138  
FXUS02 KWBC 120619  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
218 AM EDT WED SEP 12 2018  
 
VALID 12Z SAT SEP 15 2018 - 12Z WED SEP 19 2018  
 
...HURRICANE FLORENCE COULD BRING CATASTROPHIC FLASH FLOODING AND  
SIGNIFICANT RIVER FLOODING TO PORTIONS OF THE CAROLINAS AND  
SOUTHERN APPALACHIAN STATES THIS WEEKEND INTO EARLY NEXT WEEK...  
 
   
..HURRICANE FLORENCE
 
 
POWERFUL HURRICANE FLORENCE IS MOST LIKELY TO MAKE LANDFALL AS A  
MAJOR HURRICANE ALONG THE SOUTHERN HALF OF NORTH CAROLINA OR  
NORTHEASTERN SOUTH CAROLINA ON FRIDAY. THE MODELS AND ENSEMBLES  
CONTINUE TO WAVER ON THE EXPECTED LANDFALL LOCATION (OR  
NEAR-LANDFALL) AND TIMING OF WHEN FLORENCE MAY SLOW DOWN OR  
STALL/MEANDER JUST INLAND. IN SPITE OF ONGOING GUIDANCE SPREAD FOR  
TRACK/TIMING SPECIFICS, CONFIDENCE IS GRADUALLY INCREASING THAT AN  
EXTREME RAINFALL AND FLOODING EVENT MAY UNFOLD ALONG AND  
NORTH/EAST OF THE TRACK OF FLORENCE, AIDED BY INFLOW OFF THE GULF  
STREAM. CURRENTLY EXPECT AN AREA OF 15-25 INCH ACCUMULATIONS WITH  
ISOLATED LOCAL MAXIMUM TOTALS TO 35 INCHES.  
UPSLOPE/TERRAIN/COASTAL ENHANCEMENT WOULD EXACERBATE 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 COUPLE OF DAYS (IN THE SHORT RANGE) BUT THE  
SIGNAL FOR AN EXTREME/CATASTROPHIC EVENT HAS BEEN MORE CONSISTENT.  
THIS WILL BE A MULTI-DAY AND MULTI-FACETED EVENT THAT WILL CHANGE  
THROUGH TIME AS FLORENCE IS FORECAST TO BE A COHERENT ENTITY  
THROUGH AT LEAST NEXT TUESDAY (THOUGH IN WEAKENED FORM). 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
 
 
EVOLVING UPPER RIDGING NEAR ALASKA WILL CONTINUE TO FAVOR  
POSITIVELY TILTED TROUGHING FROM THE PACIFIC NORTHWEST/EASTERN  
PACIFIC INTO CENTRAL CANADA/HUDSON BAY. UPPER RIDGING IS FORECAST  
TO STRETCH FROM THE FOUR CORNERS REGION NORTHEASTWARD INTO NEW  
ENGLAND AND INTO THE NW ATLANTIC, HELPING TO GUIDE A WEAKENED  
FLORENCE WESTWARD NEAR THE NC/SC BORDER PER THE LATEST NHC TRACK  
FORECAST.  
 
THE ENSEMBLE TREND IN THE PACNW HAS BEEN DECIDEDLY SLOWER AND FOR  
A STRONGER SHORTWAVE OR CLOSED LOW NEAR COASTAL WASHINGTON THAT  
WILL SLIDE EAST-NORTHEASTWARD AND DRAG A WELL-DEFINED COLD FRONT  
THROUGH THE HIGH PLAINS/UPPER MIDWEST NEXT WEEK. THIS FRONT, AS  
CURRENTLY FORECAST, WOULD THEN PICK UP THE REMNANTS OF FLORENCE AS  
IT LIFTS NORTHWARD.  
 
USED A BLEND OF THE RECENT 12Z GUIDANCE WITH MORE WEIGHT TOWARD  
THE ECMWF AND ITS ENSEMBLE MEAN BY THE END OF THE PERIOD DUE TO  
THEIR BETTER HANDLING OF UPSTREAM FLOW OVER THE PACIFIC. THE  
INITIAL BLEND WAS ADJUSTED FOR THE NHC TRACK OF FLORENCE AS NEEDED  
OUT THROUGH MIDWEEK.  
 
   
..WEATHER HIGHLIGHTS/HAZARDS
 
 
REGARDING PRECIPITATION AWAY FROM FLORENCE, PORTIONS OF TEXAS  
(MOST LIKELY THE SOUTHERN HALF OF THE STATE) MAY SEE SIGNIFICANT  
RAINFALL IN ASSOCIATION WITH A SURFACE TROUGH/UPPER LOW EMERGING  
OVER THE SOUTHERN GULF, CROSSING BETWEEN SHORT RANGE AND MEDIUM  
RANGE. CONSULT LATEST NHC TROPICAL WEATHER OUTLOOKS AS CONDITIONS  
COULD BECOME MORE FAVORABLE FOR DEVELOPMENT AS THE FEATURE HEADS  
INTO THE WESTERN GULF. SLOWER GUIDANCE TRENDS WITH THE WEST COAST  
UPPER TROUGH HAVE DELAYED THE DRYING TREND OVER THE NORTHWEST,  
WITH LIGHT TO MODERATE ACTIVITY LIKELY TO PERSIST THROUGH MOST OF  
THE WEEKEND. TEMPERATURES OVER/NEAR THE NORTHERN ROCKIES MAY BE  
COLD ENOUGH TO SUPPORT SOME HIGHER ELEVATION SNOW BUT THE BEST  
SNOW POTENTIAL WILL BE TO THE NORTH OF THE US/CANADIAN BORDER.  
 
ASIDE FROM A MODEST DELAY PER GUIDANCE TRENDS WITH THE UPPER  
PATTERN, EXPECT THE EMPHASIS FOR COOLEST TEMPERATURES (HIGHS 5-15F  
BELOW NORMAL) TO TRANSITION FROM THE WEST COAST STATES INTO THE  
NORTHERN PLAINS. OVER THE LATTER AREA THERE MAY BE DECENT COVERAGE  
OF MINUS 10-15F ANOMALIES FOR HIGHS BY NEXT WEEK (AIR TEMPERATURES  
IN THE 50S). OTHERWISE MOST OF THE CENTRAL/EASTERN STATES SHOULD  
SEE ABOVE NORMAL TEMPERATURES, ESPECIALLY FOR OVERNIGHT MINS.  
WARMEST MORNING LOWS VERSUS NORMAL (SOME 15F+ ANOMALIES) SHOULD  
EXTEND FROM THE CENTRAL PLAINS INTO UPPER GREAT LAKES, AHEAD OF  
THE NORTHERN PLAINS FRONT, AND OVER MIDDLE/NORTHERN LATITUDES OF  
THE EAST AS FLORENCE PULLS IN WARM/HUMID AIR. SOME LOWS MAY BE  
NEAR OR EXCEED RECORD WARM VALUES ON 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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