236  
FXUS02 KWBC 281600  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1200 PM EDT SUN OCT 28 2018  
 
VALID 12Z WED OCT 31 2018 - 12Z SUN NOV 04 2018  
   
..PATTERN OVERVIEW
 
 
THE OVERWHELMING MAJORITY OF GUIDANCE MAINTAINS GOOD  
AGREEMENT/CONTINUITY FOR EVOLUTION OF THE MEAN PATTERN BUT STILL  
DISPLAYS MEANINGFUL DISAGREEMENT FOR SOME IMPORTANT EMBEDDED  
DETAILS. AN AMPLIFIED MEAN TROUGH WILL BECOME ESTABLISHED OVER  
THE CENTRAL U.S./SOUTHERN CANADA BY MID-LATE WEEK AS A STRONG  
EASTERN PACIFIC RIDGE DRIFTS TOWARD THE WEST COAST AND ANOTHER  
RIDGE BUILDS OVER THE WESTERN ATLANTIC. INCOMING PROGRESSIVE  
NORTHERN PACIFIC FLOW WILL LIKELY DAMPEN THE WEST COAST RIDGE  
SOMEWHAT AFTER EARLY FRI, WHICH SHOULD LEAD TO A BROADER MEAN  
TROUGH OVER THE LOWER 48 DURING NEXT WEEKEND. PROMINENT  
UNCERTAINTIES WITHIN THIS LARGE SCALE PATTERN INVOLVE THE WAVE  
AFFECTING THE EASTERN HALF OF THE U.S. DURING THE LATTER HALF OF  
THE WEEK (PLUS ANY TRAILING FRONTAL WAVES), A WEAK CLIPPER  
DROPPING INTO THE NORTHERN STATES/MIDWEST LATE WEEK, AND UPSTREAM  
ENERGY/SURFACE WAVES ARRIVING FROM THE PACIFIC.  
   
..GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
STARTING WITH YESTERDAY'S (OCT 27) ECMWF RUN, THERE HAS BEEN A  
PRONOUNCED SLOWER SHIFT IN THE GUIDANCE FOR THE SHORTWAVE ENERGY  
FORECAST TO BE NEAR THE SOUTHERN ROCKIES AS OF EARLY WED AND  
SURFACE WAVE THAT SHOULD TRACK NORTHEASTWARD FROM THE SOUTHERN  
PLAINS/LOWER MISSISSIPPI VALLEY DURING THE LATTER HALF OF THE  
WEEK. AS OF THE 12Z THU VALID TIME, ENSEMBLE MEMBERS SHOW BETTER  
CLUSTERING THAN 24 HOURS AGO AND ADD SUPPORT FOR THE SLOWER TREND.  
HOWEVER ONLY SOME CMC MEMBERS AND ABOUT 10 PERCENT OF ECMWF  
MEMBERS ARGUE FOR TIMING AS SLOW AS THE 00Z CMC. ENSEMBLE SPREAD  
WIDENS RAPIDLY AFTER 12Z THU SO CONFIDENCE IN ANY SPECIFIC  
SOLUTION REMAINS WELL BELOW AVERAGE. OVERALL PREFERENCE  
INCORPORATES IDEAS FROM NON-00Z CMC GUIDANCE AND A LITTLE  
CONTINUITY TO REFLECT THE SLOWER TREND BUT STAY CLOSE TO THE  
MIDDLE OF THE SPREAD GIVEN THE DIVERSE/INCONSISTENT BEHAVIOR OF  
GUIDANCE THUS FAR.  
 
THERE IS ALSO A FAIR AMOUNT OF SPREAD FOR THE REACHING THE  
NORTHERN STATES/MIDWEST BY THU-FRI BUT THE BLEND USED FOR  
RESOLVING THE EASTERN SYSTEM PROVIDES FAIRLY GOOD CONTINUITY INTO  
DAY 5 FRI. AFTER THAT TIME THE ENSEMBLE MEANS AND GFS RUNS  
INDICATE A SLOWER TREND CORRESPONDING TO UPSTREAM AGREEMENT AMONG  
THIS CURRENTLY PREFERRED CLUSTER TOWARD PACIFIC SHORTWAVE ENERGY  
NOT BEING AS AMPLIFIED AS THE 00Z ECMWF/CMC.  
 
REGARDING THIS INCOMING PACIFIC ENERGY AROUND FRI THE 00Z ECMWF IN  
PARTICULAR IS FAIRLY EXTREME WITH ITS AMPLITUDE WITH SOME HEIGHT  
CONTOURS FALLING NEAR OR JUST OUTSIDE THE FULL ENVELOPE OF ALL 00Z  
GEFS/ECMWF/CMC ENSEMBLE MEMBERS OVER PARTS OF WESTERN CANADA AND  
NORTHWESTERN U.S. CORRESPONDING SURFACE LOW PRESSURE IN THE 00Z  
ECMWF ALSO TRACKS FARTHER SOUTH THAN MOST SOLUTIONS. THEN BY DAY  
7 SUN INTERACTION WITH CANADIAN FLOW ALOFT RESULTS IN A  
U.S./CANADIAN BORDER SYSTEM THAT IS STRONGER/NORTHWARD VERSUS  
CONSENSUS AND VERY DIFFERENT FROM THE PREVIOUS ECMWF RUN.  
UPSTREAM PACIFIC ENERGY IS PROGRESSIVE WITH LOW AMPLITUDE, WHICH  
LENDS ITSELF TO WELL BELOW AVERAGE PREDICTABILITY.  
 
