660  
FXUS02 KWBC 141601  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1200 PM EDT FRI SEP 14 2018  
 
VALID 12Z MON SEP 17 2018 - 12Z FRI SEP 21 2018  
 
...HURRICANE FLORENCE TO BRING CATASTROPHIC FLASH FLOODING AND  
PROLONGED SIGNIFICANT RIVER FLOODING TO PARTS OF THE CAROLINAS AND  
APPALACHIANS...  
 
   
..HURRICANE FLORENCE
 
 
HURRICANE FLORENCE MADE LANDFALL THIS MORNING OVER SOUTHEASTERN  
NORTH CAROLINA AND THE NATIONAL HURRICANE CENTER FORECAST HAS THE  
VERY DANGEROUS SYSTEM GRADUALLY WEAKENING ON AN ARC ACROSS SOUTH  
CAROLINA OVER THE NEXT COUPLE OF DAYS BEFORE LIFTING ROUGHLY UP  
ALONG/WEST OF THE SOUTHERN AND CENTRAL APPALACHIANS MONDAY. THE  
WET POST-TROPICAL SYSTEM IS FORECAST TO FURTHER ACCELERATE  
NORTHEASTWARD OVER THE NORTHERN MID-ATLANTIC/UPPER OHIO VALLEY  
THROUGH THE NORTHEAST AND CANADIAN MARITIMES TUESDAY INTO  
WEDNESDAY. FLOODING REMAINS THE MAIN THREAT OWING TO HEAVY AND  
PERSISTENT RAINFALL AS IT INTERACTS WITH MOUNTAINOUS TERRAIN AND  
WITH THE THREAT OF PROLONGED/TRAINING OF CELLS. WHILE THE MAJORITY  
OF PROLIFIC RAINFALL OCCURS PRIOR TO MEDIUM RANGE TIME SCALES, A  
SWATH OF ENHANCED RAIN WILL FOLLOW ALONG THE TRACK OF FLORENCE  
INTO MIDWEEK. PLEASE CONSULT THE LATEST ADVISORIES AND WARNINGS  
FROM THE NATIONAL HURRICANE CENTER, THE WEATHER PREDICTION CENTER  
AND LOCAL NWS FORECAST OFFICES AND AGENCIES FOR UPDATED  
INFORMATION.  
 
   
..PATTERN OVERVIEW AND GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
THE FORECAST PERIOD BEGINS ON MONDAY WITH A TROUGH CENTERED NEAR  
THE WEST COAST, AN UPPER LEVEL RIDGE OVER THE SOUTHERN PLAINS, AND  
A BROAD TROUGH OVER THE OHIO VALLEY 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 WITH AN  
INCREASINGLY PROGRESSIVE FLOW PATTERN SLOWLY DEVELOPING OVER THE  
NORTHERN TIER OF THE NATION.  
 
THE 00 UTC ECMWF ENSEMBLE MEANS WAS USED AS A STARTING POINT FOR  
WPC SURFACE FRONTS AND PRESSURES PROGS DAYS 3-5 IN A PERIOD OF  
SMALLER FORECAST SPREAD EXCLUDING FLORENCE. THIS SOLUTION MESHED  
BEST WITH THE LATEST NHC TRACK FOR FLORENCE THAT WAS INSERTED. THE  
00 UTC ECMWF AND TO A LESSER EXTENT ECMWF ENSEMBLES THOUGH SEEM TO  
BREAK CONTINUITY AND TRANSITION TO A PROGRESSIVE FLOW PATTERN  
ACROSS THE U.S. NORTHERN TIER MUCH MORE QUICKLY THAN MOST OTHER  
GUIDANCE. THEREFORE, INSTEAD BASED DAY 6/7 WPC GUIDANCE MORE IN  
LINE WITH THE SEEMINGLY MORE REASONABLE 06 UTC GEFS MEAN AMID  
GROWING FORECAST SPREAD.  
 
   
..WEATHER HIGHLIGHTS/HAZARDS
 
 
FLORENCE WILL SPREAD A LARGE AREA OF SIGNIFICANT RAINFALL  
NORTHEASTWARD NEXT WEEK DESPITE ITS ACCELERATION AHEAD OF AN  
APPROACHING COLD FRONT. FLOODING MAY BE A CONCERN ALONG THE I-81  
TO I-95 CORRIDORS BUT WILL BE A FUNCTION OF FORWARD SPEED. BACK TO  
THE WEST, AS TROUGHING FINALLY MOVES OUT OF THE NORTHWEST, A COLD  
FRONT WILL USHER IN COOLER FALL AIR TO THE HIGH PLAINS AND UPPER  
MIDWEST/GREAT LAKES TUE-WED AND THEN TO THE EAST COAST NEXT  
THURSDAY AS EX-FLORENCE SHOULD RACE WELL INTO ATLANTIC CANADA  
THEN.EXPECT RECORD HIGH MINIMUM TEMPERATURES ALONG/AHEAD OF THE  
MOIST TRACK OF FLORENCE.  
 
MEANHWILE, A NEW AREA OF PRECIPITATION WILL TAKE SHAPE AND EXPAND  
OVER THE NORTHERN PLAINS AND UPPER MIDWEST NEXT WEEK AS WAVES AND  
ATTENDANT PRECIPTATION FOCUS ALONG/NEAR THE FRONT EXITS THE  
FOOTHILLS OF THE ROCKIES.  
 
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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