Extended Forecast Discussion NWS Weather Prediction Center College Park MD 251 AM 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 wavy 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 lead upper low initially offshore of southern California will also eject northeastward ahead of the main upper trough and produce some midweek precipitation over the central Great Basin/Rockies. ...Guidance Evaluation/Preferences... The WPC medium range product suite was primarily derived from a blend of best clustered guidance from the 18 UTC GFS/GEFS mean and the 12 UTC ECMWF/ECMWF ensemble mean along with the 01 UTC National Blend of Models for days 3/4 (Wed/Thu). Weighted the GFS/ECMWF heavily during this period. Despite continued variance with embedded weather features through the period, the overall pattern evolution still seems to offer decent predictability into days 5-7 (Fri/next weekend). In the blend, weighted the compatible GEFS/ECMWF ensemble means heavily during this period consistent with growing forecast spread. This maintains reasonable WPC product continuity overall, but did reasonably trend slightly slower with heavier rainfall lifting over the East into a Northeast high given the closed/blocky nature of the larger scale flow. Guidance is slowly improving variances and a composite of later 00 UTC guidance remains generally in line. ...Weather and Potential Hazards... Widespread areas of rain and thunderstorms will accompany a wavy frontal system that 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 through much of the week, with an uncertain but 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. 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 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. 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