Extended Forecast Discussion NWS Weather Prediction Center College Park MD 155 PM EDT Sat Oct 02 2021 Valid 12Z Tue Oct 05 2021 - 12Z Sat Oct 09 2021 ...Southeast/Southern Appalachians/Mid-Atlantic Heavy Rain Threat mid to late next week... ...Overview... The upper level flow during the medium range period should become more amplified and blocky 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 wavy front and closed upper trough/low support will spread rainfall of varying intensity and duration across the eastern third of the country. Meanwhile an eastern Pacific upper trough should steadily amplify and become more elongated through the period, pushing a cold front into the Northwest and California. A lead upper low initially offshore of southern California will eject northeastward ahead of the upper trough and may produce some areas of rainfall over the Southwest/Four Corners region/Rockies. ...Guidance Evaluation/Preferences... The latest suite of guidance generally agrees on the overall pattern amplification, but continues to show variability with the smaller scale, hard to resolve at these time scales, features. There continues to be some uncertainty in terms of placement for the upper low as it drops into the Southeast/Tennessee Valley on Tuesday and Wednesday, and eventually lifts back north into the Ohio Valley later in the week. The GFS has consistently been displaced south of the rest of the guidance, so WPC favors a non-GFS consensus blend for this feature. The amplifying eastern Pacific upper trough off the West Coast shows reasonable larger scale continuity, but with some continued spread with both timing and how the energy will be distributed, especially next Friday into Saturday. This cycle of the WPC medium range suite was derived from a blend of the deterministic models (less weighting of the GFS because of Southeast closed low issues) for days 3-5, and then a general blend of the ECMWF/GFS and ensemble means by the second half of the period to help mitigate some of the late period detail uncertainties. This also resulted in a forecast consistent with the previous WPC forecast. ...Weather and Potential Hazards... Areas of rain and thunderstorms will accompany a wavy frontal system that slows across the Southeast/Mid-Atlantic next week, with enhanced focus as a closed upper low rotates over the region. This should keep rainfall over parts of the Southeast/East through much of next week, with an uncertain but gradual focus from the Southeast into the Mid-Atlantic. The 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 will be the Southern Appalachians where terrain enhancement will be a factor. A lead upper low ejecting northeastward into the Southwest is expected to generate a period of rainfall over parts of the Southwest, Four Corners, and parts of the Rockies mainly around Tuesday-Wednesday. A subsequent and wavy cold front digging slowly but robustly into the West will spread areas of rain gradually southeastward with time into mid-later next week. Most activity should be in the light-moderate range, but a few pockets of heavier rain could be possible along with snow in the higher elevations. The West Coast states 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. 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