359  
FXUS02 KWBC 200522  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
121 AM EDT MON AUG 20 2018  
 
VALID 12Z THU AUG 23 2018 - 12Z MON AUG 27 2018  
   
..PATTERN OVERVIEW AND GUIDANCE EVALUATION/PREFERENCES
 
 
MODELS/ENSEMBLES CONTINUE TO SHOW GOOD CONSENSUS THAT A PATTERN  
CHANGE WILL OCCUR DURING THE MEDIUM RANGE, WITH A SHIFT AWAY FROM  
THE WESTERN RIDGE/EASTERN TROUGH FLOW PATTERN THAT HAS DOMINATED  
FOR SEVERAL WEEKS. AN UPPER-LEVEL RIDGE IS EXPECTED TO AMPLIFY  
ACROSS THE NORTH PACIFIC, BETWEEN ALASKA AND HAWAII, BY MID-WEEK.  
THE DOWNSTREAM EFFECTS OF THIS WILL RESULT IN LOWERING HEIGHTS  
FROM WESTERN CANADA INTO THE PACIFIC NORTHWEST, AND GRADUALLY  
RISING HEIGHTS FROM THE MIDWEST/OHIO VALLEY TO THE EASTERN  
SEABOARD AS BROAD UPPER RIDGING EXPANDS EASTWARD.  
 
AT THE START OF DAY 3 (THU), AN UPPER TROUGH WILL BE IN THE  
PROCESS OF LIFTING OUT OF THE EASTERN U.S., WITH SHORTWAVE ENERGY  
INITIALLY ACROSS THE INTERIOR NORTHWEST MOVING EAST ACROSS THE  
NORTHERN TIER. CONSENSUS SURROUNDING BOTH OF THESE FEATURES IS  
HIGH ENOUGH TO JUSTIFY USE OF A MULTI-MODEL DETERMINISTIC BLEND  
(INCLUDING THE ECMWF/GFS/UKMET/CMC) AS A STARTING POINT FOR THE  
FORECAST DURING DAYS 3-5. BY DAYS 6-7 (SUN-MON), SPREAD AMONG THE  
GUIDANCE BEGINS TO INCREASE RATHER QUICKLY. ADDITIONAL SHORTWAVE  
ENERGY DIVING SOUTHWARD ACROSS BRITISH COLUMBIA SHOULD BEGIN TO  
LOWER HEIGHTS SIGNIFICANTLY ACROSS THE PACIFIC NORTHWEST, BUT  
MODELS SHOW QUITE A BIT OF VARIANCE AS TO EXACTLY HOW MUCH  
AMPLIFICATION OCCURS. THE ECMWF FROM 12Z SUN WAS THE MOST  
AMPLIFIED DETERMINISTIC SOLUTION ALONG THE WEST COAST BY NEXT MON,  
WHILE THE 12Z CMC WAS THE LEAST AMPLIFIED, WITH THE GFS ALONG WITH  
THE ENSEMBLE MEANS IN THE MIDDLE. GIVEN THE DEGREE OF FLOW  
AMPLIFICATION EXPECTED TO OCCUR ACROSS THE NORTH PACIFIC, A MORE  
AMPLIFIED TROUGH ALONG THE WEST COAST, SUCH AS SHOWN BY THE ECMWF,  
IS CERTAINLY POSSIBLE. INCREASED TROUGH AMPLIFICATION ALONG THE  
WEST COAST WOULD ALSO LIKELY TRANSLATE DOWNSTREAM TO STRONGER  
RIDGING ACROSS THE EAST-CENTRAL STATES, AND THIS BEARS OUT AMONG  
THE DETERMINISTIC GUIDANCE WITH THE ECMWF SHOWING THE STRONGEST  
UPPER RIDGE BY DAY 7 CENTERED OVER THE LOWER MISSISSIPPI VALLEY.  
GIVEN THE SPREAD AS WELL AS THE TIME FRAME UNDER CONSIDERATION  
(DAYS 6-7) WAS NOT CONFIDENT ENOUGH TO BITE OFF ON ANY ONE  
SOLUTION. THEREFORE, WENT WITH A MAJORITY OF ENSEMBLE MEANS  
(ECENS/NAEFS) IN THE FORECAST BLEND FOR DAYS 6-7, WITH CONTINUED  
MINORITY COMPONENTS OF THE DETERMINISTIC ECMWF AND GFS.  
 
   
..WEATHER HIGHLIGHTS AND HAZARDS
 
 
A LINGERING STATIONARY FRONTAL BOUNDARY ALONG THE SOUTHEAST U.S.  
COAST THROUGH MOST OF THE MEDIUM RANGE 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 TO THE MEDIUM RANGE WITH HIGH TEMPERATURES 5 TO 10 DEG F  
BELOW AVERAGE BEFORE THE UPPER TROUGH LIFTS OUT. TEMPERATURES  
ACROSS THESE AREAS WILL GRADUALLY MODERATE BY FRI-SAT, HOWEVER.  
EXPECT HIGHS 5 TO 10 DEG F ABOVE AVERAGE ACROSS MUCH OF THE  
CENTRAL U.S. BY THE WEEKEND AS THE UPPER RIDGE EXPANDS OVERHEAD.  
FINALLY, AS HEIGHTS FALL ACROSS THE NORTHWEST BY LATE WEEK INTO  
THE WEEKEND, EXPECT TEMPERATURES TO FALL TO BELOW AVERAGE LEVELS,  
WITH HIGHS BY SUN-MON 5 TO 10 DEG BELOW AVERAGE.  
 
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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