Extended Forecast Discussion NWS Weather Prediction Center College Park MD 251 PM EDT Sun Oct 03 2021 Valid 12Z Wed Oct 06 2021 - 12Z Sun Oct 10 2021 ...Southeast/Southern Appalachians/Mid-Atlantic Heavy Rain Threat... ...Overview... The flow pattern amplifies for the coming week as an upper low closes off over the east-central U.S. and an upper ridge/high builds over the northern tier states and southern Canada. A leading front and closed upper trough/low support will spread an enhanced rainfall threat across the eastern third of the country. Meanwhile, an eastern Pacific upper trough should steadily amplify and become more elongated/split through this period, pushing a wavy cold front into the West and north-central U.S. along with increasing chances for moderate precipitation. A leading shortwave originating from offshore of southern California will also push northeastward ahead of the main upper trough and produce some midweek precipitation over the central Great Basin/Rockies. ...Guidance Evaluation/Preferences... The latest suite of guidance continues to offer fairly good agreement on the large scale, with variability in individual features and details. The upper low across the Southeast into the Ohio Valley is showing much better agreement than in previous days and so a general model blend works well for this feature. Out West, amplified troughing along the coast may split with northern stream energy driving a cold front into the Northwest U.S. and the central U.S., and southern stream energy dropping south and eventually moving into the Southwest. It's with the detail of additional energy rotating around the larger scale trough and into the southern stream feature that models struggle, especially the second half of the period. The ensemble means at this point seem to offer a reasonable and more agreeable starting point. The WPC forecast was based on a blend of the deterministic guidance for days 3-5, with increasing contributions from the ensemble means (blended with the GFS and ECMWF) to help smooth out the details by day 6 and 7. This approach maintains good continuity with the previous shift. ...Weather and Potential Hazards... Widespread areas of rain and thunderstorms will accompany a frontal system as it slows across the Southeast/Mid-Atlantic this week, with enhanced focus as a closed upper low rotates over the region. This should keep rainfall over parts of the Southeast/East for several days, with a gradual focus from the Southeast into the Mid-Atlantic. Guidance continues to signal that some locations could see some significant local totals, whether from an episode of heavy rain and/or a multi-day accumulation due to persistence of the pattern. Guidance variability and spread up to this point keep confidence in specifics fairly low, but one area to monitor for a threat of runoff issues will be the Southern Appalachians where terrain enhancement will also be a factor. Lighter precip should also eventually work its way into the Great Lakes and the Northeast next weekend. Northeastward ejection of a lead upper trough is expected to generate modest precipitation into midweek for the central Great Basin/Rockies. A subsequent and wavy cold front digging slowly but robustly into the West will spread areas of rain gradually southeastward mid-later week and into the weekend, eventually working into the north-central states. Most activity should be in the light-moderate range, along with some snow at higher elevations. The West should trend cooler with time as the upper trough approaches and the cold front makes its way through the region. Daytime highs by later next week could be 5-15 degrees below normal for some locations. Downstream upper ridging traversing the north-central U.S. will support warmer than average temperatures across the Northern Plains and vicinity next week with some plus 10-20F anomalies that equate to highs in the upper 70s and 80s. Above normal temps should also expand into the southern Plains and Midwest by the weekend. Persistence of clouds and areas of rain with the emerging closed upper low/trough and lead frontal system over the Southeast/East should keep daytime temperatures near normal, but also maintain decent coverage of plus 10F and greater anomalies for morning lows. Santorelli/Schichtel 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