440  
FXUS02 KWBC 201551  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1151 AM EDT FRI JUL 20 2018  
 
VALID 12Z MON JUL 23 2018 - 12Z FRI JUL 27 2018  
 
16Z UPDATE:  
 
THE MODELS AND ENSEMBLE MEANS REMAIN IN GOOD OVERALL SYNOPTIC  
SCALE AGREEMENT ON THE PATTERN OVER THE CONTINENTAL U.S. THROUGH  
THE END OF NEXT WEEK. THE 12Z CMC INDICATED A GREATER DEGREE OF  
RETROGRESSION WITH THE UPPER TROUGH/CLOSED LOW OVER THE SOUTHEAST  
U.S. COMPARED TO THE MODEL CONSENSUS, AND ALSO FASTER WITH AN  
UPPER LOW NEAR GREENLAND. THERE ARE MODEST DIFFERENCES REGARDING  
THE EVOLVING UPPER LOW OVER SOUTH-CENTRAL CANADA BY THE END OF THE  
WEEK, AND SOME OF THE ENSEMBLE MEANS WERE INCORPORATED FOR THAT  
PORTION OF THE FORECAST. THERE HAS ALSO BE A TREND IN THE  
GUIDANCE OVER THE PAST 24 HOURS REGARDING THE UPPER LOW OVER THE  
GULF OF ALASKA FOR THE MIDDLE OF NEXT WEEK, AND THIS IS ALSO  
RESULTING IN SOME WESTWARD ADJUSTMENTS IN THE UPPER RIDGE AXIS  
OVER WESTERN CANADA. A BLEND OF THE DETERMINISTIC GFS/ECMWF/UKMET  
WAS USED THROUGH WEDNESDAY FOR PRESSURES AND FRONTS, AND THEN SOME  
OF THE EC AND GEFS MEANS FOR THURSDAY AND FRIDAY. THE PREVIOUS  
DISCUSSION IS APPENDED BELOW FOR REFERENCE.  
 
D. HAMRICK  
__________________________________________  
   
..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
 
 
IT REMAINS THE CASE THAT THE MAIN MID-UPPER LEVEL 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/SOUTHEAST NEXT WEEK, EVENTUALLY  
REPLACED BY ANOTHER WELL DEFINED UPSTREAM TROUGH DIGGING INTO THE  
GREAT LAKES BY WED-FRI. A MAIN CONVECTIVE STORM TRACK REMAINS  
AROUND A HOT SOUTHERN PLAINS/SOUTHWEST RIDGE AND INTO THE  
AFOREMENTIONED AND DYNAMIC/UNSTABLE MAIN TROUGHS. THE LEAD MEAN  
TROUGH POSITION SETTLED INTO THE EASTERN/SOUTHEASTERN U.S. ALSO  
HAS AMPLE ACCESS TO A TROPICAL MOISTURE PLUME THAT BACKS IN OFF  
THE ATLANTIC, SETTING THE STAGE FOR A MORE WIDESPREAD HEAVY  
RAINFALL THREAT.  
 
UNDERNEATH THE RIDGE, THE HEAT WAVE IN THE SOUTHERN PLAINS THIS  
WEEKEND EXPANDS WESTWARD ACROSS THE SOUTHWEST QUARTER OF THE  
NATION NEXT WEEK WHERE SOME RECORD HOT TEMPERATURES ARE EXPECTED.  
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  
AND ENSEMBLES 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. ACCORDINGLY, THE WPC MEDIUM RANGE FORECAST SUITE WAS  
MAINLY DERIVED FROM A COMPOSITE BLEND OF THE LATEST GFS/ECMWF,  
GEFS AND ECMWF ENSEMBLE MEANS, THE NBM, AND WPC CONTINUITY.  
   
..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 100S 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.  
 
SCHICHTEL  
 
WPC MEDIUM RANGE 500 MB HEIGHTS, SURFACE SYSTEMS, WEATHER GRIDS,  
QUANTITATIVE PRECIPITATION, WINTER WEATHER OUTLOOK PROBABILITIES  
AND HEAT INDEXES ARE FOUND AT:  
 
HTTPS://WWW.WPC.NCEP.NOAA.GOV/MEDR/5DAYFCST500_WBG.GIF  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/MEDR/5DAYFCST_WBG_CONUS.GIF  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/5KM_GRIDS/5KM_GRIDSBODY.HTML  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/QPF/DAY4-7.SHTML  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/WWD/PWPF_D47/PWPF_MEDR.PHP?DAY=4  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/HEAT_INDEX.SHTML  
 

 
 
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