812  
FXUS02 KWBC 110627  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
227 AM EDT TUE SEP 11 2018  
 
VALID 12Z FRI SEP 14 2018 - 12Z TUE SEP 18 2018  
 
...HURRICANE FLORENCE TO BRING DEVASTATING AND LIFE-THREATENING  
FLOODING RAINFALL TO PORTIONS OF NORTH CAROLINA AND VIRGINIA LATER  
THIS WEEK...  
 
...HEAVY RAINFALL THREAT MAY EXTEND WELL INLAND AS FLORENCE IS  
FORECAST TO SLOW TO A CRAWL THIS WEEKEND...  
 
   
..HURRICANE FLORENCE
 
 
PER THE LATEST NATIONAL HURRICANE CENTER FORECAST, POWERFUL  
HURRICANE FLORENCE IS FORECAST TO MAKE LANDFALL AS A MAJOR  
HURRICANE ALONG THE NORTH CAROLINA OR NORTHEAST SOUTH CAROLINA  
COAST LATE THURSDAY OR EARLY FRIDAY (JUST BEFORE THE MEDIUM RANGE  
PERIOD). THE MODELS AND ENSEMBLES CONTINUED TO WAVER ON THE  
EXPECTED LANDFALL LOCATION (OR NEAR-LANDFALL) AND TIMING OF WHEN  
FLORENCE MAY SLOW DOWN AND STALL/MEANDER JUST INLAND. THOUGH  
UNCERTAIN IN EXACT LOCATION BY SUNDAY, IT IS INCREASINGLY CLEAR  
THAT AN EXTREME RAINFALL AND FLOODING EVENT WILL UNFOLD ALONG AND  
EAST OF THE TRACK OF FLORENCE WITH INFLOW OFF THE GULF STREAM.  
RAINFALL WILL LIKELY BE MEASURED IN EXCESS OF 10" OVER A LARGE  
AREA WITH LOCAL MAXIMA OVER 2 FEET. ANY UPSLOPE/TERRAIN/COASTAL  
ENHANCEMENT WOULD ONLY INCREASE THE SERIOUSNESS OF THE  
RAINFALL/FLOODING THREAT. THESE FORECAST RAINFALL AMOUNTS HAVE  
RARELY BEEN RECORDED IN THE PAST CENTURY AND CAN RESULT IN  
FLOODING WHERE IT HAS NOT BEEN OBSERVED FROM PAST EVENTS. THE  
AREAS OF GREATEST IMPACT AND HIGHEST RAINFALL WILL BE REFINED OVER  
THE NEXT FEW DAYS (IN THE SHORT RANGE) BUT THE ONGOING AND  
INCREASINGLY LOUD SIGNAL FROM THE MODEL GUIDANCE (AND  
INGREDIENTS-BASED SETUP FOR THE PATTERN EVOLUTION) HAS BEEN  
UNWAVERING. THIS WILL BE A LONG-DURATION EVENT OVER MANY DAYS LONG  
AFTER THE STRONG WINDS HAVE SUBSIDED. PLEASE CONSULT THE NATIONAL  
HURRICANE CENTER FOR THE LATEST INFORMATION ON FLORENCE AS WELL AS  
YOUR LOCAL NWS FORECAST OFFICE.  
 
   
..PATTERN OVERVIEW AND GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
UPPER RIDGING OVER THE NORTH PACIFIC WILL FAVOR POSITIVELY TILTED  
TROUGHING FROM THE PACIFIC NORTHWEST/EASTERN PACIFIC INTO CENTRAL  
CANADA. FARTHER EAST AN ANOMALOUSLY STRONG CLOSED UPPER HIGH  
BETWEEN BERMUDA AND THE NORTHEAST WILL LINGER IN PLACE AND HELP  
STEER HURRICANE FLORENCE INTO THE EAST COAST PER THE LATEST NHC  
TRACK. BY THE WEEKEND INTO NEXT WEEK EXPECT SOME OF THE ENERGY  
WITHIN THE OVERALL MEAN TROUGH TO EJECT ACROSS SOUTHERN CANADA,  
JUST FAR ENOUGH NORTH OF THE WEAKENING FLORENCE TO LEAVE THE  
SYSTEM IN A REGION OF WEAK STEERING FLOW. THE PATTERN APPEARS TO  
START TO BREAK DOWN EARLY NEXT WEEK AS TROUGHING REPLACES RIDGING  
OVER ALASKA, SENDING QUASI-ZONAL FLOW INTO WESTERN CANADA WHILE  
RIDGING REMAINS ATOP EX-FLORENCE NEXT WEEK. DUE TO THE COMPLEX  
MULTI-STREAM FLOW OUT OF THE PACIFIC INTO THE US WEST COAST, A  
BLENDED/ENSEMBLE SOLUTION WAS PREFERRED BY NEXT MON/TUE. THE ECMWF  
AND ITS ENSEMBLES WERE SLIGHTLY MORE PREFERRED OVER THE GFS/GEFS  
IN THE WEST AND WITH FLORENCE AS WELL (THOUGH NOT A PERFECT  
MATCH).  
 
   
..WEATHER HIGHLIGHTS/HAZARDS
 
 
SOUTHERN TEXAS MAY SEE SIGNIFICANT RAINS DUE TO A TROPICAL WAVE  
INTERACTING WITH A LINGERING WEAKNESS ALOFT THOUGH NOT ALL MODELS  
WERE ON BOARD. NHC IS MONITORING THIS AREA (NOW IN THE  
NORTHWESTERN CARIBBEAN SEA) FOR POSSIBLE TROPICAL DEVELOPMENT.  
PRECIPITATION OVER THE PACIFIC NORTHWEST WILL TEND TO BE LIGHT TO  
LOCALLY MODERATE, WITH A DRIER TREND LIKELY NEXT WEEK AS HEIGHTS  
RISE. TEMPERATURES OVER/NEAR THE NORTHERN ROCKIES MAY BE COLD  
ENOUGH TO SUPPORT SOME HIGHER ELEVATION SNOW, BUT ESPECIALLY NORTH  
OF THE US/CANADIAN BORDER.  
 
AS THE UPPER PATTERN SLOWLY EVOLVES, EXPECT THE EMPHASIS FOR BELOW  
NORMAL TEMPERATURES (MINUS 5-15F ANOMALIES FOR DAYTIME HIGHS) TO  
SHIFT FROM THE WEST COAST STATES/NORTHERN ROCKIES LATE IN THE WEEK  
TOWARD THE EXTREME NORTHERN PLAINS BY SUN-TUE. THE MAJORITY OF THE  
CENTRAL/EASTERN STATES SHOULD SEE ABOVE NORMAL TEMPERATURES,  
ESPECIALLY FOR OVERNIGHT MINS. PARTICULAR EMPHASIS WILL BE WITHIN  
AN AXIS OF WARM LOWS AHEAD OF A WAVY FRONT EXTENDING SOUTHWEST  
FROM THE NORTHERN PLAINS, AND OVER PARTS OF THE EAST WHICH WILL  
SEE THE ARRIVAL OF TROPICAL AIR ASSOCIATED WITH FLORENCE. SOME  
LOWS MAY BE NEAR OR EXCEED RECORD WARM VALUES ONE OR MORE DAYS.  
MAXIMUM TEMPERATURES WOULD BE HELD DOWN NEAR FLORENCE AND ITS  
CLOUD SHIELD.  
 
FRACASSO/RAUSCH  
 
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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