Extended Forecast Discussion NWS Weather Prediction Center College Park MD 211 AM 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... Model consensus remains high that a pattern change will be ongoing at the start of the medium range (Fri) across the contiguous U.S., with an upper-level ridge expanding form the Great Basin to the southern plains, and amplification of a trough across the Great Lakes/Ohio Valley. Expect a surface low pressure system to accompany the amplifying upper trough across the Midwest/Great Lakes Fri through the weekend, with a surface frontal boundary trailing into the southern plains. Additional shortwave energy crossing the Pacific Northwest and western Canada Fri-Sat will bring a Pacific cold front into the northern Great Basin and Rockies, reaching the plains by next Tue. A look at hemispheric teleconnections shows support for this pattern evolution across North America, further increasing confidence in the impending pattern change. An anomalous and persistent Rex block across eastern Asia favors upper ridging across the southwestern and south central U.S., with a relatively strong vortex across the Arctic, and an equatorward extension in the form of favored troughing across Hudson Bay and the Great Lakes. Given the persistent nature of the aforementioned Rex block, would expect that the upcoming pattern shift across North America will not be a transient one. ...Guidance Evaluation and Preferences... Models showed good agreement with respect to the large scale flow pattern changes described above, and differences were largely confined to timing/amplitude differences with individual features. Thus, a multi-model deterministic blend (including the ECMWF/GFS/UKMET/CMC) served as a good basis for the forecast through day 5 (Sun). One of the bigger areas of disagreement among the guidance is in the Pacific Northwest by early-mid next week, with some question as to how much influence the upper ridge will continue to exert, and whether additional Pacific shortwave energy will be able to lower heights once again (as shown by the GFS). By this time period (days 6-7) weighting of ensemble means (ECENS/NAEFS) was increased to comprise a slight majority of the forecast, to account for moderate spread among the guidance by that time. ...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. The heat across the southern plains will be one of the bigger stories during the medium range. 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 over 110 deg F for many locations. Low temperatures near 80 deg for many areas will add to the potentially hazardous impacts of the heat. 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