706  
FXUS02 KWBC 250651  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
250 AM EDT MON JUN 25 2018  
 
VALID 12Z THU JUN 28 2018 - 12Z MON JUL 02 2018  
   
..OVERVIEW
 
 
ONCE THE AMPLIFIED WESTERN TROUGH/EASTERN RIDGE PATTERN BECOMES  
ESTABLISHED LATE THIS WEEK, GUIDANCE HAS DISPLAYED INCREASED  
DIVERGENCE OVER THE PAST DAY OR SO REGARDING SPECIFICS OF HOW/WHEN  
WESTERN TROUGH ENERGY EJECTS DOWNSTREAM. THUS CONFIDENCE HAS  
DECLINED FOR THE CORRESPONDING SURFACE EVOLUTION OVER THE CENTRAL  
U.S. BY THE SAT-MON TIME FRAME. MEANWHILE GUIDANCE IS DIVERSE AND  
VARIABLE WITH WHAT TYPE OF SURFACE REFLECTION COULD EVOLVE  
NEAR/OFFSHORE THE EAST COAST IN CONNECTION WITH ENERGY THAT  
ORIGINATES FROM AN UPSTREAM CONVECTIVE COMPLEX IN THE SHORT RANGE.  
ONCE THE CORE OF EASTERN RIDGING SETTLES OVER THE OHIO VALLEY/MID  
ATLANTIC REGION, ADDITIONAL ENERGY ALOFT AND CORRESPONDING SURFACE  
FEATURE COULD EVOLVE TO THE SOUTH NEAR THE SOUTHEAST  
COAST/FLORIDA.  
 
   
..GUIDANCE EVALUATION/PREFERENCES
 
 
ONE SOURCE OF THE GUIDANCE SPREAD FOR EJECTION OF WESTERN TROUGH  
ENERGY LIES WITH THE TIMING OF AN UPPER LOW INITIALLY NEAR THE  
EASTERN ALEUTIANS/ALASKA PENINSULA. THE 12Z ECMWF/ECMWF MEAN WERE  
THE FASTEST TO BRING THIS FEATURE AROUND THE STRONG EASTERN  
PACIFIC RIDGE, LIKELY AIDING IN FASTER EJECTION DOWNSTREAM AND  
RESULTING IN A BROADER/MORE SHALLOW MEAN TROUGH ACROSS THE  
NORTHERN U.S. BY THE LATTER HALF OF THE PERIOD. CMC RUNS ARE  
SLOWER WITH THE UPSTREAM UPPER LOW BUT USE OTHER SHEARED ENERGY  
FLOWING AROUND THE RIDGE TO KICK OUT LEADING WESTERN TROUGH  
ENERGY. ECMWF-BASED GUIDANCE HAS ACTUALLY TRENDED TOWARD FASTER  
PROGRESSION OF HEIGHT FALLS INTO THE UPPER MS VALLEY AND GREAT  
LAKES LATE IN THE PERIOD--IN CONTRAST TO THE GEFS MEAN WHOSE PAST  
DAY OF RUNS HAVE TRENDED STRONGER WITH THE UPPER RIDGE COVERING  
THE NORTHEAST QUADRANT OF THE CONTIGUOUS U.S. GFS RUNS HAVE BEEN  
FAIRLY PERSISTENT IN DEPICTING THE MOST AMPLIFIED TROUGH-RIDGE  
PATTERN. CONTINUITY LEANED ABOUT 2/3 IN THE GFS/GEFS/NAEFS  
DIRECTION AND SEE NO REASON TO ALTER THE PHILOSOPHY AT THIS TIME  
GIVEN THE FURTHER INCREASE IN GUIDANCE SPREAD SINCE 12-24 HOURS  
AGO AND FASTER THAN CONSENSUS TIMING OF THE EASTERN ALEUTIANS/AK  
PENINSULA UPPER LOW IN THE ECMWF/ECMWF MEAN. IN ADDITION THE 12Z  
CMC MEAN WAS CLOSER TO THE GFS/GEFS SCENARIO THAN THE CMC/ECMWF  
IDEA. HOWEVER NOTE THAT TELECONNECTIONS RELATIVE TO THE CORE OF  
POSITIVE HEIGHT ANOMALIES OVER THE EASTERN PACIFIC DO OFFER  
EVENTUAL POTENTIAL FOR THE EASTERN RIDGE TO BE SOMEWHAT LESS  
PRONOUNCED THAN FORECAST BY THE GFS/GEFS. ONE LAST NOTE, LOOKING  
AT GUIDANCE JUST BEYOND DAY 7 MON SUGGESTS THAT EARLY NEXT WEEK  
MAY FEATURE A RETROGRESSION OF THE EASTERN PACIFIC/WESTERN U.S.  
PATTERN. THIS TRANSITION MAY LEAD TO A PERIOD OF INCREASED  
UNCERTAINTY AS WELL.  
 
