267  
FXUS21 KWNC 161902  
PMDTHR  
US HAZARDS OUTLOOK  
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD  
300 PM EST NOVEMBER 16 2017  
 
SYNOPSIS: A SURFACE LOW AND TRAILING COLD FRONT ARE FORECAST TO EXIT THE  
EASTERN U.S. ON NOV 19. A SERIES OF SURFACE LOW PRESSURE SYSTEMS ARE LIKELY TO  
AFFECT THE WESTERN U.S. DURING THE NEXT WEEK. A SURFACE LOW IS EXPECTED TO  
STRENGTHEN AS IT TRACKS INTO THE NORTHERN GULF OF ALASKA EARLY IN THE PERIOD,  
WHILE ARCTIC HIGH PRESSURE PREVAILS ACROSS EASTERN MAINLAND ALASKA. DURING  
WEEK-2, AN AREA OF MID-LEVEL LOW PRESSURE IS EXPECTED TO PERSIST OFFSHORE OF  
THE WEST COAST WITH MID-LEVEL HIGH PRESSURE CENTERED OVER THE WEST-CENTRAL U.S.  
 
HAZARDS  
 
HEAVY PRECIPITATION ACROSS PARTS OF NORTHERN CALIFORNIA AND THE PACIFIC  
NORTHWEST, SUN-THU, NOV 19-23.  
 
MUCH BELOW-NORMAL TEMPERATURES ACROSS EASTERN MAINLAND ALASKA, SUN-THU, NOV  
19-23.  
 
HIGH WINDS FOR THE ALEUTIANS AND ALASKA PENINSULA, SUN-MON, NOV 19-20.  
 
HIGH WINDS FOR SOUTHEAST MAINLAND ALASKA AND THE ALASKA PANHANDLE, SUN-WED, NOV  
19-22.  
 
MODERATE RISK OF MUCH BELOW-NORMAL TEMPERATURES FOR PORTIONS OF EASTERN  
MAINLAND ALASKA, FRI, NOV 24.  
 
SLIGHT RISK OF MUCH BELOW-NORMAL TEMPERATURES FOR PORTIONS OF THE ALASKA  
PANHANDLE AND MAINLAND ALASKA, FRI-SAT, NOV 24-25.  
 
SEVERE DROUGHT ACROSS PARTS OF THE LOWER AND MIDDLE MISSISSIPPI VALLEY, GREAT  
PLAINS, NORTHERN ROCKIES, ARIZONA, AND HAWAII.  
 
DETAILED SUMMARY  
 
FOR SUNDAY NOVEMBER 19 - THURSDAY NOVEMBER 23: A REX BLOCK IS FORECAST TO  
PERSIST ACROSS THE NORTH PACIFIC WITH A HIGHLY AMPLIFIED RIDGE ANCHORED OVER  
THE ALEUTIANS AND BERING SEA. ANOTHER UPPER-LEVEL RIDGE IS EXPECTED TO  
STRENGTHEN NEAR THE DAVIS STRAIT. THE COMBINATION OF THESE HIGH AMPLITUDE  
RIDGES LEADS TO A NEGATIVE ARCTIC OSCILLATION (AO) INDEX DURING MID-NOVEMBER.  
THIS LONGWAVE PATTERN OVER THE NORTHERN HEMISPHERE RESULTS IN CONTINUED  
HAZARDOUS PRECIPITATION ACROSS PARTS OF THE WESTERN U.S. AND BELOW-NORMAL  
TEMPERATURES FOR THE EASTERN U.S.  
 
