508  
FXUS02 KWBC 080503  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
103 AM EDT WED AUG 08 2018  
 
VALID 12Z SAT AUG 11 2018 - 12Z WED AUG 15 2018  
   
..PATTERN OVERVIEW AND GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
MODELS AND ENSEMBLES SHOW REASONABLY GOOD CLUSTERING THROUGH MUCH  
OF PERIOD WITH THE UPPER FLOW ACROSS THE CONUS AS AN UPPER LOW  
MOVES THROUGH SOUTHERN BRITISH COLUMBIA SATURDAY AND ANOTHER UPPER  
LOW SINKS SOUTHWARD TOWARD THE OHIO VALLEY. AS THE CANADIAN TROUGH  
APPROACHES THE GREAT LAKES IT WILL KICK OUT THE MIDWEST SYSTEM  
INTO NEW ENGLAND AS BOTH RACE TOWARD THE NORTHWESTERN ATLANTIC.  
UTILIZED A BLEND OF MOSTLY THE 12Z/18Z GFS AND 12Z ECMWF AS THEY  
PAIRED WELL WITH THEIR ENSEMBLE MEANS. HOWEVER, A RATHER  
SIGNIFICANT CHANGE WAS MADE FOR THE FORECAST NEXT TUESDAY IN THE  
NORTHWEST AS UPPER RIDGING IS NOW FAVORED JUST OFF THE COAST WITH  
THE TROUGH MUCH FARTHER EAST LIKE THE ECMWF'S SOLUTION 24 HRS AGO  
(WHICH WAS UNLIKE NEARLY ALL ENSEMBLES). TO THE EAST, THE GFS HAS  
BEEN MORE CONSISTENT WITH THE UPPER LOW IN THE MIDWEST, SO A BLEND  
SUFFICED ON BOTH ACCOUNTS GIVEN LINGERING/TYPICAL UNCERTAINTY.  
 
   
..WEATHER HIGHLIGHTS AND HAZARDS
 
 
NEAR OR ABOVE RECORD HIGH TEMPERATURES OVER THE INTERIOR  
NORTHWEST/NORTHERN ROCKIES/HIGH PLAINS SATURDAY WILL TREND COOLER  
SUNDAY/MONDAY AS THE UPPER TROUGH AND SURFACE COLD FRONT PUSH  
THROUGH. SOUTHWESTWARD EXTENSION OF THE UPPER LOW OVER OHIO ON  
SUNDAY/MONDAY WILL HELP FUEL ORGANIZED RAIN/STORMS ACROSS TEXAS  
WESTWARD TO THE FOUR CORNERS REGION, ESPECIALLY NM/CO JUST TO THE  
EAST OF THE UPPER HIGH. GUIDANCE CONTINUES TO WAVER ON PLACEMENT  
BUT SEVERAL INCHES OF RAIN ARE LIKELY OVER CENTRAL TEXAS DURING  
THE SAT-WED TIME PERIOD.  
 
WAVY FRONT WILL LINGER ACROSS THE U.S. SOUTHERN TIER DURING THE  
PERIOD WHICH WILL HELP SPARK AFTERNOON SHOWERS/STORMS AND LOCALLY  
HEAVY RAINFALL IN THE WARM/HUMID AIRMASS. AS THE UPPER TROUGH  
(EVENTUAL CLOSED LOW) DIGS/SHARPENS OVER THE EAST-CENTRAL U.S.,  
MORE WIDESPREAD PRECIPITATION MAY BE FAVORED TO ITS EAST ALONG THE  
APPALACHIANS TO THE I-95 CORRIDOR. THIS COULD DEVELOP INTO A MORE  
WIDESPREAD HEAVY RAINFALL THREAT CONSIDERING SLOW SYSTEM  
TRANSLATION AND AMPLITUDE WITH A SOUTHERLY FETCH OF AMPLE MOISTURE  
OVER SEVERAL DAYS. CLOSED LOWS IN THE SUMMER CAN BE UNDERESTIMATED  
IN THE MEDIUM RANGE IN TERMS OF PRECIPITATION INTENSITY/AMOUNTS.  
 
FINALLY, APPROACHING FRONTAL BOUNDARY OVER THE HIGH PLAINS/UPPER  
MIDWEST NEXT WEEK WILL ACT AS ANOTHER FOCUS FOR SHOWER/STORMS JUST  
TO ITS SOUTH AS THE LEFTOVER WEAKNESS ALOFT GETS ABSORBED INTO THE  
NEXT TROUGH.  
 
FRACASSO  
 
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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