604  
FXUS02 KWBC 190601  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
200 AM EDT THU JUL 19 2018  
 
VALID 12Z SUN JUL 22 2018 - 12Z THU JUL 26 2018  
 
...DANGEROUS MULTI-DAY HEAT WAVE EXPECTED FOR THE SOUTHERN  
PLAINS...  
 
...HEAVY RAINS POSSIBLE ACROSS PORTIONS OF THE EASTERN U.S. SUNDAY  
INTO NEXT WEEK...  
 
   
..OVERVIEW AND GUIDANCE EVALUATION/PREFERENCES
 
 
THE UPPER-LEVEL PATTERN IS EXPECTED TO REMAIN RELATIVELY UNCHANGED  
THROUGH THE MEDIUM RANGE, WITH PERSISTENT UPPER RIDGING FROM THE  
SOUTHWEST TO THE SOUTHERN PLAINS, AND THE TENDENCY FOR THE  
DEVELOPMENT OF DEEP TROUGHING IN THE EAST. INDICATIONS REMAIN THAT  
THIS PATTERN CHANGE WILL NOT BE SHORT-LIVED, WITH SIGNIFICANT  
HEMISPHERIC TELECONNECTIONS BETWEEN THE EVOLVING FLOW PATTERN  
ACROSS NORTH AMERICA AND A PERSISTENT REX BLOCK ACROSS EASTERN  
ASIA (WHICH SHOWS NO SIGNS OF BREAKING DOWN ANY TIME SOON).  
 
AFTER WAVERING IN RECENT DAYS, MODELS HAVE RESUMED THEIR TREND  
TOWARD FURTHER AMPLIFICATION OF THE UPPER TROUGH ACROSS THE  
EASTERN U.S. SUN INTO NEXT WEEK, NOW EVEN SUGGESTING AN INCREASING  
PROBABILITY FOR SOME ENERGY TO CUTOFF ACROSS THE SOUTHEAST BY NEXT  
TUE-WED. AT THE HIGHER LATITUDES, MODELS CONTINUE TO VARY IN TERMS  
OF TIMING AND INTENSITY WITH ADDITIONAL SHORTWAVE ENERGY CROSSING  
CANADA AND SKIRTING THE U.S. NORTHERN TIER SUN-TUE, AND WITH THE  
ASSOCIATED SURFACE LOW PRESSURE SYSTEM. THE WAVE THEN APPEARS SET  
TO UNDERGO ADDITIONAL AMPLIFICATION ACROSS THE GREAT LAKES  
WED-THU, RENEWING THE TROUGHING ACROSS THE EASTERN U.S.  
 