DOWNSTREAM OF THE UPPER-LEVEL RIDGE OVER THE NORTH PACIFIC, AN UPPER-LEVEL  
TROUGH AND ONSHORE FLOW ARE LIKELY TO PROMOTE A SERIES OF SURFACE LOW PRESSURE  
SYSTEMS AFFECTING THE PACIFIC NORTHWEST AND NORTHERN CALIFORNIA FROM NOV 19 TO  
23. THEREFORE, PERIODS OF HEAVY PRECIPITATION (LIQUID EQUIVALENT AMOUNTS OF 1.5  
INCHES OR MORE PER 24 HOURS) ARE FORECAST FOR THESE AREAS THROUGH THE MIDDLE  
PART OF NEXT WEEK. THE DETERMINISTIC 0Z ECMWF MODEL INDICATES TOTAL  
PRECIPITATION AMOUNTS, LIQUID EQUIVALENT, OF 4 TO 10 INCHES ALONG THE COASTAL  
RANGES, CASCADES, AND NORTHERN SIERRA NEVADA FROM NOV 19 TO 23. SNOW LEVELS ARE  
LIKELY TO RISE DURING THIS PERIOD WITH SNOW LIMITED TO THE HIGHEST ELEVATIONS  
OF THE SIERRA NEVADAS BY EARLY NEXT WEEK.  
 
MODEL SOLUTIONS HAVE CONVERGED ON A SURFACE LOW DEEPENING TO 968-HPA AS IT  
ENTERS THE NORTHERN GULF OF ALASKA ON NOV 19. THE TRACK OF THIS LOW FAVORS HIGH  
WINDS BEGINNING ACROSS THE ALEUTIANS AND SHIFTING EASTWARD TO THE ALASKA  
PENINSULA ON NOV 19 AND 20. 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 INTO MAINLAND ALASKA. BASED ON THE PREFERRED 0Z ECMWF MODEL, MUCH  
BELOW-NORMAL TEMPERATURES ARE FORECAST ACROSS EASTERN MAINLAND ALASKA, AS  
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 AS 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. A HIGH  
WIND HAZARD IS INDICATED FOR PARTS OF SOUTHEAST MAINLAND ALASKA AND THE  
NORTHERN ALASKA PANHANDLE FROM NOV 19 TO 22.  
 
FOR FRIDAY NOVEMBER 24 - THURSDAY NOVEMBER 30: THE DURATION OF THE NEGATIVE AO  
INDEX IS UNCERTAIN SINCE MANY GFS ENSEMBLE MEMBERS INDICATE THAT THE AO INDEX  
BECOMES MORE NEUTRAL BY THE FINAL WEEK OF NOVEMBER. ALSO, THE DETERMINISTIC  
ECMWF MODEL AND ITS ENSEMBLE MEAN INDICATE A WEAKENING OF THE PERSISTENT  
500-HPA RIDGE OVER THE BERING SEA. ALTHOUGH THERE IS MODERATE SPREAD AMONG  
ECMWF ENSEMBLE MEMBERS, MOST AGREE WITH LESS AMPLIFIED FLOW OVER THE NORTH  
PACIFIC AND RIDGING ACROSS THE WEST-CENTRAL CONUS. THIS LONGWAVE PATTERN FAVORS  
A MILDER TEMPERATURE PATTERN FOR THE END OF NOVEMBER WITH AT LEAST A TEMPORARY  
BREAK IN ARCTIC INTRUSIONS INTO THE NORTHERN ROCKIES AND PLAINS. LARGE ENSEMBLE  
SPREAD AND A RELATIVELY WARMER ECMWF MODEL SOLUTION PRECLUDE DESIGNATION OF A  
SLIGHT RISK OF MUCH BELOW NORMAL TEMPERATURES ACROSS THE EASTERN U.S., DESPITE  
THE GEFS TEMPERATURE TOOL SUPPORTING A SLIGHT RISK.  
 
ARCTIC HIGH PRESSURE IS EXPECTED TO PROLONG MUCH BELOW-NORMAL TEMPERATURES  
ACROSS EASTERN MAINLAND THROUGH THE EARLY PART OF WEEK-2. THE GEFS TEMPERATURE  
TEMPERATURE TOOL ALONG WITH GUIDANCE FROM THE ECMWF MODEL SUPPORT A SLIGHT (NOV  
24 AND 25) TO MODERATE (NOV 24) RISK OF MUCH BELOW NORMAL TEMPERATURES FOR  
PARTS OF MAINLAND ALASKA.  
 
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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