888  
FXUS02 KWBC 270645  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
245 AM EDT SAT OCT 27 2018  
 
VALID 12Z TUE OCT 30 2018 - 12Z SAT NOV 03 2018  
   
..PATTERN OVERVIEW
 
 
A MODERATELY PROGRESSIVE FLOW PATTERN ACROSS THE U.S. EARLY IN THE  
MEDIUM RANGE PERIOD WILL BECOME INCREASINGLY AMPLIFIED AND SLOWER  
BY THE MIDDLE OF NEXT WEEK. GUIDANCE AGREES THAT THE UPPER RIDGE  
WHICH HAS BEEN PREVALENT OFF THE WEST COAST WILL REBUILD 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
 
 
A BLEND OF THE ECMWF/UKMET/FV3 WAS USED AS A STARTING POINT FOR  
THE FORECAST DURING DAYS 3-4 (TUE-WED). MODELS SHOWED GOOD  
AGREEMENT WITH RESPECT TO AN UPPER TROUGH EXITING THE NORTHEAST  
DURING THAT TIME PERIOD, BUT SHOWED SOMEWHAT GREATER SPREAD WITH  
ANOTHER SHORTWAVE MOVING FROM THE NORTHERN ROCKIES INTO THE  
NORTHERN PLAINS AND UPPER MIDWEST. THE 12Z/18Z RUNS OF THE GFS  
WERE ON THE FAST SIDE OF THE SPREAD WITH RESPECT TO THIS FEATURE,  
AND WERE EXCLUDED FROM THE FORECAST AS FAST OUTLIERS. THE 00Z GFS  
HAS MOVED TOWARD THE SLOWER CONSENSUS. FROM DAY 5 (THU) ONWARD, AS  
A LARGE SCALE UPPER-LEVEL TROUGH IS CARVED OUT ACROSS THE CENTRAL  
U.S. THROUGH THE PASSAGE OF NUMEROUS SMALLER-SCALE SHORTWAVES,  
MODEL SPREAD BEGINS TO INCREASE, PARTICULARLY WITH RESPECT TO THE  
DETAILS/TIMING/AMPLITUDE OF THESE SMALLER SCALE FEATURES. THE  
GFS/FV3/UKMET HAVE CONSISTENTLY BEEN MORE AMPLIFIED WITH SHORTWAVE  
ENERGY CROSSING THE CENTRAL ROCKIES/PLAINS AND REACHING THE  
MID-MISSISSIPPI VALLEY BY THU, WHILE THE ECMWF/CMC HAVE BEEN LESS  
AMPLIFIED. THE EXACT INTENSITY OF THIS WAVE AS IT CROSSES THE  
PLAINS WILL PLAY A ROLE IN THE GENERATION/INTENSITY/TIMING OF A  
WAVE ALONG A SURFACE FRONT CROSSING THE LOWER/MID MISSISSIPPI  
VALLEY. AN ADDITIONAL UPPER SHORTWAVE WILL DIVE SOUTHEASTWARD FROM  
THE CANADIAN ROCKIES INTO THE CENTRAL U.S. THU NIGHT/FRI. MODELS  
SHOW A RANGE OF SOLUTIONS FOR THIS FEATURE BY SAT, RANGING FROM A  
DEEP CUTOFF UPPER LOW OVER THE MID-MISSISSIPPI VALLEY TO A MORE  
PROGRESSIVE TROUGH. DUE TO LOWER FORECAST CONFIDENCE FROM DAY 5  
ONWARD, WEIGHTING OF ENSEMBLE MEANS WAS BOOSTED GRADUALLY THROUGH  
DAY 7 (SAT).  
 
   
..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.  
THE RESULT WILL BE POTENTIALLY WIDESPREAD RAIN AND THUNDERSTORMS,  
WITH THE POTENTIAL FOR AREAS OF HEAVY RAINFALL FROM THE  
LOWER/MID-MISSISSIPPI VALLEY AND OHIO VALLEY TUE-WED TO THE  
MID-ATLANTIC AND NORTHEAST THU-FRI. THE PACIFIC NORTHWEST AND  
NORTHERN ROCKIES WILL ALSO SEE MULTIPLE ROUNDS OF PRECIPITATION  
(RAIN AND HIGH ELEVATION SNOW) AS WEAKENING SHORTWAVES/FRONTAL  
SYSTEMS TRAVERSE THE NORTHERN PERIPHERY OF THE PACIFIC RIDGE.  
INITIALLY ABOVE AVERAGE TEMPERATURES ACROSS MUCH OF THE CENTRAL  
U.S. WILL QUICKLY DECREASE BY THE MIDDLE OF NEXT WEEK AS THE UPPER  
TROUGH AMPLIFIES AND A SURFACE COLD FRONT SPREADS A POLAR AIR MASS  
SOUTHWARD. HIGH TEMPERATURES FROM 5 TO 15 DEG F BELOW AVERAGE ARE  
EXPECTED FOR MUCH OF THE CENTRAL U.S. AND ROCKIES FROM THE MIDDLE  
TO END OF NEXT WEEK. MEANWHILE, AREAS EAST OF THE FRONT WILL SEE  
WARMER CONDITIONS, WITH HIGH TEMPERATURES POTENTIALLY 5 TO 10  
DEGREES ABOVE AVERAGE ACROSS THE MISSISSIPPI/OHIO VALLEYS ON TUE  
AND SOUTHEAST/MID-ATLANTIC ON WED.  
 
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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