633  
FXUS02 KWBC 240655  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
154 AM EST SAT NOV 24 2018  
 
VALID 12Z TUE NOV 27 2018 - 12Z SAT DEC 1 2018  
 
SYNOPTIC SCALE OVERVIEW  
-----------------------  
THE MEDIUM RANGE PERIOD BEGINS ON TUESDAY WITH A LARGE SCALE UPPER  
LEVEL GYRE CENTERED OVER THE GREAT LAKES REGION WITH MULTIPLE  
EMBEDDED SHORTWAVES PIVOTING AROUND IT, AND BROAD TROUGHING OVER  
THE EASTERN PACIFIC WITH A DOWNSTREAM RIDGE AXIS OVER THE  
INTERMOUNTAIN WEST. THE LARGE CLOSED LOW SLOWLY EXITS THE  
NORTHEAST U.S. ON THURSDAY WITH QUASI-ZONAL FLOW LIKELY BECOMING  
ESTABLISHED OVER MUCH OF THE CENTRAL U.S. AND A LARGE SCALE TROUGH  
IS EXPECTED TO MOVE INLAND ACROSS THE WESTERN U.S. ONCE THE  
TROUGH AXIS REACHES THE PLAINS, A DOWNSTREAM RIDGE OVER THE  
EASTERN U.S. MAY TRY TO BUILD BY NEXT WEEKEND.  
 
MODEL GUIDANCE AND UNCERTAINTY ASSESSMENT  
-----------------------------------------  
DETERMINISTIC MODELS AND THE ENSEMBLE MEANS ARE IN RELATIVELY  
DECENT SYNOPTIC SCALE AGREEMENT ON TUESDAY ACROSS THE CONTINENTAL  
U.S. HOWEVER, NOTEWORTHY DIFFERENCES IN THE DETERMINISTIC  
GUIDANCE BECOME APPARENT AS EARLY AS WEDNESDAY, WITH THE 12Z CMC  
INDICATING A MUCH STRONGER UPPER TROUGH CROSSING THE WESTERN U.S.  
WHERE THE OTHER MODELS ARE INDICATING QUASI-ZONAL FLOW, AND THEN  
IS OUT OF PHASE ACROSS THE CENTRAL U.S. BY FRIDAY WHERE THE  
REMAINING MODEL CONSENSUS IS INDICATIVE OF WEAK UPPER LEVEL  
RIDGING. THE 12Z ECMWF BECOMES MORE AMPLIFIED WITH A SHORTWAVE  
PIVOTING AROUND THE NORTHEAST U.S. UPPER LEVEL GYRE, AND AFFECTS  
THE OHIO VALLEY TO THE NORTHEAST U.S. WITH A STRONGER SURFACE LOW  
ON THURSDAY MORNING NEAR MAINE.  
 
BY THE END OF THE MEDIUM RANGE PERIOD FRIDAY AND INTO SATURDAY,  
SPEED AND AMPLITUDE DIFFERENCES ARE APPARENT AS THE UPPER TROUGH  
FROM THE WESTERN U.S. REACHES THE CENTRAL PLAINS, WITH THE 12Z  
ECMWF BEING SLOWER AND MORE AMPLIFIED, AND THE GFS MORE  
PROGRESSIVE BY A FEW HUNDRED MILES WITH ITS TROUGH AXIS. THE 18Z  
GEFS MEAN APPEARS TO OFFER A MORE REALISTIC DEPICTION OF THE  
TROUGH, WHEREAS THE EC MEAN IS CONSIDERABLY BROADER. 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 LEADING TO BELOW AVERAGE  
CONFIDENCE IN THE FORECAST BY THE END OF THE FORECAST PERIOD.  
 
TAKING THESE CONSIDERATIONS INTO ACCOUNT, THE WPC FORECAST WAS  
BASED PRIMARILY ON THE ECMWF/GFS/UKMET THROUGH THURSDAY MORNING,  
AND THEN PRIMARILY GEFS MEAN/EC MEAN WITH SOME DETERMINISTIC  
GFS/ECMWF FOR FRIDAY AND INTO SATURDAY.  
 
SENSIBLE WEATHER  
----------------  
THE DEPARTING STORM SYSTEM OVER THE NORTHEASTERN U.S. EARLY IN THE  
FORECAST PERIOD IS EXPECTED TO PRODUCE LAKE EFFECT SNOW THROUGH  
WEDNESDAY EVENING, WITH HEAVY SNOW POSSIBLE DOWNWIND OF LAKE ERIE  
AND LAKE ONTARIO, AND LIGHTER SNOW FOR NORTHERN MICHIGAN. THE  
HEAVIEST PRECIPITATION 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 AN  
ATMOSPHERIC RIVER EVENT APPEARING LIKELY ON TUESDAY FOR NORTHERN  
CALIFORNIA. 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 THROUGH FRIDAY NIGHT.  
 
TEMPERATURES ARE EXPECTED TO BE ON THE ORDER OF 10 TO 20 DEGREES  
BELOW AVERAGE FROM THE NORTHERN PLAINS TO THE OHIO VALLEY ON  
TUESDAY AND WEDNESDAY BEFORE A MODERATION TREND ENSUES BY LATER IN  
THE WEEK. AREAS WITH FRESH SNOW COVER ACROSS THE MIDWEST IN THE  
WAKE OF THE WINTER STORM MAY HAVE OVERNIGHT LOWS DROPPING WELL  
INTO THE SINGLE DIGITS FOR THE EARLY TO MIDDLE PART OF THE  
UPCOMING WORK WEEK, WITH SOME READINGS POSSIBLY FALLING BELOW ZERO  
WHERE WINDS ARE LIGHT 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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