Extended Forecast Discussion NWS Weather Prediction Center College Park MD 245 AM EDT Mon Oct 04 2021 Valid 12Z Thu Oct 07 2021 - 12Z Mon Oct 11 2021 ...Southeast/Southern Appalachians/Mid-Atlantic Heavy Rain Threat... ...Overview... Guidance overall agrees that the upper flow amplifies this week as an upper low closes off over the east-central U.S. and an upper ridge builds over the north-central states and southern Canada. A leading front and upper support will spread an enhanced rainfall threat across the eastern third of the country. Meanwhile, an eastern Pacific upper trough will amplify and become more elongated/split and work inland through this period, pushing a wavy cold front over the West and across the Rockies into the north-central U.S. along with moderate precipitation chances. ...Guidance Evaluation/Preferences... Guidance continues to offer fairly good agreement on the large scale, bolstering forecast confidence, albeit with continued variability in individual features and details. There is improving forecast clustering with the difficult upper low meadering over the east-central U.S. into later this week and out West, amplified troughing along the coast splits with northern stream energy driving a cold front over the Northwest/West then central U.S. as southern stream energy energy ejects over the Southwest and over the southern Rockies to the Southern Plains late period. The WPC medium range product suite was mainly derived from a composite blend of the 18 UTC GFS/GEFS mean, the 12 UTC ECMWF/ECMWF ensemble mean/Canadian and the 01 UTC National Blend of Models. This maintains max WPC continuity. Latest and compatible 00 UTC models and ensembles overall remain in line with the aforementioned composite blend. ...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 heavier local totals, whether from an episode of heavy rain and/or a multi-day accumulation due to persistence of the pattern. One area to monitor for a threat of runoff issues in particular 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 by the weekend. A wavy cold front digging slowly but robustly across the West will spread areas of rain gradually southeastward mid-later week and the weekend, eventually working across the Rockies into the north-central states. Most activity should be in the light-moderate range, along with 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. 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