Extended Forecast Discussion NWS Weather Prediction Center College Park MD 422 PM EDT Fri Oct 01 2021 Valid 12Z Mon Oct 04 2021 - 12Z Fri Oct 08 2021 ...Overview... The latest guidance continues to highlight an amplification of the medium range weather pattern next week. The flow should become more blocky 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 the Four Corners region/Rockies. ...Guidance Evaluation/Preferences... The latest models and ensemble means generally agree on the overall pattern evolution, but continue to show run to run varability in the details. For the upper low dropping from the Ohio Valley to the Southeast, exact placement continues to be an issue, with the last two runs of the GFS showing a farther south solution, and the ECMWF/UKMET/CMC favoring something farther north. The ensemble means generally follow suit with their deterministic counterpart, but do serve as an ok middle ground solution. Meanwhile, the amplifying eastern Pacific upper trough nearing the West Coast and leading closed low ejection from offshore CA offer reasonable continuity, with some spread for how the energy will be distributed and also overall timing of the trough axis by late in the week. This cycle of the WPC progs used a blend of the latest deterministic guidance (which more weighting towards the ECMWF, UKMET, and CMC over the GFS) for days 3-5, with increasing ensemble mean contribution thereafter. This approach maintained good continuity with the overnight WPC forecast. ...Weather and Potential Hazards... Areas of rain and thunderstorms will accompany the wavy frontal system that works across the eastern U.S. early next week and then possibly stalls near the Southeast coast by midweek. Moderate to locally heavy rainfall should linger over the Northeast on Monday, with an increasing focus towards the Southwest, especially as the upper low settles 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 significant 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 may be a factor. Expect the upper low ejecting northeastward into the West to generate a period of rainfall over parts of the Southwest, Four Corners, 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 with snow in the higher elevations. 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. 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 Hazards: - Heavy rain across portions of the Northeast, Mon, Oct 4. - Heavy rain across portions of the Southeast and Southern Appalachians Mon-Tue, Oct 4-Oct 5. - Heavy rain across portions of the Southern and Central Appalachians and the Mid-Atlantic, Wed-Thu, Oct 6-Oct 7. - Flooding occurring or imminent across portions of the Great Lakes. 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