BASED ON COMBINED FORECAST CONSIDERATIONS THE UPDATED FORECAST  
STARTED WITH A CONSENSUS OF 00-06Z GFS AND 00Z ECMWF/UKMET ON DAY  
3 WED BUT THEN REDUCED ECMWF WEIGHT WHILE INCLUDING A LITTLE  
CONTINUITY FOR DAYS 4-5 THU-FRI. BY DAYS 6-7 FRI-SAT THE 06Z  
GEFS/00Z ECMWF MEANS AND CONTINUITY PROVIDED MAJORITY INPUT. SOME  
GFS DETAILS BECAME A LITTLE QUESTIONABLE BUT OVERALL EVOLUTION WAS  
CLOSE ENOUGH TO THE MEANS TO FAVOR HOLDING ONTO A MODEST COMPONENT  
OF THEIR 00-06Z RUNS, AND HELPED TO PROVIDE A TAD MORE DETAIL FOR  
POSSIBLE FRONTAL WAVINESS OFF THE MID-ATLANTIC/NEW ENGLAND COAST  
LATE IN THE PERIOD.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
THE SHORTWAVE ENERGY ROUNDING THE CENTRAL U.S. MEAN TROUGH ALOFT  
MID-LATE WEEK AND ASSOCIATED SURFACE LOW PRESSURE/FRONTAL SYSTEM  
WILL PULL A SIGNIFICANT AMOUNT OF GULF OF MEXICO MOISTURE INTO THE  
EASTERN HALF OF THE COUNTRY. SO FAR THE GUIDANCE HAS BEEN DIVERSE  
AND INCONSISTENT FOR THE PRECISE STRENGTH/TIMING OF THE SURFACE  
SYSTEM, BUT THERE HAS BEEN A MORE STABLE OVERALL SIGNAL FOR  
HEAVIEST RAINFALL TO OCCUR WITHIN AN AREA FROM THE MID-LOWER  
MISSISSIPPI VALLEY NORTHEAST INTO THE TENNESSEE VALLEY/OHIO VALLEY  
AND EASTERN GREAT LAKES. ACROSS THESE REGIONS EXPECT POTENTIALLY  
WIDESPREAD RAIN AND SOME THUNDERSTORMS, WITH MULTI-INCH RAINFALL  
TOTALS POSSIBLE OVER THE AFOREMENTIONED REGIONS. SOME CONVECTION  
OVER SOUTHERN AREAS MAY BE ON THE STRONG SIDE SO CONSULT SPC  
OUTLOOKS AS SPECIFICS BECOME MORE CLEAR CLOSER TO THE EVENT. SOME  
RAIN WILL LIKELY EXTEND TO THE EAST COAST BUT WITH LESSER AMOUNTS  
THAN EXPECTED FARTHER WEST. THE NORTHEAST MAY SEE SOMEWHAT  
GREATER RAINFALL THAN LOCATIONS TO THE SOUTH, ESPECIALLY IF  
TRAILING FRONTAL WAVINESS DEVELOPS NEXT WEEKEND. THE NORTHERN  
U.S./MIDWEST MAY SEE A PERIOD OF RAIN/POSSIBLY NORTHERN SNOW LATE  
WEEK WITH THE CLIPPER SYSTEM TRACKING THROUGH THE AREA. MEANWHILE  
THE PACIFIC NORTHWEST AND NORTHERN ROCKIES WILL SEE MULTIPLE  
PERIODS OF RAIN AND HIGH ELEVATION SNOW AS ENERGY ALOFT FLOWS  
AROUND THE NORTHERN PERIPHERY OF THE EASTERN PACIFIC RIDGE ALOFT.  
THERE IS SOME UNCERTAINTY REGARDING PRECISE AMPLITUDE OF THIS  
ENERGY AND THUS SOUTHWARD EXTENT OF PRECIPITATION.  
 
THE DEVELOPING EASTERN U.S. SYSTEM WILL BRING WARM TEMPERATURES  
(ESPECIALLY FOR MORNING LOWS, WITH SOME PLUS 10-20F ANOMALIES)  
INTO THE EAST MID-LATE WEEK WHILE PULLING BELOW NORMAL READINGS  
INTO THE CENTRAL U.S. LOCATIONS WEST OF THE ROCKIES WILL TEND TO  
SEE ABOVE NORMAL TEMPERATURES. BROADENING OF THE MEAN TROUGH  
ALOFT BY NEXT SUN SHOULD LEAD TO A MODERATING TREND SOMEWHAT  
CLOSER TO NORMAL OVER MANY AREAS AT THAT TIME.  
 
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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