295  
FXUS02 KWBC 171600  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1200 PM EDT SUN JUN 17 2018  
 
VALID 12Z WED JUN 20 2018 - 12Z SUN JUN 24 2018  
 
...HEAVY RAIN THREAT FOR SOUTHERN TEXAS AND THE NORTH-CENTRAL  
U.S...  
   
..PATTERN OVERVIEW AND GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
ENERGY FROM A SHORT RANGE CLOSED LOW OVER THE WESTERN U.S. EJECTS  
EASTWARD INTO THE PLAINS BY THE START OF MEDIUM RANGE PERIOD  
(WED-SUN) AND CONTINUES TRACKING EASTWARD INTO THE MISSISSIPPI  
VALLEY BY LATE WEEK. MEANWHILE, STRONG RIDGING OVER THE PACIFIC  
NORTHWEST SHIFTS EASTWARD AS A SHORTWAVE TROUGH ENTERS THE  
NORTHWEST BY FRIDAY. ACROSS THE EASTERN STATES, THE NORTHERN  
PERIPHERY OF RIDGING OVER THE EAST/SOUTHEAST SHOULD BE ERODED BY  
NORTHERN STREAM CYCLONIC FLOW OVER THE GREAT LAKES AND THE  
NORTHEAST, WHICH SHOULD SUPPRESS THE OPPRESSIVE HEAT AND HUMIDITY  
EXPECTED EARLY THIS WEEK.  
 
OVERALL, THE MODELS AND ENSEMBLES CONTINUE TO SHOW A REASONABLY  
SIMILAR LARGE SCALE PATTERN OVER THE NEXT WEEK, WITH SMALLER  
MID-SCALE EMBEDDED FEATURES STILL OFFERING A BIT OF FORECAST  
SPREAD. THERE ARE MINOR TIMING AND INTENSITY DIFFERENCES REGARDING  
THE WESTERN U.S. SHORTWAVE, WITH THE LATEST RUN OF THE ECMWF  
SHOWING A MUCH FASTER PROGRESSION INTO THE PACIFIC NORTHWEST COAST  
THAN THE DETERMINISTIC GFS OR THE ENSEMBLE MEANS. THE OTHER AREA  
OF CONCERN IS WITH A UPPER LEVEL TROPICAL WEAKNESS LIFTING INTO  
THE TEXAS GULF COAST AND LINGERING ACROSS THE RIO GRANDE VALLEY  
MID-WEEK. THE OPERATIONAL ECMWF AND UKMET BOTH SHOW A DEFINED LOW  
SOMEWHERE NEAR FAR SOUTH TEXAS, WHILE THE GFS AND ENSEMBLE MEANS  
WOULD SUPPORT SOMETHING WEAKER. REGARDLESS, THIS POSES A  
HEAVY/EXCESSIVE RAINFALL THREAT FOR PORTIONS OF THE TEXAS COAST  
AND FAR SOUTH TEXAS.  
 
THIS CYCLE OF THE WPC MEDIUM RANGE SUITE WAS MAINLY DERIVED FROM A  
BLEND OF THE LATEST RUNS OF THE GFS/ECMWF WITH THEIR ENSEMBLE  
MEANS, WITH MORE WEIGHTING TOWARDS THE ENSEMBLES BY THE END OF THE  
PERIOD. THIS BLEND PROVIDES A GOOD MIDDLE GROUND SOLUTION TO ALL  
SYSTEMS MENTIONED ABOVE AND DE-EMPHASIZES THE LESS PREDICTABLE  
SMALLER SCALE FEATURES WHICH STILL REMAIN HIGHLY UNCERTAIN. THIS  
ALSO MAINTAINS GOOD CONTINUITY WITH THE PREVIOUS SHIFT.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
A MAIN AND PERSISTENT/WAVY FRONTAL BOUNDARY REMAINS  
QUASI-STATIONARY THIS WEEK FROM THE PLAINS TO THE OHIO VALLEY AND  
MID-ATLANTIC. EXPECT PERIODS OF LOCALLY EXCESSIVE CONVECTION TO  
FOCUS ALONG THIS BOUNDARY AND IN THE WARM SECTOR AIDED BY EARLY  
PERIOD TROPICAL MOISTURE INFLUX FROM THE GULF OF MEXICO. THE BEST  
POTENTIAL FOR HEAVY TO EXCESSIVE RAINFALL HOWEVER SHOULD BE OVER  
SOUTHERN TEXAS, DUE TO A POTENTIAL TROPICAL WAVE/DISTURBANCE NOW  
BEING FOLLOWED BY NHC. AS THE TROUGH AND FRONT SHIFTS SLOWLY  
EASTWARD INTO LATER WEEK, THE HEAVIEST RAINFALL MAY INCREASINGLY  
SPREAD ALONG THE SURFACE BOUNDARY AS ENHANCED BY ANY POTENTIAL  
WAVE DEVELOPMENTS/CONVECTIVE SYSTEM PROPAGATIONS INTO THE  
MIDWEST/OHIO VALLEY STATES AND MID-ATLANTIC.  
 
SANTORELLI  
 
WPC MEDIUM RANGE FORECASTS OF 500 MB HEIGHTS, SURFACE SYSTEMS,  
WEATHER GRIDS, QUANTITATIVE PRECIPITATION, AND WINTER WEATHER  
OUTLOOK PROBABILITIES CAN BE 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  
 

 
 
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