696  
FXUS02 KWBC 210545  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
145 AM EDT FRI SEP 21 2018  
 
VALID 12Z MON SEP 24 2018 - 12Z FRI SEP 28 2018  
   
..PATTERN OVERVIEW
 
 
UPPER-LEVEL RIDGING IS EXPECTED TO STRENGTHEN OFF THE WEST COAST  
OF NORTH AMERICA DURING THE MEDIUM RANGE, AS SUPPORTED BY A  
CONSENSUS OF MODEL/ENSEMBLE SOLUTIONS. DOWNSTREAM OF THE  
AMPLIFYING RIDGE, AN UPPER-LEVEL TROUGH WILL ALSO BECOME  
INCREASINGLY AMPLIFIED FROM THE NORTHERN PLAINS AND MIDWEST TO THE  
GREAT LAKES AS THE PASSAGE OF MULTIPLE SHORTWAVES PROGRESSIVELY  
LOWERS HEIGHTS. A LOW PRESSURE SYSTEM ACCOMPANYING THE FIRST  
SHORTWAVE WILL CROSS THE MIDWEST MON-TUE, WITH ANOTHER  
(REMINISCENT OF A WINTER-LIKE CLIPPER SYSTEM) QUICKLY FOLLOWING IN  
ITS WAKE BY WED-THU.  
 
   
..GUIDANCE EVALUATION/PREFERENCES
 
 
MODEL/ENSEMBLE CONSENSUS WAS ABOVE AVERAGE WITH RESPECT TO THE  
LARGE SCALE PATTERN EVOLUTION DESCRIBED ABOVE. THEREFORE, A  
MULTI-MODEL DETERMINISTIC BLEND (INCLUDING THE GFS/ECMWF/UKMET)  
WAS USED AS A BASIS FOR THE WPC FORECAST DURING DAYS 3-5  
(MON-WED). THE MOST SIGNIFICANT DIFFERENCE DURING THIS TIME PERIOD  
INVOLVED SOME SOLUTIONS (THE CMC/UKMET AS WELL AS A NUMBER OF  
ECMWF ENSEMBLE MEMBERS) HANGING ENERGY BACK ACROSS THE GREAT  
BASIN/ROCKIES FOR A LONGER PERIOD OF TIME RELATIVE TO THE OTHER  
GUIDANCE. WITH RESPECT TO THE NORTHERN STREAM, CONFIDENCE WAS  
RELATIVELY HIGH, HOWEVER. MODELS HAVE SHOWN SOME RUN-TO-RUN  
VARIABILITY WITH RESPECT TO THE TIMING AND AMPLITUDE OF THE SECOND  
SIGNIFICANT SHORTWAVE CROSSING THE NORTHERN TIER DAYS 5-6  
(WED-THU), BUT IN GENERAL THE TREND HAS BEEN TOWARD MORE  
AMPLIFICATION WITH THIS WAVE AND A SOMEWHAT STRONGER SURFACE LOW  
PRESSURE SYSTEM. GIVEN THE RUN-TO-RUN VARIABILITY WITH  
SMALLER-SCALE FEATURES BY LATER IN THE PERIOD, WEIGHTING OF  
ENSEMBLE MEANS (ECENS AND NAEFS) WAS GRADUALLY INCREASED FROM DAY  
5 ONWARD, WITH MEANS COMPRISING A MAJORITY OF THE BLEND BY DAYS  
6-7.  
 
   
..SENSIBLE WEATHER HIGHLIGHTS
 
 
A WEAKENING SURFACE WARM FRONT ALONG WITH ANOTHER APPROACHING COLD  
FRONT WILL SUPPORT AREAS OF POTENTIALLY HEAVY RAIN ACROSS MUCH OF  
THE MISSISSIPPI AND OHIO VALLEYS MON-WED, WITH SOME POTENTIAL FOR  
HEAVY RAINS ACROSS PORTIONS OF THE MID-ATLANTIC AS WELL. THE FRONT  
WILL LINGER FROM THE SOUTHERN PLAINS TO THE SOUTHEAST THROUGH LATE  
IN THE WEEK, WITH LOCALLY HEAVY CONVECTION POSSIBLE THROUGH  
FRIDAY.  
 
IN TERMS OF TEMPERATURES, THE MOST SIGNIFICANT STORY WILL BE THE  
ARRIVAL OF A SUBSTANTIALLY COOLER AIR MASS FROM THE NORTHERN  
ROCKIES TO THE MIDWEST AS THE UPPER TROUGH AMPLIFIES THROUGH THE  
WEEK. HIGH TEMPERATURES IN THE WAKE OF THE FIRST COLD FRONT  
MON-TUE WILL BE 5 TO 10 DEG F BELOW AVERAGE, WITH TEMPERATURES  
PERHAPS DROPPING TO 10-20 DEG BELOW AVERAGE BEHIND THE SECOND COLD  
FRONT LATER IN THE WEEK. MODELS SHOW A RANGE OF SOLUTIONS WITH  
RESPECT TO HOW STRONG THE CANADIAN SURFACE HIGH WILL BE BY LATE  
THIS WEEK, AND THUS HOW ANOMALOUSLY COLD THE AIR MASS WILL BE  
ACROSS THE NORTHERN ROCKIES/PLAINS. MEANWHILE, AREAS ACROSS THE  
SOUTHWEST, GREAT BASIN, AND WEST COAST WILL REMAIN 5 TO 10 DEG  
ABOVE AVERAGE WITH THE BUILDING UPPER RIDGE JUST OFFSHORE.  
 
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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