Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1126 AM EDT Wed Jul 18 2018 Valid 12Z Sat Jul 21 2018 - 12Z Wed Jul 25 2018 ...Dangerous multi-day heat wave expected for the southern plains... ...16Z update... Minor changes from the overnight package as the models/ensembles remained in rather good agreement. Refreshed the 500mb and surface fronts/pressures with the 00Z/06Z guidance, using a deterministic blend to start (GFS/ECMWF/UKMET/Canadian) trending toward a 50/50 deterministic/ensemble blend by next Wednesday. The 00Z ECMWF/Canadian forged the best cluster with the ensemble means across southern Canada early next week as the 00Z/06Z GFS runs appeared too far north with the sfc low, but were within tolerance farther south with the frontal timing. Upper low in the east should linger through the period with no place to go, blocked by downstream building upper ridging in the Atlantic. The intense heat centered over Texas will be the biggest threat during the period with many days near/above record high temperatures (and record warm minimum temperatures). Fracasso Previous discussion below... ...Overview... Confidence is high that significant upper-level flow amplification will occur across the CONUS by day 3 (Sat), with a strong and expansive upper ridge taking hold from the Great Basin to the southern plains and lower Mississippi Valley, and a deepening trough axis across the Great Lakes/Ohio Valley. A surface low pressure system is expected to accompany the amplifying trough across the eastern third of the country Sat-Mon, before eventually becoming stationary and washing out. Additional shortwave energy crossing the Pacific Northwest and western Canada on Sat will bring a Pacific cold front into the northern Great Basin and Rockies, reaching the plains by early next week. Models show general consensus that this wave will undergo additional amplification Tue-Wed as it reaches the Upper Midwest and Great Lakes, setting the stage for an amplified trough across the eastern U.S. once again. Hemispheric teleconnections continue to suggest that this pattern change will not be short-lived, with a persistent Rex block across eastern Asia favoring upper ridging across the southwest and south central U.S., a relatively strong vortex across the Arctic, and an equatorward extension in the form of favored troughing across Hudson Bay and the Great Lakes. Models/ensembles showed good consensus with respect to the large scale pattern, and differences with respect to smaller scale features were primarily confined to timing/amplitude through much of the forecast period. As a result, a multi-model deterministic blend (including the ECMWF/GFS/UKMET/CMC) served as a basis for the forecast during days 3-5 (Sat-Mon). The 12Z CMC was too weak with the southern tier upper ridge relative to consensus by days 4-5, resulting in the amplification of shortwave energy/lowering of heights across the Pacific Northwest - and this solution was weighted less relative to the other guidance after day 3 as a result. During days 6-7, as spread gradually increased among the guidance, weighting of ensemble means (ECENS/NAEFS) was boosted to comprise a slight majority of the forecast. Models differ as to exactly how quickly the upper trough will begin to lift out of the eastern U.S. ahead of the next system crossing the the northern tier. Additionally, the ECMWF became a bit faster than consensus with the progression of the second wave across the Upper Midwest Tue-Wed, and a solution a bit closer to the GFS and ensemble means was preferred. ...Weather Highlights/Threats... Scattered to numerous showers and thunderstorms with locally heavy rainfall will be possible Sat-Sun from the Great Lakes to the Southeast and southern Mid-Atlantic region in association with the amplifying upper trough and surface frontal system. 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 Sat will gain access to deeper moisture by the time it reaches the northern plains Sun-Tue, bringing locally heavy convection to portions of the plains and 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 Sat into next week, with a number of record high temperatures and record high minimum temperatures potentially in jeopardy. Relatively high dew points (near 70 deg F for some areas) will combine with the hot temperatures to produce dangerous heat index values 110-115 deg F for many locations. 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 from 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