430  
FXUS02 KWBC 251553  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1053 AM EST SUN NOV 25 2018  
 
VALID 12Z WED NOV 28 2018 - 12Z SUN DEC 02 2018  
   
..PATTERN OVERVIEW
 
 
THE WPC MEDIUM RANGE PERIOD BEGINS ON WEDNESDAY WITH A LARGE  
CLOSED UPPER LEVEL VORTEX 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. AS THE TROUGH REACHES THE PLAINS NEXT WEEKEND,  
LOW PRESSURE SYSTEM GENESIS AND DEEPENING WILL LIKELY BRING DEEP  
GULF MOISTURE INLAND TO FUEL WIDESPREAD PRECIPITATION OVER THE  
EAST-CENTRAL U.S. UPPER LEVEL RIDGING IS EXPECTED TO DEVELOP AHEAD  
OF THIS DEVELOPING STORM SYSTEM 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  
ECMWF ENSEMBLE MEAN BEING SLOWER AND MORE AMPLIFIED, AND THE GFS  
AND CMC MORE PROGRESSIVE WITH THE TROUGH. 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. GIVEN UNCERTAINTY,  
THE WPC FORECAST WAS PRIMARILY BASED ON A BLEND OF THE 06 UTC  
GEFS/00 UTC ECMWF ENSEMBLE MEANS. HOWEVER, ALSO ADDED THE 00 UTC  
ECMWF INTO THE COMPOSITE BLEND TO INFUSE A BIT MORE DETAIL/SYSTEM  
DEPTH. THE ECMWF SEEMS THE MOST CONSISTENT DETERMINISTIC MODEL  
WITH THE ENSEMBLES AND RUN TO RUN.  
 
   
.WEATHER HIGHLIGHTS/THREATS
 
 
A DEPARTING STORM SYSTEM OVER THE NORTHEASTERN U.S. IS EXPECTED TO  
PRODUCE LAKE EFFECT SNOWS INTO THURSDAY. THE HEAVIEST  
PRECIPITATION DURING THIS FORECAST PERIOD IS EXPECTED FROM WESTERN  
WASHINGTON TO CENTRAL CA AS TWO PACIFIC STORM SYSTEMS MOVE ONSHORE  
AND BRING ENHANCED MOISTURE, WITH MULTIPLE INCHES OF RAIN POSSIBLE  
FOR NORTHWEST CALIFORNIA AND SOUTHWEST OREGON. HEAVY SNOW IS  
LIKELY FOR THE HIGHER MOUNTAINS FROM THE SIERRA NEVADA TO THE  
CENTRAL AND NORTHERN ROCKIES, WITH SEVERAL FEET OF SNOW POSSIBLE.  
ANOTHER AREA OF HEAVY PRECIPITATION WILL LIKELY DEVELOP FROM THE  
GULF COAST TO THE MIDWEST A A CENTRAL PLAINS SURFACE LOW  
DEEPENS/ORGANIZES AND LEAD FLOW ADVECTS COPIOUS MOISTURE NORTHWARD  
NEXT WEEKEND. THIS SYSTEM ALSO PRESENTS ANOTHER SIGNIFICANT HEAVY  
SNOW/ICE RISK ON THE COOLED NORTHERN PERIPHERY OF THE ENLARGING  
PRECIPITION SHIELD AND SOME THREAT FOR WARM SECTOR SEVERE WEATHER  
THAT IS BEING FOLLOWED BY SPC.  
 
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 5-10 DEGREES OF  
CLIMATOLOGICAL AVERAGES FOR THE WESTERN THIRD OF THE NATION.  
 
D. HAMRICK/SCHICHTEL  
 
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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