIAL FOR HEAVY SNOW DUE TO DECREASED ODDS OF A LAKE-PARALLEL  
BAND SETTING UP AND REDUCED FETCH ACROSS THE LAKES. A SMALLER AREA IS NOW  
HIGHLIGHTED ON THE 20TH FOR UP TO 6 INCHES OF LAKE-EFFECT SNOW, WITH THIS  
REGION SHIFTED SLIGHTLY SOUTHWARD RELATIVE TO THE PREVIOUS OUTLOOK.  
 
PERIODIC HEAVY PRECIPITATION ODDS CONTINUE TO BE FAVORED ACROSS MUCH OF THE  
WEST FROM THE 17TH THROUGH 20TH. THIS TRANSLATES TO SOME DAYS WHERE LIQUID  
EQUIVALENT PRECIPITATION TOTALS COULD BE 2 INCHES OR GREATER. SNOW LEVELS ARE  
FORECAST TO RISE THROUGHOUT THIS PERIOD DUE TO THE SOUTHERLY COMPONENT OF THE  
LOW-LEVEL FLOW, WITH SNOW EVENTUALLY BECOMING CONFINED TO ONLY THE HIGHEST  
PEAKS AND ACROSS THE NORTHERN CASCADES. RELATIVE TO YESTERDAY, MODEL GUIDANCE  
IS BRINGING MORE OF THE PACIFIC MOISTURE IN THIS SCENARIO INTO CANADA.  
 
UNCERTAINTY AMONG MODEL GUIDANCE APPEARS ESPECIALLY HIGH DURING THE OUTLOOK  
PERIOD FOR ALASKA. BOTH THE GFS AND ECMWF FEATURE A SYSTEM DROPPING  
SOUTH-SOUTHEASTWARD FROM THE BERING SEA INTO THE GULF OF ALASKA EARLY IN THE  
FORECAST PERIOD. THE 0Z ECMWF INTENSIFIES THIS SYSTEM MUCH DEEPER THAN THE GFS  
(SUB 950-HPA VERSUS AROUND 970-HPA), WHILE THE GFS KEEPS THIS SYSTEM FURTHER TO  
THE SOUTH AND WEST RELATIVE TO THE EUROPEAN. HEAVY PRECIPITATION WOULD BE  
POSSIBLE LATE IN WEEK-1 FOR THE ALASKA PANHANDLE WITH THE ECMWF TRACK, BUT  
GIVEN MODEL UNCERTAINTY AT THIS TIME NO HAZARD IS INDICATED. A HIGH WIND HAZARD  
IS FORECAST FROM THE ALEUTIANS THROUGH THE NORTHERN ALASKA PANHDNLE FOR THE  
18TH THROUGH 21ST, WHERE WINDS IN EXCESS OF 50 KNOTS ARE ANTICIPATED. THE  
INITIAL WIND THREAT FOR THE WESTERN PORTION OF THIS HAZARD IS IN ASSOCIATION  
WITH THE AFOREMENTIONED LOW TRANSITIONING INTO THE GULF OF ALASKA WHILE  
DEEPENING. THE EASTERLY WIND TO THE NORTH OF THIS LOW IS FORECAST TO HELP BRING  
BITTERLY COLD AIR WESTWARD OUT OF THE YUKON AND ACROSS MUCH OF MAINLAND ALASKA  
AND PORTIONS OF THE PANHANDLE, WHERE MUCH BELOW-NORMAL TEMPERATURES ARE  
FORECAST FROM THE 19TH THROUGH 21ST. WIDESPREAD MINIMUM TEMPERATURE ANOMALIES  
OF -20 DEGREES F OR GREATER ARE POSSIBLE, WITH THE POTENTIAL FOR THE FIRST -40  
DEGREE READINGS IN ALASKA THIS SEASON. HIGH WINDS ARE ALSO POSSIBLE SOUTH OF  
THE ALASKA RANGE THROUGH THE 21ST IN ASSOCIATION WITH KATABATIC FLOW. LASTLY, A  
HIGH WIND THREAT EXISTS FOR PORTIONS OF THE NORTHERN GULF OF ALASKA COASTLINE  
SURROUNDING YAKUTAT FOR THE 18TH THROUGH 21ST. HERE A POTENTIAL GAP WIND EVENT  
FROM THE COLD AIR RUSHING THROUGH THE VALLEYS COULD SEE LOCALIZED GUSTS  
APPROACHING 100 MPH.  
 
