514  
FXUS02 KWBC 281601  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1100 AM EST WED NOV 28 2018  
 
VALID 12Z SAT DEC 01 2018 - 12Z WED DEC 05 2018  
   
..OVERVIEW
 
 
THE FORECAST CONTINUES TO PRESENT SOME SIGNIFICANT UNCERTAINTIES  
WITHIN A MULTI-DAY MEAN PATTERN EVOLUTION THAT SHOWS BETTER  
CONSENSUS. THE MORE AGREEABLE LARGER SCALE EVOLUTION DEPICTS  
BROAD MEAN TROUGHING ALOFT THAT SHOULD SUPPORT AN EXPANDING AREA  
OF BELOW NORMAL TEMPERATURES OVER THE LOWER 48, WITH EASTERN  
PACIFIC/NORTHWESTERN NORTH AMERICA MEAN RIDGING EVENTUALLY LEADING  
TO MORE CANADIAN FLOW REACHING THE NORTHERN TIER STATES.  
MEANWHILE SEPARATING TROUGH ENERGY OVER THE EASTERN PACIFIC SHOULD  
FORM AN UPPER LOW THAT MAY APPROACH CALIFORNIA BY THE MIDDLE OF  
NEXT WEEK. THE MAJOR QUESTION MARKS INVOLVE TIMING OF THE  
MISSISSIPPI VALLEY/GREAT LAKES SYSTEM DURING THE WEEKEND AND EXACT  
DETAILS OF THE EASTERN PACIFIC RIDGE ALOFT. THE LATTER WILL  
AFFECT THE TIMING/COVERAGE OF MOISTURE THAT WOULD REACH THE WEST  
COAST AFTER MON AS WELL AS SPECIFICS OF DOWNSTREAM FLOW AND  
CHARACTER OF A SYSTEM THAT COULD AFFECT THE EASTERN U.S. TOWARD  
TUE-WED.  
 
   
..GUIDANCE EVALUATION/PREFERENCES
 
 
AFTER GOOD CLUSTERING AS OF EARLY DAY 3 SAT, MODELS/ENSEMBLE  
MEMBERS STILL RAPIDLY DIVERGE FOR THE SYSTEM INITIALLY OVER THE  
EASTERN PLAINS/MISSISSIPPI VALLEY REGION. SOLUTIONS STILL OCCUPY  
SIMILAR PARTS OF THE ENVELOPE FROM YESTERDAY, WITH GFS-BASED  
GUIDANCE ON THE FAST SIDE AND THE 00Z ECMWF/ECMWF MEAN/CMC ON THE  
SLOW SIDE. THE FORECAST FOR THIS SYSTEM IS STILL VERY SENSITIVE  
WITH FAIRLY SMALL SCALE DETAILS ALONG EITHER SIDE OF THE CANADIAN  
BORDER HAVING A SIGNIFICANT INFLUENCE ON SYSTEM PROGRESSION. FAST  
GFS PROGRESSION CORRESPONDS TO THAT MODEL BEING QUICKEST TO  
AMPLIFY ONTARIO SHORTWAVE ENERGY INTO THE UPPER GREAT LAKES. THE  
00Z FV3 GFS AND CMC MEAN PROVIDE ADDED SUPPORT FOR TIMING THAT IS  
SLOWER THAN OPERATIONAL GFS RUNS WHILE ALSO MAINTAINING POTENTIAL  
FOR A LITTLE MORE PROGRESSION THAN THE SLOW END OF THE SPREAD.  
THERE CONTINUES TO BE SOMEWHAT BETTER CLUSTERING AS LOW PRESSURE  
REACHES THE VICINITY OF THE CANADIAN MARITIMES BY DAY 5 MON.  
 
STARTING WITH YESTERDAY'S (NOV 27) 12Z CYCLE, SOME GUIDANCE--MOST  
NOTABLY THE ECMWF--HAS BEEN DEPICTING CLOSURE OF THE NORTHERN PART  
OF THE EASTERN PACIFIC RIDGE INTO A HIGH. THIS CHANGES THE  
DETAILS OF SHORTWAVE ENERGY AMPLIFYING INTO THE WESTERN HALF OF  
THE LOWER 48, MAKING IT SOMEWHAT MORE ROUNDED AND LEADING TO LESS  
EASTERN U.S. SURFACE DEVELOPMENT UNTIL HEIGHT FALLS ALOFT APPROACH  
THE EAST COAST AND SUPPORT OFFSHORE DEVELOPMENT. THE 00Z ECMWF  
BACKED OFF A BIT FROM ITS VERY FLAT 12Z/27 RUN, WHILE IN VARIOUS  
FORMS GFS RUNS (OPERATIONAL AND FV3)/GEFS MEANS/CMC MEANS STILL  
SHOW SOME SURFACE OF DEVELOPMENT WEST OF THE APPALACHIANS AS OF  
EARLY DAY 6 TUE, AND MORE BROADLY THERE STILL APPEARS TO BE ENOUGH  
SIGNATURE OF A SHORTWAVE ALOFT TO HAVE SOME EVIDENCE OF A SURFACE  
REFLECTION. THAT SAID, THE SIGNAL IS CERTAINLY WEAKER NOW THAN IN  
YESTERDAY'S 00Z-06Z CYCLE. THUS FOR THE DETERMINISTIC FORECAST  
PREFER TO WEAKEN THE SYSTEM A BIT BUT STILL MAINTAIN ITS PRESENCE.  
 
CONSENSUS STILL LOOKS BEST FOR THE EASTERN PACIFIC/WEST COAST  
REGION. THERE IS A MODERATE DEGREE OF SPREAD FOR THE UPPER LOW  
THAT FORMS WELL OFFSHORE CALIFORNIA AND APPROACHES THE STATE LATE  
IN THE PERIOD. THE ONE EXTREME SOLUTION BY DAY 7 WED IS THE 00Z  
CMC THAT STRAYS TO THE SOUTHWEST OF OTHER SOLUTIONS AND THUS  
SIGNIFICANTLY DELAYS ARRIVAL OF ANY MOISTURE. EVEN WITH THE  
DETAIL UNCERTAINTIES, TELECONNECTIONS RELATIVE TO THE BEST DEFINED  
FEATURES PROVIDE REASONABLE SUPPORT FOR SOME DEGREE OF INCREASING  
MOISTURE OVER CALIFORNIA AND RELATIVELY DRY CONDITIONS OVER THE  
PACIFIC NORTHWEST.  
 
FROM DAY 3 SAT INTO DAY 5 MON THE PRIMARY NOTE IS THAT THE 00Z FV3  
GFS SERVED AS THE GFS INPUT FOR THE MULTI-MODEL BLEND. FOR THE  
REST OF THE FORECAST THE BLEND STARTED WITH SOME 00Z FV3 GFS/ECMWF  
WEIGHT WITH THE REST CONSISTING OF THE 06Z GEFS/00Z ECMWF MEANS.  
 
   
..WEATHER HIGHLIGHTS/HAZARDS
 
 
THE STORM SYSTEM TRACKING NORTHEASTWARD FROM THE EASTERN  
PLAINS/MID-UPPER MISSISSIPPI VALLEY DURING THE WEEKEND WILL BRING  
AN ARRAY OF SIGNIFICANT WEATHER TO THE EASTERN HALF OF THE  
COUNTRY. SHOWERS AND THUNDERSTORMS IN THE WARM SECTOR OF THE  
SYSTEM WILL PRODUCE SOME AREAS OF HEAVY RAINFALL AND POTENTIALLY  
STRONG/SEVERE CONVECTION. CONSULT SPC OUTLOOKS FOR THE LATEST  
INFORMATION ON SEVERE THREATS. LOCATIONS TO THE NORTHWEST OF THE  
SURFACE LOW TRACK MAY SEE A BAND OF HEAVY SNOW ALONG WITH BRISK TO  
STRONG WINDS. LOW PRESSURE/UPPER TROUGHING THAT INITIALLY MOVES  
INTO THE WEST WILL PRODUCE AREAS OF RAIN AND HIGHER ELEVATION  
SNOW. AS SHORTWAVE ENERGY EJECTS EASTWARD THERE IS CONSIDERABLE  
UNCERTAINTY OVER COVERAGE/INTENSITY/TYPE OF PRECIPITATION OVER THE  
EASTERN HALF OF THE COUNTRY BY EARLY-MID WEEK. SOUTHERN STREAM  
TROUGHING/UPPER LOW APPROACHING CALIFORNIA LATER IN THE PERIOD  
WILL LIKELY BRING AN AREA OF RAIN AND MOUNTAIN SNOW BY TUE-WED.  
 
EXPECT HIGH TEMPERATURES OF 10-20F ABOVE NORMAL FROM THE GULF  
COAST TO GREAT LAKES DURING THE WEEKEND, AHEAD OF THE STRONG STORM  
SYSTEM AFFECTING THE EASTERN HALF OF THE COUNTRY. SOME MORNING  
LOWS MAY EXCEED 20F ABOVE NORMAL. MUCH COLDER WEATHER WILL SETTLE  
IN ACROSS THE WESTERN U.S. COURTESY OF THE AMPLIFYING UPPER LEVEL  
TROUGH AND THEN EXPAND EASTWARD. ANOMALIES OF 10-20F BELOW NORMAL  
FOR HIGHS WILL EXPAND FROM THE GREAT BASIN THROUGH THE ROCKIES AND  
INTO THE PLAINS. MUCH OF THE LOWER 48 SHOULD SEE BELOW NORMAL  
TEMPERATURES BY THE MIDDLE OF NEXT WEEK.  
 
RAUSCH  
 
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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