115  
FXUS02 KWBC 101559  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1159 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 HEAVY RAINFALL THREAT CENTERED OVER 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 FROM THE PACIFIC NORTHWEST INTO THE  
EASTERN PACIFIC AS WELL AS OVER 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. BY THE WEEKEND INTO  
NEXT WEEK EXPECT SOME OF THE ENERGY WITHIN THE OVERALL MEAN TROUGH  
TO EJECT ACROSS SOUTHERN CANADA, JUST FAR ENOUGH NORTH OF THE  
WEAKENING FLORENCE TO LEAVE THE SYSTEM 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.  
DEPENDING ON THE EXACT POSITION OF THE SURFACE LOW, THERE MAY ALSO  
BE TERRAIN ENHANCEMENT ALONG THE EASTERN SLOPES OF THE  
APPALACHIANS. 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
 
 
FOR THE OVERALL FORECAST ACROSS THE CONTIGUOUS U.S. AND VICINITY,  
A BLEND OF OPERATIONAL MODELS (00Z PARALLEL GFS, ECMWF, UKMET,  
CMC) PROVIDED A REASONABLE STARTING POINT TO REPRESENT THE MOST  
COMMON IDEAS OF GUIDANCE DURING THE FIRST HALF OF THE PERIOD.  
FROM THE WEEKEND INTO EARLY NEXT WEEK MODEL/ENSEMBLE SPREAD  
INCREASES FAIRLY QUICKLY WITH RESPECT TO HOW INDIVIDUAL SHORTWAVES  
WITHIN THE CANADA/PAC NW/PACIFIC UPPER TROUGH EJECT EASTWARD, DUE  
IN PART TO UNCERTAINTY OVER DETAILS OF HOW THE PACIFIC INTO ALASKA  
UPPER RIDGE WEAKENS. THIS SPREAD AND UNCERTAINTY FAVOR A FAIRLY  
RAPID TREND TOWARD MAJORITY INPUT FROM THE 06Z GEFS/00Z ECMWF  
ENSEMBLE MEANS BY DAYS 6-7 SUN-MON. THE MOST NOTICEABLE TREND  
OVER THE PAST 24-36 HOURS IS TOWARD SLOWER PROGRESSION OF HEIGHT  
FALLS ACROSS SOUTHERN CANADA, WHICH HOLDS BACK NORTHERN U.S.  
FRONTAL PROGRESSION BY DAY 6 SUN. THE 06Z GFS HAS REVERSED THAT  
TREND SOMEWHAT, WHILE THE 00Z ECMWF EVENTUALLY EJECTS MORE WEST  
COAST TROUGH ENERGY THAN CONSENSUS.  
 
REGARDING HURRICANE FLORENCE, THERE IS STILL GREATER SPREAD THAN  
DESIRED CONSIDERING THE MAGNITUDE OF IMPACTS INVOLVED AND  
DIFFERENCES IN THESE IMPACTS RELATIVE TO EXACT TRACK/MOTION OF THE  
SYSTEM. THE ECMWF CONTINUES TO LIE TOWARD THE WESTERN/LEFT SIDE  
OF THE SPREAD AND MOST GFS RUNS ON THE EAST SIDE, HOWEVER THE 00Z  
GFS PULLS THE SURFACE LOW TO THE MIDDLE AND THEN THE WESTERN SIDE  
OF THE ENVELOPE TOWARD SUN-MON. AT THE VERY LEAST THERE IS  
INCREASING CONFIDENCE TOWARD THE IDEA THAT DEEP LAYER RIDGING TO  
THE NORTH OF FLORENCE WILL LIKELY LEAD TO SIGNIFICANT DECELERATION  
OF FLORENCE AFTER ARRIVAL AND A TRACK THAT MAY NOT REACH MUCH  
FARTHER THAN THE WESTERN HALF OF THE NC/VA BORDER BY DAY 7 MON.  
 
MEANWHILE MOST GUIDANCE SHOWS AN AREA OF WEAK TROUGHING/POSSIBLE  
EMBEDDED LOW PRESSURE REACHING AND POSSIBLY LINGERING NEAR THE  
TEXAS COAST. THIS FEATURE COULD PROMOTE ANOTHER AREA OF  
SIGNIFICANT RAINFALL IN ITS VICINITY. EXACT DEPICTION WILL BE  
COORDINATED IN TODAY'S NHC/WPC EXTENDED RANGE CALL.  
 
   
..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 EXTREME NORTHERN PLAINS  
INTO UPPER GREAT LAKES IN THE VICINITY OF A WAVY FRONTAL ZONE,  
ALIGNED WITH THE SOUTHWESTERLY UPPER FLOW CONTINUING INTO THE  
WEEKEND. ALSO THE WESTERN GULF COAST REGION MAY BE AT RISK FOR  
AREAS OF ENHANCED RAINFALL WITH THE COMBINATION OF AN ARRIVING  
(AND POSSIBLY LINGERING) TROPICAL WAVE/UPPER WEAKNESS.  
PRECIPITATION OVER THE NORTHWEST WILL TEND TO BE LIGHT TO LOCALLY  
MODERATE, WITH A DRIER TREND LIKELY BY SUN-MON. TEMPERATURES  
OVER/NEAR THE NORTHERN ROCKIES MAY BE COLD ENOUGH TO SUPPORT SOME  
HIGHER ELEVATION SNOW.  
 
AS THE UPPER PATTERN SLOWLY EVOLVES, EXPECT THE EMPHASIS FOR BELOW  
NORMAL TEMPERATURES (MINUS 5-15F ANOMALIES FOR DAYTIME HIGHS) TO  
SHIFT FROM THE WEST COAST STATES/NORTHERN ROCKIES LATE IN THE WEEK  
TOWARD THE EXTREME NORTHERN PLAINS BY SUN-MON. THE MAJORITY OF  
THE CENTRAL/EASTERN STATES SHOULD SEE ABOVE NORMAL TEMPERATURES,  
ESPECIALLY FOR MINS. PARTICULAR EMPHASIS WILL BE WITHIN AN AXIS  
OF WARM LOWS AHEAD OF A WAVY FRONT EXTENDING SOUTHWEST FROM THE  
NORTHERN PLAINS, AND OVER PARTS OF THE EAST WHICH WILL SEE THE  
ARRIVAL OF TROPICAL AIR ASSOCIATED WITH FLORENCE. SOME LOWS MAY  
BE NEAR OR EXCEED RECORD WARM VALUES 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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