229  
FXUS02 KWBC 201600  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1159 AM EDT MON AUG 20 2018  
 
VALID 12Z THU AUG 23 2018 - 12Z MON AUG 27 2018  
   
..PATTERN OVERVIEW AND GUIDANCE EVALUATION/PREFERENCES
 
 
MODELS AND ENSEMBLES THIS MORNING CONTINUE TO SHOW VERY GOOD  
AGREEMENT ON THE SYNOPTIC PATTERN EVOLUTION ACROSS THE U.S.  
THROUGH NEXT MONDAY. AN UPPER-LEVEL TROUGH IS EXPECTED TO LIFT  
OUT OF THE EASTERN U.S. LATE THIS WEEK AS AN UPPER-LEVEL TROUGH  
APPEARS TO BECOME MORE AMPLIFIED OVER THE PACIFIC NORTHWEST DURING  
THE WEEKEND INTO MONDAY. DISCREPANCIES SHOWN IN THE MODEL  
SOLUTIONS ARE RELATIVELY SMALL EVEN OUT TO DAY 7 NEXT MONDAY.  
THEREFORE, A GENERAL MODEL COMPROMISE WAS ADOPTED TO GENERATE THE  
WPC MEDIUM-RANGE GRID FIELDS. FOR THE SEA-LEVEL PRESSURE FIELD, A  
50-50 BLEND OF THE 00Z ECMWF WITH THE 06Z GFS WAS USED FOR DAY 3,  
TRENDING TOWARD A 30%-70% BLEND OF THEIR ENSEMBLE MEANS TO THEIR  
DETERMINISTIC SOLUTIONS FROM DAYS 5 THROUGH 7. THIS HAS KEPT GOOD  
CONTINUITY WITH THE PREVIOUS WPC MEDIUM-RANGE PACKAGE.  
 
   
..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 FROM THE NORTHERN/CENTRAL PLAINS TO THE  
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 5 TO 10 DEG F  
BELOW AVERAGE BEFORE THE UPPER TROUGH LIFTS OUT. 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 THE CENTRAL U.S. TOWARD THE EASTERN U.S.  
DURING THE WEEKEND. FINALLY, HEIGHT FALLS ACROSS THE NORTHWEST  
WILL LEAD TO A COOL TREND WITH TEMPERATURES FALLING TO MORE THAN  
10 DEG BELOW AVERAGE IN THE INTERIOR BY SUNDAY INTO MONDAY.  
 
KONG/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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