Extended Forecast Discussion NWS Weather Prediction Center College Park MD 410 PM EDT Sat Sep 12 2020 Valid 12Z Tue Sep 15 2020 - 12Z Sat Sep 19 2020 ...Tropical Storm Sally is forecast to intensify to hurricane strength as it approaches central Gulf Coast mid-week... ...Some welcome rains for Northwest/Northern California next week... ...Guidance/Predictability Assessment... Models today have decidedly trended toward a much better-defined circulation associated with the newly upgraded Tropical Storm Sally as it was emerging into the eastern Gulf from southern Florida. But more importantly, there is a decided model trend toward a sharp recurvature of Sally toward the northeast after landfall as it will likely interact with a stationary front and become extratropical by the end of the week. On the other hand, the latest 12Z ECMWF now offers a much more inland track toward Tennessee and the southern Appalachians as the broad upper trough from Canada is forecast to dip further down toward the eastern U.S. and interact with Sally. This will obviously have a large influence on the eventual rainfall distribution associated with Sally. Another area of note is off to the west of the Pacific Northwest where an upper low is forecast to form, spreading precipitation inland. Today's model guidance has come into better agreement on its speed and timing of this upper low heading onshore near the end of the week. The 06Z GFS appears too fast in bringing this system onshore. The 06Z GEFS and 00Z EC mean are quite agreeable with each other regarding this system. Therefore, the WPC medium range suite begins with a even blend of the 00 UTC ECMWF and 06Z GFS together with some of their ensemble means, then trends toward a blend of the 00Z EC ensemble mean and the 06 UTC GEFS mean, together with a small portion of the 00Z NAEFS. Good WPC continuity is maintained despite a pattern with above average forecast spread, especially with northern stream systems. ...Overview and Weather/Hazard Highlights... With Sally forecast to head toward the central Gulf Coast and intensify into a hurricane, the threat of heavy rainfall, damaging winds, and storm surge have continued to increase along the Gulf Coast during mid-week. A lingering and slow moving front draped atop will likely help to focus very heavy rainfall along the eastern Gulf Coast into the Florida Panhandle as well as along the western Gulf Coast well into next week. But the highest threat of heavy rain in addition to high winds and storm surge may ultimately be near and just east of where Sally makes landfall. Deep moisture and slow system translation suggest that heavy rainfall threat may work inland later next week through the MS/TN Valleys depending on the degree of interaction with the aforementioned broad upper trough. Meanwhile, a complex evolution over the northeastern Pacific should yield a fairly deep trough/closed low that is slated to bring some welcome rainfall and cooler temperatures into the fire stricken Northwest and northern California by early-mid next week, lingering into late next week in a wet late summer pattern. Kong/Schichtel 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