125  
FXUS02 KWBC 140656  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
255 AM 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 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 00 UTC NAEFS/ECMWF ENSEMBLE MEANS WERE USED AS A STARTING  
POINT FOR WPC SURFACE FRONTS/PRESSURES PROGS DAYS 3-5 IN A PERIOD  
OF SMALLER FORECAST SPREAD EXCLUDING FLORENCE. THIS BLEND LEFT  
ROOM FOR INSERTION OF FLORENCE AS PER THE LATEST NHC FORECAST THAT  
DID NOT EXPLICITLY MATCH ANY PARTICULAR DETERMINISTIC MODEL.  
 
LATER, THE UPPER PATTERN FROM THE PACIFIC ACROSS NORTH AMERICA  
WILL MOSTLY REMAIN STAGNANT WITH MEANDERING RIDGING FROM SOUTHWEST  
ALASKA TO THE ALEUTIANS. THIS MAINTAINS A TROUGH POSITION TO ITS  
SOUTHEAST DOWN TOWARD THE PACIFIC NORTHWEST. THERE REMAINS  
DIFFERENCES CONCERNING TROUGHING NEXT WEDNESDAY/THURSDAY INTO THE  
NORTHWESTERN U.S. THE SOMEWHAT MORE AMPLIFIED ECMWF ENSEMBLES SEEM  
MORE REASONABLE THAN THE MORE PROGRESSIVE GEFS SOLUTIONS GIVEN  
UPSTREAM FLOW AMPLITUDE AND WPC CONTINUITY.  
 
   
..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 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.  
 
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 SOME RECORD HIGH MINIMUM TEMPERATURES ALONG/AHEAD OF  
THE MOIST TRACK OF FLORENCE.  
 
DEEPENING TROPICAL MOISTURE OVER THE GULF OF MEXICO AND EXPECTED  
TRACK OF A TROPICAL WAVE INTO SOUTHERN TEXAS AND THE RIO GRANDE  
VALLEY WILL MEANWHILE OFFER A THREAT OF LOCALLY HEAVY RAINFALL FOR  
THE AREA FROM THE WEEKEND INTO EARLY NEXT WEEK.  
 
MEANWHILE, A NEW AREA OF PRECIPITATION WILL TAKE SHAPE AND EXPAND  
OVER THE NORTHERN PLAINS AND UPPER MIDWEST NEXT WEEK AS WAVES AND  
ATTENDANT PRECIPITATION FOCUS ALONG/NEAR THE FRONT EXITS THE  
FOOTHILLS OF THE ROCKIES.  
 
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  
 

 
 
CLICK HERE TO GO TO PREVIOUS BULLETINS.

The Nexlab HPC Page Main Text Page