823  
FXUS02 KWBC 280601  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
201 AM EDT SUN OCT 28 2018  
 
VALID 12Z WED OCT 31 2018 - 12Z SUN NOV 04 2018  
   
..PATTERN OVERVIEW
 
 
A MODERATELY PROGRESSIVE FLOW PATTERN ACROSS THE U.S. WILL BECOME  
INCREASINGLY AMPLIFIED DURING THE EXTENDED FORECAST PERIOD. THE  
UPPER RIDGE WHICH HAS BEEN PREVALENT OFF THE WEST COAST OVER THE  
PAST FEW MONTHS IS EXPECTED TO BECOME REESTABLISHED BY MIDWEEK,  
WITH A BROAD AND AMPLIFIED MEAN TROUGH SETTING UP OVER THE CENTRAL  
U.S. THIS FLOW PATTERN WILL SET THE STAGE FOR TRANSPORT OF CHILLY  
AIR SOUTHWARD INTO THE CENTRAL U.S., WHILE WET CONDITIONS DEVELOP  
ON THE EASTERN SIDE OF THE EVOLVING TROUGH, ESPECIALLY FROM THE  
LOWER MISSISSIPPI VALLEY/OHIO VALLEY TO THE EASTERN SEABOARD.  
 
   
..GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
WHILE MODELS SHOW BROAD AGREEMENT ON THE GENERAL EVOLUTION OF THE  
LARGE SCALE CENTRAL U.S. TROUGH THROUGH THE PERIOD, THEY CONTINUE  
TO STRUGGLE WITH THE DETAILS OF SMALLER SCALE SHORT WAVES  
TRAVERSING THE FLOW. A NUMBER OF SOLUTIONS (INCLUDING A SIZABLE  
NUMBER OF ENSEMBLE MEMBERS) HAVE TRENDED TOWARD A MORE AMPLIFIED  
SHORTWAVE EXITING THE CENTRAL/SOUTHERN ROCKIES INTO THE PLAINS ON  
DAY 3 (WED). AFTER LAGGING THIS TREND, THE 12Z ECMWF IS NOW AMONG  
THE MORE AMPLIFIED DETERMINISTIC SOLUTIONS, ESPECIALLY BY THU (DAY  
4) BY THE TIME THE SYSTEM REACHES THE MISSISSIPPI VALLEY, SHOWING  
A RELATIVELY DEEP SURFACE LOW ACROSS THE LOWER MISSISSIPPI VALLEY  
BY 12Z THU. AT THIS TIME, A BLEND OF DETERMINISTIC GUIDANCE WAS  
FAVORED DURING DAYS 3-4 TO SMOOTH OUT THESE DIFFERENCES, INCLUDING  
THE GFS/ECMWF/CMC.  
 
FROM DAY 5 (FRI) ONWARD, THINGS BECOME EVEN MORE MURKY AS  
ADDITIONAL SHORTWAVE ENERGY TRAVERSES THE TROUGH ACROSS THE NORTH  
CENTRAL U.S. AND AMPLIFIES AS IT REACHES THE OHIO/TENNESSEE  
VALLEYS. THE 12Z ECMWF MADE A SIGNIFICANT CHANGE FROM PREVIOUS  
RUNS (AND OTHER GUIDANCE), EJECTING THE MAIN UPPER-LEVEL TROUGH  
NORTHEASTWARD INTO EASTERN CANADA MUCH MORE QUICKLY, WITH HEIGHTS  
RISING AGAIN ACROSS MUCH OF THE SOUTHEASTERN U.S. THE GFS HAS  
SHOWN A GREATER DEGREE OF RUN-TO-RUN CONSISTENCY BY THIS TIME  
FRAME, AND HAS A SIGNIFICANT DEGREE OF ENSEMBLE SUPPORT FOR A  
SLOWER PROGRESSION OF THIS SECOND WAVE, KEEPING THE ENERGY NEAR  
THE NORTHEAST U.S. COAST INTO DAY 7 (SUN). ALSO BY NEXT WEEKEND,  
ADDITIONAL FAST-MOVING/LOW AMPLITUDE SHORTWAVE ENERGY ARRIVING  
FROM THE PACIFIC SHOULD REACH THE CENTRAL U.S., PERHAPS  
REINFORCING THE LARGE SCALE TROUGH. GIVEN THE TIME RANGE AND THE  
SMALL SCALE OF THESE FEATURES, MODEL SPREAD IS, AS WOULD BE  
EXPECTED, LARGE. GIVEN THE DESCRIBED DIFFERENCES SHOWN BY THE 12Z  
ECMWF, IT WAS EXCLUDED FROM THE WPC FORECAST STARTING ON DAY 5,  
AND WEIGHTING OF THE ECENS/GEFS ENSEMBLE MEANS WAS GRADUALLY  
INCREASED THROUGH DAY 7 GIVEN THE QUICKLY INCREASING SPREAD.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
THE AMPLIFYING TROUGH ACROSS THE CENTRAL U.S. WILL FAVOR TRANSPORT  
OF GULF OF MEXICO MOISTURE AND INSTABILITY NORTHWARD INTO PORTIONS  
OF THE CENTRAL/EASTERN U.S. ALONG/AHEAD OF A SURFACE COLD FRONT  
AND UPPER SHORTWAVE ENERGY. THE RESULT WILL BE POTENTIALLY  
WIDESPREAD RAIN AND THUNDERSTORMS, WITH THE POSSIBILITY FOR HEAVY  
MULTI-INCH RAINFALL TOTALS FROM THE LOWER/MID-MISSISSIPPI VALLEY  
TO THE OHIO VALLEY TUE-WED. WIDESPREAD RAINS ARE ALSO EXPECTED TO  
REACH THE EASTERN SEABOARD, WITH AMOUNTS LIKELY NOT AS HEAVY AS  
FARTHER WEST. THE PACIFIC NORTHWEST AND NORTHERN ROCKIES WILL ALSO  
SEE MULTIPLE ROUNDS OF PRECIPITATION (RAIN AND MOUNTAIN SNOW) AS  
WEAKENING SHORTWAVES/FRONTAL SYSTEMS TRAVERSE THE NORTHERN  
PERIPHERY OF THE PACIFIC RIDGE. A CHILLY, POLAR AIR MASS WILL  
ACCOMPANY THE AMPLIFYING UPPER TROUGH ACROSS THE CENTRAL U.S. AND  
PORTIONS OF THE ROCKIES, WITH HIGH TEMPERATURES FROM 5 TO 15 DEG F  
BELOW AVERAGE FROM MID TO LATE WEEK. MEANWHILE, AREAS EAST OF THE  
FRONT WILL SEE WARMER CONDITIONS, WITH HIGH TEMPERATURES EXPECTED  
TO REACH 5 TO 10 DEGREES ABOVE AVERAGE ACROSS THE SOUTHEAST AND  
MID-ATLANTIC WED-THU. COOLER TEMPERATURES WILL ARRIVE IN THESE  
AREAS BY LATE IN THE WEEK AS THE SURFACE COLD FRONT PASSES.  
TEMPERATURES WILL ALSO BE ABOVE AVERAGE ALONG MUCH OF THE WEST  
COAST THROUGH THE WEEK JUST DOWNSTREAM OF THE PACIFIC UPPER RIDGE.  
 
RYAN  
 
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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