610  
FXUS02 KWBC 100633  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
233 AM EDT MON SEP 10 2018  
 
VALID 12Z THU SEP 13 2018 - 12Z MON SEP 17 2018  
 
...HURRICANE FLORENCE TO BRING DANGEROUS CONDITIONS TO PORTIONS OF  
THE SOUTHEAST AND MID-ATLANTIC STATES LATER THIS WEEK...  
 
...VERY SERIOUS RAINFALL THREAT FOR PORTIONS OF NORTH CAROLINA AND  
VIRGINIA THROUGH THE WEEKEND AND PERHAPS INTO NEXT WEEK...  
 
   
..PATTERN OVERVIEW
 
 
PERSISTENT BUT SHIFTING RIDGING OVER THE NORTH PACIFIC WILL FAVOR  
POSITIVELY TILTED TROUGHING IN THE PACIFIC NORTHWEST AND ACROSS  
CENTRAL/NORTHEASTERN CANADA. 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. BY THE WEEKEND INTO NEXT WEEK, ANY TROUGHING COMING OUT  
OF THE WEST WILL LIFT THROUGH EASTERN CANADA JUST TO THE NORTH OF  
A WEAKENING FLORENCE WHICH WILL LEAVE IT IN A REGION OF WEAK  
STEERING FLOW. THIS MAY SET UP A POTENTIALLY EXTREME RAINFALL  
EVENT FOR PORTIONS OF NORTH CAROLINA AND VIRGINIA AS THE INFLOW  
OFF THE WARM GULF STREAM PROVIDES COPIOUS AMOUNTS OF MOISTURE. THE  
POTENTIALLY DEVASTATING IMPACTS FROM SUSTAINED RAINFALL OVER  
SEVERAL DAYS CANNOT BE UNDERSTATED. PLEASE CONSULT THE NATIONAL  
HURRICANE CENTER FOR THE LATEST INFORMATION ON FLORENCE.  
   
..GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
MODELS/ENSEMBLES CONTINUE TO STRUGGLE IN THE WEST DUE TO UPSTREAM  
UNCERTAINTY OVER THE PACIFIC AND HOW TO HANDLE THE RIDGE BREAKDOWN  
OVER ALASKA AND SUBSEQUENT SHORTWAVE TRAJECTORIES/AMPLITUDES  
THROUGH THE TROUGH. THE GFS/GEFS WERE GENERALLY QUICKER/WEAKER  
THAN THE ECMWF AND ITS ENSEMBLES WITH SUCH SHORTWAVES, BUT THERE  
HAS BEEN NO DEFINED ENSEMBLE TREND AS OF LATE, SO A BLENDED  
SOLUTION OF THE RECENT ENSEMBLES SHOULD SUFFICE FOR NOW UNTIL THE  
PACIFIC GETS SORTED OUT.  
 
IN THE EAST, TREND HAS BEEN FOR HIGHER HEIGHTS IN THE EAST ATOP  
FLORENCE AS IT MOVES INLAND LATER THIS WEEK. THIS RESULTS IN  
INCREASED CONFIDENCE THAT IT WILL NOT GET PICKED UP INTO THE FLOW  
BUT RATHER LINGER NEAR THE NC/VA BORDER (PER NHC/WPC  
COORDINATION). FORECAST TRACK PER NHC ALIGNED RATHER WELL WITH THE  
18Z PARALLEL GFS (FV3 VERSION) THROUGH MOST OF THE PERIOD. THE  
SPEED OF THE SYSTEM TOWARD/INTO THE COAST AND TIMING OF WHEN THE  
UPPER FLOW DIMINISHES WILL DICTATE WHERE THE SYSTEM WILL GO, BUT  
THE NARROWING ENSEMBLE SPREAD MOSTLY SHOWS THE SYSTEM LINGERING  
JUST INLAND INTO NEXT WEEK.  
   
..WEATHER HIGHLIGHTS/HAZARDS
 
 
ASIDE FROM THE INITIAL WIND/WAVE HAZARDS WITH A POTENTIALLY MAJOR  
LANDFALLING HURRICANE, THE HEAVY RAIN AND FLOODING THREAT WILL  
CONTINUE FOR SEVERAL DAYS DUE TO THE COMBINATION OF RARE EVENTS --  
STALLED TROPICAL SYSTEM NEAR THE COAST WITH INFLOW ACROSS THE WARM  
GULF STREAM AT THE ATLANTIC'S WARMEST TIME OF YEAR. THOUGH NWP  
RAINFALL SHOULD NOT BE TAKEN LITERALLY, INFORMATION IS CONTAINED  
WITHIN THE OUTPUT AND THE RAINFALL SIGNAL IS STRIKING WHEN TAKEN  
IN CONCERT WITH THE ENSEMBLES. THE LOCATION OF HEAVY/EXTREME  
RAINFALL WILL NOT BE KNOWN UNTIL THE SHORT RANGE, BUT THE  
PROBABILITY OF IT OCCURRING IN THIS SCENARIO IS HIGH AND THE  
PROBABILITY OF IT OCCURRING OVER LAND CONTINUES TO INCREASE. ANY  
UPSLOPE/TERRAIN ENHANCEMENT WOULD ONLY INCREASE THE SERIOUSNESS OF  
THE RAINFALL THREAT.  
 
A SECOND AREA OF RAIN WILL EXIST OVER THE NORTHERN TIER (MN/WI AND  
THE UP OF MICHIGAN) IN THE VICINITY OF A WAVY FRONTAL ZONE,  
ALIGNED WITH THE SW UPPER FLOW THU-SUN. FINALLY, A TROPICAL WAVE  
IN THE GULF MAY COMBINE WITH THE IN-SITU WEAKNESS ALOFT OVER THE  
UPPER GULF COAST/TX TO PRODUCE MODEST RAIN FOR THE TEXAS COAST  
LATER THIS WEEK UNTIL UPPER RIDGING REASSERTS ITSELF.  
 
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 IN THE TROPICAL AIRMASS). MAXIMUM TEMPERATURES  
WOULD BE HELD DOWN CLOSER TO FLORENCE AND ITS CLOUD SHIELD. THOSE  
COOLER TEMPERATURES IN THE NORTHWEST MAY SPREAD EASTWARD THROUGH  
MONTANA THIS WEEKEND WHICH WOULD SUPPORT SOME HIGHER ELEVATION  
SNOW.  
 
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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