063  
FXUS02 KWBC 250657  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
156 AM EST SUN NOV 25 2018  
 
VALID 12Z WED NOV 28 2018 - 12Z SUN DEC 2 2018  
   
..PATTERN OVERVIEW
 
 
THE MEDIUM RANGE PERIOD BEGINS ON WEDNESDAY WITH A LARGE CLOSED  
UPPER LEVEL LOW CENTERED OVER NEW ENGLAND WITH MULTIPLE EMBEDDED  
SHORTWAVES PIVOTING AROUND IT, AND QUASI-ZONAL FLOW ACROSS THE  
WESTERN U.S. TO THE CENTRAL PLAINS. THE LARGE CLOSED LOW SLOWLY  
EXITS THE NORTHEAST U.S. ON THURSDAY, AND A LARGE SCALE TROUGH IS  
EXPECTED TO MOVE INLAND ACROSS THE WESTERN U.S. BY THE END OF THE  
WEEK. ONCE THE TROUGH AXIS REACHES THE PLAINS BY THE WEEKEND, A  
DEVELOPING LOW PRESSURE SYSTEM WILL LIKELY BRING WIDESPREAD  
PRECIPITATION FROM THE GULF COAST TO THE UPPER MIDWEST. UPPER  
LEVEL RIDGING IS EXPECTED TO DEVELOP AHEAD OF THIS DEVELOPING  
STORM SYSTEM FOR NEXT WEEKEND.  
 
   
..MODEL GUIDANCE AND UNCERTAINTY ASSESSMENT
 
 
DETERMINISTIC MODELS AND THE ENSEMBLE MEANS ARE IN RELATIVELY  
DECENT SYNOPTIC SCALE AGREEMENT ON WEDNESDAY ACROSS THE  
CONTINENTAL U.S. WITH THE QUASI-ZONAL FLOW PATTERN ACROSS MUCH OF  
THE CONTINENTAL U.S. ON THURSDAY, THE DETERMINISTIC MODELS ARE  
INDICATING SOME DIFFERENCES WITH REGARD TO A SUBTLE SHORTWAVE  
FEATURE CROSSING THE PLAINS, WITH THE 00Z UKMET SHOWING THE  
GREATEST AMPLITUDE WITH THAT. THE MODELS REMAIN IN DECENT  
AGREEMENT REGARDING A PACIFIC STORM SYSTEM REACHING CALIFORNIA  
LATE ON THURSDAY, WITH THE 00Z GFS SLIGHTLY FASTER.  
 
BY THE END OF THE MEDIUM RANGE PERIOD NEXT WEEKEND, SPEED AND  
AMPLITUDE DIFFERENCES ARE APPARENT AS THE UPPER TROUGH FROM THE  
WESTERN U.S. REACHES THE CENTRAL PLAINS, WITH THE ECMWF AND THE EC  
MEAN BEING SLOWER AND MORE AMPLIFIED, AND THE GFS AND CMC MORE  
PROGRESSIVE WITH ITS TROUGH AXIS. THERE IS GOOD MODEL SUPPORT FOR  
A DOWNSTREAM RIDGE EAST OF THE MISSISSIPPI RIVER, WITH THE ECMWF  
SHOWING A RATHER PRONOUNCED RIDGE AXIS COMPARED TO THE BROADER GFS  
RIDGE. DIFFERENCES ARE EVEN MORE APPARENT ACROSS SOUTHERN CANADA  
IN THE NORTHERN STREAM FLOW. TAKING THESE CONSIDERATIONS INTO  
ACCOUNT, THE WPC FORECAST WAS BASED PRIMARILY ON THE  
ECMWF/GFS/UKMET THROUGH FRIDAY, AND THEN PRIMARILY NAEFS MEAN/EC  
MEAN WITH SOME DETERMINISTIC GFS/ECMWF FOR THE WEEKEND.  
 
   
.WEATHER HIGHLIGHTS/THREATS
 
 
THE DEPARTING STORM SYSTEM OVER THE NORTHEASTERN U.S. EARLY IN THE  
FORECAST PERIOD IS EXPECTED TO PRODUCE LAKE EFFECT SNOW ON  
WEDNESDAY, WITH HEAVY SNOW POSSIBLE DOWNWIND OF LAKE ERIE AND LAKE  
ONTARIO, AND LIGHTER SNOW FOR NORTHERN MICHIGAN. THE HEAVIEST  
PRECIPITATION DURING THIS FORECAST PERIOD IS EXPECTED FROM CENTRAL  
CALIFORNIA TO WESTERN WASHINGTON AS TWO PACIFIC STORM SYSTEMS ARE  
EXPECTED TO MOVE ONSHORE AND BRING ENHANCED MOISTURE INTO THESE  
AREAS, WITH SEVERAL INCHES OF RAIN POSSIBLE FOR NORTHWESTERN  
CALIFORNIA AND SOUTHWESTERN OREGON. HEAVY SNOW IS LIKELY FOR THE  
HIGHER MOUNTAINS FROM THE SIERRA NEVADA TO THE CENTRAL AND  
NORTHERN ROCKIES, WITH 1 TO 2 FEET OF SNOW POSSIBLE FOR THE  
HIGHEST ELEVATIONS THROUGH FRIDAY NIGHT. ANOTHER AREA OF HEAVY  
RAINFALL WILL LIKELY DEVELOP FROM THE GULF COAST TO THE OHIO  
VALLEY AS THE CENTRAL PLAINS SURFACE LOW ADVECTS COPIOUS MOISTURE  
NORTHWARD NEXT WEEKEND.  
 
TEMPERATURES ARE EXPECTED TO BE ON THE ORDER OF 10 TO 20 DEGREES  
BELOW AVERAGE FROM THE MIDWEST TO THE APPALACHIANS FOR WEDNESDAY  
AND THURSDAY BEFORE A MODERATION TREND ENSUES BY THE END OF THE  
WEEK. AREAS WITH FRESH SNOW COVER ACROSS THE MIDWEST IN THE WAKE  
OF THE UPCOMING WINTER STORM WILL LIKELY HAVE MUCH COLDER  
OVERNIGHT LOWS EARLY IN THE FORECAST PERIOD, WITH A FEW SUBZERO  
READINGS POSSIBLE WHERE WINDS SUBSIDE AND SKIES CLEAR OUT.  
TEMPERATURES SHOULD GENERALLY BE WITHIN 5 DEGREES OF  
CLIMATOLOGICAL AVERAGES FOR THE WESTERN THIRD OF THE NATION.  
 
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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