Extended Forecast Discussion NWS Weather Prediction Center College Park MD 344 PM 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 presents a late period threat to the Canadian Maritimes/parts of New England... ...Guidance/Predictability Assessment... 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 some 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. The ECMWF has been very consistent over the past day or two with a more amplified Northeast U.S. trough, helping to steer a still powerful Teddy (likely beginning extra tropical transition by then) into the Canadian Maritimes by day 6/Wednesday. The past run or two of the GFS/CMC/UKMET has also trended increasingly towards the ECMWF, albeit a bit slower, which boosts confidence a bit on Teddy's eventual track compared to recent days. The ensemble means also would support a track into the Canadian Maritimes as well. The WPC blend for today relies heavily on the agreeable deterministic models for days 3-4, with increasing contributions from the ensemble means to help mitigate the differences with Teddy, a shortwave traversing the Canadian/US border day 6-7, and also potential tropical development in the Gulf of Mexico (which shows considerable spread late in the period). This approach also helps maintain good continuity from the previous overnight WPC forecast. ...Weather/Hazard Highlights... Confidence has increased some in the recent days on what is currently Hurricane Teddy over the central Atlantic eventually tracking towards/into the Canadian Maritimes. This would increase the threat for wrapback moderate rains/unsettled conditions into parts of New England by next Tuesday into Wednesday. A cold front pushing offshore to begin the period will allow strong high pressure to settle into the East bringing well below normal temperatures and a few record values this weekend into early next week, with gradual moderation from west to east. This should also help to confine deeper moisture and convective rain potential mainly to Florida and the Gulf Coast near a slow moving trailing front which looks to linger over the next week. In addition, NHC continues to monitor likely development of a slow moving tropical disturbance in the southwest Gulf of Mexico, and models seem set to bring moderate to heavy rainfall into the western to west-central Gulf Coast beginning as early as this weekend. It should be noted there remains considerable spread amongst the models and ensembles on whether this potential low tracks more towards south Texas or farther north into the west-central Gulf Coast, of course having strong implications on if/when/where the heaviest rainfall sets up. The potential is there and so the coastal threat continues to be highlighted both in the latest WPC medium range QPF and day 3-7 hazards chart. Elsewhere, temperatures should remain above to much above normal throughout the period across the north-central U.S. and back into the interior West. There is also growing signal in the models and ensembles to develop an amplified northeast Pacific upper trough. The resultant deepened storm could force a lead plume of much needed and welcomed moisture and enhanced rainfall into the Pacific Northwest mid to later next week. Santorelli 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 Hazards: - Heavy rain across portions of the Pacific Northwest and the Northern Great Basin, Wed-Thu, Sep 23-Sep 24. - Heavy rain across portions of the Southern Plains, the Lower Mississippi Valley, and the Southeast, Sun-Thu, Sep 20-Sep 24. - Flooding possible across portions of the Southeast, the Mid-Atlantic, the Southern Appalachians, and the Tennessee Valley. - Flooding occurring or imminent across portions of the Southeast. - Flooding likely across portions of the Southeast. - Much below normal temperatures across portions of the Great Lakes, the Ohio Valley, the Central Appalachians, the Mid-Atlantic, and the Northeast, Sun-Tue, Sep 20-Sep 22. 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