Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 PM EDT Wed Sep 29 2021 Valid 12Z Sat Oct 02 2021 - 12Z Wed Oct 06 2021 ...Overview... The slow-moving front extending across the Great Plains should begin to move eastward as the medium range period begins on Saturday, spreading scattered showers/thunderstorms and possibly embedded heavy rain eastward across the Ohio Valley on Sunday and Monday, and into the East Coast Tuesday and Wednesday. Over the Pacific Northwest, the next moisture plume is forecast to slowly edge into the region by early next week. Meanwhile, some monsoonal moisture appears to make a return across the Four-Corners region by early next week. ...Guidance Evaluation/Preferences... Model guidance are coming into better agreement with respect to shifting the slow-moving frontal boundary across the Plains progressively eastward beginning on Saturday. Both of the ECMWF and GFS have reached relatively good agreement on this eastward shift, with a more typical slightly faster GFS verses the slightly slower ECMWF. The slower ECMWF favors a weaker low pressure wave moving across the central Plains behind a main low pressure center forecast to drift southeastward across the Great Lakes by both models. By next Tuesday, the 00Z ECMWF develops this system over New England ahead of an amplifying upper low over the Great Lakes--a scenario that is not yet supported by its ensemble mean. Therefore, a consensus of the 00Z ECMWF/00Z EC mean with the 06Z GFS/GEFS was adopted to handle this system, with more weights given to the EC mean for Days 5-7 relative to its deterministic component. This blend should provide a proper handle to the next system of interest moving into the Pacific Northwest. The 00Z CMC has not been included in the blend due to significant deviation in the general synoptic pattern across the U.S. through much of the medium-range period. ...Weather and Potential Hazards... Scattered showers and thunderstorms can be expected to spread gradually eastward across the Ohio Valley on Sunday and Monday ahead of a frontal system, with some embedded heavy rain possible. Steady rain is now forecast for New England Sunday and Monday north of a stationary front. The rain could linger into Tuesday as a low pressure center drifts across the Great Lakes along the stationary front toward the northern Mid-Atlantic region. The scattered showers and storms should then affect much of the eastern U.S. Monday and Tuesday and possibly into Wednesday as the front slowly moves across and could reach the East Coast on Wednesday. Meanwhile, some monsoonal moisture appears to make a return across the Four-Corners region by early next week as an upper low off the southern California coast is forecast to move onshore. Over the Pacific Northwest, the next moisture plume ahead of a positively-tilted trough is forecast to slowly edge into the region by early next week. Warmer than average temperatures are forecast for the Midwest and Ohio Valley especially in terms of lows, but even highs should be around 10 degrees above normal on Saturday, though are likely to moderate to near normal by Monday. The Northern Rockies/Northern Plains can expect highs around 10 to 15 degrees above average for the first half of next week underneath the upper high forming. Meanwhile, temperatures a few degrees below normal are forecast for the Four Corners states and into southern/central parts of the Plains through the period. Kong/Tate 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