778  
FXUS02 KWBC 220649  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
249 AM EDT FRI JUN 22 2018  
 
VALID 12Z MON JUN 25 2018 - 12Z FRI JUN 29 2018  
   
..OVERVIEW AND GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
THE MAJORITY OF MODEL AND ENSEMBLE GUIDANCE AGREES FAIRLY WELL  
WITH THE LARGE SCALE EVOLUTION DURING NEXT WEEK. EXPECT UPPER  
RIDGING INITIALLY OVER LOWER LATITUDES OF THE CONTIGUOUS U.S. TO  
EXPAND ITS INFLUENCE OVER THE EASTERN HALF OF THE COUNTRY WITH  
TIME, FIRST WITH THE DEPARTURE OF A NORTHEAST/MID ATLANTIC TROUGH  
AND THEN BEHIND AN UPPER LOW/TROUGH TRACKING NORTHEASTWARD FROM  
THE HIGH PLAINS. A VIGOROUS SYSTEM ENTERING WESTERN CANADA WILL  
CONTINUE EASTWARD WHILE A RIDGE BUILDING OVER THE EASTERN PACIFIC  
BY THE LATTER HALF OF THE WEEK WILL SUPPORT AN UPPER TROUGH THAT  
DEVELOPS ALONG OR MORE LIKELY A BIT INLAND FROM THE WEST COAST.  
 
FOR THE DAYS 3-5 MON-WED PERIOD AN OPERATIONAL MODEL BLEND (18Z  
GFS/12Z ECMWF AND SOMEWHAT LESSER WEIGHT OF THE 12Z UKMET/12Z CMC)  
EITHER REPRESENTED CONSENSUS OR PROVIDED A GOOD INTERMEDIATE  
SOLUTION IN THE CASE OF MORE PRONOUNCED DIFFERENCES. THE LATTER  
SITUATION APPLIED TO THE DEPARTING EAST COAST TROUGH. THE 12Z  
ECMWF AND MANY OF ITS ENSEMBLE MEMBERS SHOWED A FASTER DEPARTURE  
THAN OTHER GUIDANCE WHILE THE GFS HAD ADJUSTED TO THE SLOWER SIDE  
OF THE SPREAD. BOTH MODELS HAVE DISPLAYED SOME RUN TO RUN  
VARIABILITY SO A COMPROMISE APPEARED BEST. NEW 00Z RUNS THUS FAR  
SEEM TO SUPPORT THAT IDEA WITH THE GFS TRENDING FASTER THAN ITS  
12Z/18Z RUNS BUT THE GFS/UKMET/CMC REMAINING SLOWER THAN THE 12Z  
ECMWF TO VARYING DEGREES. GUIDANCE BEHAVIOR AND SPREAD HOLDS  
CONFIDENCE DOWN TO MODERATE LEVELS FOR THIS FEATURE. DIFFERENCES  
FOR THE SYSTEM TRACKING FROM THE PLAINS INTO THE GREAT LAKES AND  
UPSTREAM SOUTHERN CANADA LOW PRESSURE/FRONT TRAILING INTO THE  
NORTHERN PLAINS-WESTERN U.S. WERE GENERALLY WITHIN TYPICAL  
GUIDANCE ERROR FOR DAYS 3-5 FORECASTS.  
 
BY DAYS 6-7 THU-FRI THE FORECAST INCORPORATED 30-60 PERCENT  
ENSEMBLE GUIDANCE BY WAY OF THE 18Z GEFS/12Z ECMWF MEAN AS DETAIL  
UNCERTAINTY INCREASED, THOUGH THE OPERATIONAL RUNS WERE CLOSE  
ENOUGH TO THE MEANS TO MAINTAIN SOME WEIGHT THROUGH THE END OF THE  
FORECAST. THE 12Z CMC WAS PHASED OUT AFTER THU DUE TO DEVELOPING  
A STRONGER THAN CONSENSUS EASTERN CANADA/GREAT LAKES UPPER  
TROUGH--A TRAIT THAT HAS CARRIED INTO ITS NEW 00Z RUN. REGARDING  
THE DEVELOPING WESTERN TROUGH, RECENT ECMWF RUNS HAVE TENDED TO  
SHOW THE AXIS NEAR THE WEST COAST BUT TELECONNECTIONS RELATIVE TO  
THE UPSTREAM RIDGE'S POSITIVE HEIGHT ANOMALY CENTER SUGGEST THE  
TROUGH AXIS COULD END UP A LITTLE FARTHER EASTWARD. THERE ARE  
STILL SHORTWAVE DETAIL QUESTIONS THAT LOWER CONFIDENCE IN  
SPECIFICS WITHIN THE MORE AGREEABLE MEAN TROUGH.  
 
