235  
FXUS02 KWBC 140816  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION...UPDATED  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
414 AM EDT FRI SEP 14 2018  
 
VALID 12Z MON SEP 17 2018 - 12Z FRI SEP 21 2018  
 
***HURRICANE FLORENCE TO BRING FLASH FLOODING AND PROLONGED  
SIGNIFICANT RIVER FLOODING TO PARTS OF THE APPALACHIANS INTO EARLY  
NEXT WEEK***  
 
   
..HURRICANE FLORENCE
 
 
THE NATIONAL HURRICANE CENTER FORECASTS HURRICANE FLORENCE TO MAKE  
LANDFALL OVER SOUTHEASTERN NORTH CAROLINA FRIDAY MORNING. THE  
CYCLONE SHOULD GRADUALLY WEAKEN AS IT TRACKS ACROSS NORTHERN SOUTH  
CAROLINA BEFORE LIFTING NORTHWARD, ROUGHLY ALONG AND JUST WEST OF  
THE APPALACHIANS THROUGH TUESDAY AS IT ACCELERATES NORTHEASTWARD.  
FLOODING WILL BE THE MAIN THREAT OWING TO HEAVY AND PERSISTENT  
RAINFALL AS IT INTERACTS WITH THE MOUNTAINOUS TERRAIN. ALTHOUGH  
THE MAJORITY OF THE PROLIFIC RAINFALL WILL HAPPEN BEFORE THE  
MEDIUM RANGE PERIOD BEGINS, A SWATH OF ENHANCED RAIN WILL FOLLOW  
ALONG THE TRACK OF FLORENCE AS IT BECOMES EXTRATROPICAL. THIS  
WILL BE A MULTI-DAY AND MULTI-FACETED EVENT, AND THE NATIONAL  
HURRICANE CENTER AND LOCAL NWS FORECAST OFFICES HAVE ADDITIONAL  
INFORMATION PERTAINING TO THIS EVENT.  
 
   
..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 A  
PROGRESSIVE FLOW PATTERN ACROSS THE NORTHERN TIER OF THE NATION.  
THE TROUGH OVER THE WESTERN THIRD OF NATION BECOMES RE-ESTABLISHED  
BY THE END OF THE WEEK.  
 
THE FORECAST WAS BASED PRIMARILY ON A BLEND OF THE DETERMINISTIC  
GFS, ECMWF, AND THEIR RESPECTIVE ENSEMBLE MEANS, WITH A GREATER  
PERCENTAGE OF THE MEANS BY NEXT THURSDAY AND FRIDAY OWING TO  
TIMING DIFFERENCES NOTED WITH THE WEST COAST TROUGH AXIS. THE  
UPPER LEVEL RIDGE HAS BETTER MODEL AGREEMENT OVERALL.  
 
   
..WEATHER HIGHLIGHTS/HAZARDS
 
 
FLORENCE IS FORECAST TO SPREAD A LARGE AREA OF SIGNIFICANT  
RAINFALL ACROSS THE CENTRAL AND NORTHERN APPALACHIANS EARLY NEXT  
WEEK DESPITE ITS ACCELERATION AHEAD OF AN APPROACHING COLD FRONT.  
RAINFALL AMOUNTS ON THE ORDER OF 2 TO 4 INCHES APPEAR LIKELY WITH  
ISOLATED HIGHER TOTALS OF 6 INCHES POSSIBLE. FLASH FLOODING WILL  
BE POSSIBLE FOR FLOOD PRONE AREAS AND THIS IS SOMETHING THAT WPC  
WILL CONTINUE TO MONITOR AS THE EVENT ENTERS THE SHORT RANGE  
FORECAST PERIOD.  
 
FARTHER TO THE WEST, WITH THE UPPER LEVEL TROUGH MOVING OUT OF THE  
NORTHWEST, A COLD FRONT WILL ADVECT A COOLER AIR MASS TO THE  
NORTHERN PLAINS AND UPPER MIDWEST/GREAT LAKES THROUGH MID WEEK AS  
A CANADIAN SURFACE HIGH BUILD INTO THE REGION, AND THEN TO THE  
EAST COAST NEXT THURSDAY. ABOVE NORMAL TEMPERATURES ARE EXPECTED  
FOR THE SOUTHERN TIER OF THE U.S. COURTESY OF THE UPPER RIDGE IN  
PLACE, WITH THE POTENTIAL FOR SOME DAILY RECORD HIGHS AND RECORD  
HIGH MINIMUMS.  
 
HAMRICK/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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