866  
FXUS02 KWBC 200658  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
258 AM EDT THU SEP 20 2018  
 
VALID 12Z SUN SEP 23 2018 - 12Z THU SEP 27 2018  
 
PATTERN OVERVIEW AND MODEL EVALUATION  
_____________________________________  
 
A QUASI-ZONAL PATTERN ACROSS THE NORTHERN TIER OF THE U.S. ON  
SUNDAY IS FORECAST TO TRANSITION INTO A MORE AMPLIFIED PATTERN BY  
TUESDAY AS A DEEP UPPER LEVEL TROUGH BECOMES ESTABLISHED OVER THE  
NORTH-CENTRAL STATES. UPPER LEVEL RIDGES ARE EXPECTED OVER BOTH  
THE WESTERN ATLANTIC AND THE EASTERN PACIFIC AND MAY LEAD TO A  
BLOCKED FLOW PATTERN DEVELOPING. SHORTWAVE IMPULSES ARE EXPECTED  
TO TRACK AROUND THE BASE OF THE SYNOPTIC SCALE TROUGH AND THESE  
WILL RESULT IN A SERIES OF SURFACE LOWS AND A RELATIVELY STRONG  
COLD FRONT CROSSING THE CENTRAL AND EASTERN U.S. NEXT WEEK. THERE  
IS ALSO A COLD AIR DAMMING SIGNATURE IN THE SURFACE PRESSURE  
PATTERN ACROSS THE MID-ATLANTIC AND APPALACHIANS EARLY IN THE WEEK.  
 
THERE IS GOOD OVERALL MODEL AGREEMENT ON THE SYNOPTIC SCALE  
THROUGH MONDAY REGARDING THE EVOLUTION OF THE UPPER TROUGH OVER  
THE ROCKIES AND THE NORTHERN/CENTRAL PLAINS. THE CMC IS NOTABLY  
SLOWER WITH THE PROGRESSION OF THE FRONT ACROSS THE PLAINS OWING  
TO ITS PLACEMENT OF THE DOWNSTREAM RIDGE OVER THE EASTERN U.S.,  
WHEREAS THE OTHER MODELS ARE FARTHER EAST WITH THE HIGH AND  
INDICATE A QUICKER PROGRESSION. COMPARED TO THE PAST COUPLE OF  
DAYS, THE ECMWF HAS TRENDED SLOWER AND CLOSER TO THE MODEL  
CONSENSUS REGARDING THE MAIN TROUGH AXIS OVER THE NORTH-CENTRAL  
U.S. EARLY NEXT WEEK. THERE HAS BEEN A QUICKER TREND WITH THE GFS  
WITH THE SHORTWAVE IMPULSE CROSSING THE GREAT LAKES DURING THE  
MIDDLE OF THE WEEK, AND THE 12Z ECMWF IS STRONGER WITH THE SURFACE  
LOW LIFTING NORTHWARD ACROSS ONTARIO. THE LATEST WPC FORECAST IS  
MORE PROGRESSIVE THAN THE PREVIOUS ONE, AND INCORPORATED MAINLY  
UKMET/GFS/ECMWF THROUGH TUESDAY, AND THEN MORE TOWARDS THE EC MEAN  
ALONG WITH SOME OF THE GEFS MEAN AND ECMWF.  
 
SENSIBLE WEATHER AND POTENTIAL HAZARDS  
______________________________________  
 
THE POTENTIAL FOR HEAVY RAIN AND STRONG THUNDERSTORMS EXISTS  
ACROSS PARTS OF THE MIDDLE MISSISSIPPI VALLEY THROUGH EARLY  
TUESDAY AS DEEP MOISTURE CONTINUES TO SURGE NORTH FROM THE GULF OF  
MEXICO, ALONG WITH REMNANT MOISTURE FROM FORMER TROPICAL  
DEPRESSION NINETEEN-E. SIGNIFICANT MODEL DIFFERENCES REMAIN WITH  
THE AXIS OF HEAVIEST QPF, WHICH MAY BE ON THE ORDER OF A FEW  
INCHES IN SOME AREAS. A SWATH OF ENHANCED RAINFALL MAY EXTEND  
EASTWARD TO THE MID-ATLANTIC REGION, ALBEIT LIGHTER IN INTENSITY.  
BEHIND THE COLD FRONT ACROSS MONTANA AND WYOMING, LIGHT TO  
MODERATE RAIN IS LIKELY AND SOME SNOW IN THE HIGHEST ELEVATIONS OF  
THE ROCKIES. TEMPERATURES ARE EXPECTED TO BE ABOVE NORMAL ACROSS  
MUCH OF THE CENTRAL AND SOUTHERN U.S. BEFORE THE COLD FRONT BRINGS  
A RETURN TO MORE AUTUMNAL CONDITIONS FOR MUCH OF THE NATION BY THE  
MIDDLE TO END OF NEXT WEEK. BELOW NORMAL READINGS ARE EXPECTED  
ACROSS THE NORTHERN TIER OF THE NATION WITH HIGHS IN THE 50S AND  
60S.  
 
D. HAMRICK  
 
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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