   
..WEATHER THREATS/HIGHLIGHTS
 
 
AN AREA EXTENDING FROM THE NORTHERN-CENTRAL PLAINS INTO THE  
WESTERN GREAT LAKES WILL BE ONE FOCUS FOR HEAVY RAINFALL DURING  
THE PERIOD. THE MAJORITY OF CONVECTION WILL BE SUPPORTED BY THE  
SYSTEM TRACKING FROM THE PLAINS INTO GREAT LAKES MON-WED. SOME  
MOISTURE WILL EVENTUALLY EXTEND INTO THE EAST BUT RAIN SHOULD  
TREND LIGHTER AS UPPER DYNAMICS WEAKEN/LIFT FARTHER NORTH. THE  
FRONT MOVING INTO THE WEST EARLY IN THE WEEK MAY BRING  
LIGHT/SCATTERED PRECIPITATION TO EXTREME NORTHERN PARTS OF THE  
WEST AND THEN SOME LIGHT-MODERATE RAINFALL OVER THE NORTHERN  
PLAINS. AS THIS FRONT STALLS, IT MAY BECOME A MORE PROMINENT  
FOCUS FOR CONVECTION OVER THE NORTHERN TIER STATES TOWARD THE END  
OF THE WEEK. THIS LATTER POTENTIAL EPISODE WILL HAVE TO BE  
MONITORED FOR ANY OVERLAP WITH LOCATIONS RECEIVING SIGNIFICANT  
RAINFALL EARLIER IN THE WEEK. MEANWHILE SOME AREAS OVER THE  
SOUTHEAST/FLORIDA MAY SEE ONE OR MORE DAYS OF LOCALLY HEAVY  
RAINFALL IN DIURNALLY FAVORED CONVECTION. A COLD FRONT DROPPING  
INTO THE SOUTHEAST AND EVENTUALLY WEAKENING WILL LIKELY ENHANCE  
SHOWERS/THUNDERSTORMS AT TIMES.  
 
AFTER EASTERN U.S. SURFACE HIGH PRESSURE DEPARTS INTO THE ATLANTIC  
AFTER TUE, THE EXPECTED PATTERN EVOLUTION AT THE SURFACE AND ALOFT  
SHOULD PROMOTE AN EXPANDING AREA OF HEAT/HUMIDITY OVER THE  
CENTRAL/EASTERN STATES. GREATEST ANOMALIES FOR MIN/MAX  
TEMPERATURES (PLUS 10-15F) BY THU-FRI SHOULD EXTEND FROM PARTS OF  
THE CENTRAL PLAINS THROUGH THE MS VALLEY INTO PORTIONS OF THE  
GREAT LAKES/OH VALLEY. DEW POINTS REACHING WELL INTO THE 70'S F  
OVER SOME AREAS MAY CONTRIBUTE TO HIGH HEAT INDEX VALUES. ANOTHER  
AREA OF PERSISTENT WARMTH ALBEIT WITH LOWER HUMIDITY SHOULD BE  
OVER THE GREAT BASIN/FOUR CORNERS REGION, WITH MIN TEMPS TENDING  
TO BE SOMEWHAT MORE ABOVE NORMAL THAN MAXES. THE INITIAL HIGH  
PLAINS SYSTEM WILL LEAD TO COOL DAYTIME HIGHS ON MON. THE  
NORTHWEST WILL SEE NEAR TO BELOW NORMAL HIGHS BEHIND THE EARLY  
WEEK FRONT AND THEN WITH UPPER TROUGHING MOVING IN LATE IN THE  
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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