497  
FXUS02 KWBC 091518  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1118 AM EDT SUN SEP 09 2018  
 
VALID 12Z WED SEP 12 2018 - 12Z SUN SEP 16 2018  
 
...HURRICANE FLORENCE LIKELY TO IMPACT A PORTION OF THE EAST COAST  
LATER THIS WEEK...  
 
...POTENTIALLY SIGNIFICANT RAINFALL THREAT FOR PORTIONS OF THE  
SOUTHEAST AND MID-ATLANTIC...  
 
   
..PATTERN OVERVIEW
 
 
A RATHER STAGNANT UPPER PATTERN STARTS THE MEDIUM RANGE PERIOD  
(WEDNESDAY) WITH STRONG RIDGING OVER THE NORTHERN PACIFIC AND THE  
NORTHWESTERN ATLANTIC OFF THE NEW ENGLAND COAST. THIS FAVORS THE  
MAINTENANCE OF A POSITIVELY TILTED TROUGH ALONG THE WEST COAST OF  
THE LOWER 48 AND BROAD SW FLOW OUT OF THE CENTRAL US INTO  
SOUTHEASTERN CANADA. AS THE PACIFIC RIDGING BREAKS DOWN LATER THIS  
WEEK, A BUCKLE TO THE FLOW WILL EVENTUALLY WORK ITS WAY INTO THE  
PAC NW NEXT WEEK. HURRICANE FLORENCE IS FORECAST TO STRENGTHEN TO  
A MAJOR HURRICANE OVER THE NEXT FEW DAYS AND IS LIKELY TO IMPACT  
AT LEAST PART OF THE SOUTHEAST/MID-ATLANTIC STARTING AROUND  
THURSDAY BUT PERHAPS CARRYING INTO NEXT WEEKEND, EVEN IF THE  
CENTER STAYS OFFSHORE. PLEASE CONSULT THE NATIONAL HURRICANE  
CENTER FOR THE LATEST INFORMATION ON FLORENCE.  
   
..GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
OVERALL, THE AMPLIFIED PATTERN REMAINS FAIRLY WELL-HANDLED BY THE  
ENSEMBLE GUIDANCE ESPECIALLY EAST OF THE ROCKIES WITH UNCERTAINTY  
ASSOCIATED WITH THE RIDGE BREAKDOWN OVER THE ALEUTIANS. ENSEMBLE  
CONSENSUS APPROACH SEEMS BEST SUITED IN THE WEST WITH THE  
DETERMINISTIC RUNS TOO VARIED TO INSTILL CONFIDENCE. TO THE EAST,  
DOMINANT RIDGING WILL HELP STEER FLORENCE TOWARD THE US SOUTHEAST  
COAST AND PERHAPS INLAND INTO THE CAROLINAS PER THE LATEST NHC  
TRACK. BY NEXT FRI-SUN, THE STEERING FLOW OVER THE EAST  
ESSENTIALLY VANISHES WHICH LEAVES FLORENCE WITH NO REAL EXIT  
STRATEGY. WHERE THIS MAY HAPPEN WILL HAVE ENORMOUS CONSEQUENCES ON  
THE WIND/RAIN THREAT TO COASTAL AND THEN INLAND AREAS. FARTHER  
WEST, A WEAKNESS ALOFT OVER TEXAS ANS THE LOWER MS VALLEY WILL  
DRIFT WESTWARD THROUGH THE PERIOD.  
   
..WEATHER HIGHLIGHTS/HAZARDS
 
 
BIGGEST IMPACTS WILL BE IN THE EAST SURROUNDING FLORENCE. WITH AN  
INCREASE IN THE LIKELIHOOD THAT THE SYSTEM STALLS JUST INLAND OR  
JUST OFFSHORE, THE HEAVY RAIN THREAT WILL CONTINUE VIA AN  
UNLIMITED MOISTURE SOURCE. MAXIMUM POTENTIAL RAINFALL PER SOME  
MODEL GUIDANCE HAS BEEN BEYOND EXTREME -- BUT REMAINS JUST MODEL  
OUTPUT FOR NOW. MANY FACTORS WILL DETERMINE  
LOCATION/AMOUNTS/TIMING BUT THAT WON'T BE RESOLVED FOR A COUPLE  
MORE DAYS AT THE LEAST; HOWEVER, THE RAINFALL/FLOODING THREAT IS A  
REAL CONCERN EVEN AT THIS TIME RANGE.  
 
A SECOND AREA OF RAIN WILL EXIST OVER THE NORTHERN AND  
NORTHWESTERN GULF COAST UNDER A WEAKNESS ALOFT THAT WILL  
EVENTUALLY FADE TO THE WEST. AN INCH OR SO OF RAIN IS POSSIBLE  
OVER THE EARLIER PORTIONS OF THE MEDIUM RANGE 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). THOSE COOLER TEMPERATURES MAY SPREAD EASTWARD  
THROUGH MONTANA THIS WEEKEND WHICH WOULD SUPPORT SOME HIGHER  
ELEVATION SNOW.  
 
FRACASSO/RYAN  
 
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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