149  
FXUS02 KWBC 131523  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1122 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 GREAT BASIN/FOUR CORNERS REGION AND A SERIES  
OF UPPER WAVES GRADUALLY CARVING OUT A TROUGH ACROSS THE GREAT  
LAKES/NORTHEAST. HEIGHTS ARE 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  
00Z CANADIAN/UKMET WERE DISPLACED FROM THE CONSENSUS BY ABOUT NEXT  
WEDNESDAY OVER WESTERN CANADA (UKMET) AND SOUTHEASTERN CANADA  
(CANADIAN). BASED ON THE GRADUALLY INCREASING FLOW AMPLIFICATION  
THROUGH THE PERIOD, THE PREFERENCE WAS TO GO NEAR OR SLIGHTLY  
SLOWER/DEEPER THAN THE ENSEMBLE MEAN CONSENSUS AND THIS WAS  
REFLECTED IN THE WPC FORECAST. UNCERTAINTY INCREASES BY NEXT  
THU/FRI WITH THE AMPLITUDE AND EVOLUTION OF TROUGHING INTO THE  
OHIO VALLEY (GFS MOST AMPLIFIED) BUT THE UPSTREAM/DOWNSTREAM  
RIDGING OVER THE SOUTHERN ROCKIES/WESTERN ATLANTIC, RESPECTIVELY,  
WOULD SUPPORT A SLOWER/DEEPER TROUGH OR POSSIBLY A CLOSED LOW  
ALONG 85-90W BY THE END OF THE PERIOD INTO NEXT WEEKEND. HOWEVER,  
MODELS CAN HAVE A TENDENCY TO OVERAMPLIFY THE LONGER RANGE AND  
TREND WEAKER AS LEAD TIME DECREASES.  
 
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 NEXT FRI ON SHOWING A MEAN TROUGH AXIS ACROSS BRITISH COLUMBIA  
WHILE THE DETERMINISTIC MODELS (AND MANUAL FORECAST) MOSTLY SHOWED  
A CLOSED LOW.  
 
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 (ECMWF AND GEFS) 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 ESPECIALLY  
EARLY IN THE WEEK (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  
(WHICH ARE NEAR/AT THE MAXIMUM FOR THE YEAR). 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.  
 
FRACASSO/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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