575  
FXUS02 KWBC 260640  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
240 AM EDT TUE JUN 26 2018  
 
VALID 12Z FRI JUN 29 2018 - 12Z TUE JUL 03 2018  
   
..OVERVIEW AND GUIDANCE EVALUATION/PREFERENCES
 
 
ENERGY FLOWING AROUND A STRONG EASTERN PACIFIC RIDGE WILL HELP TO  
EJECT AN AMPLIFIED TROUGH FORECAST TO BE OVER THE WEST AS OF DAY 3  
FRI. CYCLONIC MEAN FLOW ACROSS THE NORTHERN TIER STATES WILL  
TREND BROADER AND LESS AMPLIFIED AS THIS OCCURS. BY THE FIRST  
HALF OF NEXT WEEK GUIDANCE AGREES IN PRINCIPLE THAT THE PACIFIC  
RIDGE WILL BEGIN TO RETROGRADE AND SET THE STAGE FOR DOWNSTREAM  
TROUGH RELOADING THAT SHOULD ALIGN FARTHER WEST THAN WITH THE  
PRECEDING TROUGH. FARTHER EAST/SOUTH, A STRONG EASTERN RIDGE  
ALOFT WILL LIKELY CONTAIN A HIGH CENTER THAT SHOULD TRACK FROM THE  
OHIO VALLEY NORTHEASTWARD INTO OR NEAR SOUTHERN NEW ENGLAND.  
RIDGING ACROSS THE SOUTH WILL LIKELY BUILD ACROSS THE SOUTHERN  
PLAINS/ROCKIES EARLY NEXT WEEK AS THE PACIFIC/WEST COAST PATTERN  
ADJUSTMENT BEGINS.  
 
GUIDANCE TRENDS OVER THE PAST DAY HAVE DRAMATICALLY LOWERED THE  
SPREAD FOR WHAT HAPPENS TO THE INITIAL WESTERN TROUGH. THE  
ECMWF/ECMWF MEAN HAVE BACKED OFF A BIT ON THE TIMING OF HEIGHT  
FALLS REACHING THE GREAT LAKES REGION BY EARLY NEXT WEEK BUT A  
FASTER PROGRESSION IN THE GFS/GEFS/CMC MEAN PLAYED A MUCH GREATER  
ROLE IN ACHIEVING THE CURRENT CLUSTERING. IN SPITE OF THIS BETTER  
AGREEMENT IN PRINCIPLE THERE IS STILL MEANINGFUL SPREAD FOR  
DETAILS OF THE EJECTING ENERGY AND TO A GREATER EXTENT FOR  
UPSTREAM FLOW FEEDING INTO THE MEAN TROUGH.  
 
IN ADDITION GUIDANCE STILL SHOWS A VARIETY OF POSSIBLE SOLUTIONS  
FOR AN UPPER LOW THAT IS CURRENTLY NEAR THE ALEUTIANS. MOST  
SOLUTIONS INDICATE THIS FEATURE WILL BECOME THE CORE OF THE TROUGH  
THAT RELOADS NEAR THE WEST COAST AROUND THE END OF THE PERIOD. AS  
HAS BEEN THE CASE IN RECENT RUNS THE ECMWF IS ON THE FAST SIDE OF  
THE SPREAD (BUT AT LEAST WITH THE 12Z RUN SLOWER THAN ITS  
PREDECESSOR) WITH THE ALEUTIANS LOW. GFS RUNS ARE KEEPING THE LOW  
OFFSHORE THROUGH DAY 7 TUE WHILE THE CMC HAS ADJUSTED FROM A  
COMPROMISE IN THE 12Z RUN TO BEING CLOSER TO THE GFS IN THE NEW  
00Z RUN. AN AVERAGE OF THE ENSEMBLE MEANS OFFERS A REASONABLE  
INTERMEDIATE APPROACH GIVEN CURRENT GUIDANCE SPREAD. ONCE THE  
RETROGRADING PACIFIC RIDGE SETTLES INTO PLACE, TELECONNECTIONS  
WITH THE ASSOCIATED POSITIVE HEIGHT ANOMALIES SUGGEST THE AXIS OF  
THE DEVELOPING WEST COAST TROUGH SHOULD ULTIMATELY DRIFT A LITTLE  
OFFSHORE.  
 
THE FORECAST REMAINS AMBIGUOUS TO THE SOUTH OF THE UPPER HIGH OVER  
THE EAST. THERE CONTINUES TO BE A GENERAL SIGNAL THAT SOME ENERGY  
FLOWING AROUND THE HIGH MAY REACH THE SOUTHEAST BUT WITH POOR  
AGREEMENT/CONSISTENCY FOR SPECIFICS. THE 12Z ECMWF WAS STRONGER  
THAN MOST OTHER GUIDANCE WITH THIS ENERGY BY LATE IN THE PERIOD.  
 