IN GENERAL, CONSENSUS AMONG THE DETERMINISTIC GUIDANCE WAS  
SUFFICIENT TO USE A MULTI-MODEL DETERMINISTIC BLEND (INCLUDING THE  
ECMWF/GFS/UKMET) AS A BASIS FOR THE FORECAST FOR DAYS 3-5  
(SUN-TUE). THE CMC WAS EXCLUDED FROM THE BLEND AS IT CONTINUES TO  
BE TOO WEAK WITH THE SOUTHWEST/SOUTH CENTRAL U.S. RIDGE, WHICH IN  
TURN ALLOWS HEIGHTS TO FALL ACROSS THE PACIFIC NORTHWEST AS SOME  
ENERGY SEPARATES FROM THE HIGHER LATITUDE WAVE. FOR DAYS 6-7  
(WED-THU) WEIGHTING OF ENSEMBLE MEANS (ECENS/NAEFS) WAS INCREASED  
TO COMPRISE A SLIGHT MAJORITY OF THE FORECAST. MODEL SOLUTIONS  
DIFFER WITH RESPECT TO THE DEGREE OF TROUGH AMPLIFICATION ACROSS  
THE GREAT LAKES/OHIO VALLEY BY THIS TIME PERIOD, BUT A TREND  
TOWARD ENSEMBLE MEANS SHOULD BEST REPRESENT THE CURRENT MODEL  
CONSENSUS.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
THE AMPLIFIED UPPER TROUGH, AND ASSOCIATED DECAYING FRONTAL SYSTEM  
IN PLACE ACROSS THE EASTERN U.S. AT THE START OF THE FORECAST  
PERIOD WILL BRING SCATTERED TO NUMEROUS SHOWERS AND THUNDERSTORMS  
TO AREAS FROM THE SOUTHEAST AND OHIO VALLEY TO THE EASTERN  
SEABOARD. THE TREND TOWARD GREATER AMPLIFICATION IN RECENT  
MODEL/ENSEMBLE RUNS HAS INCREASED THE POTENTIAL FOR AREAS OF  
HEAVY/EXCESSIVE RAINFALL ACROSS AREAS OF THE MID-ATLANTIC AND  
NORTHEAST SUN-THU, WITH THE POTENTIAL FOR MORE SIGNIFICANT  
NORTHWARD TRANSPORT OF DEEP MOISTURE. A NUMBER OF DETERMINISTIC  
SOLUTIONS NOW SHOW THE POTENTIAL FOR AREAS OF MULTI-INCH RAINFALL  
TOTALS ACROSS THESE AREAS ACROSS MULTIPLE DAYS, WITH A MODERATE  
DEGREE OF ENSEMBLE SUPPORT AS WELL. THE ARRIVAL OF A SECOND COLD  
FRONT AND AMPLIFYING TROUGH IN THE EAST BY THE MIDDLE OF NEXT WEEK  
WILL ONLY BRING A CONTINUATION OF THE CONVECTION AND LOCALLY HEAVY  
RAINFALL. FARTHER WEST, THE COLD FRONT TRAILING THE LOW PRESSURE  
SYSTEM CROSSING CENTRAL CANADA WILL BRING SCATTERED CONVECTION AND  
LOCALLY HEAVY RAINFALL TO PORTIONS OF THE CENTRAL/NORTHERN PLAINS  
SUN-MON. THE TRAILING END OF THE FRONT APPEARS LIKELY TO STALL  
FROM THE CENTRAL PLAINS INTO THE ROCKIES ON TUE, BRINGING A  
CONTINUED THREAT FOR LOCALLY HEAVY CONVECTION.  
 
THE OTHER STORY CONTINUES TO BE THE HEAT ACROSS THE SOUTHERN  
PLAINS. WITH THE UPPER RIDGE BUILDING OVERHEAD AND A STRONG  
SUBSIDENCE INVERSION LIKELY IN PLACE, TEMPERATURES WILL SOAR WELL  
PAST THE CENTURY MARK FOR MANY AREAS. HIGH TEMPERATURES ARE  
EXPECTED TO BE 5 TO 15 DEG F ABOVE AVERAGE ACROSS MUCH OF THE  
SOUTHERN PLAINS FROM SUN PERHAPS INTO THE MIDDLE OF NEXT WEEK,  
WITH A NUMBER OF RECORD HIGH TEMPERATURES AND RECORD HIGH MINIMUM  
TEMPERATURES POTENTIALLY IN JEOPARDY. RELATIVELY HIGH DEW POINTS  
(NEAR 70 DEG F FOR SOME AREAS) WILL COMBINE WITH THE HOT  
TEMPERATURES TO PRODUCE DANGEROUS HEAT INDEX VALUES 110-115 DEG F  
FOR MANY LOCATIONS. LOW TEMPERATURES NEAR OR EVEN ABOVE 80 DEG FOR  
MANY AREAS WILL AMPLIFY THE POTENTIALLY HAZARDOUS IMPACTS OF THE  
HEAT. THE POTENTIAL ARRIVAL OF A COLD FRONT ACROSS THE SOUTHERN  
PLAINS BY NEXT TUE-WED COULD BRING SOME SLIGHT RELIEF FROM THE  
HEAT DUE TO INCREASES IN CLOUD COVER AND ISOLATED CONVECTION.  
 
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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