447  
FXUS02 KWBC 202311  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
710 PM EDT THU SEP 20 2018  
 
VALID 12Z SUN SEP 23 2018 - 12Z THU SEP 27 2018  
   
..PATTERN OVERVIEW
 
 
A SIGNIFICANT TROUGH IS FORECAST TO DIG INTO THE CENTRAL STATES  
EARLY NEXT WEEK AS UPPER RIDGING LUMBERS TOWARD A POSITION JUST  
OFF THE PACIFIC NORTHWEST COAST. THIS WILL TAKE A LEAD SYSTEM  
NORTHEASTWARD OUT OF THE HIGH PLAINS MONDAY AND INTO EASTERN  
CANADA AS ITS ATTENDANT COLD FRONT MOVES SOUTHWARD AND EASTWARD,  
BRINGING IN COLDER AIR IN ITS WAKE AND A SWATH OF RAIN ALONG AND  
AHEAD OF THE BOUNDARY.  
   
..GUIDANCE EVALUATION/PREFERENCES
 
 
MODEL/ENSEMBLE AGREEMENT WAS BETTER THAN AVERAGE AS THE PATTERN  
TRANSITIONS TOWARD ROBUST AMPLIFICATION. THE 00Z CANADIAN AND ITS  
ENSEMBLES DEPARTED FROM THE GFS/ECMWF ENSEMBLE CLUSTERING IN THE  
WEST AS THE INITIAL TROUGH MOVES OUT OF THE WEST LATE THIS WEEKEND  
BEING SLOWER/DEEPER WITH THE NORTHERN STREAM PORTION OF THE TROUGH  
RATHER THAN ALLOWING IT SLIDE EASTWARD (AS THE TREND HAS BEEN  
QUICKER). THE 00Z/06Z GFS WERE JUST A BIT QUICKER WITH THE TROUGH  
PROGRESSION THAN THE ECMWF/UKMET AND ECMWF/GEFS MEANS BUT A BROAD  
BLEND OFFERED A REASONABLE STARTING POINT. THE 00Z PARALLEL GFS  
WAS USED BY THE END OF THE PERIOD WITH THE ECMWF AS THE  
DETERMINISTIC PARTS TO ADD SOME DETAIL TO THE ENSEMBLE  
COUNTERPART.  
   
..SENSIBLE WEATHER HIGHLIGHTS
 
 
THE POTENTIAL FOR LOCALLY HEAVY RAIN AND CONVECTION EXISTS ACROSS  
PARTS OF THE MIDDLE MISSISSIPPI VALLEY EARLY NEXT WEEK AS DEEP  
MOISTURE SURGES NORTH FROM THE GULF OF MEXICO ATOP A LINGERING  
STATIONARY SURFACE FRONT. AS THE FRONT MOVES EASTWARD, RAINFALL  
MAY EXPAND ACROSS THE OHIO VALLEY INTO THE NORTHEAST/MID-ATLANTIC  
VIA A SURGE OF MOISTURE FROM THE EASTERN GULF AND ALSO OFF THE  
ATLANTIC AS A WEAK WAVE MOVES TOWARD THE SOUTHEAST COAST.  
 
TEMPERATURES WILL TREND COOLER THAN AVERAGE FROM THE  
NORTHWEST/HIGH PLAINS SUN/MON TO THE PLAINS/MS VALLEY TUE-THU.  
AHEAD OF THE FRONT, TEMPERATURES WILL REMAIN ABOVE AVERAGE,  
ESPECIALLY MIN TEMPERATURES (ABOUT +5 TO +15F) THANKS TO A HUMID  
AIRMASS, IN THE SOUTHEAST TO THE NORTHEAST.  
 
FRACASSO  
 
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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