Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 PM EDT Sun Sep 13 2020 Valid 12Z Wed Sep 16 2020 - 12Z Sun Sep 20 2020 ...Sally is expected to spread heavy rainfall northward into the Tennessee Valley and the southern Appalachians Wednesday night through Friday morning... ...Welcome rains for the Northwest, northern California, and the northern Rockies this week into the weekend... ...Guidance/Predictability Assessment... Lead by the 12Z ECMWF from yesterday, models today have generally trended toward a more inland recurvature of Sally across the interior Southeast during the latter part of this week. Both the 00Z UKMET and 00Z CMC still preferred a sharper recurvature of Sally across southern Alabama and southern Georgia but their latest 12Z runs have also shifted toward a more gradual turn to the northeast in agreement with the GFS and ECMWF. The latest 12Z ECMWF did track Sally slight farther south but that is within the realm of reasonable uncertainty. Elsewhere, models have come into better agreement on the timing of an upper low moving into the Pacific Northwest late this week. The GFS appears to be too fast in bringing upper level energy into the High Plains by this weekend, leading to too much surface trough/low pressure development. Therefore, the morning WPC medium-range prognostic charts were derived using an even blend of the 06 UTC GFS and the 00Z ECMWF with some of their ensemble means, then gradually eliminating the GFS solution toward day 7, and with larger weights toward the 00Z EC mean. ...Overview and Weather/Hazard Highlights... With Sally's forecast track farther inland into the interior Southeast after landfall, the threat of heavy rainfall has increased across the Tennessee Valley and the southern Appalachians. The timing of the heavy rain should be from Wednesday night through Friday morning. Sally is expected to become extratropical over the interior Southeast as it merges with a stationary front there. A longwave trough forecast to dip into the eastern U.S. should then push the system relatively quickly off to the east and into the Atlantic by the weekend. Meanwhile, a complex evolution over the northeastern Pacific should yield a fairly deep trough/closed low that is slated to slowly bring some welcome rainfall and cooler temperatures into the fire stricken Northwest and northern California by midweek, lingering into late week in a wet late summer pattern. Kong 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