Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1200 PM EDT Tue Jul 17 2018 Valid 12Z Fri Jul 20 2018 - 12Z Tue Jul 24 2018 ...Dangerous heat wave expected for the southern plains by later this week... ...Overview and Guidance Evaluation/Preferences... The latest model guidance and a quick look at hemispheric teleconnections continues to increase the confidence that a pattern change will be ongoing at the start of the medium range period (Friday) across the contiguous U.S.. Models continue to show an upper-level ridge expanding eastward from the Great Basin/Southwest to the southern Plains, and amplification of a trough across the Great Lakes/Ohio Valley and into the central Appalachians/Mid-Atlantic. Expect a surface low pressure system to accompany the eastern trough, with a surface frontal boundary trailing back into the southern Plains. Additional shortwave energy crossing the Pacific Northwest and western Canada late this week will drive a cold front eastward into the northern Great Basin and Rockies, which reaches the Plains and upper Midwest by early next week. Models showed very good agreement with respect to the overall large scale flow through day 7, with any differences mostly confined to the usual timing/amplitude disagreements with individual features mainly late in the period. There continues to be some question with whether or not shortwave energy in the Pacific Northwest will lower heights again out west, possibly breaking down the Southwest ridge slightly, as shown by the last few runs of the GFS. This cycle of the WPC medium range 500/surface progs used a blend of the latest runs of the deterministic models (including the ECMWF/GFS/UKMET/CMC) for days 3-4, with increasing contributions from the ensemble means (ECENS/GEFS) by days 5-7 to account for the usual late period differences with smaller scale features. ...Weather Highlights/Threats... The frontal system crossing the Great Lakes/Ohio Valley Fri-Sat will be accompanied by scattered to numerous showers and thunderstorms, some of which could produce locally heavy rainfall. A lingering stationary/warm front will also produce convection across the Southeast. The strongest signal among the guidance for heavy rainfall continues to be along the immediate Southeast coastline Fri-Sat, in close proximity to the frontal boundary. 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 Fri-Sat will gain access to deeper moisture by the time it reaches the northern plains Sun-Mon, bringing locally heavy convection to the central/northern plains and back into the 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 Fri into next week, with a number of record high temperatures and record high minimum temperatures potentially in jeopardy. High dew points will combine with the hot temperatures to produce dangerous heat index values 110-115 deg F for many locations. Low temperatures near 80 deg for many areas will add to the potentially hazardous impacts of the heat. Santorelli/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