Extended Forecast Discussion NWS Weather Prediction Center College Park MD 219 AM EDT Sun Sep 08 2019 Valid 12Z Wed Sep 11 2019 - 12Z Sun Sep 15 2019 ...Potential for record heat to continue for the lower Mississippi Valley and Southeast... ...Overview and Guidance Evaluation/Preferences... An upper-level ridge is forecast to remain rather entrenched from the lower Mississippi Valley to the Southeast through much of the medium range. Meanwhile, initially splitting upper flow from the eastern North Pacific into western North America will consolidate as a stream of relatively vigorous upper shortwaves traverse the CONUS northern tier. Farther south, an easterly wave is forecast to approach Florida and the coastal southeastern U.S. by the upcoming weekend, traversing the southern periphery of the expansive upper ridge. A blend of the 12Z ECMWF/CMC/18Z GFS was used as a forecast starting point during days 3-4 (Wed-Thu). This blend represented consensus well with the vigorous upper shortwave moving from the Rockies toward the central/northern High Plains during that time, with surface cyclogenesis expected across the Northern Plains late Wed into early Thu. Less GFS was used from day 5 (Fri) onward due to growing differences from consensus on the timing of the central U.S. system, as well as a faster than consensus solution with the next energetic shortwave trough approaching the Pacific Northwest by that time. This trough reaching the Northwest from Fri onward poses the biggest forecast problem of the period, with models diverging significantly on whether the wave moves inland quickly (GFS) or closes off and progresses much more slowly (CMC). The ECMWF was in the middle if not perhaps a bit toward the faster side of the spread with this feature. Based on flow upstream across the North Pacific and in the vicinity of Alaska that is quite amplified, the preference was to lean away from the faster GFS, toward the middle of if not even a bit toward the slower side of the solution spread. Thus, while weighting of ensemble means was boosted during days 5-7 (Fri-Sun) to account for the increased spread, minority components of the ECMWF and CMC were also included. ...Weather Highlights/Threats... Heavy rain will be possible across much of the Northern Plains and Upper Midwest Wed-Thu in association with a deepening surface low pressure system crossing the region. Models suggest the potential for at least a couple inches of rainfall across a relatively wide area. The easterly wave approaching Florida and the Southeast U.S. by late in the week is expected to increase shower and thunderstorm activity, with locally heavy rains possible. Increasing rain, potentially moderate to heavy, will also be possible across the mountain ranges of the Pacific Northwest by late week as upper troughing progresses into the region. Hot conditions are expected to persist across portions of the south central and southeastern U.S. through much of the upcoming week. High temperatures are forecast to be 5 to 15 deg F above average across a large area from the Ohio Valley to the lower Mississippi Valley and the Southeast. Several record high temperatures are possible each day, especially from the lower Mississippi Valley to the Southeast where highs may soar well into the 90s, perhaps approaching 100 deg at a couple locations. Ryan Additional 3-7 Day Hazards information can be found on the WPC medium range hazards chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation, winter weather outlook probabilities and heat indices are at: https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml