420  
FXUS02 KWBC 271601  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1100 AM EST TUE NOV 27 2018  
 
VALID 12Z FRI NOV 30 2018 - 12Z TUE DEC 04 2018  
   
..OVERVIEW
 
 
THE FORECAST PERIOD FROM THIS FRI THROUGH NEXT TUE WILL BE ACTIVE  
WITH TWO DOMINANT SYSTEMS OF INTEREST. ONE VIGOROUS SHORTWAVE  
OVER THE WEST AS OF EARLY FRI WILL PROGRESS INTO THE  
PLAINS/MISSISSIPPI VALLEY BY SAT AND POSSIBLY CLOSE OFF AN UPPER  
LOW, ACCOMPANIED BY A STRONG SURFACE LOW PRESSURE SYSTEM.  
GUIDANCE DIVERGES REGARDING SYSTEM PROGRESSION AFTER EARLY SAT.  
ANOTHER FAIRLY STRONG SYSTEM WILL REACH THE WEST COAST BY EARLY  
SAT. ASSOCIATED UPPER TROUGHING SHOULD AMPLIFY/BROADEN OVER THE  
WEST DURING THE WEEKEND, FOLLOWED BY EJECTION OF LEADING ENERGY  
WHICH WILL MAY GENERATE ANOTHER GREAT LAKES STORM SYSTEM BY NEXT  
TUE. DURING THE FIRST HALF OF NEXT WEEK THE SPECIFICS OF THE  
EASTERN PACIFIC PATTERN BECOME SOMEWHAT UNCERTAIN WITH INDIVIDUAL  
MODELS/ENSEMBLES DIFFERING OVER HOW FLOW MAY SEPARATE WITHIN THE  
OVERALL TROUGH THAT WILL BE HEADING INTO THE UPPER RIDGE THAT  
APPROACHES THE WEST COAST.  
 
   
..GUIDANCE EVALUATION/PREFERENCES
 
 
ONE MAJOR UNCERTAINTY IN THE FORECAST INVOLVES THE PROGRESSION OF  
THE STRONG SYSTEM REACHING THE EASTERN PLAINS/MISSISSIPPI VALLEY  
AS OF EARLY SAT. THERE IS DECENT CLUSTERING UP TO THAT TIME BUT  
THEN A PRONOUNCED DIVERGENCE DUE TO DIFFERENCES IN FLOW DETAILS  
ALONG/NORTH OF THE CANADIAN BORDER. ONE CLUSTER INCLUDING THE  
ECMWF/CMC/UKMET SHOWS SOME EAST-WEST UPPER RIDGING TO THE NORTH,  
HELPING TO HOLD THE PLAINS/MS VALLEY SYSTEM RATHER FAR WESTWARD.  
IN CONTRAST THE GFS/GEFS MEAN SHOW VERY LITTLE OF THIS NORTHERN  
RIDGE AND THUS BECOME QUITE PROGRESSIVE. INTERESTINGLY THE 00Z  
GFS/ECMWF FROM YESTERDAY (NOV 26) WERE MUCH MORE SIMILAR AND  
POSITIONED NEAR THE MIDDLE OF THE CURRENT SPREAD. AN INTERMEDIATE  
SOLUTION LOOKS BEST HERE AS THE 06Z GFS HAS TRENDED A BIT SLOWER  
THAN THE 00Z RUN WHILE THE 00Z ECMWF MEAN/CMC END UP BEING FASTER  
THAN THE OPERATIONAL 00Z ECMWF. UPSTREAM TROUGHING SEEMS TO  
SUPPORT SOMEWHAT GREATER PROGRESSION OVER EASTERN NORTH AMERICA  
THAN SEEN IN THE 00Z ECMWF AS WELL.  
 
FROM THE LARGE SCALE PERSPECTIVE THE NEXT SYSTEM, REACHING THE  
WEST COAST AS OF EARLY SAT, IS ACTUALLY BETTER BEHAVED THROUGH THE  
FORECAST PERIOD. SOME DETAIL DIFFERENCES ALOFT LEAD TO NOTICEABLE  
SPREAD AT THE SURFACE OVER THE INTERIOR WEST AND PLAINS AROUND  
SUN-MON BUT BY DAY 7 TUE THERE IS BETTER THAN AVERAGE AGREEMENT  
(FOR A DAY 7 FORECAST) AMONG THE MODELS/MEANS FOR SURFACE LOW  
PRESSURE IN THE OHIO VALLEY/GREAT LAKES.  
 
THE OTHER POTENTIALLY SIGNIFICANT UNCERTAINTY ARISES LATE IN THE  
PERIOD WITH RESPECT TO EASTERN PACIFIC TROUGH ENERGY HEADING INTO  
THE UPPER RIDGE THAT APPROACHES THE WEST COAST EARLY NEXT WEEK.  
MODELS AND INDIVIDUAL ENSEMBLE MEMBERS BECOME QUITE DIVERSE WITH  
POSSIBLE IDEAS FOR HOW THE TROUGH ENERGY WILL SEPARATE AND AFFECT  
THE WEST COAST RIDGE. THE CURRENT ARRAY OF GUIDANCE SUGGESTS THE  
00Z ECMWF MAY PULL OF ITS SOUTHERN CLOSED LOW TO FAR SOUTHWESTWARD  
WHILE BRINGING TOO MUCH SHORTWAVE ENERGY THROUGH THE NORTHEASTERN  
PACIFIC. LATEST GFS/CMC RUNS ALIGN WELL WITH THE 06Z GEFS MEAN  
WHILE OTHER ENSEMBLE MEANS SHOW A LESS PRONOUNCED HINT AT FLOW  
SEPARATION. IN THESE FLOW SEPARATION CASES THE OPERATIONAL  
GUIDANCE TENDS TO LEAD THE ENSEMBLE MEANS SO PREFER TO MAINTAIN  
SOME OPERATIONAL INFLUENCE, WITH THE GFS/CMC PREFERRED OVER THE  
ECMWF.  
 
THE FORECAST BLEND EMPLOYED A MULTI-MODEL OPERATIONAL BLEND DURING  
THE FIRST HALF OF THE PERIOD TO REPRESENT CONSENSUS OR  
INTERMEDIATE SOLUTION AS APPROPRIATE, FOLLOWED BY A TRANSITION TO  
ABOUT HALF MODEL/HALF ENSEMBLE MEAN TO ACCOUNT FOR TYPICAL  
INCREASE IN DETAIL UNCERTAINTY AT EXTENDED TIME FRAMES. INITIAL  
MASS FIELDS/GRIDS WERE ADJUSTED A BIT MORE TOWARD OPERATIONAL  
GUIDANCE NEAR THE WEST COAST TOWARD THE END OF THE PERIOD, WITH  
SLOWER PROGRESSION OF HEIGHT FALLS/MOISTURE APPROACHING THAT  
REGION.  
 
   
..SENSIBLE WEATHER HIGHLIGHTS/THREATS
 
 
THE SYSTEM TRACKING FROM THE WEST INTO THE PLAINS EARLY IN THE  
PERIOD WILL BRING AREAS OF SNOW TO MOUNTAINOUS TERRAIN, TAPERING  
OFF AFTER LATE THIS WEEK, AND MULTIPLE HAZARDS FARTHER EASTWARD.  
THE CENTRAL PLAINS AND UPPER MIDWEST HAVE THE BEST POTENTIAL TO  
SEE A SIGNIFICANT SNOWFALL WHILE HEAVY WARM SECTOR RAINFALL IS  
POSSIBLE FROM THE GULF COAST REGION NORTHWARD/NORTHEASTWARD.  
THERE IS ALSO POTENTIAL FOR SEVERE WEATHER OVER PARTS OF THE SOUTH  
SO CHECK SPC OUTLOOKS FOR THE LATEST INFORMATION IN THAT REGARD.  
THE SYSTEM WILL LIKELY BE INTENSE ENOUGH TO PRODUCE FAIRLY STRONG  
WINDS AT SOME LOCATIONS. SOME PRECIPITATION OF VARIOUS TYPES WILL  
LIKELY REACH NEW ENGLAND BY SUN. THE NEXT SYSTEM IN THE SERIES  
WILL BRING RAIN AND HIGHER ELEVATION SNOW INTO THE WEST FROM LATE  
FRI/EARLY SAT ONWARD THROUGH THE WEEKEND. THEN EARLY NEXT WEEK  
EXPECT ANOTHER EPISODE OF POTENTIALLY HEAVY SNOW FROM THE CENTRAL  
PLAINS NORTHEASTWARD ALONG WITH HEAVY RAINFALL WITHIN PORTIONS OF  
THE WARM SECTOR.  
 
PATTERN EVOLUTION/SYSTEM PROGRESSION WILL FAVOR  
EXPANSION/STRENGTHENING OF NEGATIVE TEMPERATURE ANOMALIES FROM THE  
WEST INTO THE PLAINS OVER THE COURSE OF THE PERIOD. EXPECT HIGH  
TEMPERATURES TO DECLINE TO AT LEAST 10F BELOW NORMAL OVER THE  
GREAT BASIN BY SAT, WITH MINUS 10-20F ANOMALIES FOR HIGHS  
PERSISTING OVER THAT REGION INTO NEXT WEEK WHILE SPREADING ACROSS  
THE ROCKIES AND INTO THE PLAINS. ON THE OTHER HAND THE STORM  
TRACK WILL PROMOTE ABOVE NORMAL TEMPERATURES OVER THE SOUTH AND  
EAST WITH SOME LOCATIONS SEEING MORNING LOWS 20F OR MORE ABOVE  
NORMAL ONE OR MORE DAYS. SUCH READINGS MAY APPROACH OR EXCEED  
DAILY RECORDS FOR WARM LOWS.  
 
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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