Extended Forecast Discussion NWS Weather Prediction Center College Park MD 244 AM EDT Sun Oct 24 2021 Valid 12Z Wed Oct 27 2021 - 12Z Sun Oct 31 2021 ...Storm system to bring a heavy rainfall threat from the Midwest/South to the Eastern U.S. Wednesday-Saturday... ...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 show good agreement on the large scale evolution/track of this system, with a possible southern stream closed low and deep associated surface low. Lingering uncertainties remain in the details however, especially late period. The ECMWF continues to be slightly south of the rest of the global guidance, but since yesterday, the trend has been to the south so the ECMWF is not unreasonable. Blended with the 18z (yesterday) GFS, it provides a reasonable forecast. Upstream, the next shortwave moves into the Pacific Northwest on Friday which should bring another threat for heavy rain/mountain snows to the Northwest later this week. 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. Across the board, WPC relied on a general model blend between the GFS, ECMWF, and smaller amounts of the CMC for days 3-4. After this, amidst growing forecast spread in the details and timing of individual systems, the forecast trended increasingly towards the ensemble means, with more emphasis on the GEFS over the ECENS mean because the GEFS had a better southern stream/closed low separation for the eastern U.S. system. This maintains decent continuity with the previous WPC forecast with mostly minor changes needed. ...Weather/Hazards Highlights... The bulk of the rainfall on the backside of the low exiting the Northeast on Wednesday should be offshore by the time the period begins, but some lingering rainfall is possible on Wednesday across New England. The biggest weather threat during the medium range period will be with a deep low pressure system moving into the east-central U.S. late this week and into next weekend. Ample Gulf of Mexico moisture ahead of this system will present a lead rainfall/convective focus with widespread moderate to locally heavy rains possible across much of the lower/middle Mississippi Valley and central Gulf Coast regions on Wednesday. The Storm Prediction Center also is highlighting the potential for severe weather across the central Gulf Coast on Wednesday. Closed upper low development and a slowed frontal system progression will allow rainfall to expand eastward into the Ohio Valley/Appalachians and eventually the Mid-Atlantic/Northeast Thursday-Saturday. Out West, a shortwave entering the Northwest mid to late this week will direct a plume of moisture into the West Coast bringing 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. 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