136  
FXUS02 KWBC 150659  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
258 AM EDT SAT SEP 15 2018  
 
VALID 12Z TUE SEP 18 2018 - 12Z SAT SEP 22 2018  
   
..PATTERN OVERVIEW AND GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
THE FORECAST PERIOD BEGINS ON TUESDAY WITH A TROUGH CENTERED NEAR  
THE WEST COAST, AN UPPER LEVEL RIDGE OVER THE SOUTH-CENTRAL U.S.,  
AND A MID-UPPER LEVEL TROUGH OVER THE APPALACHIANS THAT CAPTURES  
THE REMNANTS OF FLORENCE. THE UPPER RIDGE EXPANDS ACROSS THE  
SOUTH-CENTRAL U.S. AND THEN THE SOUTHEAST STATES BY THE END OF THE  
WEEK, FOLLOWED BY A BUILDING TROUGH OVER THE NORTHEAST U.S. THAT  
BREAKS DOWN THE RIDGE AND RESULTS IN A COLD FRONT PASSAGE BY NEXT  
WEEKEND.  
 
THERE IS DECENT SYNOPTIC SCALE MODEL AGREEMENT REGARDING THE LARGE  
SCALE PATTERN THROUGH THE MIDDLE OF THE WEEK, WITH THE GREATEST  
DIFFERENCES NOTED IN REGARDS TO THE EVOLVING UPPER LEVEL TROUGH  
NEAR THE WEST COAST. THE CMC APPEARS TO HAVE MORE DOWNSTREAM  
RIDGING AND ITS SURFACE LOW TRACKS FARTHER TO THE NORTHWEST ACROSS  
THE CENTRAL PLAINS COMPARED TO THE MODEL CONSENSUS. THE MOST  
UNCERTAIN ASPECT OF THE FORECAST IS ACROSS THE PACIFIC NORTHWEST  
BY FRIDAY AND SATURDAY, WITH WIDELY DIFFERING MODEL SOLUTIONS  
PERTAINING TO A POTENTIAL SURFACE LOW OVER THE NORTHEAST PACIFIC.  
 
   
..WEATHER HIGHLIGHTS/HAZARDS
 
 
IN TERMS ON WHAT WILL BE THE REMNANTS OF FORMER HURRICANE  
FLORENCE, A BAND OF HEAVY RAINFALL IS EXPECTED FROM UPSTATE NEW  
YORK TO THE NEW ENGLAND COAST DURING THE DAY 3 PERIOD ON TUESDAY  
AFTER HAMMERING THE CAROLINAS AND PARTS OF THE APPALACHIANS.  
SINCE IT WILL BECOME CAUGHT UP INTO THE WESTERLIES, THE REMNANTS  
ARE EXPECTED TO PROGRESS EASTWARD QUICKLY AND DRIER WEATHER IS  
EXPECTED TO COMMENCE ON WEDNESDAY. THE 12Z ECMWF WAS HEAVIER AND  
TO THE SOUTH OF THE MORE MODEST AMOUNTS DEPICTED BY THE 00Z GFS.  
SOME INSTANCES OF FLASH FLOODING WILL BE POSSIBLE OVER THIS REGION  
GIVEN THE POTENTIAL FOR 2 TO LOCALLY 4 INCHES OF RAIN,  
PARTICULARLY OVER AREAS THAT HAVE RECEIVED HEAVY RAINFALL RECENTLY.  
 
ANOTHER AREA OF SIGNIFICANT RAINFALL IS BECOMING INCREASINGLY  
LIKELY FROM THE DAKOTAS TO THE UPPER MIDWEST FOR THE MIDDLE OF  
NEXT WEEK AND THEN REACHING THE GREAT LAKES BY THURSDAY IN  
ASSOCIATION WITH A DEVELOPING SURFACE LOW ACROSS THE CENTRAL  
PLAINS. THE POTENTIAL EXISTS FOR A FEW INCHES OF RAINFALL IN SOME  
AREAS. MUCH COOLER WILL FOLLOW BY THE END OF THE WEEK FOR MUCH OF  
THE CENTRAL AND NORTHERN TIER OF THE U.S. AS A CANADIAN SURFACE  
HIGH BUILDS INTO THE REGION AND BRINGS A PRISTINE, AUTUMNAL  
AIRMASS.  
 
TEMPERATURES ARE EXPECTED TO BE WELL ABOVE NORMAL ACROSS MUCH OF  
THE CENTRAL AND EASTERN U.S. AHEAD OF THE COLD FRONT, WITH HIGHS  
IN THE UPPER 80S TO MIDDLE 90S LIKELY ACROSS MUCH OF THE  
SOUTHEASTERN QUADRANT OF THE NATION. COOLER THAN NORMAL  
CONDITIONS ARE EXPECTED FROM THE NORTHERN ROCKIES TO THE UPPER  
MIDWEST, WITH THE POTENTIAL FOR FROST AND FREEZE CONDITIONS BY THE  
END OF THE WEEK FOR PARTS OF MONTANA, NORTH DAKOTA, AND MINNESOTA.  
 
D. HAMRICK  
 
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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