359  
FXUS02 KWBC 030659  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
258 AM EDT MON SEP 03 2018  
 
VALID 12Z THU SEP 06 2018 - 12Z MON SEP 10 2018  
   
..PATTERN OVERVIEW AND GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
GUIDANCE CONTINUES TO ADVERTISE RELATIVELY PROGRESSIVE MEAN FLOW  
ACROSS THE NORTHERN CONTIGUOUS U.S. AND SOUTHERN CANADA,  
DOWNSTREAM FROM A NORTHEAST PACIFIC TROUGH THAT EXISTS IN RESPONSE  
TO A VERY STRONG BERING SEA RIDGE. WITHIN THIS RIDGE, LATEST  
GFS/ECMWF RUNS SUGGEST POTENTIAL FOR HEIGHTS THAT COULD REACH 3-4  
STANDARD DEVIATIONS ABOVE CLIMATOLOGY LATE IN THE WEEK. MEANWHILE  
FLOW AROUND A WEAKENING EASTERN U.S. RIDGE ALOFT SHOULD GUIDE  
POTENTIAL TROPICAL CYCLONE SEVEN INTO THE SOUTHERN PLAINS BY  
THU-FRI, AND FARTHER EAST BRING AN INVERTED UPPER TROUGH FROM THE  
BAHAMAS INTO FLORIDA.  
 
WITHIN AND DOWNSTREAM FROM THE NORTHEAST PACIFIC TROUGH THERE HAS  
BEEN DECENT AGREEMENT/CONTINUITY FOR SYNOPTIC-SCALE SYSTEMS OVER  
THE PAST COUPLE DAYS. THE ONE EXCEPTION COMES FROM CMC RUNS WHICH  
HAVE VARIED BETWEEN BEING EITHER MORE OPEN/PROGRESSIVE WITH  
NORTHEAST PACIFIC FLOW OR FARTHER WESTWARD THAN CONSENSUS WITH AT  
LEAST ONE OF TWO LIKELY EMBEDDED CLOSED LOWS. THE 12Z CMC MEAN  
REFLECTED THE OPEN/PROGRESSIVE OPERATIONAL RUN. THE ONE  
INSTABILITY IN THE GFS IS WITH OCCASIONAL RUNS PULLING BACK THE  
SECOND CLOSED LOW (FROM ENERGY DROPPING SOUTH ACROSS ALASKA) A  
LITTLE WEST OF CONSENSUS. THE MAJORITY CLUSTER HAS ONE UPPER  
LOW/SHORTWAVE WELL OFFSHORE THE NORTHWEST AT THE START OF THE  
PERIOD THU. THIS ENERGY SHOULD EJECT INLAND BY SAT WITH  
ASSOCIATED SOUTHERN CANADA LOW PRESSURE ULTIMATELY PUSHING A  
TRAILING FRONT THROUGH THE NORTHWEST AND INTO THE PLAINS/MIDWEST.  
THE SECOND CLOSED LOW SHOULD SOMEWHAT OFFSHORE VANCOUVER ISLAND BY  
THE START OF DAY 7 MON BUT A LEADING FRONTAL SYSTEM MAY APPROACH  
THE PACIFIC NORTHWEST. BY LATE IN THE PERIOD TELECONNECTIONS  
RELATIVE TO THE UPSTREAM RIDGE FAVOR FLAT TO SLIGHTLY CYCLONIC  
MEAN FLOW ACROSS THE NORTHWEST VERSUS THE RIDGE THAT BUILDS INTO  
THE REGION IN THE 12Z-18Z GFS RUNS. THUS WOULD PREFER TO DOWNPLAY  
THE OPERATIONAL GFS RUNS AT THAT TIME.  
 
CONSULT NATIONAL HURRICANE CENTER PRODUCTS FOR THE LATEST INFO  
REGARDING POTENTIAL TROPICAL CYCLONE SEVEN, EXPECTED TO TRACK  
THROUGH THE FLORIDA KEYS AND INTO THE CENTRAL GULF COAST IN THE  
SHORT RANGE TIME FRAME AND THEN CONTINUE NORTHWESTWARD INTO THE  
SOUTHERN/SOUTH-CENTRAL PLAINS. THUS FAR GFS RUNS HAVE BEEN WEAKER  
THAN OTHER GUIDANCE WITH THE REFLECTION OF THIS SYSTEM. THE NEW  
00Z UKMET IS QUITE CLOSE TO THE 12Z ECMWF IN PRINCIPLE. THE 00Z  
CMC IS AS WELL FOR A TIME BEFORE STRAYING FASTER WITH ITS  
RECURVATURE.  
 
