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