188  
FXUS02 KWBC 191603  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1202 PM EDT THU JUL 19 2018  
 
VALID 12Z SUN JUL 22 2018 - 12Z THU JUL 26 2018  
   
..HEAT WAVE TO INTENSIFY OVER THE SOUTHWEST STATES NEXT WEEK
 
 
...HEAVY RAIN / FLASH FLOODING EXPECTED OVER THE EASTERN U.S. NEXT  
WEEK...  
   
..OVERVIEW AND GUIDANCE EVALUATION/PREFERENCES
 
 
TROUGHS ARE OF MEDIUM TO LONG WAVELENGTH AND FAIRLY SLOW TO EVOLVE  
GIVEN THE SEASON. THEY ARE ALSO RELATIVELY WELL DEFINED, WITH ONE  
CLOSED LOW CENTER EXPECTED IN THE EAST EARLY NEXT WEEK, EVENTUALLY  
REPLACED BY ANOTHER WELL DEFINED UPSTREAM TROUGH DIGGING INTO THE  
GREAT LAKES BY WED/THU. SO THE MAIN STORM TRACK IS UP AND OVER THE  
SOUTHERN PLAINS RIDGE, AND THIS MEAN TROUGH IN THE  
NORTHERN/EASTERN U.S. HAS AMPLE ACCESS TO A TROPICAL MOISTURE  
PLUME THAT BACKS IN OFF THE ATLANTIC INTO THE EASTERN QUARTER OF  
THE NATION. MEANWHILE BENEATH THE RIDGE, THE HEAT WAVE IN THE  
SOUTHERN PLAINS THIS WEEKEND THEN EXPANDS WESTWARD AS THE RIDGE  
BECOMES REPOSITIONED, WITH THE 594 DECAMETER 500-MB HEIGHT  
COVERING A VERY BROAD EXPANSE BY TUESDAY. IN SOME PLACES LIKE  
NORTHERN MEXICO AND SOUTHERN AZ/CA, WHERE MONSOON SEASON IS MORE  
COMMONLY IN FULL SWING BY NOW, THESE HEIGHTS PEAK AT 3.5 STANDARD  
DEVIATIONS ABOVE CLIMATOLOGY. STANDARD DEVIATIONS OF +2.0 EXTEND  
AS FAR NORTH AS SAN FRANCISCO AND SALT LAKE CITY.  
 
MUCH OF THIS MEDIUM RANGE FORECAST IS OF HIGH CONFIDENCE DURING  
THIS CYCLE, WITH STRONG AGREEMENT / CLUSTERING AMONG THE MODELS  
REGARDING THESE WELL DEFINED AND SLOWLY EVOLVING FEATURES. MOST OF  
THE MORE NOTICEABLE MODEL DIFFERENCES ARE ACROSS THE VERY HIGH  
LATITUDES, WHEREAS THE FORECAST OVER THE CONUS SHOWS LESS SPREAD.  
WE THEREFORE BEGAN WITH A SIMPLE GFS/ECMWF BLEND, BUT BEGAN TO  
WORK IN MORE OF THE GEFS AND ECMWF ENSEMBLE MEAN. THE OPERATIONAL  
00Z AND 06Z GFS WERE THE FIRST TO BE DROPPED FROM THE BLEND AS  
THEY WERE QUITE AGGRESSIVE IN DIGGING THE CLOSED LOW SOUTHWESTWARD  
ALONG THE GULF COAST D4-5. THE OPERATIONAL ECMWF SAW MORE ENSEMBLE  
SUPPORT IN THIS REGARD, AND PERHAPS THE GFS SOLUTIONS IS BEING  
DRIVEN MORE BY CONVECTIVE FEEDBACK ONTO THE SYNOPTICS. BY DAYS 5-7  
WE WERE USING ROUGHLY A 40/40/20 BLEND OF GEFS MEAN / EC MEAN /  
OPERATIONAL ECMWF.  
   
..WEATHER HIGHLIGHTS/THREATS
 
 
SIGNIFICANT HAZARDS EXPECTED OVER BROAD AREAS OF THE COUNTRY THE  
NEXT SEVEN DAYS. HEAT STRESS WILL BE A MAJOR STORY IN THE WEST.  
FORECAST HIGHS ARE IN THE UPPER 100 AND TEENS (110-119) IN THE  
DESERT AREAS (INCLUDING PHOENIX, VEGAS) FOR SEVERAL CONSECUTIVE  
DAYS. 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 THERE, BUT THE HEAT IN  
THE SOUTHWEST LOOKS TO BE A LENGTHY AND POTENTIALLY DANGEROUS  
EVENT.  
 
FLASH FLOODING AND PERHAPS EVEN LONGER TERM, LARGER SCALE FLOODING  
(E.G., RIVER FLOODING) WILL BE POSSIBLE IN BROAD SWATHS OF THE  
EASTERN STATES. THE CLOSED LOW DROPPING THROUGH THE OH/TN VALLEYS  
TOWARD THE GULF COAST ANCHORS A WELL DEFINED TROUGH, WITH  
PRECIPITABLE WATER OF 2.0 INCHES OR GREATER FORMING AN AXIS THAT  
OVERLAPS WITH MANY MOUNTAINOUS AREAS IN THE APPALACHIANS AND NEW  
ENGLAND. THESE AREAS ARE PRONE TO FLASH FLOODING, AND THE  
COMBINATION OF THE SYNOPTIC SUPPORT ALONG WITH SEASONABLY STRONG  
SUN ANGLE WILL LIKELY YIELD POCKETS OF INSTABILITY SUFFICIENT TO  
SUPPORT AN OFF AND ON HEAVY RAINFALL THREAT.  
 
BURKE/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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