485  
FXUS02 KWBC 210738  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
335 AM EDT TUE AUG 21 2018  
 
VALID 12Z FRI AUG 24 2018 - 12Z TUE AUG 28 2018  
   
..PATTERN OVERVIEW AND GUIDANCE EVALUATION/PREFERENCES
 
 
MODEL GUIDANCE CONTINUES TO SHOW STRONG SUPPORT FOR A  
STRENGTHENING UPPER RIDGE ACROSS THE NORTHEAST PACIFIC DURING THE  
MEDIUM RANGE, WHICH SUPPORTS A DOWNSTREAM MEAN TROUGH ACROSS THE  
NORTHWESTERN U.S., AND AN EXPANDING RIDGE FROM THE  
LOWER/MID-MISSISSIPPI VALLEY TO THE MID-ATLANTIC. DETERMINISTIC  
SOLUTIONS SHOWED RELATIVELY MODEST TIMING/AMPLITUDE DIFFERENCES  
WITH SEVERAL SHORTWAVES TRAVERSING THE NORTHERN TIER FRI-SUN, AND  
A MULTI-MODEL DETERMINSITIC BLEND (INCLUDING THE  
ECMWF/GFS/UKMET/CMC) SERVED AS A BASIS FOR THE FORECAST FOR DAYS  
3-5. BY DAYS 6-7 (MON-TUE), MODELS CONTINUE TO SHOW SOME SPREAD AS  
TO THE DEGREE OF WESTERN U.S. TROUGH AMPLIFICATION THAT WILL  
OCCUR, AND ALSO WITH THE DOWNSTREAM RIDGE AMPLIFICATION ACROSS THE  
MISSISSIPPI VALLEY. GIVEN THE DEGREE OF FLOW AMPLIFICATION  
EXPECTED ACROSS THE NORTH PACIFIC, AS WELL AS THE EXTRATROPICAL  
TRANSITION OF TYPHOON SOULIK ACROSS EASTERN ASIA, WOULD EXPECT A  
GREATER DEGREE OF FLOW AMPLIFICATION TO OCCUR ACROSS THE CONUS AS  
WELL. GIVEN THE RELATIVE INCREASE IN MODEL SPREAD BY DAYS 6-7,  
WEIGHTING OF ECENS/GEFS MEANS WAS BOOSTED TO A MAJORITY OF THE  
FORECAST DURING THAT TIME PERIOD.  
 
   
..WEATHER HIGHLIGHTS AND HAZARDS
 
 
A LINGERING STATIONARY FRONTAL BOUNDARY ALONG THE SOUTHEAST U.S.  
COAST THROUGH MOST OF THE MEDIUM RANGE PERIOD WILL RESULT IN  
SCATTERED TO NUMEROUS SHOWERS AND THUNDERSTORMS WITH LOCALLY HEAVY  
RAINFALL POSSIBLE. AREAS OF POTENTIALLY HEAVY CONVECTION WILL ALSO  
BE POSSIBLE ACROSS THE MID/UPPER MISSISSIPPI VALLEY AND UPPER  
MIDWEST THROUGH MUCH OF THE PERIOD AS A COUPLE WAVES OF LOW  
PRESSURE TRAVERSE A FRONTAL BOUNDARY ACROSS THE REGION. CONVECTION  
ACROSS THE CENTRAL U.S. MAY BE CONFINED MORE TO THE MIDWEST AND  
GREAT LAKES BY THE WEEKEND AS UPPER-LEVEL HEIGHTS BEGIN TO RISE  
ACROSS THE CENTRAL U.S., INCREASING THE LIKELIHOOD OF A CAPPED  
ENVIRONMENT FARTHER SOUTH. SCATTERED AFTERNOON/EVENING SHOWERS AND  
STORMS ARE ALSO EXPECTED TO PERSIST THROUGH THE WEEK ACROSS THE  
FOUR CORNERS REGION, WITH MONSOONAL MOISTURE REMAINING IN PLACE.  
 
AREAS FROM THE MID-MISSISSIPPI VALLEY TO THE MID-ATLANTIC WILL  
BEGIN THE MEDIUM RANGE PERIOD WITH HIGH TEMPERATURES 4 TO 8 DEG F  
BELOW AVERAGE, BUT AS THE UPPER RIDGE EXPANDS OVERHEAD, A GRADUAL  
WARMING TREND WILL FOLLOW AS HIGH TEMPERATURES 5 TO 10 DEG F ABOVE  
AVERAGE ARE FORECAST TO EXPAND EASTWARD FROM THE CENTRAL U.S.  
TOWARD THE EASTERN U.S. DURING THE WEEKEND. FINALLY, HEIGHT FALLS  
ACROSS THE NORTHWEST WILL LEAD TO A COOLING TREND WITH  
TEMPERATURES FALLING TO MORE THAN 10 DEG BELOW AVERAGE IN THE  
INTERIOR BY SUNDAY INTO MONDAY.  
 
RYAN  
 
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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