321  
FXUS02 KWBC 021601  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1201 PM EDT SUN SEP 02 2018  
 
VALID 12Z WED SEP 05 2018 - 12Z SUN SEP 09 2018  
   
..PATTERN OVERVIEW AND GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
TO COMMENCE THE PERIOD, A STRONG MID/UPPER RIDGE CENTERED OVER THE  
EASTERN U.S. WILL REMAIN A FIXTURE IN THE FORECAST. INITIALLY  
AROUND 2 TO 2.5 SIGMA ABOVE CLIMATOLOGY, IT SHOULD GRADUALLY  
WEAKEN IN RESPONSE TO AMPLIFIED NORTHERN STREAM FLOW GRAZING NEW  
ENGLAND. THIS WILL ALLOW A BACKDOOR FRONT TO DROP SOUTH INTO THE  
MID-ATLANTIC DURING THE DAY 5-7, SEPTEMBER 7-9 WINDOW. THIS  
CONTINUES TO BE ONE OF THE MORE AGREED UPON FEATURES ON THE MAP.  
TO THE SOUTH OF THE PERSISTENT RIDGE, WHAT IS NOW DEEMED INVEST  
91L CENTERED JUST NORTH OF HISPANOLA SHOULD TRACK WESTWARD ACROSS  
THE FLORIDA PENINSULA TOWARD THE NORTHERN GULF OF MEXICO. THE  
NATIONAL HURRICANE CENTER HAS RAISED TROPICAL CYCLOGENESIS  
PROBABILITIES TO 80 PERCENT DURING THE NEXT 5 DAYS. REGARDING THE  
GUIDANCE, THE PAST SEVERAL GFS RUNS REMAIN THE MOST CONSERVATIVE  
SHOWING VERY LITTLE SURFACE REFLECTION. WHILE THE 00Z ECMWF HAS  
TRENDED DOWNWARD WITH INTENSITY RELATIVE TO YESTERDAY'S RUN, THE  
00Z CMC/UKMET ARE NEARLY IN LINE WITH THE ECMWF SOLUTIONS. AT THIS  
TIME, THE DEEPEST SOLUTION IS THE 00Z UKMET WHICH REMAINS CLOSE TO  
YESTERDAY'S 00Z ECMWF. BEYOND DAY 5/FRIDAY, THE FORECAST BECOMES  
COMPLICATED WITH THE 00Z CMC/UKMET EACH SHOWING FURTHER  
STRENGTHENING DESPITE BEING INLAND. SUFFICE TO SAY, IT REMAINS AN  
AREA TO MONITOR GIVEN THE HEAVY RAINFALL POTENTIAL.  
 
ACROSS WESTERN NORTH AMERICA, A BROAD LONGWAVE TROUGH LURKS  
OFFSHORE WITH THE PAST FOUR ENSEMBLES SHOWING GREATER STABILITY  
THAN PRIOR DAYS. GRADUALLY THIS SYSTEM WILL PUSH INLAND ON FRIDAY  
WITH THE 00Z CMC LAGGING BEHIND STAYING WITH A MORE OFFSHORE  
SOLUTION. WHILE THESE HEIGHT FALLS ACCELERATE EASTWARD TOWARD THE  
NORTH-CENTRAL U.S., THE SYNOPTIC-SCALE FLOW ATTAINS A MORE  
QUASI-ZONAL APPEARANCE INTO THE WEEKEND. ON ITS HEELS, A MORE  
POWERFUL UPPER LOW DROPS DOWN FROM THE GULF OF ALASKA WITH AN  
EVENTUAL POSITION WELL OFFSHORE OF THE WEST COAST BY DAY  
7/SEPTEMBER 9. AT THIS POINT, OUTSIDE OF THE 00Z CMC/GFS, THE  
GUIDANCE IS WELL CLUSTERED WITH THIS EVENTUAL CONFIGURATION.  
 
WITH SUFFICIENT MODEL CLUSTERING AMONGST THE 06Z GFS AND 00Z  
GFS-PARALLEL/ECMWF/UKMET FORECASTS, WAS ABLE TO UTILIZE A  
COMBINATION OF THESE MODELS THROUGH LATE THURSDAY. DECIDED TO  
MANUALLY LOWER PRESSURES WITH THE TROPICAL INVEST GIVEN THE  
GFS/GFS-PARALLEL SOLUTIONS WASH OUT THE SIGNAL. BEYOND THIS  
PERIOD, BEGAN INCORPORATING A HEALTHY AMOUNT OF THE ENSEMBLE  
MEANS, 06Z GEFS/00Z ECMWF TO BE EXACT. CONFIDENCE IS A BIT ABOVE  
AVERAGE IN MOST SECTORS OF THE MAP BUT STILL MORE NEBULOUS WITH  
TROPICAL CONSIDERATIONS ALONG THE WESTERN/CENTRAL GULF COASTS.  
 
   
..WEATHER HIGHLIGHTS/HAZARDS
 
 
WEDNESDAY AND THURSDAY WILL PROVE TO BE RATHER WARM DAYS ACROSS  
THE LOWER GREAT LAKES/OHIO VALLEY INTO THE NORTHEASTERN U.S.  
GENERALLY SPEAKING, DEPARTURES FROM EARLY SEPTEMBER CLIMATOLOGY  
SIT AROUND THE 10 TO 15 DEGREE MARK AS HIGHS REACH 90 DEGREES AS  
FAR NORTH AS 40N LATITUDE. OVERNIGHT LOWS MAY BE WARM ENOUGH TO  
SUPPORT THE BREAKING OF SEVERAL DAILY RECORDS. SOME IMPROVEMENT  
CAN ANTICIPATED BY FRIDAY AS THE INFLUENCE OF THE BACKDOOR FRONT  
BRINGS COOLER MARITIME AIR INTO THE PICTURE. ELSEWHERE, THE CENTER  
OF THE COUNTRY SHOULD SIT ON THE OTHER SIDE OF THE SPECTRUM WITH  
HIGHS AROUND 5 TO 10 DEGREES BELOW AVERAGE. THROUGH FRIDAY, DAILY  
PRECIPITATION CHANCES ACCOMPANIED BY ENHANCED CLOUD COVER SHOULD  
HELP REDUCE INSOLATION OVER THE REGION.  
 
HEAVY RAINFALL POTENTIAL EXISTS FROM THE CENTRAL PLAINS THROUGH  
THE MIDDLE MISSISSIPPI VALLEY AND INTO THE GREAT LAKES. A SLOW  
MOVING BOUNDARY WILL BE THE FOCAL POINT FOR MULTIPLE DAYS OF  
RAINFALL WHICH COULD PROVE TO ENHANCE THE THREAT FOR HYDROLOGIC  
ISSUES. ALONG THE GULF COAST, ENHANCED TROPICAL MOISTURE  
ACCOMPANYING THE MENTIONED INVEST WILL PROVE TO BE A HEAVY  
RAINFALL MAKER. EXACTLY TO WHAT EXTENT REMAINS UNKNOWN BUT THE  
SITUATION SHOULD CONTINUE TO BE MONITORED IN THE UPCOMING DAYS.  
 
RUBIN-OSTER  
 
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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