197  
FXUS02 KWBC 230647  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
246 AM EDT SAT JUN 23 2018  
 
VALID 12Z TUE JUN 26 2018 - 12Z SAT JUN 30 2018  
   
..OVERVIEW AND GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
OVER THE COURSE OF THE PERIOD THE MAJORITY OF GUIDANCE IS FAIRLY  
CONSISTENT AND AGREEABLE IN SHOWING THE ESTABLISHMENT OF A WESTERN  
U.S. TROUGH WHILE A MEAN RIDGE BECOMES MORE PROMINENT OVER THE  
EASTERN HALF OF THE COUNTRY. AFTER PASSAGE OF A NORTHERN  
PLAINS/UPPER MS VALLEY THROUGH NEW ENGLAND SHORTWAVE DURING THE  
FIRST HALF OF THE PERIOD, THE EXPANDING EASTERN RIDGE SHOULD  
DEFLECT THE WESTERLIES WELL NORTHWARD, INTO THE GREAT LAKES/NEW  
ENGLAND. AT THAT TIME THIS AREA WOULD ALSO REPRESENT THE  
SOUTHEAST PERIPHERY OF A BROAD CYCLONIC CIRCULATION COVERING THE  
EASTERN HALF OF CANADA.  
 
THE GOOD CONTINUITY OF THE ENSEMBLE MEANS THUS FAR, AND  
TELECONNECTION SUPPORT FOR THE WESTERN TROUGH (AND TO SOME DEGREE  
FLOW DOWNSTREAM) RELATIVE TO THE BUILDING EASTERN PACIFIC RIDGE,  
SUGGEST THE PATTERN HAS ABOVE AVERAGE PREDICTABILITY. HOWEVER  
SOME INDIVIDUAL MODEL RUNS AND ENSEMBLE MEMBERS MAINTAIN THE  
POTENTIAL FOR A SOMEWHAT DIFFERENT OUTCOME. THE 12Z GFS SEEMED TO  
PROVIDE ONE OF THE LESS LIKELY SOLUTIONS, WITH A FASTER  
LATE-PERIOD EJECTION OF WESTERN TROUGH ENERGY INTO THE PLAINS  
VERSUS ALL OTHER 12Z GEFS/ECMWF/CMC ENSEMBLE MEMBERS AT SOME  
LOCATIONS. 18Z-00Z GFS RUNS HAVE ADJUSTED BACK TO THE CONSENSUS  
TROUGH AXIS BUT WITH AN EMBEDDED UPPER LOW/TROUGH CORE SOMEWHAT  
DEEPER AND/OR SOUTH OF THE MAJORITY CLUSTER BY DAYS 6-7 FRI-SAT.  
ON THE OTHER HAND CMC RUNS HAVE TENDED TO BE SOMEWHAT FLATTER WITH  
THE OVERALL EASTERN PACIFIC/WESTERN U.S. PATTERN. THE 00Z VERSION  
COMPARES BETTER TO OTHER GUIDANCE THAN THE PRIOR RUN THOUGH.  
UNCERTAINTY OVER THE PRECISE CHARACTER OF THE WESTERN TROUGH LEADS  
TO SOME SPREAD FOR DOWNSTREAM FLOW, WITH ENSEMBLE SPREAD FOR  
576-582DM HEIGHT CONTOURS COVERING MUCH OF SOUTHEASTERN CANADA AND  
NORTHEASTERN U.S. BY LATE IN THE PERIOD. QUESTIONABLE WESTERN  
TROUGH PROGRESSION IN THE 12Z GFS LED TO CORRESPONDING  
QUESTIONABLY HIGH HEIGHTS OVER EASTERN NORTH AMERICA LATE IN THE  
FORECAST.  
 
ANOTHER FORECAST UNCERTAINTY DURING THE PERIOD WILL INVOLVE THE  
ULTIMATE EVOLUTION OF THE CLOSED LOW INITIALLY OVER WESTERN CANADA  
AND FLOW TO ITS EAST/SOUTHEAST. SOLUTIONS SHOW MEANINGFUL  
DIVERGENCE FOR DETAILS FROM ABOUT DAY 5 THU ONWARD. THIS WILL  
AFFECT THE HANDLING OF SURFACE FRONT(S) AFFECTING THE NORTHERN  
STATES. AHEAD OF THIS FEATURE THERE IS BETTER THAN AVERAGE  
AGREEMENT FOR THE NORTHERN PLAINS/UPPER MS VALLEY SYSTEM THAT  
OPENS UP AS IT PROGRESSES EAST-NORTHEASTWARD.  
 
