Extended Forecast Discussion NWS Weather Prediction Center College Park MD 245 AM EDT Fri Oct 01 2021 Valid 12Z Mon Oct 04 2021 - 12Z Fri Oct 08 2021 ...Overview... Guidance continues to highlight an amplification of the medium range weather pattern next week. The flow will take on a more blocky appearance as an upper low closes off over the east-central U.S. while 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. An upper low initially offshore of California will eject northeastward ahead of the upper trough and may produce some areas of rainfall over parts of the Interior West/Rockies. ...Guidance Evaluation/Preferences... Latest models and ensembles means generally agree on the general pattern evolution, but show marginal run to run continuity that details of the east-central U.S. upper low. Meanwhile, the amplifying eastern Pacific upper trough nearing the West Coast and lead closed low ejection from offshore CA offer reasonable continuity, but there is some spread and variability for how energy will be distributed. This leads to some continuity adjustments for specifics of a leading cold front and embedded waves. Overall, a model and ensemble mean composite blend along with the National Blend of Models was used as a reasonable basis for the WPC product suite. This maintains reasonably good WPC continuity. ...Weather and Potential Hazards... Areas of rain and thunderstorms will accompany the wavy frontal system that works across the central and eastern U.S. early next week and then possibly stalls near the Southeast coast by midweek. Best focus for some areas of locally moderate to heavy rainfall will linger over the Northeast Monday, but meanwhile increasingly focus over the Southeast. Then the upper low settling over the east-central U.S. may keep areas of rainfall over parts of the Southeast/East all next week with an uncertain/gradual focus translation from the Southeast to the Mid-Atlantic. Some locations could see significant totals, whether from an episode of heavy rain and/or 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 may be a factor. Expect the upper low ejecting northeastward into the West to generate a period of rainfall over parts of the Southwest, Great Basin, and parts of the Rockies mainly around Tuesday-Wednesday. The front pushing into the Pacific Northwest and northern California will spread areas of rain gradually southeastward with time mid-later next week. Most activity should be in the light-moderate range, but a few pockets of heavier rain could be possible. A few high elevations may see some snow. The West Coast will see moderately below normal daytime highs beginning around midweek as the upper trough approaches and the cold front makes its way through the region. 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 moderate daytime temperatures, but should 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 Hazards: - Heavy rain across portions of the Tennessee Valley, the Ohio Valley, and the Great Lakes, Sun, Oct 3. - Heavy rain across portions of the Northeast, Mon, Oct 4. - Heavy rain across portions of the Southern Appalachians and Piedmont, Tue-Wed, Oct 5-Oct 6. - Flooding occurring or imminent across portions of the Great Lakes.