837  
FXUS02 KWBC 160657  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
256 AM EDT SUN SEP 16 2018  
 
VALID 12Z WED SEP 19 2018 - 12Z SUN SEP 23 2018  
   
..PATTERN OVERVIEW AND WEATHER HIGHLIGHTS/HAZARDS
 
 
AT THE START OF THE PERIOD, WHAT IS CURRENTLY TROPICAL STORM  
FLORENCE WILL HAVE ALREADY BECOME EXTRATROPICAL IN NATURE AS IT  
MOVES ACROSS THE WESTERN ATLANTIC ON WEDNESDAY. MID-LEVEL HEIGHTS  
SHOULD GRADUALLY BUILD ACROSS THE EASTERN SEABOARD IN RESPONSE TO  
A PROGRESSIVE SHORTWAVE DIGGING THROUGH THE NORTH-CENTRAL U.S. ON  
THURSDAY. THIS PARTICULAR REGION OF AMPLIFICATION HAS THE  
POTENTIAL TO DRIVE A FAIRLY IMPRESSIVE SURFACE CYCLONE ACROSS  
QUEBEC BY FRIDAY. ULTIMATELY, THIS WOULD FAVOR A WELL DEFINED  
BAROCLINIC ZONE ACROSS THE GREAT LAKES AND EVENTUALLY INTO THE  
NORTHEASTERN U.S. ON DAYS 5-7, SEPTEMBER 21-23. A STRONG 594-DM  
MID-LEVEL RIDGE CENTERED OVER THE SOUTHEASTERN U.S. SHOULD HELP  
FOCUS THE EVENTUAL STALLED BOUNDARY UP TOWARD THE MID-ATLANTIC  
REGION. BACK ACROSS THE EASTERN PACIFIC, A BROAD RIDGE EXTENDING  
UP INTO THE GULF OF ALASKA WILL HELP MAINTAIN LOWER HEIGHTS ALONG  
THE WEST COAST FOR MUCH OF THE PERIOD. AN INITIAL SHORTWAVE IS  
FORECAST TO SLIDE INTO THE PACIFIC NORTHWEST LATE THURSDAY BEFORE  
EVENTUAL ABSORPTION INTO THE ACTIVE CANADIAN NORTHERN STREAM. ON  
ITS HEELS SHOULD BE A MORE IMPRESSIVE AXIS OF HEIGHT FALLS WHICH  
FOCUSES A LONGWAVE TROUGH OVER THE FAR EASTERN PACIFIC NEXT  
WEEKEND. FINALLY, ACROSS THE DESERT SOUTHWEST, AN AREA OF  
DISTURBED WEATHER LIFTING OUT OF THE SUBTROPICAL PACIFIC SHOULD  
ENHANCE LOW/MID-LEVEL MOISTURE ACROSS THE REGION. THE NATIONAL  
HURRICANE CENTER CURRENTLY SUGGESTS AN 80 PERCENT CHANCE FOR  
TROPICAL CYCLOGENESIS DURING THE NEXT 5 DAYS.  
 
REGARDING THE MODELS HANDLING OF THESE MENTIONED SYSTEMS, THE  
DYNAMIC SURFACE LOW EJECTING OUT OF THE GREAT LAKES SEEMS HIGHLY  
PROBABLE BUT WITH MUCH SPATIO-TEMPORAL UNCERTAINTIES. GIVEN THE  
EXTENT OF FORCING ACCOMPANYING THIS SHORTWAVE, THE DEEP WAVE  
SCENARIO SEEMS REASONABLE. RUN-TO-RUN CONTINUITY REMAINS A BIT  
SUSPECT WITH THE 00Z UKMET JUMPING BACK TO THE NORTH TO JOIN THE  
00Z CMC. MEANWHILE, THE 00Z/18Z GFS AND 12Z ECMWF SIT ALONG THE  
SOUTHERN FLANK OF THE SPREAD. CONSIDERABLE THICKNESS PACKING  
ACCOMPANYING THIS FRONTAL PASSAGE SUGGESTS A FEEL OF AUTUMN IN THE  
AIR ALTHOUGH THE BOUNDARY WILL STALL AS THE BETTER FORCING LIFTS  
WELL INTO EASTERN CANADA. ACROSS WESTERN NORTH AMERICA, THERE HAS  
DEFINITELY BECOME A LARGE TREND IN THE ENSEMBLE SPAGHETTI PLOTS  
FAVORING A LONGWAVE TROUGH FOR LATE IN THE PERIOD. WHILE ENSEMBLES  
CLUSTER WELL, THE AMOUNT OF CHANGE BETWEEN RUNS IS QUITE NOTABLE  
SO FUTURE CYCLES COULD EXHIBIT SIMILAR BEHAVIOR.  
 
