510  
FXUS02 KWBC 141600  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1159 AM EDT SAT JUL 14 2018  
 
VALID 12Z TUE JUL 17 2018 - 12Z SAT JUL 21 2018  
   
..OVERVIEW
 
 
THE FLOW PATTERN ACROSS THE CONTIGUOUS U.S. WILL BECOME A BIT MORE  
AMPLIFIED DURING THE MEDIUM RANGE PERIOD. EXPECT TWO SEPARATE  
UPPER RIDGES, CENTERED NEAR CALIFORNIA AND THE SOUTHERN PLAINS AS  
OF EARLY TUE, TO MERGE WITH THE MORE DOMINANT WESTERN CENTER  
TRACKING ACROSS THE FOUR CORNERS AND THEN SETTLING OVER THE  
SOUTHERN ROCKIES/PLAINS (WHILE ALSO STRENGTHENING/EXPANDING) BY  
FRI-SAT. MEANWHILE ONE OR MORE UPPER WAVES ORIGINATING FROM THE  
EASTERN PACIFIC WILL BE ROUNDING THE UPPER RIDGE AT THE START OF  
THE PERIOD AND SHOULD GRADUALLY CARVE OUT A TROUGH ACROSS THE  
EASTERN U.S. A LEADING UPPER TROUGH WILL CROSS EASTERN NORTH  
AMERICA EARLY-MID PERIOD. HEIGHTS ALOFT WILL FALL ACROSS THE  
PACIFIC NORTHWEST AND EVENTUALLY EASTWARD ACROSS THE NORTHERN TIER  
FROM MIDWEEK ONWARD AS A NORTHEASTERN PACIFIC UPPER TROUGH WITH  
EMBEDDED LOW DIGS SOUTHEASTWARD AND THEN CONTINUES EASTWARD.  
 