FOR WEDNESDAY NOVEMBER 22 - TUESDAY NOVEMBER 28: THE WEEK-2 CIRCULATION IS  
FAIRLY SIMILAR AMONG THE ENSEMBLE MEANS OF THE GEFS, ECMWF, AND CANADIAN  
GUIDANCE. EACH CONTINUES TO FEATURE THE BLOCK IN THE NORTH PACIFIC, RIDGING  
OVER THE WEST, TROUGHING IN THE VICINITY OF THE ATLANTIC, AND ANOTHER BLOCK  
BUILDING OVER THE DAVIS STRAIT. SPAGHETTI PLOTS FOR THE RESPECTIVE MODEL SUITES  
REVEAL A DIFFERENT PERSPECTIVE THOUGH, WITH SUBSTANTIAL SPREAD ACROSS MUCH OF  
THE NORTHERN HEMISPHERE. THIS UNCERTAINTY LIMITS POTENTIAL FOR DETERMINISTIC  
HAZARDS DURING WEEK-2.  
 
THE 0Z DETERMINISTIC AND ENSEMBLE ECMWF FORECASTS SUGGEST WIDESPREAD 24-HOUR  
LIQUID EQUIVALENT PRECIPITATION OF 2 INCHES OR MORE ON THE 22ND FROM NORTHERN  
CALIFORNIA THROUGH THE CANADIAN BORDER. THE GFS AND GEFS ARE FURTHER NORTH WITH  
THIS PRECIPITATION, INSTEAD FOCUSED OVER COASTAL BRITISH COLUMBIA. CANADIAN  
MODEL GUIDANCE IS A BIT OF A COMPROMISE BETWEEN THE TWO, INSTEAD FOCUSED OVER  
COASTAL WASHINGTON AND OREGON, ALBEIT WITH LESSER PRECIPITATION TOTALS. GIVEN  
THE UNCERTAINTY REGARDING ANY PRECIPITATION HAZARDS, NO DETERMINISTIC HAZARD IS  
ASSOCIATED WITH THIS EVENT AT THIS TIME.  
 
THE SLIGHT RISK OF MUCH BELOW-NORMAL TEMPERATURES FOR THE EAST IS DROPPED FROM  
YESTERDAY'S OUTLOOK, AS ECMWF GUIDANCE HAS TRENDED TOWARDS A LESS AMPLIFIED  
TROUGH THAT IS NOW SITUATED OVER THE CANADIAN MARITIMES, WELL TO THE EAST OF  
YESTERDAY'S FORECAST. THE GEFS PROBABILISTIC EXTREMES TOOL ALSO FAILS TO  
PRODUCE TEMPORALLY AND SPATIALLY COHERENT SIGNALS FOR A 20% CHANCE OF MINIMUM  
TEMPERATURES BEING BELOW THE 15TH CLIMATOLOGICAL PERCENTILE, DESPITE STILL  
PRODUCING A WEEKLY MEAN ANOMALOUS TROUGH CENTERED CLOSER TO THE ATLANTIC COAST.  
 
THE MUCH BELOW-NORMAL TEMPERATURE THREAT DOES APPEAR TO CONTINUE FOR ALASKA  
DURING WEEK-2. MUCH OF THE STATE IS HIGHLIGHTED FOR A SLIGHT RISK OF MUCH  
BELOW-NORMAL TEMPERATURES THROUGH THE 26TH. A MODERATE RISK FOR MUCH  
BELOW-NORMAL TEMPERATURES IS FORECAST THROUGH THE 24TH FOR PORTIONS OF  
SOUTH-CENTRAL MAINLAND ALASKA WHERE ECMWF REFORECAST GUIDANCE HAS THE GREATEST  
COLD SIGNAL DURING THE 6-10 DAY PERIOD.  
 
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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