123  
FXUS02 KWBC 130520  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
119 AM EDT FRI JUL 13 2018  
 
VALID 12Z MON JUL 16 2018 - 12Z FRI JUL 20 2018  
   
..OVERVIEW
 
 
THE FLOW PATTERN ACROSS THE CONTIGUOUS U.S. WILL BECOME A BIT MORE  
AMPLIFIED DURING THE MEDIUM RANGE PERIOD WITH AN UPPER-LEVEL RIDGE  
EXPANDING ACROSS THE SOUTHERN GREAT BASIN/FOUR CORNERS REGION, AND  
A SERIES OF UPPER WAVES GRADUALLY CARVING OUT A TROUGH ACROSS THE  
GREAT LAKES/NORTHEAST. HEIGHTS ARE ALSO EXPECTED TO FALL ACROSS  
THE PACIFIC NORTHWEST BY LATE NEXT WEEK AS AN UPPER LOW DIGS SOUTH  
FROM THE GULF OF ALASKA TOWARD THE REGION.  
 
   
..GUIDANCE EVALUATION AND PREFERENCES
 
 
WITH SHORTWAVE ENERGY PASSING SOUTH OF HUDSON BAY AND SKIRTING THE  
UPPER MIDWEST/GREAT LAKES DAYS 3-5, MODELS SHOWED A RANGE OF  
SOLUTIONS WITH RESPECT TO BOTH TIMING AND AMPLITUDE. THE GFS AND  
ECMWF WERE GENERALLY WELL CENTERED WITHIN THE SPREAD, WHILE THE  
12Z CMC WAS ON THE SLOW/DEEP SIDE AND THE UKMET WAS ON THE  
FASTER/WEAKER SIDE. BASED ON THE GRADUALLY INCREASING FLOW  
AMPLIFICATION THROUGH THE PERIOD, THE PREFERENCE WAS TO GO NEAR OR  
SLIGHTLY SLOWER/DEEPER THAN THE CONSENSUS, AND THIS WAS REFLECTED  
IN THE WPC FORECAST. MODELS SHOW ADDITIONAL SHORTWAVE ENERGY  
CROSSING THE NORTHERN/CENTRAL PLAINS AND MIDWEST BY WED-THU. THE  
18Z BECAME MORE AMPLIFIED THAN THE CONSENSUS WITH THIS FEATURE  
ACROSS THE MISSISSIPPI VALLEY BY THU-FRI. MODELS SUGGEST THE  
POTENTIAL FOR THIS ENERGY TO INTERACT WITH ADDITIONAL ENERGY  
FARTHER NORTH ACROSS CANADA TO AMPLIFY A BROADER TROUGH ACROSS THE  
GREAT LAKES/OHIO VALLEY BY LATE NEXT WEEK, ALTHOUGH CONFIDENCE IS  
LOW IN THE DETAILS WITH MANY POTENTIAL FACTORS AFFECTING THE  
EVENTUAL OUTCOME, SO AN ENSEMBLE APPROACH WAS PREFERRED BY THAT  
TIME. FARTHER WEST, WITH A SHORTWAVE/UPPER LOW DIVING SOUTHEAST  
ACROSS THE GULF OF ALASKA MID TO LATE NEXT WEEK AND LOWERING  
HEIGHTS ACROSS THE PACIFIC NORTHWEST, THERE IS UNCERTAINTY AS TO  
HOW DEEP THE FEATURE WILL BE AND WHETHER IT WILL DIG FARTHER SOUTH  
OR MOVE MORE QUICKLY EASTWARD. ENSEMBLE MEANS WERE IN GENERAL  
AGREEMENT AT 12Z FRI ON SHOWING A MEAN TROUGH AXIS ACROSS BRITISH  
COLUMBIA. GIVEN THE RELATIVELY LOW CONFIDENCE IN THE DETAILS, AN  
ENSEMBLE MEAN APPROACH WAS PREFERRED HERE AS WELL.  
 
THE WPC FORECAST INITIALLY BASED ON A MULTI-MODEL DETERMINISTIC  
BLEND DURING DAYS 3-5 (MON-WED), WITH MOST WEIGHT PLACED TOWARD  
THE ECMWF AND GFS. BY DAYS 6-7 (THU-FRI), WEIGHTING OF ENSEMBLE  
MEANS (ECENS AND NAEFS) WAS GRADUALLY INCREASED TO COMPRISE A  
MAJORITY OF THE WEIGHTING.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
SHOWERS AND STORMS WITH LOCALLY HEAVY RAIN WILL ACCOMPANY A COLD  
FRONT ACROSS THE OHIO/TENNESSEE VALLEYS TO THE  
SOUTHEAST/MID-ATLANTIC/NORTHEAST MON-WED. MONSOONAL MOISTURE WILL  
ALSO PRODUCE SCATTERED AFTERNOON/EVENING THUNDERSTORMS ACROSS FROM  
THE SOUTHERN GREAT BASIN/SOUTHWEST TO THE SOUTHERN/CENTRAL ROCKIES  
THROUGH THE NEXT WEEK, WITH LOCALLY HEAVY RAIN POSSIBLE. THE  
BUILDING UPPER-LEVEL RIDGE MAY SOMEWHAT REDUCE THE COVERAGE OF  
CONVECTION BY LATER NEXT WEEK. TEMPERATURES WILL INITIALLY BE 5 TO  
15 DEG F ABOVE AVERAGE ACROSS THE NORTHEAST AHEAD OF THE COLD  
FRONT EARLY NEXT WEEK, WITH A DECREASE CLOSER TO SEASONAL AVERAGES  
BY MID TO LATE NEXT WEEK. IN THE WAKE OF THE COLD FRONT, HIGH  
TEMPERATURES ARE EXPECTED TO BE 5 TO 10 DEG BELOW AVERAGE ACROSS  
THE CENTRAL/NORTHERN PLAINS AND PORTIONS OF THE MIDWEST THROUGH  
MUCH OF NEXT WEEK. FINALLY, HOT TEMPERATURES ACROSS THE PACIFIC  
NORTHWEST (HIGHS 5 TO 15 DEG ABOVE AVERAGE) SHOULD MODERATE SOME  
BY LATER NEXT WEEK AS A PACIFIC COLD FRONT MOVES INLAND.  
 
RYAN  
 
WPC MEDIUM RANGE 500 MB HEIGHTS, SURFACE SYSTEMS, WEATHER GRIDS,  
QUANTITATIVE PRECIPITATION, WINTER WEATHER OUTLOOK PROBABILITIES  
AND HEAT INDEXES ARE FOUND AT:  
 
HTTP://WWW.WPC.NCEP.NOAA.GOV/MEDR/5DAYFCST500_WBG.GIF  
HTTP://WWW.WPC.NCEP.NOAA.GOV/MEDR/5DAYFCST_WBG_CONUS.GIF  
HTTP://WWW.WPC.NCEP.NOAA.GOV/5KM_GRIDS/5KM_GRIDSBODY.HTML  
HTTP://WWW.WPC.NCEP.NOAA.GOV/QPF/DAY4-7.SHTML  
HTTP://WWW.WPC.NCEP.NOAA.GOV/WWD/PWPF_D47/PWPF_MEDR.PHP?DAY=4  
HTTP://WWW.WPC.NCEP.NOAA.GOV/HEAT_INDEX.SHTML  
 

 
 
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