543  
FXUS02 KWBC 261524  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1123 AM EDT SUN AUG 26 2018  
 
VALID 12Z WED AUG 29 2018 - 12Z SUN SEP 02 2018  
   
..LATE SUMMER HEAT TO EXIT THE NORTHEAST LATER THIS WEEK
 
 
   
..WEATHER PATTERN HIGHLIGHTS AND HAZARDS
 
 
A SERIES OF SHORTWAVE TROUGHS WILL DIG OVER THE NORTHEASTERN  
PACIFIC TO SPLIT OFF THE WEST COAST, WITH ENERGETIC FLOW AND  
MODEST PRECIPITATION POTENTIAL WORKING INTO THE PACIFIC  
NORTHWEST/WEST BEFORE PROGRESSING EAST-NORTHEASTWARD THROUGH  
SOUTHERN CANADA AND THE U.S. NORTHERN TIER. WAVY FRONTAL SYSTEMS  
UNDERNEATH WILL COMBINE WITH DEEPENED MOISTURE AND INSTABILITY TO  
FUEL PERIODS OF HEAVY CONVECTION, FOCUSED FROM THE EASTERN  
PLAINS/MISSISSIPPI VALLEY TO THE MIDWEST/MID-SOUTH AND  
NORTHEAST/MID-ATLANTIC.  
 
EXPECT A THREAT FOR LOCALLY EXCESSIVE, BUT LESS ORGANIZED,  
THUNDERSTORMS/HEAVY DOWNPOURS WITHIN THE HOT/HUMID SECTOR IN THE  
SOUTHERN/EASTERN U.S. WHICH MAY ALIGN WITH MESO-BOUNDARIES AND  
LOCAL CONVERGENCE EFFECTS. THIS POTENT LATE SUMMER HOT SECTOR  
UNDER RIDGING ALOFT WILL OFFER SPOTTY RECORD MAXIMUM TEMPERATURES  
OVER THE NORTHEAST/MID-ATLANTIC WEDNESDAY WITH NUMEROUS RECORD  
HIGH MINIMUM TEMPERATURES MAINLY WEDNESDAY INTO THURSDAY WITH HIGH  
DEW POINTS INTO THE UPPER 60S AND LOW 70S. COLD FRONT WILL PUSH  
THROUGH THE NORTHEAST EARLY THURSDAY AND THE MID-ATLANTIC  
THEREAFTER, SETTLING INTO THE CAROLINAS WHERE RELIEF WILL BE MUCH  
MORE MARGINAL. HOT SECTOR OVER THE U.S. SOUTHERN TIER/PLAINS WILL  
GENERALLY HOLD FIRM FOR THE NEXT WEEK.  
   
..GUIDANCE AND UNCERTAINTY ASSESSMENT
 
 
THE LARGER SCALE FLOW PATTERN IS REASONABLY WELL-DEPICTED WITHIN  
THE GUIDANCE OVER THE LOWER 48 BUT WAS MUCH MORE UNCERTAIN  
UPSTREAM OVER ALASKA AND THE NORTH PACIFIC. SEE THE PMDAK  
DISCUSSION FOR MORE DETAILS. THAT UNCERTAINTY TRANSLATES INTO  
WESTERN CANADA AND THE PAC NW BY NEXT WEEKEND WITH LARGE TIMING  
DIFFERENCES AMONG THE ENSEMBLES. ECMWF ENSEMBLES HAVE TRENDED  
TOWARD INCREASED TROUGHING SOUTH OF ALASKA WHEREAS THE  
GEFS/CANADIAN MEMBERS MAINTAIN STRONGER RIDGING THROUGH NEXT  
WEEKEND, WHICH MODULATES THE LONGITUDE OF DOWNSTREAM TROUGHING IN  
THE WEST. TELECONNECTIONS SUPPORT A TROUGH AXIS FARTHER WEST THAN  
THE ECMWF ENSEMBLE MEAN ALONG 120W RATHER THAN 110W GIVEN THE  
MAINTENANCE OF UPPER RIDGING IN THE EAST. PREFERRED A BLEND THAT  
TRENDED TOWARD THE 00Z NAEFS AND 06Z GEFS MEAN BUT STILL WITH SOME  
MINORITY WEIGHTING OF THE 00Z ECMWF ENSEMBLE MEAN AS IT WAS THE  
BETTER PERFORMER IN RECENT DAYS.  
 
FRACASSO/SCHICHTEL  
 
WPC MEDIUM RANGE 500 MB HEIGHTS, SURFACE SYSTEMS, WEATHER GRIDS,  
QUANTITATIVE PRECIPITATION, WINTER WEATHER OUTLOOK PROBABILITIES  
AND HEAT INDEXES ARE FOUND AT:  
 
HTTPS://WWW.WPC.NCEP.NOAA.GOV/MEDR/5DAYFCST500_WBG.GIF  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/MEDR/5DAYFCST_WBG_CONUS.GIF  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/5KM_GRIDS/5KM_GRIDSBODY.HTML  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/QPF/DAY4-7.SHTML  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/WWD/PWPF_D47/PWPF_MEDR.PHP?DAY=4  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/HEAT_INDEX.SHTML  
 

 
 
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