264  
FXUS02 KWBC 130559  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
159 AM EDT THU SEP 13 2018  
 
VALID 12Z SUN SEP 16 2018 - 12Z THU SEP 20 2018  
 
...DANGEROUS HURRICANE FLORENCE TO BRING CATASTROPHIC FLASH  
FLOODING AND PROLONGED SIGNIFICANT RIVER FLOODING TO PORTIONS OF  
THE CAROLINAS AND THE SOUTHERN AND CENTRAL APPALACHIANS INTO NEXT  
WEEK...  
 
   
..HURRICANE FLORENCE
 
 
THE NATIONAL HURRICANE CENTER FORECASTS FLORENCE TO MAKE LANDFALL  
NEAR THE COASTAL NORTH CAROLINA/SOUTH CAROLINA BORDER FRIDAY. BY  
THE START OF THE MEDIUM RANGE (EARLY SUNDAY) A WEAKENING FLORENCE  
SHOULD BEGIN MAKING ITS LARGE ARC THROUGH UPSTATE SC AND  
NORTHEASTERN GEORGIA AS IT FINALLY ESCAPES THE REGION OF WEAK  
STEERING FLOW. HOWEVER, THE THREAT FROM HEAVY AND PERSISTENT  
RAINFALL WILL REMAIN FOR FLOODING WILL REMAIN AS IT INTERACTS WITH  
THE TERRAIN OF THE SOUTHERN APPALACHIANS. MUCH OF THE RAIN WILL  
FALL BEFORE THE MEDIUM RANGE PERIOD BEGINS, BUT A SIGNIFICANT AREA  
OF RAIN WILL FOLLOW ALONG AND TO THE EAST OF THE CENTER UP THE  
SPINE OF THE APPALACHIANS NEXT WEEK. THIS WILL BE A MULTI-DAY AND  
MULTI-FACETED EVENT THAT TRANSITIONS THROUGH THE PERIOD. PLEASE  
CONSULT THE NATIONAL HURRICANE CENTER AND YOUR LOCAL NWS FORECAST  
OFFICE FOR THE LATEST INFORMATION.  
   
..PATTERN OVERVIEW AND GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
OVERALL UPPER PATTERN FROM THE PACIFIC ACROSS NORTH AMERICA WILL  
MOSTLY REMAIN STAGNANT WITH MEANDERING RIDGING OVER SOUTHWESTERN  
ALASKA INTO/ACROSS THE ALEUTIANS. THIS WILL MAINTAIN THE TROUGH  
POSITION TO ITS EAST/SOUTHEAST ALONG THE WEST COAST (WA/OR) BUT BY  
NEXT THURSDAY THE ENSEMBLES POINT TO MORE ZONAL FLOW OUT OF THE  
WESTERN CONUS. THERE REMAINS DIFFERENCES BETWEEN THE ECMWF  
ENSEMBLES AND THE GEFS IN THE AMOUNT OF TROUGHING NEXT TUE/WED  
INTO NORCAL/OR BUT MORE WEIGHT WAS GIVEN TO THE ECMWF IDEA DUE TO  
ITS SLOWER HANDLING OF EX-FLORENCE IN THE EAST MORE IN LINE WITH  
THE NHC FORECAST.  
 
TO THE EAST, FLORENCE WILL EVENTUALLY LIFT NORTH/NORTHEASTWARD AS  
ENOUGH HEIGHT FALLS DIP SOUTH TO GUIDE IT THROUGH THE CENTRAL  
APPALACHIANS THEN INTO THE NORTHEAST/NEW ENGLAND. THE 12Z ECMWF  
SERVED AS THE BEST PROXY TO THE NHC FORECAST (SLOWER SYSTEM EXIT  
GIVEN ITS TREND/HISTORY SEEMED REASONABLE, BUT THIS SYSTEM HAS HAD  
ITS SHARE OF AUDIBLES IN THE MEDIUM RANGE).  
   
..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.  
 
TEMPERATURES WILL BE MILD IN THE TROPICAL AIRMASS IN THE EAST TO  
START THE WEEK WITH COOLER TEMPERATURES BY THE END OF THE PERIOD  
(AND LESS HUMID). WARM/HOT TEMPERATURES IN THE HIGH PLAINS SUNDAY  
(MID OR UPPER 90S) WILL 60S NEXT WED/THU BEHIND THE COLD FRONT.  
THE SOUTHWEST AND SOUTHERN TIER WILL REMAIN NEAR TO ABOVE AVERAGE  
WITH MOSTLY 80S/90S BUT LOWER 100S IN THE DESERTS.  
 
A NEW AREA OF PRECIPITATION MAY START TO TAKE SHAPE AND EXPAND IN  
THE CENTRAL OR NORTHERN PLAINS BY NEXT WED/THU AS A WAVE ALONG THE  
FRONT EXITS THE FOOTHILLS OF THE ROCKIES. OTHERWISE, THE REST OF  
THE WEST SHOULD FINALLY SEE A DRIER PERIOD.  
 
FRACASSO  
 
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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