771  
FXUS02 KWBC 121600  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1200 PM EDT WED SEP 12 2018  
 
VALID 12Z SAT SEP 15 2018 - 12Z WED SEP 19 2018  
 
...DANGEROUS FLORENCE TO BRING CATASTROPHIC FLASH FLOODING AND  
PROLONGED SIGNIFICANT RIVER FLOODING TO PORTIONS OF THE CAROLINAS  
AND FOR THE SOUTHERN TO CENTRAL APPALACHIANS THROUGH THE WEEKEND  
AND WELL INTO NEXT WEEK...  
 
   
..HURRICANE FLORENCE AND GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
POWERFUL FLORENCE IS FORECAST BY THE NATIONAL HURRICAN E CENTER TO  
MAKE LANDFALL AS A MAJOR HURRICANE INTO SOUTHERN NORTH  
CAROLINA/NORTHEASTERN SOUTH CAROLINA FRIDAY. IN SPITE OF ONGOING  
GUIDANCE SPREAD FOR TRACK/TIMING SPECIFICS, CONFIDENCE IS  
INCREASING THAT AN EXTREME RAINFALL AND FLOODING EVENT MAY UNFOLD  
MAINLY ALONG AND NORTH/EAST OF THE TRACK OF FLORENCE, AIDED BY  
DEEP TROPICAL MOISTURE INFLOW. CURRENTLY EXPECT AN AREA OF 15-25  
INCH ACCUMULATIONS WITH ISOLATED LOCAL STORM TOTAL MAXIMUM TOTALS  
UP TO 40 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, BUT THE  
SIGNAL FOR AN EXTREME/CATASTROPHIC EVENT HAS BEEN MORE CONSISTENT.  
THIS WILL BE A MULTI-DAY AND MULTI-FACETED EVENT AS FLORENCE IS  
FORECAST TO BE A COHERENT ENTITY IN WEAKENED FORM INTO NEXT  
MIDWEEK. PLEASE CONSULT THE NATIONAL HURRICANE CENTER AND YOUR  
LOCAL NWS FORECAST OFFICE FOR LATEST FLORENCE INFORMATION.  
 
   
..PATTERN OVERVIEW
 
 
EVOLVING UPPER RIDGING NEAR ALASKA FAVORS POSITIVELY TILTED AND  
REASONABLY AMPLIFIED TROUGHING ALOFT 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 FLORENCE. A COMPOSITE OF REASONABLY CLUSTERED GUIDANCE  
FROM THE 00 UTC GFS/ECMWF AND THE TRACK OF FLORENCE AS PER THE  
NATIONAL HURRICANE CENTER WAS USED FOR THIS WEEKEND INTO EARLY  
NEXT WEEK, QUICKLY TRANITIONING INTO A COMBINATION OF ECMWF  
ENSEMBLES AND THE NHC TRACK THEREAFTER.  
 
   
..WEATHER HIGHLIGHTS/HAZARDS
 
 
REGARDING PRECIPITATION AWAY FROM FLORENCE, SOUTHERN TEXAS 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.  
 
EXPECT THE EMPHASIS FOR COOLEST TEMPERATURES (HIGHS 5-15F BELOW  
NORMAL) TO TRANSITION FROM THE WEST COAST STATES INTO THE NORTHERN  
PLAINS AS HIGH PRESSURE BUILDS IN THE WAKE OF FRONTAL PASSAGE.  
OVER THE LATTER AREA THERE MAY BE DECENT COVERAGE OF MINUS 10-15F  
ANOMALIES FOR HIGHS BY NEXT WEEK (MAXIMUM TEMPERATURES INTO THE  
50S). 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 MUCH OF THE EAST AS  
FLORENCE PULLS IN WARM/HUMID AIR. SOME MINIMUM TEMPERATURES MAY  
NEAR OR EXCEED RECORD WARM VALUES. MAXIMUM TEMPERATURES WOULD BE  
HELD DOWN NEAR FLORENCE AND ITS CLOUD SHIELD.  
 
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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