A CONTINUING DISCREPANCY IN THE GUIDANCE INVOLVES THE SURFACE  
EVOLUTION AHEAD OF SHORTWAVE ENERGY EXPECTED TO MIGRATE ACROSS THE  
CENTRAL U.S. LATE IN THE WEEK AND THEN CONTINUE NORTHEASTWARD.  
THE CMC WITH ITS FAST RECURVATURE OF POTENTIAL T.C. SEVEN, AND GFS  
WITH STRONG CONCENTRATION OF ENERGY ALOFT AFTER THE SHORTWAVE  
EXITS THE PLAINS, ARE ON THE AGGRESSIVE SIDE OF GUIDANCE WITH A  
POSSIBLE SURFACE WAVE TRACKING THROUGH THE GREAT LAKES AND INTO  
STRONG DOWNSTREAM SURFACE HIGH PRESSURE. RECENT GEFS MEANS HAVE  
REFLECTED GFS IDEAS BUT IN A MORE SUBDUED FORM. 12Z ECMWF TREND  
TOWARD A LITTLE MORE WAVINESS THAN ITS PRIOR RUN SUGGESTS THE  
ANSWER MAY LIE BETWEEN THE TWO EXTREMES BUT MORE LIKELY SOMEWHAT  
CLOSER TO THE ECMWF GIVEN THE STRENGTH OF THE LEADING HIGH  
PRESSURE. THE 00Z UKMET THROUGH LATE SAT IS SIMILAR TO THE ECMWF  
IN PRINCIPLE. AHEAD OF THIS EVOLUTION, THERE IS GOOD AGREEMENT  
REGARDING THE FRONT THAT DROPS FROM THE NORTHEAST INTO  
MID-ATLANTIC AND STALLS FOR A TIME FROM THE EAST COAST INTO THE  
PLAINS. ALBEIT WITH UNCERTAIN WAVE DETAILS AS DESCRIBED ABOVE,  
THERE IS GENERAL AGREEMENT THAT THIS FRONT WILL LIFT NORTHEAST AS  
A WARM FRONT AHEAD OF THE SYSTEM THAT TRACKS ACROSS CANADA DURING  
THE WEEKEND AND EARLY NEXT WEEK.  
 
COMBINING THE ABOVE CONSIDERATIONS, THE STARTING BLEND FOR THE  
FORECAST LEANED SOMEWHAT MORE IN THE DIRECTION OF THE 12Z  
ECMWF/ECMWF MEAN RELATIVE TO THE 18Z GFS/GEFS MEAN WITH THE GEFS  
MEAN FAVORED MORE THAN THE GFS COMPONENT BY DAYS 6-7 SUN-MON.  
 
   
..WEATHER HIGHLIGHTS/HAZARDS
 
 
EXPECT A PERIOD OF HEAVY RAINFALL POTENTIAL OVER PARTS OF THE  
SOUTHERN PLAINS AND VICINITY NEAR THE PATH OF POTENTIAL TROPICAL  
CYCLONE SEVEN. THIS MOISTURE WILL FEED INTO A LARGER SCALE BAND  
OF MOISTURE EXTENDING FROM THE SOUTHERN PLAINS NORTHEASTWARD,  
INTERACTING WITH THE FRONT THAT STALLS OVER MID-LATITUDES OF THE  
LOWER 48 AND THEN LIFTS NORTHEASTWARD AS A WARM FRONT.  
SIGNIFICANT DETAILS HAVE YET TO BE RESOLVED SO CONFIDENCE IS NOT  
YET AS HIGH AS DESIRED FOR PINPOINTING THE BEST FOCUS FOR HIGHEST  
RAINFALL AMOUNTS WITHIN A FAVORED ZONE FROM THE SOUTHERN-CENTRAL  
PLAINS THROUGH THE MIDWEST INTO PARTS OF THE NORTHERN  
MID-ATLANTIC/NORTHEAST. MID LEVEL ENERGY AND A SURFACE TROUGH  
REACHING FLORIDA FROM THE BAHAMAS MAY ENHANCE RAINFALL AT TIMES  
OVER/NEAR THAT STATE, WHILE LINGERING MOISTURE MAY AID SOME  
ACTIVITY ALONG PORTIONS OF THE GULF COAST. THE PACIFIC NORTHWEST  
SHOULD SEE A PERIOD OF LIGHT-MODERATE PRECIPITATION LATE WEEK INTO  
THE WEEKEND WITH POTENTIALLY ANOTHER AROUND THE START OF NEXT WEEK.  
 
EXPECT VERY WARM AND HUMID CONDITIONS OVER THE EAST LATE THIS WEEK  
AHEAD OF THE BACKDOOR FRONT THAT REACHES INTO THE MID-ATLANTIC.  
THE BEST SIGNAL FOR MOST EXTREME ANOMALIES (AT LEAST PLUS 10-15F)  
CONTINUES TO BE FOR MIN TEMPS FROM THE OHIO VALLEY/APPALACHIANS  
INTO MID-ATLANTIC/NORTHEAST WITH SOME READINGS POSSIBLY EXCEEDING  
DAILY RECORD WARM MIN VALUES. CLOUDS/RAINFALL WILL TEMPER HIGH  
TEMPERATURES OVER THE EAST AFTER FRI BUT KEEP THE LOWS FAIRLY  
WARM. MEANWHILE THE COOLEST ANOMALIES DURING THE PERIOD (MINUS  
10F OR SO) WILL BE OVER THE CENTRAL PLAINS LATE THIS WEEK.  
PROGRESSION OF FEATURES ACROSS THE WESTERN HALF OF THE NORTHERN  
TIER STATES WILL LEAD TO A WARMING TREND FOLLOWED BY COOLER  
TEMPERATURES.  
 
RAUSCH  
 
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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