THE UPDATED FORECAST BLEND YIELDED ONLY THE TYPICAL RUN TO RUN  
DETAIL VARIABILITY, WITH GOOD CONTINUITY FOR SIGNIFICANT FEATURES.  
THE 18Z GFS AND 12Z ECMWF/UKMET/CMC PROVIDED INPUT FOR ABOUT THE  
FIRST HALF OF THE PERIOD. THEN THE FORECAST INCORPORATED 50-60  
PERCENT TOTAL OF THE 18Z GEFS/12Z ECMWF MEANS BY DAYS 6-7 FRI-SAT  
GIVEN THEIR RECENT STABILITY AND TO DOWNPLAY LOWER CONFIDENCE  
OPERATIONAL MODEL SPECIFICS. POOR COMPARISON TO CONSENSUS LED TO  
REMOVAL OF THE CMC FROM THE BLEND AFTER DAY 5 THU.  
 
   
..WEATHER THREATS/HIGHLIGHTS
 
 
THE SYSTEM TRACKING EAST-NORTHEAST FROM THE NORTHERN PLAINS/UPPER  
MS VALLEY TUE ONWARD WILL LIKELY PRODUCE ONE OR MORE EPISODES OF  
LOCALLY HEAVY CONVECTION. BEST POTENTIAL FOR AREAS OF ENHANCED  
RAINFALL SHOULD EXIST FROM THE UPPER-MID MS VALLEY THROUGH THE  
GREAT LAKES AND OHIO VALLEY. SOME POCKETS OF MODERATE TO HEAVY  
RAINFALL MAY EXTEND INTO THE NORTHEAST BUT SHOULD GENERALLY BE  
MORE SCATTERED AS THE UPPER SYSTEM WEAKENS. THE TRAILING FRONT  
PROGRESSING ACROSS THE NORTHERN TIER STATES WILL STALL OVER THE  
PLAINS WITH AT LEAST A PORTION LIKELY TO RETURN NORTH AS A WARM  
FRONT BY FRI-SAT. THE COMBINATION OF THIS LOW LEVEL FOCUS AND  
INCREASING TENDENCY FOR SHORTWAVE IMPULSES TO EJECT FROM THE  
EVOLVING WESTERN TROUGH ALOFT MAY PROMOTE PERIODS OF  
CONVECTION/RAINFALL--SOME POTENTIALLY HEAVY--FROM THE NORTHERN  
ROCKIES EASTWARD BY LATE WEEK/WEEKEND. THE WESTERN TROUGH SHOULD  
ALSO BRING UNSETTLED WEATHER TO OTHER PARTS OF THE NORTHERN HALF  
OF THE WEST THOUGH WITH LIGHTER AMOUNTS. MEANWHILE EXPECT PERIODS  
OF CONVECTION OVER THE SOUTHEAST/FLORIDA DURING THE PERIOD WITH  
SOME HEAVY ACTIVITY POSSIBLE. A GRADUALLY WEAKENING FRONT OVER  
THE REGION WILL PROVIDE SOME ADDED LOW LEVEL FOCUS DURING THE  
FIRST HALF OF THE PERIOD.  
 
THE FORECAST IS STILL ON TRACK FOR HOT/HUMID CONDITIONS TO BECOME  
ESTABLISHED OVER AN INCREASINGLY LARGE PORTION OF THE  
CENTRAL/EASTERN STATES. LOCATIONS FROM THE CENTRAL PLAINS TO MID  
ATLANTIC/NORTHEAST WILL HAVE THE BEST POTENTIAL FOR SEEING ONE OR  
MORE DAYS WITH MIN AND/OR MAX READINGS REACHING AS HIGH AS 10-15F  
ABOVE NORMAL. ADDING IN THE EFFECTS OF HUMIDITY, HIGHEST HEAT  
INDEX VALUES SHOULD EXPAND FROM THE SOUTH EARLY IN THE PERIOD TO A  
LARGE AREA CENTERED OVER/NEAR THE MID MS VALLEY AS WELL AS  
SOUTHERN MID ATLANTIC/SOUTHEAST. THE GREAT BASIN/FOUR CORNERS  
REGION WILL SEE WARM TEMPERATURES BUT WITH SOME GRADUAL COOLING  
LATER IN THE PERIOD WITH THE ARRIVAL OF THE WESTERN TROUGH ALOFT.  
THIS FEATURE SHOULD BRING COOLEST ANOMALIES (5-15F BELOW NORMAL  
FOR HIGHS) INTO THE NORTHERN PARTS OF THE WEST BY NEXT SAT.  
 
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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