695  
FXUS02 KWBC 171600  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1200 PM EDT TUE JUL 17 2018  
 
VALID 12Z FRI JUL 20 2018 - 12Z TUE JUL 24 2018  
 
...DANGEROUS HEAT WAVE EXPECTED FOR THE SOUTHERN PLAINS BY LATER  
THIS WEEK...  
 
   
..OVERVIEW AND GUIDANCE EVALUATION/PREFERENCES
 
 
THE LATEST MODEL GUIDANCE AND A QUICK LOOK AT HEMISPHERIC  
TELECONNECTIONS CONTINUES TO INCREASE THE CONFIDENCE THAT A  
PATTERN CHANGE WILL BE ONGOING AT THE START OF THE MEDIUM RANGE  
PERIOD (FRIDAY) ACROSS THE CONTIGUOUS U.S.. MODELS CONTINUE TO  
SHOW AN UPPER-LEVEL RIDGE EXPANDING EASTWARD FROM THE GREAT  
BASIN/SOUTHWEST TO THE SOUTHERN PLAINS, AND AMPLIFICATION OF A  
TROUGH ACROSS THE GREAT LAKES/OHIO VALLEY AND INTO THE CENTRAL  
APPALACHIANS/MID-ATLANTIC. EXPECT A SURFACE LOW PRESSURE SYSTEM TO  
ACCOMPANY THE EASTERN TROUGH, WITH A SURFACE FRONTAL BOUNDARY  
TRAILING BACK INTO THE SOUTHERN PLAINS. ADDITIONAL SHORTWAVE  
ENERGY CROSSING THE PACIFIC NORTHWEST AND WESTERN CANADA LATE THIS  
WEEK WILL DRIVE A COLD FRONT EASTWARD INTO THE NORTHERN GREAT  
BASIN AND ROCKIES, WHICH REACHES THE PLAINS AND UPPER MIDWEST BY  
EARLY NEXT WEEK.  
 
MODELS SHOWED VERY GOOD AGREEMENT WITH RESPECT TO THE OVERALL  
LARGE SCALE FLOW THROUGH DAY 7, WITH ANY DIFFERENCES MOSTLY  
CONFINED TO THE USUAL TIMING/AMPLITUDE DISAGREEMENTS WITH  
INDIVIDUAL FEATURES MAINLY LATE IN THE PERIOD. THERE CONTINUES TO  
BE SOME QUESTION WITH WHETHER OR NOT SHORTWAVE ENERGY IN THE  
PACIFIC NORTHWEST WILL LOWER HEIGHTS AGAIN OUT WEST, POSSIBLY  
BREAKING DOWN THE SOUTHWEST RIDGE SLIGHTLY, AS SHOWN BY THE LAST  
FEW RUNS OF THE GFS. THIS CYCLE OF THE WPC MEDIUM RANGE  
500/SURFACE PROGS USED A BLEND OF THE LATEST RUNS OF THE  
DETERMINISTIC MODELS (INCLUDING THE ECMWF/GFS/UKMET/CMC) FOR DAYS  
3-4, WITH INCREASING CONTRIBUTIONS FROM THE ENSEMBLE MEANS  
(ECENS/GEFS) BY DAYS 5-7 TO ACCOUNT FOR THE USUAL LATE PERIOD  
DIFFERENCES WITH SMALLER SCALE FEATURES.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
THE FRONTAL SYSTEM CROSSING THE GREAT LAKES/OHIO VALLEY FRI-SAT  
WILL BE ACCOMPANIED BY SCATTERED TO NUMEROUS SHOWERS AND  
THUNDERSTORMS, SOME OF WHICH COULD PRODUCE LOCALLY HEAVY RAINFALL.  
A LINGERING STATIONARY/WARM FRONT WILL ALSO PRODUCE CONVECTION  
ACROSS THE SOUTHEAST. THE STRONGEST SIGNAL AMONG THE GUIDANCE FOR  
HEAVY RAINFALL CONTINUES TO BE ALONG THE IMMEDIATE SOUTHEAST  
COASTLINE FRI-SAT, IN CLOSE PROXIMITY TO THE FRONTAL BOUNDARY. THE  
UPPER TROUGH IS EXPECTED TO LINGER ACROSS THE EASTERN U.S. INTO  
EARLY NEXT WEEK AS THE FRONTAL BOUNDARY BEGINS TO WASH OUT,  
KEEPING SHOWERS AND THUNDERSTORMS A POSSIBILITY FOR MUCH OF THE  
EAST. FARTHER WEST, THE FRONTAL SYSTEM CROSSING THE NORTHERN  
ROCKIES FRI-SAT WILL GAIN ACCESS TO DEEPER MOISTURE BY THE TIME IT  
REACHES THE NORTHERN PLAINS SUN-MON, BRINGING LOCALLY HEAVY  
CONVECTION TO THE CENTRAL/NORTHERN PLAINS AND BACK INTO THE  
CENTRAL ROCKIES.  
 
THE BIGGEST STORY IN THE MEDIUM RANGE PERIOD 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 FRI INTO NEXT WEEK, WITH A  
NUMBER OF RECORD HIGH TEMPERATURES AND RECORD HIGH MINIMUM  
TEMPERATURES POTENTIALLY IN JEOPARDY. HIGH DEW POINTS WILL COMBINE  
WITH THE HOT TEMPERATURES TO PRODUCE DANGEROUS HEAT INDEX VALUES  
110-115 DEG F FOR MANY LOCATIONS. LOW TEMPERATURES NEAR 80 DEG FOR  
MANY AREAS WILL ADD TO THE POTENTIALLY HAZARDOUS IMPACTS OF THE  
HEAT.  
 
SANTORELLI/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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