Extended Forecast Discussion NWS Weather Prediction Center College Park MD 432 PM EDT Tue Sep 22 2020 Valid 12Z Fri Sep 25 2020 - 12Z Tue Sep 29 2020 ...Overview... Models and ensembles indicate a high-confidence upper pattern transition toward much more amplification with a strong western ridge/upper high and eastern trough/low. However within this developing configuration, by Sun-Tue confidence is lower for embedded shortwaves and surface fronts that will influence sensible weather. The amplified pattern will bring another warm-up for much of the West (and perhaps near record highs in California/Arizona) versus mid-fall temperatures for the East along with a heavy rain threat next week. 1987 and 1991 were the most comparable recent years showing up in composite analog matches (late Sept/early Oct time frame) for the expected D+8 pattern in the multi-day means. ...Guidance/Predictability Assessment... The updated forecast maintained a multi-model blend for about the first half of the period and then transitioned to a model/ensemble mean composite. This approach represented consensus showing a stronger western U.S. ridge--likely including a 594 dm contour around days 5-6 Sun-Mon--versus the GFS/GEFS mean that keep the core of the ridge below that threshold. Early in the period there are significant differences with the remnants of Beta combined with a compact shortwave dropping through the Midwest into the start of the forecast early Fri. The small scale of the shortwave lends itself to low predictability and favors a conservative depiction aloft between the slow/strong 00Z UKMET (ECMWF only slightly less so) and fast/weak GFS/CMC. The favored solution led to a mere trough for the corresponding surface reflection. New 12Z guidance still displays a fair amount of spread but with some nudges toward intermediate ideas for timing or strength. By Sun-Tue the model-mean consensus yielded some adjustments for the series of shortwaves/surface fronts rounding the West Coast upper ridge and continuing into the eastern mean trough. As is typically the case for such features, confidence is lower than for the larger scale pattern and further changes are likely. One or more of the surface lows anchoring the fronts may track over areas between the Northern Plains and Great Lakes. ...Weather/Hazard Highlights... Remnants of Beta combined with an upper-level trough will continue to bring rain, perhaps locally heavy, to areas from northern Georgia/southern Appalachians into parts of the Carolinas and possibly Mid-Atlantic through Fri and maybe into the weekend. Current guidance indicates high uncertainty regarding precise rainfall amounts and northward extent. A cold front emerging from the Plains will promote an expanding area of rain initially over the Upper Midwest into the Great Lakes during the weekend and then reaching the Northeast. As the upper flow ahead of the amplifying trough turns more north-south oriented, Atlantic moisture will stream northward along the coast and may enhance rainfall from the Mid-Atlantic to the Northeast. The tail-end of the front through the Southeast may have more limited Gulf moisture in the wake of Beta. A rapid sequence of fronts behind the first one should produce additional rainfall from the far northern Plains into mainly the northeastern quadrant of the lower 48, though possibly farther south at times. Depending on exact track one or more of the lows anchoring these fronts could provide added rainfall focus. In the West, Pacific Northwest/extreme northern Rockies rainfall continuing into late week should gradually diminish/retreat northward thereafter as West Coast upper ridging strengthens. The upper ridge will also favor dry conditions for other areas west of the Plains through the period. Temperatures initially cooler than normal (by 5-10F or so) in the Pacific Northwest Friday will moderate into next week as upper ridging builds in. Coverage of plus 5-15F anomalies for highs will increase with time west of the Rockies. Some locations in California and the southwest corner of Oregon will see the best potential for max temperatures on the higher end of that range with offshore flow and an increased risk of fire conditions. Also record highs/warm lows may again be approached/exceeded especially over parts of California and Arizona. The East will trend cooler in time after an initial warm-up ahead of the leading cold front. Greatest anomalies for warm sector daytime highs from late week into the weekend (at least plus 10F) should be from Plains into the Great Lakes/Northeast. By next Tuesday expect moderately below normal highs from the northern-central Plains into the Great Lakes and Ohio/Tennessee Valleys. Rausch/Fracasso Additional 3-7 Day Hazards information can be found on the WPC medium range hazards chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php Hazards: - Heavy rain across portions of the Pacific Northwest and the Northern Great Basin, Fri-Sat, Sep 25-Sep 26. - Heavy rain across portions of the Southeast, the Mid-Atlantic, and the Southern Appalachians, Fri, Sep 25. - Heavy rain across portions of the Lower Great Lakes, the Mid-Atlantic, and the Northeast, Sun-Mon, Sep 27-Sep 28. - Flooding possible across portions of the Lower Mississippi Valley and the Southern Plains. - Flooding occurring or imminent across portions of the Southeast. - Much above normal temperatures across portions of California, the Central Great Basin, the Pacific Northwest, and the Northern Great Basin, Sun-Tue, Sep 27-Sep 29. - Much above normal temperatures across portions of the Central Plains, the Northern Plains, the Central Rockies, the Middle Mississippi Valley, the Upper Mississippi Valley, and the Southern Plains, Fri, Sep 25. - Much above normal temperatures across portions of the Southern Plains, Sat, Sep 26. 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