THROUGH DAY 4, THURSDAY, A MULTI-MODEL CONSENSUS OF THE 12Z  
GFS/ECMWF/CMC/UKMET WAS UTILIZED BEFORE GRADUALLY INCORPORATING  
ENSEMBLE MEANS INTO THE MIX. FROM DAY 5/FRIDAY ONWARD, REMOVED ANY  
CONTRIBUTION OF THE 12Z CMC/UKMET FROM THE PICTURE AND REPLACED  
THEM WITH THE 12Z NAEFS/ECMWF ENSEMBLE MEANS. WITH UNCERTAINTY  
LOOMING NEXT WEEKEND, ONLY KEPT ABOUT 30 PERCENT OF OPERATIONAL  
GUIDANCE IN THE BLEND WHILE BEING LED BY ENSEMBLE INFLUENCES.  
 
   
..GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
A HEAVY RAINFALL THREAT LOOMS ACROSS SECTIONS OF THE UPPER  
MIDWEST/GREAT LAKES THROUGH EARLY FRIDAY AS THE MENTIONED SURFACE  
CYCLONE TRAVERSES THE REGION. WHILE MODELS KEEP THE HIGHER AMOUNTS  
JUST NORTH OF THE AREA WHICH HAS BEEN WET THE PAST MONTH, A LOT  
COULD CHANGE IN THE UPCOMING MODEL CYCLES. FLASH FLOODING ISSUES  
COULD ENSUE GIVEN THE MAGNITUDE OF RAINFALL NOTED IN MANY OF THE  
MORE RELIABLE PIECES OF GUIDANCE. 2 TO 3 INCH 24-HOUR AREAL  
AVERAGES ARE HIGHLY POSSIBLE WITH ISOLATED HEAVIER AMOUNTS OF  
COURSE POSSIBLE. ACROSS THE SOUTHERN HALF OF THE COUNTRY, DIURNAL  
CONVECTION IS EXPECTED EACH DAY WITH A BETTER FOCUS ALONG THE GULF  
COAST GIVEN ENHANCEMENT ALONG COASTAL BOUNDARIES. BACK ACROSS THE  
FOUR CORNERS, A SURGE OF MOISTURE ACCOMPANYING A TROPICAL  
DISTURBANCE COULD SPREAD SHOWERS AND THUNDERSTORMS NORTH OF THE  
INTERNATIONAL BORDER WITH MEXICO. AND ACROSS THE PACIFIC  
NORTHWEST, A SERIES OF SHORTWAVE PASSAGES SHOULD KEEP CONDITIONS  
WET THROUGHOUT MUCH OF THE FORECAST.  
 
REGARDING TEMPERATURES, BELOW AVERAGE NUMBERS ARE ANTICIPATED FROM  
THE PACIFIC NORTHWEST EASTWARD INTO THE NORTHERN ROCKIES AND  
ADJACENT GREAT PLAINS. THIS REGION THE FROM THE ROCKIES EASTWARD  
COULD SEE HIGHS AROUND 10 TO 15 DEGREES BELOW CLIMATOLOGY WHICH  
WOULD OFFER HIGHS IN THE UPPER 50S TO LOWER 60S. TO THE SOUTH OF  
THE BAROCLINIC ZONE, EXPECTED HIGHS SHOULD BE WELL ABOVE AVERAGE,  
PARTICULARLY FROM THE CENTRAL ROCKIES EASTWARD ACROSS THE MIDDLE  
MISSISSIPPI VALLEY AND INTO THE OHIO VALLEY/GREAT LAKES REGION.  
SIMILAR ANOMALIES ARE IN THE FORECAST WHICH SUPPORTS HIGHS IN THE  
80S FOR THE MOST PART.  
 
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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