Extended Forecast Discussion NWS Weather Prediction Center College Park MD 240 PM EDT Sun Oct 24 2021 Valid 12Z Wed Oct 27 2021 - 12Z Sun Oct 31 2021 ...Major storm to focus a heavy rainfall/runoff threat from the Midwest/Southeast Thursday to the Mid-Atlantic/Northeast into next weekend... ...Overview and Guidance/Predictability Assessment... As one deep cyclone exits the Northeast to start the period on Wednesday, the next one will be spinning up over the southern Plains, keeping the weather pattern quite active across the eastern half of the nation into next weekend. Guidance continues to trend less progressive and more amplified with this system as it tracks from the Southern Plains into the Mid-Atlantic/Northeast by next weekend. The latest models actually now show much better agreement on the evolution/track of this system overall, bolstering forecast confidence, with a possible southern stream closed low and deep associated surface low. Lingering uncertainties remain in the details however, especially late period. The ECMWF/ECMWF ensembles continues to be slightly south/less progressive than the rest of the global guidance, but the trend has been to the south so the ECMWF is not unreasonable. Well upstream, a favorable shortwave and upper jet pattern back into the Pacific Northwest mid-late week will foster another multi-day threat for heavy rain/mountain snows for the Northwest. Guidance shows reasonable agreement with this setup, though timing and strength differences arise once the system moves inland across the northern Rockies and into the northern Plains next weekend. Accordingly overall, a blend of the ECMWF/ECMWF ensemble mean along with WPC continuity and the National Blend of Models was primarily used to produce the WPC product suite. ...Weather/Hazards Highlights... The bulk of the rainfall on the backside of the low exiting the Northeast should push offshore Wednesday, but some lingering wrapback activity will remain possible across New England. The biggest weather threat during the medium range period will be with a deep low pressure system working across the central to eastern U.S. later this week that is slated to slowly lift into the Northeast into next weekend. Ample Gulf of Mexico moisture ahead of this system will present a protracted rainfall/convective focus with widespread moderate to locally heavy rains possible. The Storm Prediction Center also is highlighting the potential for severe weather across the central Gulf Coast. Closed upper low development and a slowed frontal system progression will allow rainfall to expand into the Ohio Valley/Appalachians then Mid-Atlantic/Northeast into next weekend as aided by Atlantic moisture feed. Repeat/duration activity and favored terrain lift will focus the runoff threat. Out West, shortwave energies progressing entering the Northwest mid to late this week will direct a plume of moisture to support another round of heavy rain/mountain snows to mainly parts of the Pacific Northwest, but also farther inland across the northern Rockies. Conditions should finally dry out by next weekend as upper ridging works into the region. Schichtel/Santorelli Additional 3-7 Day Hazard information can be found on the WPC medium range hazards outlook 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