   
..GUIDANCE EVALUATION AND PREFERENCES
 
 
MOST OF THE LATEST MODEL/ENSEMBLE GUIDANCE AGREES WELL WITH THE  
OVERALL EVOLUTION ALOFT DURING THE PERIOD. CANADIAN ENSEMBLES ARE  
A BIT OF AN EXCEPTION AS THEY ARE SOMEWHAT SLOWER WITH THE  
NORTHEASTERN PACIFIC INTO CANADA UPPER TROUGH/LOW--COUNTER TO THE  
GENERAL TREND OF THE GEFS/ECMWF MEANS TOWARD SLIGHTLY FASTER  
PROGRESSION. OPERATIONAL RUNS HAVE VARIED ON TIMING THOUGH, AND  
THE FULL ENVELOPE OF GEFS/ECMWF MEMBERS WOULD SUGGEST A  
SIGNIFICANT DEGREE OF UNCERTAINTY ON SPECIFICS ACROSS THE WESTERN  
HALF OF THE NORTHERN U.S./SOUTHERN CANADA. THERE IS ALSO A  
REASONABLY HIGH LEVEL OF UNCERTAINTY IN EXACTLY HOW THE THU-SAT  
UPPER TROUGH OVER THE EAST WILL EVOLVE WITH CORRESPONDING EFFECTS  
AT THE SURFACE. BOTH THE GFS AND ECMWF EASILY FIT INTO THE  
EXPECTED MEAN PATTERN BUT THE DEEP/CLOSED 00Z ECMWF ALOFT  
GENERATES A RATHER STRONG GREAT LAKES SURFACE LOW BY DAY 7 SAT  
WHILE THE 00Z-06Z GFS WEAK/OPEN WAVE DEPICTION YIELDS A VERY  
DIFFUSE SURFACE PATTERN. ULTIMATELY PREFER AN INTERMEDIATE  
SOLUTION BETWEEN THESE TWO EXTREMES, WHICH PROVIDES REASONABLE  
CONTINUITY WHILE AWAITING BETTER GUIDANCE AGREEMENT. OVERALL A  
BLEND AMONG THE 00Z/06Z OPERATIONAL MODELS REPRESENTED CONSENSUS  
WELL FOR DAYS 3-5 TUE-THU. WITH THE INCREASING DETAIL UNCERTAINTY  
LATER IN THE PERIOD THE FORECAST TRANSITIONED TOWARD A BLEND OF  
OPERATIONAL GUIDANCE WITH THE 06Z GEFS/00Z ECMWF MEANS AND WPC  
CONTINUITY.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
SHOWERS AND STORMS WITH LOCALLY HEAVY RAIN WILL ACCOMPANY A COLD  
FRONT FROM THE NORTHEAST/MID-ATLANTIC/APPALACHIANS TO THE  
SOUTHEAST TUE-WED. THE TRAILING END OF THE FRONTAL BOUNDARY WILL  
LIKELY STALL FROM THE SOUTHEAST INTO THE SOUTHERN/CENTRAL PLAINS,  
WHICH COULD PROVIDE AN ONGOING FOCUS FOR LOCALLY HEAVY SHOWERS AND  
THUNDERSTORMS THROUGH LATER NEXT WEEK. MONSOONAL MOISTURE WILL  
ALSO PRODUCE SCATTERED AFTERNOON/EVENING THUNDERSTORMS FROM THE  
SOUTHERN GREAT BASIN/SOUTHWEST TO THE SOUTHERN/CENTRAL ROCKIES  
THROUGH NEXT WEEK. EXPECT THE BEST POTENTIAL FOR LOCALLY HEAVY  
RAIN TO BE EARLY IN THE WEEK AS THE BUILDING UPPER RIDGE SHOULD  
GRADUALLY REDUCE THE COVERAGE/INTENSITY OF CONVECTION BY LATE  
WEEK/WEEKEND. THERE IS DECENT AGREEMENT IN THE GUIDANCE THAT  
SHORTWAVE ENERGY EMERGING INTO THE PLAINS SHOULD ENCOURAGE SOME  
ORGANIZED CONVECTION/RAINFALL OVER THE NORTHERN HALF OF THE PLAINS  
TUE-WED. HOWEVER AFTER THAT TIME THE UNCERTAINTY IN DETAILS AT  
THE SURFACE AND ALOFT HOLDS CONFIDENCE LOWER THAN DESIRED FOR THE  
RAINFALL FORECAST FROM THE EASTERN PLAINS INTO THE GREAT  
LAKES/APPALACHIANS. THE GENERAL EVOLUTION TOWARD UPPER TROUGHING  
SHOULD AT LEAST PROMOTE SOME INCREASE IN SHOWER/THUNDERSTORM  
ACTIVITY.  
 
THE COMBINATION OF RELATIVELY COOLER AIR BEHIND THE INITIAL  
EASTERN U.S. COLD FRONT AND CLOUDS/RAINFALL AHEAD OF THE POTENTIAL  
FEATURE EMERGING OVER THE PLAINS/EAST-CENTRAL U.S. SHOULD LEAD TO  
AN AREA OF BELOW NORMAL TEMPERATURES FROM THE NORTHERN-CENTRAL  
PLAINS INTO THE EAST. SOME LOCATIONS MAY SEE ONE OR MORE DAYS  
WITH MINUS 5-10F ANOMALIES. HOT TEMPERATURES ACROSS THE PACIFIC  
NORTHWEST (SOME HIGHS GREATER THAN 10F ABOVE AVERAGE OVER INTERIOR  
AREAS) SHOULD MODERATE BY LATER NEXT WEEK AS A PACIFIC COLD FRONT  
MOVES INLAND. EVOLUTION ALOFT WILL KEEP WARM MINS OVER A LARGE  
AREA COVERING THE WEST AND INTO THE NORTHERN PLAINS, WHILE THE  
STRENGTHENING SOUTHERN ROCKIES/PLAINS RIDGE ALOFT BY FRI-SAT WILL  
LEAD TO SOMEWHAT MORE FOCUS ON THE SOUTHERN PLAINS AND VICINITY  
FOR HIGHEST MAX TEMPERATURE ANOMALIES.  
 
RAUSCH/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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