278  
FXUS02 KWBC 230543  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
143 AM EDT THU AUG 23 2018  
 
VALID 12Z SUN AUG 26 2018 - 12Z THU AUG 30 2018  
   
..PATTERN OVERVIEW AND GUIDANCE EVALUATION/PREFERENCES
 
 
CONFIDENCE IS HIGH THAT A STRONG UPPER-LEVEL RIDGE WILL BE IN  
PLACE SOUTH OF ALASKA AT THE START OF THE MEDIUM RANGE. DOWNSTREAM  
EFFECTS OF THIS RIDGE WILL BE A MEAN TROUGH ACROSS THE  
NORTHWESTERN U.S., AND EXPANSIVE UPPER RIDGING FROM THE SOUTHERN  
PLAINS TO THE MID-ATLANTIC. AN ACTIVE FLOW REGIME WILL PERSIST  
ACROSS THE NORTHERN TIER AS A NUMBER OF SHORTWAVES EJECT FROM THE  
WESTERN MEAN TROUGH CROSS THE NORTHERN PLAINS/UPPER MIDWEST TO THE  
GREAT LAKES AND NEW ENGLAND, GENERATING MULTIPLE WAVES ALONG A  
SURFACE FRONTAL BOUNDARY.  
 
AS IN RECENT DAYS, SOLUTIONS WITH A MORE AMPLIFIED TROUGH ACROSS  
THE WESTERN STATES WERE PREFERRED GIVEN THE HIGHLY AMPLIFIED FLOW  
EXPECTED TO EVOLVE ACROSS THE NORTH PACIFIC. INITIALLY, THESE MORE  
AMPLIFIED SOLUTIONS WERE GFS/CMC, WITH THE ECMWF JOINING THE  
CONSENSUS LATER IN THE PERIOD. AS SMALLER SCALE ENERGY EJECTS  
EASTWARD IN THE NORTHERN STREAM FLOW THROUGHOUT THE PERIOD, MODELS  
SHOW EXPECTED TIMING/AMPLITUDE DIFFERENCES. THE MOST SIGNIFICANT  
PIECE OF ENERGY TO MOVE EASTWARD ACROSS THE NORTHERN PLAINS LOOKS  
TO OCCUR TUE-WED, GENERATING A WAVE ALONG THE SURFACE FRONTAL  
BOUNDARY FROM THE NORTHERN/CENTRAL PLAINS TO THE UPPER MIDWEST.  
GIVEN THESE CONSIDERATIONS, THE FORECAST FOR DAYS 3-5 WAS BASED ON  
A MULTI-MODEL DETERMINISTIC BLEND, WITH MORE WEIGHT INITIALLY  
PLACED TOWARD THE GFS/CMC. BY MID TO LATE NEXT WEEK, ADDITIONAL  
SHORTWAVE ENERGY LOOKS TO ENTER THE WESTERN MEAN TROUGH WITH  
AMPLIFICATION LIKELY TO OCCUR. MODELS BEGIN TO DIVERGE BY THIS  
TIME PERIOD AS TO THE TIMING AND EXACT CHARACTER OF THIS FEATURE,  
WITH SOME SOLUTIONS SHOWING A DEEP CLOSED UPPER LOW BEFORE THE  
SYSTEM REACHES THE NORTHWEST, AND OTHERS MORE QUICKLY MOVING THE  
ENERGY INTO THE MEAN TROUGH WITH AMPLIFICATION ACROSS THE GREAT  
BASIN. GIVEN THE INCREASE IN SPREAD, A TREND TOWARD ENSEMBLE MEANS  
(ECENS AND NAEFS) WAS PREFERRED FOR DAYS 6-7.  
 
   
..WEATHER HIGHLIGHTS AND HAZARDS
 
 
THE MOST ACTIVE WEATHER IN TERMS OF PRECIPITATION WILL BE ACROSS  
THE NORTHERN TIER DURING THE MEDIUM RANGE GIVEN THE  
TROUGH/SHORTWAVE ENERGY AND FRONTAL BOUNDARY DESCRIBED ABOVE.  
MODELS SHOW THE POTENTIAL FOR MULTIPLE AREAS OF POTENTIALLY HEAVY  
CONVECTION ACROSS MULTIPLE DAYS FROM THE NORTHERN ROCKIES TO THE  
GREAT LAKES. BY NEXT TUE-WED, SOME CONVECTION MAY SPREAD AS FAR  
EAST AS THE NORTHEAST. THE CONSENSUS OF MODEL/ENSEMBLE SOLUTIONS  
SUGGEST THAT THE GREATEST POTENTIAL FOR HEAVY RAINFALL WILL EXIST  
ACROSS PORTIONS OF THE UPPER MIDWEST AND GREAT LAKES, AS THE  
FRONTAL BOUNDARY WILL BE IN THE VICINITY THROUGH MOST OF THE  
FORECAST PERIOD. FARTHER SOUTH, SCATTERED AFTERNOON/EVENING  
SHOWERS AND THUNDERSTORMS WILL PERSIST ACROSS THE SOUTHEAST AS A  
STATIONARY SURFACE FRONTAL BOUNDARY SLOWLY DECAYS. MONSOONAL  
CONVECTION IN THE FOUR CORNERS REGION SHOULD GRADUALLY REDUCE IN  
COVERAGE/INTENSITY THROUGH THE MEDIUM RANGE AS THE LOW/MID-LEVEL  
FLOW BECOMES LESS FAVORABLE FOR TRANSPORT OF DEEP MOISTURE INTO  
THE REGION.  
 
EXPECT BELOW AVERAGE TEMPERATURES ACROSS MUCH OF THE NORTHWESTERN  
U.S. GIVEN THE PERSISTENT UPPER TROUGH OVERHEAD. HIGH TEMPERATURES  
OF 5 TO 15 DEG F BELOW AVERAGE ARE EXPECTED INTO EARLY-MID NEXT  
WEEK. FARTHER EAST, THE EXPANDING UPPER RIDGE WILL SUPPORT  
INCREASING TEMPERATURES, WITH HIGHS 5 TO 10 DEG F ABOVE AVERAGE  
ACROSS MUCH OF THE CENTRAL AND EASTERN U.S. THROUGH THE PERIOD.  
 
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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