337  
FXUS02 KWBC 301558  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1157 AM EDT THU AUG 30 2018  
 
VALID 12Z SUN SEP 02 2018 - 12Z THU SEP 06 2018  
   
..PATTERN OVERVIEW AND GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
THE MAIN FIXTURE IN THE SYNOPTIC-SCALE PATTERN WILL BE A STRONG  
POSITIVE HEIGHT ANOMALY CENTERED OVER THE EASTERN U.S. THE 00Z GFS  
ADVERTISES MID-LEVEL HEIGHTS AROUND 2 TO 2.5 SIGMA ABOVE  
CLIMATOLOGY WITH 594-DM VALUES EXTENDING FROM THE MISSISSIPPI  
RIVER EASTWARD INTO THE WESTERN ATLANTIC BY MID-WEEK. THIS IS  
RATHER WELL ESTABLISHED IN THE MODEL GUIDANCE WITH ONLY MODEST  
DIFFERENCES BETWEEN THEM. UNDERNEATH THIS DOMINANT RIDGE, MODELS  
CONTINUE TO HINT AT SOME SORT OF TROPICAL DISTURBANCE TRACKING  
WESTWARD THROUGH THE BAHAMAS AND PERHAPS INTO THE GULF OF MEXICO.  
AS HAS BEEN THE CASE THE PAST COUPLE OF DAYS, THE ECMWF SOLUTIONS  
HAVE FAVORED A WEAK SURFACE REFLECTION WITH SOME OF THE ECMWF/CMC  
ENSEMBLE MEMBERS SUPPORTING THIS. MEANWHILE, THE GFS/GEFS SHOWS  
LITTLE INDICATION OF SUCH A SCENARIO UNFOLDING AS SURFACE RIDGING  
SINKS TOWARD THE SOUTHERN GULF OF MEXICO.  
 
MOST OF THE UNCERTAINTY RESIDES ACROSS THE WESTERN/CENTRAL PARTS  
OF NORTH AMERICA. THE GUIDANCE CONTINUES TO EXHIBIT POOR  
RUN-TO-RUN CONTINUITY WITH PLENTY OF DIFFERENCES BETWEEN MODEL  
SUITES. ULTIMATELY, A WAVE TRAIN OF SHORTWAVES ARE EXPECTED TO  
SLIDE FROM THE NORTHEAST PACIFIC DOWNSTREAM INTO WESTERN NORTH  
AMERICA. SOME OF THESE IMPULSES COULD AMPLIFY MORE THAN OTHERS BUT  
DETAILS ARE SOMEWHAT NEBULOUS. EASTWARD MIGRATION OF THESE SYSTEMS  
WILL CARRY SHORTWAVE ENERGY INTO CENTRAL CANADA POSSIBLY DOWN INTO  
THE NORTH-CENTRAL U.S. SOME DEGREE OF SHEARING IS ANTICIPATED  
GIVEN THE EXTENT OF DOWNSTREAM MID/UPPER-LEVEL RIDGING IN PLACE.  
FURTHER INSTANCES OF SHORTWAVE PROGRESSION ACROSS SIMILAR REGIONS  
ARE LIKELY THROUGHOUT THE PERIOD. A FEW MODELS STAND OUT AS MORE  
AMPLIFIED IN NATURE, MOST NOTABLY THE 00Z ECMWF WITH THE LEAD WAVE  
BY DAY 5/TUESDAY AND THE 06Z GFS GIVEN ITS DEPICTION OF THE  
FOLLOWING SYSTEM. MEANWHILE, THE 00Z UKMET IS FLATTER TO START  
WITH WHICH IS PERHAPS TOO SUBDUED IN NATURE. OVERALL, ENSEMBLES  
SHOULD PLAY A MAJOR ROLE IN FORECAST PRODUCTION FOR A GOOD CHUNK  
OF NEXT WEEK.  
 
THE PREFERENCE LEANED HEAVILY ON A MULTI-OPERATIONAL MODEL  
CONSENSUS TO START WITH FEATURING A MIXTURE OF THE 00Z  
GFS/GFS-PARALLEL/ECMWF/CMC SOLUTIONS. THEREAFTER, GRADUALLY  
INCREASED INFLUENCES OF THE 00Z GEFS/ECMWF ENSEMBLE MEANS WITH  
THESE MODELS ENCOMPASSING 80 PERCENT OF THE BLEND BY DAY 6/7,  
SEPTEMBER 5/6. CONFIDENCE IS HIGHEST WITH THE PERSISTENT RIDGE  
OVER THE EASTERN U.S. WHILE LIKELY A BIT BELOW AVERAGE FOR AREAS  
TO THE WEST.  
 
   
..WEATHER HIGHLIGHTS/HAZARDS
 
 
UPPER RIDGING WILL KEEP CONDITIONS QUITE WARM GENERALLY EAST OF  
THE MISSISSIPPI RIVER WITH THE MOST ANOMALOUS READINGS EXPECTED  
NORTH OF 40N LATITUDE. FORECAST HIGHS ARE AROUND 5 TO 10 DEGREES  
ABOVE EARLY SEPTEMBER CLIMATOLOGY. SUCH DEPARTURES SHOULD BE EVEN  
HIGHER FOR OVERNIGHT LOWS AS MINIMA SIT AROUND 10 TO 15 DEGREES  
ABOVE AVERAGE. ON TUESDAY AND WEDNESDAY MORNING, SOME DAILY  
RECORDS MAY BE BROKEN GIVEN THE WARMTH OVERNIGHT. THIS WOULD  
GENERALLY FOCUS ACROSS THE OHIO VALLEY AND ALONG THE EASTERN  
SEABOARD. REGARDING PRECIPITATION, THE BEST POTENTIAL FOR HEAVY  
RAINFALL WILL RESIDE ANYWHERE FROM THE CENTRAL PLAINS UP THROUGH  
THE MIDDLE MISSISSIPPI VALLEY AND INTO THE UPPER GREAT LAKES. THIS  
REGION IS JUXTAPOSED WITH WHERE A FRONTAL BOUNDARY SHOULD STALL  
DURING THE PERIOD. ELSEWHERE, ENHANCED TROPICAL MOISTURE  
ACCOMPANYING THE TROPICAL WAVE COULD LEAD TO HEAVY RAINFALL  
ANYWHERE ALONG THE CENTRAL TO EASTERN GULF COAST. ELSEWHERE, SOME  
RETURN OF MOISTURE TO THE FOUR CORNERS SHOULD LEAD TO DAILY ROUNDS  
OF THUNDERSTORMS TO THE REGION.  
 
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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