Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM EDT Thu Sep 17 2020 Valid 12Z Sun Sep 20 2020 - 12Z Thu Sep 24 2020 ...Heavy rainfall threat for the western to west-central Gulf Coast next week associated with likely slow moving tropical system development in the Southwest Gulf of Mexico... ...Hurricane Teddy looming over the Atlantic... ...Guidance/Predictability Assessment and Weather/Hazard Highlights... Model forecasts remain pretty well clustered across the CONUS days 3/4 (Sun-Mon) and a blend of the latest GFS/ECMWF/CMC/UKMET seemed reasonable. Guidance remains in reasonable agreement with the flow pattern for much of the nation days 5-7, but there remains question on how much a shortwave moving through the Great Lakes amplifies across the Northeast, which has implications for the eventual track of Hurricane Teddy during the day 5-7 frame. Recent ECMWF runs and 12 UTC ECMWF ensembles offer a much slower and more amplified Northeast U.S. trough than guidance consensus. This would help to phase/steer a still powerful Teddy (likely beginning extra tropical transition by that point) to the Canadian Maritimes or New England next midweek. The past few GFS/CMC/GEFS runs have been faster to progress the upper trough over the Northeast which forces Teddy more offshore. The older 12 UTC ECENS ensemble mean was mostly used to create the WPC medium range product suite as it offered a compromise between the aforementioned guidance camps and best fit 03 UTC NHC guidance. This trended Teddy westward toward the Canadian Maritimes and allowed midweek moderate rains/unsettled conditions to wrap back into New England. However, the newer 00 UTC GFS and especially the 00 UTC UKMET/Canadian have trended increasingly toward the ECMWF, substantially increasing threat potential in model space. In the wake of Sally, strong high pressure settling into the East will bring well below normal temperatures including a few record values this weekend into early next week, with gradual moderation from west to east. The high pressure surge will also act to confine deeper moisture and heavy convective rain potential mainly to Florida and the Gulf of Mexico near a slow moving trailing front to linger over the next week. NHC is monitoring likely development of a slow moving tropical disturbance now over the southwest Gulf of Mexico and broad deep moisture seems set to fuel widespread moderate to heavy rainfall that could feed in onshore flow into the western to west-central Gulf Coast all next week. The latest 00 UTC UKMET/Canadian and especially the 00 UTC ECMWF now shows much more threatening development up into the western Gulf off southern TX. There is also a growing later period model and ensemble signal to develop an amplified northeast Pacific upper trough whose deepened storm could force a lead plume of moisture and enhanced rainfall into the Pacific Northwest mid-later next week in active flow. Santorelli/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