967  
FXUS02 KWBC 190549  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
148 AM EDT SUN AUG 19 2018  
 
VALID 12Z WED AUG 22 2018 - 12Z SUN AUG 26 2018  
   
..PATTERN OVERVIEW AND GUIDANCE EVALUATION/PREFERENCES
 
 
MODELS/ENSEMBLES 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 THE MIDDLE OF THE  
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.  
 
IN GENERAL, A BLEND OF THE ECMWF/GFS AND THEIR RESPECTIVE ENSEMBLE  
MEANS SERVED AS A GOOD STARTING POINT FOR THE WPC FORECAST.  
FORECAST CONFIDENCE WAS INITIALLY SUFFICIENT TO USE A MAJORITY OF  
DETERMINISTIC SOLUTIONS DURING DAYS 3-4 (WED-THU), WITH A GRADUAL  
SHIFT TOWARD HEAVIER ENSEMBLE MEAN WEIGHTING DURING DAYS 5-7  
(FRI-SUN). THESE SOLUTIONS SHOW TYPICAL TIMING/AMPLITUDE  
DIFFERENCES THE UPPER TROUGH LIFTING OUT OF THE EASTERN U.S.  
WED-THU, AS WELL AS WITH A COUPLE ADDITIONAL AREAS OF SHORTWAVE  
ENERGY TRAVERSING THE NORTHERN TIER, FROM THE PACIFIC NORTHWEST TO  
THE NORTHERN PLAINS, DURING THE FORECAST PERIOD (EACH GRADUALLY  
LOWERING HEIGHTS ACROSS THE NORTHWEST).  
 
   
..WEATHER HIGHLIGHTS AND HAZARDS
 
 
A COLD FRONT THAT IS EVENTUALLY EXPECTED TO BECOME STATIONARY BY  
THE WEEKEND NEAR THE SOUTHEAST U.S. COASTLINE WILL BRING SCATTERED  
SHOWERS AND STORMS WITH LOCALLY HEAVY RAINFALL THROUGH MUCH OF THE  
NEXT WEEK. AREAS OF SHOWERS AND THUNDERSTORMS WITH LOCALLY HEAVY  
RAIN WILL ALSO BE POSSIBLE THU-SUN ACROSS THE NORTHERN PLAINS AND  
UPPER MIDWEST IN ASSOCIATION WITH THE PASSAGE OF A COUPLE  
UPPER-LEVEL SHORTWAVES AND A SURFACE FRONTAL BOUNDARY. ACROSS THE  
FOUR CORNERS STATES, MONSOONAL MOISTURE WILL PERSIST WITH  
SCATTERED SHOWERS AND THUNDERSTORMS A DAILY OCCURRENCE.  
 
A RELATIVELY PLEASANT AIR MASS WITH HIGH TEMPERATURES 5 TO 8 DEG F  
BELOW AVERAGE WILL INITIALLY BE IN PLACE FROM THE MID-MISSISSIPPI  
VALLEY TO THE MID-ATLANTIC AND NORTHEAST IN THE WAKE OF THE COLD  
FRONT STALLING ALONG THE SOUTHEAST U.S. COAST. AS UPPER HEIGHT  
GRADUALLY RISE BY NEXT WEEKEND, TEMPERATURES ACROSS THESE AREAS  
WILL RISE, AND HIGH TEMPERATURES OF 5 TO 10 DEG F ABOVE AVERAGE  
ARE POSSIBLE BY LATE IN THE WEEK FROM THE PLAINS TO THE MIDWEST  
AND GREAT LAKES.  
 
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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