194  
FXUS02 KWBC 111600  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1159 AM EDT TUE SEP 11 2018  
 
VALID 12Z FRI SEP 14 2018 - 12Z TUE SEP 18 2018  
 
...HURRICANE FLORENCE COULD BRING CATASTROPHIC FLASH FLOODING AND  
SIGNIFICANT RIVER FLOODING TO PORTIONS OF THE CAROLINAS AND  
MID-ATLANTIC STATES LATE THIS WEEK INTO EARLY NEXT WEEK...  
 
   
..HURRICANE FLORENCE
 
 
BASED ON THE 1500 UTC NATIONAL HURRICANE CENTER ADVISORY, POWERFUL  
HURRICANE FLORENCE IS MOST LIKELY TO MAKE LANDFALL AS A MAJOR  
HURRICANE ALONG THE SOUTHERN HALF OF THE NORTH CAROLINA BY FRIDAY  
(NEAR THE START OF THE MEDIUM RANGE PERIOD). THE MODELS AND  
ENSEMBLES CONTINUE TO WAVER ON THE EXPECTED LANDFALL LOCATION (OR  
NEAR-LANDFALL) AND TIMING OF WHEN FLORENCE MAY SLOW DOWN AND  
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  
EAST OF THE TRACK OF FLORENCE, AIDED BY INFLOW OFF THE GULF  
STREAM. CURRENTLY EXPECT AN AREA OF 15-20 INCH ACCUMULATIONS WITH  
ISOLATED LOCAL MAXIMUM TOTALS TO 30 INCHES.  
UPSLOPE/TERRAIN/COASTAL ENHANCEMENT WOULD ENHANCE 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 MORE  
CONSISTENT. THIS WILL BE A LONG-DURATION EVENT, CONTINUING EVEN  
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. OVER THE PAST COUPLE DAYS GUIDANCE HAS BEEN SHOWING STEADY  
TRENDS TOWARD HOLDING TROUGH ENERGY OVER THE PACIFIC NORTHWEST FOR  
A LONGER TIME BEFORE EJECTING ACROSS CANADA AND THE EXTREME  
NORTHERN U.S. AS A RESULT LATEST CONSENSUS CONTINUES TO ADJUST  
SLOWER WITH THE EASTWARD PROGRESSION OF A WAVY FRONT OVER THE  
NORTHERN PLAINS--FINALLY BECOMING A FAIRLY STRONG COLD FRONT BY  
DAY 7 TUE. DOWNSTREAM THIS SLOWER TREND ALLOWS MORE UPPER RIDGING  
TO BUILD FROM THE MIDWEST THROUGH THE GREAT LAKES/NORTHEAST DURING  
THE WEEKEND, HELPING TO KEEP FLORENCE SUPPRESSED. ALBEIT WITH A  
BIT OF A DELAY, THE PATTERN STILL APPEARS TO START TO BREAKING  
DOWN EARLY NEXT WEEK AS TROUGHING (OR AT LEAST FLATTER MEAN FLOW)  
REPLACES RIDGING OVER ALASKA. TOWARD THE END OF THE PERIOD ECMWF  
AND GFS/GEFS-BASED GUIDANCE DIVERGES SOMEWHAT ON HOW QUICKLY  
LINGERING WEST COAST TROUGH ENERGY EJECTS IN RESPONSE TO UPSTREAM  
FLOW. PREFER A COMPROMISE/BLEND APPROACH, GIVEN OFFSETTING  
CONSIDERATIONS--THE OVERALL SLOWER TREND TO EJECT ENERGY IN  
GENERAL, VERSUS TELECONNECTIONS RELATIVE THE EVOLVING UPSTREAM  
PATTERN SUGGESTING DEVELOPMENT OF BROAD NORTHEASTERN PACIFIC  
TROUGHING THAT WOULD SERVE TO KICK OUT LEADING WEST COAST ENERGY.  
THE STARTING BLEND USED IDEAS FROM 00Z PARALLEL GFS/ECMWF/UKMET  
EARLY IN THE PERIOD FOLLOWED BY A TRANSITION TOWARD A BLEND OF  
MODELS (00Z GFSP, 00-06Z GFS, 00Z ECMWF) AND MEANS (06Z GEFS/00Z  
ECMWF MEAN). THE INITIAL BLEND WAS ADJUSTED FOR THE NHC TRACK OF  
FLORENCE AS NEEDED.  
 
   
..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. 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 MON-TUE.  
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.  
 
RAUSCH/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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