036  
FXUS02 KWBC 310658  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
258 AM EDT FRI AUG 31 2018  
 
VALID 12Z MON SEP 03 2018 - 12Z FRI SEP 07 2018  
   
..PATTERN OVERVIEW AND GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
LATEST GUIDANCE MAINTAINS REASONABLE AGREEMENT/CONTINUITY FOR THE  
OVERALL MEAN PATTERN BUT CONTINUES TO SUGGEST GREATER THAN AVERAGE  
UNCERTAINTY FOR SOME DETAILS. EXPECT THE CENTER OF A  
STRENGTHENING UPPER RIDGE OVER THE EASTERN U.S. AND WESTERN  
ATLANTIC AS OF EARLY MON TO SETTLE OVER OR NEAR THE MID-ATLANTIC  
REGION, WITH HIGHEST HEIGHTS ALOFT LIKELY AROUND MIDWEEK FOLLOWED  
BY SOME GRADUAL WEAKENING. FLOW AROUND THIS RIDGE MAY BRING ONE  
OR MORE BUNDLES OF ENERGY (AND POSSIBLY A WEAK SURFACE REFLECTION)  
ACROSS FLORIDA AND THE GULF COAST REGION. AT THE SAME TIME AN  
EVOLVING MEAN TROUGH SHOULD COVER WESTERN CANADA AND THE  
NORTHEASTERN PACIFIC. THE GREATEST FORECAST UNCERTAINTY INVOLVES  
THE SHORTWAVE DETAILS WITHIN AND DOWNSTREAM FROM THE MEAN TROUGH,  
KEEPING CONFIDENCE FAIRLY LOW FOR SPECIFIC FRONTAL DETAILS ACROSS  
MUCH OF THE NORTHERN CONTIGUOUS U.S.. DETAILS WITHIN FLOW TO THE  
SOUTH OF THE EASTERN RIDGE ALOFT ARE ALSO FAIRLY UNCERTAIN.  
 
CONSECUTIVE RUNS OF THE GFS/ECMWF AS WELL AS INDIVIDUAL 12Z/18Z  
ENSEMBLE MEMBERS ARE QUITE VARIED REGARDING THE AMPLITUDE/TIMING  
OF ONE OR MORE SHORTWAVES EJECTING FROM THE WESTERN  
CANADA/NORTHEASTERN PACIFIC MEAN TROUGH. TRENDS THROUGH THE 18Z  
CYCLE GENERALLY SHOWED A SLOWER TREND WITH THE BEST DEFINED  
SHORTWAVE FORECAST TO EJECT FROM WESTERN NORTH AMERICA AFTER MON,  
THOUGH THE 12Z UKMET/CMC WERE SOMEWHAT FASTER THAN THE MAJORITY  
CLUSTER. REINFORCING THE IDEA OF LOW PREDICTABILITY FOR THIS  
ASPECT OF THE FORECAST, THE 00Z GFS HAS REVERTED BACK TO A FASTER  
SCENARIO FOR THE SHORTWAVE. ISSUES WITH THIS SHORTWAVE ENERGY  
WILL RESULT IN VARIATIONS FOR TIMING AND SOUTHWARD EXTENT OF THE  
ASSOCIATED FRONT OVER THE NORTHERN STATES. THERE IS SOMEWHAT  
BETTER CLUSTERING OF GUIDANCE FOR A LEADING FRONT THAT PROGRESSES  
FROM THE UPPER GREAT LAKES INTO NEW ENGLAND DURING THE FIRST HALF  
OF THE PERIOD AND LIKELY LIFTS BACK AS A WARM FRONT MID-LATE WEEK.  
 