FOR DAYS 3-5 FRI-SUN GUIDANCE AGREEMENT WAS SUFFICIENTLY GOOD TO  
ALLOW FOR AN OPERATIONAL MODEL BLEND (18Z GFS/12Z ECMWF AND LESSER  
WEIGHT OF THE 12Z UKMET/CMC). DECREASING CONFIDENCE IN VARYING  
DETAILS OF THE OPERATIONAL RUNS LED TO INCORPORATING 50-70 PERCENT  
TOTAL WEIGHT OF THE 18Z GEFS AND 12Z ECMWF/NAEFS MEANS BY DAYS 6-7  
MON-TUE. THE PRIMARY ADJUSTMENT IN THE FORECAST WAS FARTHER  
EASTWARD PROGRESSION OF THE PLAINS FRONT INTO THE UPPER MS VALLEY  
AND GREAT LAKES MID-LATE PERIOD.  
 
   
..WEATHER THREATS/HIGHLIGHTS
 
 
LOW PRESSURE TRACKING NORTHEAST FROM THE PLAINS INTO CANADA FROM  
FRI ONWARD ALONG WITH ITS ASSOCIATED FRONTS AND UPPER SHORTWAVE  
SUPPORT WILL LIKELY PRODUCE CONVECTIVE EPISODES CAPABLE OF  
PRODUCING AREAS OF HEAVY RAINFALL. BEST SIGNAL FOR HIGHEST TOTALS  
IS ACROSS THE NORTHERN PLAINS/MIDWEST/UPPER MS VALLEY/UPPER GREAT  
LAKES. SOUTHWARD PROGRESSION OF THE COLD FRONT MAY BRING  
CONVECTION INTO THE CENTRAL PLAINS. SOME ACTIVITY MAY BE STRONG  
TO SEVERE AS WELL. CHECK LATEST SPC OUTLOOKS AS DETAILS BECOME  
MORE CLEAR IN COMING DAYS. EXPECT SHOWERS/THUNDERSTORMS TO EXTEND  
FARTHER EASTWARD LATER IN THE PERIOD BUT LIKELY WITH LESSER  
AMOUNTS AS BEST UPPER DYNAMICS LIFT INTO CANADA. MEANWHILE THERE  
SHOULD BE DIURNALLY FAVORED CONVECTION OVER PORTIONS OF THE  
SOUTH/FLORIDA THROUGH THE PERIOD WITH SOME LOCALLY HEAVY AMOUNTS.  
IT REMAINS A QUESTION MARK TO WHAT EXTENT UPPER ENERGY/SURFACE  
FEATURE(S) FLOWING AROUND THE DEEP LAYER HIGH TO THE NORTH MAY  
ENHANCE THIS CONVECTION.  
 
WITHIN THE LARGE AREA OF HOT AND HUMID WEATHER OVER THE  
CENTRAL/EASTERN U.S., HIGHEST MIN/MAX TEMPERATURE ANOMALIES OF  
PLUS 10-20F SHOULD EXTEND FROM THE MIDWEST THROUGH THE GREAT  
LAKES/OHIO VALLEY AND NORTHEAST. THE HEAT/HUMIDITY WILL GRADUALLY  
ERODE FROM THE WEST OVER THE COURSE OF THE PERIOD AS A PLAINS COLD  
FRONT MOVES INTO THE GREAT LAKES. THERE ARE MANY LOCATIONS THAT  
MAY SEE ONE OR MORE DAYS OF MIN TEMPS AT LEAST CLOSE TO DAILY  
RECORD WARM VALUES. DAILY RECORD HIGHS SHOULD BE LESS NUMEROUS  
AND MOST LIKELY CONFINED TO THE GREAT LAKES/OHIO VALLEY/NORTHEAST  
DURING SAT-MON. THE HUMIDITY WILL CONTRIBUTE TO HIGH HEAT INDEX  
VALUES. BELOW NORMAL TEMPERATURES WILL BE MOST COMMON OVER THE  
WEST AND HIGH PLAINS, WITH THE NORTHERN ROCKIES/HIGH PLAINS LIKELY  
TO SEE ONE OR MORE DAYS OF HIGHS 10F OR MORE BELOW NORMAL FRI-SUN.  
EJECTION OF THE INITIAL WESTERN TROUGH ALOFT WILL HELP READINGS  
TREND TO MODERATELY ABOVE NORMAL LEVELS OVER THE CENTRAL/SOUTHERN  
PARTS OF THE WEST BY EARLY NEXT WEEK.  
 
RAUSCH  
 
WPC MEDIUM RANGE FORECASTS OF 500 MB HEIGHTS, SURFACE SYSTEMS,  
WEATHER GRIDS, QUANTITATIVE PRECIPITATION, AND WINTER WEATHER  
OUTLOOK PROBABILITIES 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  
 

 
 
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