Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 AM EDT Thu Jul 19 2018 Valid 12Z Sun Jul 22 2018 - 12Z Thu Jul 26 2018 ...Dangerous multi-day heat wave expected for the southern plains... ...Heavy rains possible across portions of the eastern U.S. Sunday into next week... ...Overview and Guidance Evaluation/Preferences... The upper-level pattern is expected to remain relatively unchanged through the medium range, with persistent upper ridging from the Southwest to the southern plains, and the tendency for the development of deep troughing in the East. Indications remain that this pattern change will not be short-lived, with significant hemispheric teleconnections between the evolving flow pattern across North America and a persistent Rex block across eastern Asia (which shows no signs of breaking down any time soon). After wavering in recent days, models have resumed their trend toward further amplification of the upper trough across the eastern U.S. Sun into next week, now even suggesting an increasing probability for some energy to cutoff across the Southeast by next Tue-Wed. At the higher latitudes, models continue to vary in terms of timing and intensity with additional shortwave energy crossing Canada and skirting the U.S. northern tier Sun-Tue, and with the associated surface low pressure system. The wave then appears set to undergo additional amplification across the Great Lakes Wed-Thu, renewing the troughing across the eastern U.S. In general, consensus among the deterministic guidance was sufficient to use a multi-model deterministic blend (including the ECMWF/GFS/UKMET) as a basis for the forecast for days 3-5 (Sun-Tue). The CMC was excluded from the blend as it continues to be too weak with the southwest/south central U.S. ridge, which in turn allows heights to fall across the Pacific Northwest as some energy separates from the higher latitude wave. For days 6-7 (Wed-Thu) weighting of ensemble means (ECENS/NAEFS) was increased to comprise a slight majority of the forecast. Model solutions differ with respect to the degree of trough amplification across the Great Lakes/Ohio Valley by this time period, but a trend toward ensemble means should best represent the current model consensus. ...Weather Highlights/Threats... The amplified upper trough, and associated decaying frontal system in place across the eastern U.S. at the start of the forecast period will bring scattered to numerous showers and thunderstorms to areas from the Southeast and Ohio Valley to the Eastern Seaboard. The trend toward greater amplification in recent model/ensemble runs has increased the potential for areas of heavy/excessive rainfall across areas of the Mid-Atlantic and Northeast Sun-Thu, with the potential for more significant northward transport of deep moisture. A number of deterministic solutions now show the potential for areas of multi-inch rainfall totals across these areas across multiple days, with a moderate degree of ensemble support as well. The arrival of a second cold front and amplifying trough in the East by the middle of next week will only bring a continuation of the convection and locally heavy rainfall. Farther west, the cold front trailing the low pressure system crossing central Canada will bring scattered convection and locally heavy rainfall to portions of the central/northern plains Sun-Mon. The trailing end of the front appears likely to stall from the central plains into the Rockies on Tue, bringing a continued threat for locally heavy convection. The other story 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 Sun perhaps into the middle of 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 due to increases in cloud cover and isolated convection. 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