ENSEMBLE MEANS HAVE TRENDED STRONGER WITH THE MID-ATLANTIC RIDGE  
ALOFT OVER THE PAST DAY, ESPECIALLY IN FORECASTS VALID AROUND DAYS  
5-6 WED-THU. THIS TREND IS A LITTLE LESS PRONOUNCED IN THE  
OPERATIONAL RUNS. TO THE SOUTH OF THE RIDGE, THE 12Z ECMWF MADE A  
NOTABLE ADJUSTMENT TOWARD A WEAKER SURFACE REFLECTION OF A WAVE  
EXPECTED TO EMERGE FROM THE BAHAMAS AND POSSIBLY AFFECT FLORIDA  
AND THE GULF COAST REGION. THE MANUAL FORECAST MAINTAINS  
YESTERDAY'S WPC-NHC COORDINATED DEPICTION OF THIS FEATURE THROUGH  
DAY 6 THU. THE NEW DAY 7 FORECAST SHOWS AN EVOLUTION TOWARD A  
TROUGH ALONG/NORTHEAST OF THE TEXAS COAST TO INCORPORATE LATEST  
WEAKENING TRENDS FOR ANY LINGERING REFLECTION OF THE GULF COAST  
WAVE, WHILE ALSO INCLUDING LOW LEVEL CIRCULATION POTENTIAL FARTHER  
SOUTHWEST NEAR THE SOUTH-CENTRAL TEXAS COAST AS HINTED AT BY SOME  
12Z/18Z MODELS AND MEANS. THIS ASPECT OF THE FORECAST WILL BE  
REFINED LATER TODAY BASED ON NEWEST GUIDANCE.  
 
IN LIGHT OF THE ARRAY OF GUIDANCE FROM THE 12Z/18Z CYCLES, THE  
EARLY PART OF THE FORECAST PRIMARILY CONSISTED OF THE 18Z GFS, 12Z  
FV3 (PARALLEL) GFS, AND 12Z ECMWF WITH MODEST INCLUSION OF THE 18Z  
GEFS/12Z ECMWF MEANS. DETAIL DIVERGENCE IN THE MODELS LED TO  
INCREASING ENSEMBLE WEIGHT TO 50 PERCENT OR HIGHER MID-LATE  
PERIOD. BY LATE IN THE WEEK THE 12Z FV3 GFS COMPARED BETTER TO  
THE MEANS THAN THE 18Z GFS (POSSIBLY TOO AMPLIFIED WITH ITS  
EASTERN CANADA TROUGH/SOUTHWARD WITH THE FRONT TO THE SOUTH) AND  
12Z ECMWF (IN WHICH THE EASTERN PACIFIC TROUGH SAGGED A LITTLE  
FARTHER SOUTHWARD THAN THE MAJORITY OF SOLUTIONS).  
 
   
..WEATHER HIGHLIGHTS/HAZARDS
 
 
THE STRONG UPPER RIDGE OVER THE EAST WILL LEAD TO MULTIPLE DAYS OF  
HUMID AND VERY WARM TO HOT CONDITIONS OVER MUCH OF THE EASTERN  
HALF OF THE COUNTRY. HIGHEST ANOMALIES OF 10-15F ABOVE NORMAL ARE  
MOST LIKELY FROM THE GREAT LAKES/OHIO VALLEY INTO THE NORTHEAST,  
WITH MORNING LOWS TENDING TO HAVE SLIGHTLY GREATER ANOMALIES--AND  
BETTER CHANCE OF ACHIEVING DAILY RECORD VALUES--VERSUS THE DAYTIME  
HIGHS. A FEW RECORD HIGHS MAY STILL BE POSSIBLE THOUGH. ONE OR  
MORE DAYS OF WELL BELOW NORMAL HIGH TEMPERATURES ARE POSSIBLE  
BEHIND THE STRONGEST NORTHERN TIER FRONT, ESPECIALLY OVER THE  
EXTREME NORTHERN ROCKIES/HIGH PLAINS. MULTIPLE AREAS WILL SEE AT  
LEAST SOME POTENTIAL FOR HEAVY RAINFALL AROUND THE PERIPHERY OF  
THE UPPER RIDGE. FLORIDA AND THE GULF COAST REGION SHOULD SEE AN  
EPISODE OF ENHANCED ACTIVITY WITH WESTWARD-MOVING ENERGY  
ALOFT/POSSIBLE SURFACE REFLECTION EMERGING FROM THE BAHAMAS. A  
RETURN OF MOISTURE TO THE SOUTHERN ROCKIES/HIGH PLAINS SHOULD  
YIELD PERIODS OF DIURNALLY FAVORED CONVECTION OVER THAT REGION.  
LOCATIONS FROM THE NORTHERN-CENTRAL PLAINS INTO THE UPPER GREAT  
LAKES MAY SEE MULTIPLE HEAVY CONVECTIVE EVENTS IN THE VICINITY OF  
A MEAN FRONTAL BOUNDARY EXPECTED TO MEANDER OVER THIS AREA DURING  
THE PERIOD.  
 
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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