180  
FXUS02 KWBC 081528  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1127 AM EDT SAT SEP 08 2018  
 
VALID 12Z TUE SEP 11 2018 - 12Z SAT SEP 15 2018  
 
...FLORENCE LIKELY TO IMPACT A PORTION OF THE EAST COAST NEXT  
WEEK...  
 
   
..15 UTC UPDATE
 
 
MODELS/ENSEMBLES CONTINUE TO SHOW ABOVE AVERAGE CONSENSUS ON THE  
AMPLIFIED FLOW PATTERN ACROSS NORTH AMERICA DURING THE EXTENDED  
PERIOD. WITH RESPECT TO THE EVENTUAL TRACK OF FLORENCE, CONFIDENCE  
CONTINUES TO GROW THAT AN IMPACT SOMEWHERE ALONG THE SOUTHEAST OR  
MID-ATLANTIC COASTLINE WILL OCCUR BY MID TO LATE NEXT WEEK. AMONG  
THE GLOBAL DETERMINISTIC GUIDANCE, THE 00Z ECMWF WAS CLOSEST TO  
THE OFFICIAL NHC FORECAST TRACK THROUGH 12 UTC ON DAY 5 (THU),  
KEEPING WHAT IS FORECAST TO BE HURRICANE FLORENCE A COUPLE HUNDRED  
MILES OFF THE SOUTH CAROLINA COAST BY THAT TIME. PLEASE REFER TO  
PRODUCTS ISSUED BY THE NHC FOR THE LATEST INFORMATION ON FLORENCE.  
ELSEWHERE, THE OTHER SIGNIFICANT QUESTION BY LATE NEXT WEEK IS THE  
DEGREE OF AMPLIFICATION OF UPPER SHORTWAVE ENERGY OFF THE PACIFIC  
NORTHWEST COAST. AS MENTIONED OVERNIGHT, THE ECMWF REMAINS THE  
MOST AMPLIFIED SOLUTION, BUT HAS BEEN VERY CONSISTENT IN THIS,  
WHILE THE GFS HAS SHOWN RATHER SIGNIFICANT FLUCTUATIONS FROM RUN  
TO RUN. AT THIS TIME WILL LEAN TOWARD THE ECMWF AND ENSEMBLE MEANS  
HERE.  
 
GIVEN INITIALLY EXCELLENT CONSENSUS AMONG THE GUIDANCE INITIALLY,  
A MULTI-MODEL DETERMINISTIC BLEND INCLUDING THE  
ECMWF/UKMET/CMC/GFS WAS USED A BASIS FOR THE WPC FORECAST ACROSS  
THE CONUS ON DAYS 3-5 (TUE-THU). FROM DAY 6 ONWARD, INCREASING  
WEIGHT WAS PLACED TOWARD THE ECENS/NAEFS MEANS ALONG WITH  
CONTINUED SUBSTANTIAL WEIGHTING OF THE ECMWF GIVEN ITS PROXIMITY  
TO THE NHC TRACK FOR FLORENCE EARLIER IN THE PERIOD.  
 
RYAN  
 
PREVIOUS DISCUSSION (ISSUED AT 0532 UTC)  
   
..PATTERN OVERVIEW
 
 
STRONG UPPER RIDGING OVER THE NORTHERN PACIFIC AND THE  
NORTHWESTERN ATLANTIC OFF THE NEW ENGLAND COAST WILL FAVOR THE  
MAINTENANCE OF A POSITIVELY TILTED TROUGH ALONG THE WEST COAST OF  
THE LOWER 48. CURRENT TROPICAL STORM FLORENCE IS FORECAST TO  
STRENGTHEN TO A MAJOR HURRICANE BETWEEN THE BAHAMAS AND THE OUTER  
BANKS NEXT WEEK AND IS LIKELY TO IMPACT AT LEAST PART OF THE  
SOUTHEAST/MID-ATLANTIC STARTING AS EARLY AS LATE WEDNESDAY BUT  
PERHAPS STILL INTO NEXT WEEKEND. PLEASE CONSULT THE NATIONAL  
HURRICANE CENTER FOR THE LATEST INFORMATION.  
   
..GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
OVERALL, THE AMPLIFIED PATTERN REMAINS WELL-HANDLED BY THE  
ENSEMBLE GUIDANCE BUT WITH UNCERTAINTY ATOP THE RIDGE IN THE  
PACIFIC THAT TRANSLATES TOWARD THE PACIFIC NORTHWEST. TO THE EAST,  
THE STRENGTH AND SHAPE OF THE BLOCKING UPPER HIGH WILL DICTATE  
WHERE FLORENCE WILL TRACK, BUT THE SLOWLY NARROWING ENSEMBLE  
CONSENSUS APPEARS TO TAKE THE SYSTEM INTO THE SOUTHEASTERN US BUT  
THE RANGE OF POSSIBILITIES STILL EXTENDS FROM FLORIDA NORTHWARD TO  
AT LEAST THE MID-ATLANTIC WHICH INCLUDES A CHANCE OF RECURVING  
JUST OFFSHORE. WITH BROAD WSW FLOW FROM MID-CONTINENT TO THE  
ATLANTIC, THE TYPICAL ACCELERATION OF A TRANSITIONING TROPICAL  
SYSTEM MAY BE DELAYED AS THE LONGWAVE PATTERN REMAINS RATHER  
STUCK. A BLEND OF THE DETERMINISTIC MODELS SUFFICED FOR THE EARLY  
PORTION OF THE PERIOD BUT THEN TRANSITIONED TO AN 18Z GFS/12Z  
ECMWF/18Z GEFS BLEND TO ACCOUNT FOR THE COORDINATED PREFERENCE IN  
THE EAST BUT ALSO TO DAMPEN THE PERHAPS OVERZEALOUS ECMWF OFF THE  
PAC NW WITH SUBSEQUENT/REINFORCING TROUGHING THU/FRI.  
   
..WEATHER HIGHLIGHTS/HAZARDS
 
 
BIGGEST IMPACTS WILL BE IN THE EAST WITH THE FORECAST TAKEN  
VERBATIM. HEAVY RAIN, DAMAGING WINDS, COASTAL FLOODING, STORM  
SURGE, ETC. ARE REAL POSSIBILITIES FOR COASTAL AND THEN INLAND  
LOCATIONS. THE HEAVY RAIN THREAT WILL CONTINUE INLAND WITH A  
CONTINUED TROPICAL FETCH ON THE SYSTEM'S EAST SIDE. A TRACK INLAND  
WITHOUT A DEFINED ACCELERATION INTO THE WESTERLIES HAS THE  
POTENTIAL FOR A LARGE AREA OF 5-10"+ INCHES OF RAIN OVER A FEW  
DAYS. MANY FACTORS WILL DETERMINE LOCATION/AMOUNTS/TIMING BUT THAT  
WON'T BE RESOLVED FOR A FEW MORE DAYS AT THE LEAST.  
 
A SECOND AREA OF RAIN WILL EXIST OVER THE NORTHERN AND  
NORTHWESTERN GULF COAST UNDER A WEAKNESS ALOFT. WITH PLENTY OF  
MOISTURE TO WORK WITH, SEVERAL INCHES OF RAIN ARE POSSIBLE OVER  
THE PERIOD.  
 
TEMPERATURES WILL GENERALLY BE COOLER THAN AVERAGE IN THE  
NORTHWEST UNDER THE TROUGH BUT NEAR TO ABOVE AVERAGE ELSEWHERE  
(ESPECIALLY FOR MINIMUM TEMPERATURES WHICH MAY BE NEAR RECORD WARM  
VALUES IN THE EAST).  
 
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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