631  
FXUS02 KWBC 181527  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1126 AM EDT WED JUL 18 2018  
 
VALID 12Z SAT JUL 21 2018 - 12Z WED JUL 25 2018  
 
...DANGEROUS MULTI-DAY HEAT WAVE EXPECTED FOR THE SOUTHERN  
PLAINS...  
   
..16Z UPDATE
 
 
MINOR CHANGES FROM THE OVERNIGHT PACKAGE AS THE MODELS/ENSEMBLES  
REMAINED IN RATHER GOOD AGREEMENT. REFRESHED THE 500MB AND SURFACE  
FRONTS/PRESSURES WITH THE 00Z/06Z GUIDANCE, USING A DETERMINISTIC  
BLEND TO START (GFS/ECMWF/UKMET/CANADIAN) TRENDING TOWARD A 50/50  
DETERMINISTIC/ENSEMBLE BLEND BY NEXT WEDNESDAY. THE 00Z  
ECMWF/CANADIAN FORGED THE BEST CLUSTER WITH THE ENSEMBLE MEANS  
ACROSS SOUTHERN CANADA EARLY NEXT WEEK AS THE 00Z/06Z GFS RUNS  
APPEARED TOO FAR NORTH WITH THE SFC LOW, BUT WERE WITHIN TOLERANCE  
FARTHER SOUTH WITH THE FRONTAL TIMING. UPPER LOW IN THE EAST  
SHOULD LINGER THROUGH THE PERIOD WITH NO PLACE TO GO, BLOCKED BY  
DOWNSTREAM BUILDING UPPER RIDGING IN THE ATLANTIC. THE INTENSE  
HEAT CENTERED OVER TEXAS WILL BE THE BIGGEST THREAT DURING THE  
PERIOD WITH MANY DAYS NEAR/ABOVE RECORD HIGH TEMPERATURES (AND  
RECORD WARM MINIMUM TEMPERATURES).  
 
FRACASSO  
 
PREVIOUS DISCUSSION BELOW...  
   
..OVERVIEW
 
 
CONFIDENCE IS HIGH THAT SIGNIFICANT UPPER-LEVEL FLOW AMPLIFICATION  
WILL OCCUR ACROSS THE CONUS BY DAY 3 (SAT), WITH A STRONG AND  
EXPANSIVE UPPER RIDGE TAKING HOLD FROM THE GREAT BASIN TO THE  
SOUTHERN PLAINS AND LOWER MISSISSIPPI VALLEY, AND A DEEPENING  
TROUGH AXIS ACROSS THE GREAT LAKES/OHIO VALLEY. A SURFACE LOW  
PRESSURE SYSTEM IS EXPECTED TO ACCOMPANY THE AMPLIFYING TROUGH  
ACROSS THE EASTERN THIRD OF THE COUNTRY SAT-MON, BEFORE EVENTUALLY  
BECOMING STATIONARY AND WASHING OUT. ADDITIONAL SHORTWAVE ENERGY  
CROSSING THE PACIFIC NORTHWEST AND WESTERN CANADA ON SAT WILL  
BRING A PACIFIC COLD FRONT INTO THE NORTHERN GREAT BASIN AND  
ROCKIES, REACHING THE PLAINS BY EARLY NEXT WEEK. MODELS SHOW  
GENERAL CONSENSUS THAT THIS WAVE WILL UNDERGO ADDITIONAL  
AMPLIFICATION TUE-WED AS IT REACHES THE UPPER MIDWEST AND GREAT  
LAKES, SETTING THE STAGE FOR AN AMPLIFIED TROUGH ACROSS THE  
EASTERN U.S. ONCE AGAIN. HEMISPHERIC TELECONNECTIONS CONTINUE TO  
SUGGEST THAT THIS PATTERN CHANGE WILL NOT BE SHORT-LIVED, WITH A  
PERSISTENT REX BLOCK ACROSS EASTERN ASIA FAVORING UPPER RIDGING  
ACROSS THE SOUTHWEST AND SOUTH CENTRAL U.S., A RELATIVELY STRONG  
VORTEX ACROSS THE ARCTIC, AND AN EQUATORWARD EXTENSION IN THE FORM  
OF FAVORED TROUGHING ACROSS HUDSON BAY AND THE GREAT LAKES.  
 
MODELS/ENSEMBLES SHOWED GOOD CONSENSUS WITH RESPECT TO THE LARGE  
SCALE PATTERN, AND DIFFERENCES WITH RESPECT TO SMALLER SCALE  
FEATURES WERE PRIMARILY CONFINED TO TIMING/AMPLITUDE THROUGH MUCH  
OF THE FORECAST PERIOD. AS A RESULT, A MULTI-MODEL DETERMINISTIC  
BLEND (INCLUDING THE ECMWF/GFS/UKMET/CMC) SERVED AS A BASIS FOR  
THE FORECAST DURING DAYS 3-5 (SAT-MON). THE 12Z CMC WAS TOO WEAK  
WITH THE SOUTHERN TIER UPPER RIDGE RELATIVE TO CONSENSUS BY DAYS  
4-5, RESULTING IN THE AMPLIFICATION OF SHORTWAVE ENERGY/LOWERING  
OF HEIGHTS ACROSS THE PACIFIC NORTHWEST - AND THIS SOLUTION WAS  
WEIGHTED LESS RELATIVE TO THE OTHER GUIDANCE AFTER DAY 3 AS A  
RESULT. DURING DAYS 6-7, AS SPREAD GRADUALLY INCREASED AMONG THE  
GUIDANCE, WEIGHTING OF ENSEMBLE MEANS (ECENS/NAEFS) WAS BOOSTED TO  
COMPRISE A SLIGHT MAJORITY OF THE FORECAST. MODELS DIFFER AS TO  
EXACTLY HOW QUICKLY THE UPPER TROUGH WILL BEGIN TO LIFT OUT OF THE  
EASTERN U.S. AHEAD OF THE NEXT SYSTEM CROSSING THE THE NORTHERN  
TIER. ADDITIONALLY, THE ECMWF BECAME A BIT FASTER THAN CONSENSUS  
WITH THE PROGRESSION OF THE SECOND WAVE ACROSS THE UPPER MIDWEST  
TUE-WED, AND A SOLUTION A BIT CLOSER TO THE GFS AND ENSEMBLE MEANS  
WAS PREFERRED.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
SCATTERED TO NUMEROUS SHOWERS AND THUNDERSTORMS WITH LOCALLY HEAVY  
RAINFALL WILL BE POSSIBLE SAT-SUN FROM THE GREAT LAKES TO THE  
SOUTHEAST AND SOUTHERN MID-ATLANTIC REGION IN ASSOCIATION WITH THE  
AMPLIFYING UPPER TROUGH AND SURFACE FRONTAL SYSTEM. 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 SAT  
WILL GAIN ACCESS TO DEEPER MOISTURE BY THE TIME IT REACHES THE  
NORTHERN PLAINS SUN-TUE, BRINGING LOCALLY HEAVY CONVECTION TO  
PORTIONS OF THE PLAINS AND 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 SAT INTO 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.  
 
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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