SPREAD AND RECENT VARIABILITY AMONG MODELS/ENSEMBLES DO NOT  
INSPIRE CONFIDENCE FOR THE FORECAST NEAR THE EAST COAST. THE PAST  
TWO ECMWF RUNS WERE WILDLY DIVERGENT FOR ATLANTIC LOW PRESSURE  
ORIGINATING FROM A PLAINS CONVECTIVE COMPLEX. CMC RUNS ARE  
TRACKING LOW PRESSURE OFF THE NORTHEASTERN COAST WHILE GFS RUNS  
ARE QUITE DIFFUSE. ADDITIONAL ENERGY MAY GRAVITATE TOWARD THE  
SOUTHEAST COAST/FLORIDA AS THE CORE OF THE EASTERN RIDGE ALOFT  
REACHES THE OHIO VALLEY/NORTHERN MID ATLANTIC. MANUAL PROGS  
MAINTAIN YESTERDAY'S WPC/NHC-COORDINATED DEPICTION OF A SURFACE  
TROUGH/WEAK LOW NEAR THE SOUTHEAST COAST/FLORIDA.  
 
THE FIRST HALF OF THE FORECAST INCORPORATED PORTIONS OF THE  
12Z-18Z GFS/GEFS AND 12Z ECMWF/ECMWF MEAN/UKMET, WITH LESS THAN  
TYPICAL WEIGHT OF THE ECMWF DUE TO ITS LOW CONFIDENCE FEATURE OFF  
THE EAST COAST. THE CMC WAS EXCLUDED DUE TO NORTHEASTERN  
TROUGHING THAT BECOME FAIRLY EXTREME COMPARED TO THE ENSEMBLE  
ENVELOPE. THE NEW 00Z CMC APPEARS MORE REASONABLE WITH A TREND  
TOWARD CONSENSUS. THE REMAINDER OF THE FORECAST STARTED WITH 2/3  
TOTAL GFS/GEFS/NAEFS AND 1/3 ECMWF MEAN.  
 
   
..WEATHER THREATS/HIGHLIGHTS
 
 
A SYSTEM AFFECTING THE NORTHEAST AROUND THE START OF THE PERIOD  
WILL BE ACCOMPANIED BY AREAS OF RAINFALL, SOME OF WHICH MAY BE  
MODERATE TO HEAVY. BEHIND THIS SYSTEM THE FOCUS FOR SIGNIFICANT  
RAINFALL WILL TURN TO AN AREA ENCOMPASSING THE NORTHERN  
ROCKIES/HIGH PLAINS TO UPPER GREAT LAKES AND POSSIBLY EXTENDING  
SOUTHWARD CLOSE TO THE CENTRAL PLAINS. A PERSISTENT FRONTAL  
BOUNDARY WITH ONE OR MORE WAVES, ALONG WITH SHORTWAVE ENERGY  
EJECTING FROM THE WESTERN TROUGH ALOFT, WILL LIKELY PROMOTE  
MULTIPLE CONVECTIVE EPISODES WHICH MAY PRODUCE HEAVY RAINFALL AT  
SOME LOCATIONS. GUIDANCE SPREAD FOR DETAILS ALOFT KEEPS  
CONFIDENCE LOW FOR THE DETAILS OF TIMING/LOCATION WITHIN A BROADER  
PATTERN EVOLUTION THAT SHOULD SUPPORT THE HEAVY RAINFALL THREAT.  
FARTHER WEST THE UPPER TROUGH WILL BRING SCATTERED RAINFALL WITH  
LIGHTER AMOUNTS. MEANWHILE THE SOUTHEAST/FLORIDA SHOULD SEE AREAS  
OF LOCALLY HEAVY CONVECTION DURING THE PERIOD. UPPER LEVEL ENERGY  
AND POSSIBLE SURFACE REFLECTION TO THE SOUTH OF THE DEEP LAYER  
RIDGE OVER THE OHIO VALLEY/MID ATLANTIC MAY PROVIDE SOME  
ENHANCEMENT OF THIS ACTIVITY.  
 
EXPECT A BROAD AREA OF HOT/HUMID WEATHER ACROSS MUCH OF THE  
CENTRAL-EASTERN U.S. DURING THE PERIOD. HIGHEST TEMPERATURE  
ANOMALIES OF PLUS 10-20F SHOULD EXTEND FROM THE CENTRAL  
PLAINS/MIDWEST THROUGH THE NORTHEAST. DAILY RECORDS ARE POSSIBLE  
FOR BOTH HIGHS AND WARM LOWS, WITH WARM LOW RECORDS LIKELY TO BE  
MORE NUMEROUS. IN ADDITION HUMIDITY WILL BE SUFFICIENT TO YIELD  
VERY HIGH HEAT INDEX VALUES OVER MANY AREAS. ON THE OTHER SIDE OF  
THE TEMPERATURE SPECTRUM, MUCH OF THE WEST AND NORTHERN HIGH  
PLAINS WILL SEE MULTIPLE DAYS WITH BELOW NORMAL HIGHS. THE  
NORTHERN ROCKIES AND HIGH PLAINS SHOULD SEE THE MOST EXTREME  
ANOMALIES WITH HIGHS AS MUCH AS 10-15F BELOW NORMAL FRI-SUN.  
 
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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