Extended Forecast Discussion NWS Weather Prediction Center College Park MD 435 PM EDT Fri Sep 22 2023 Valid 12Z Mon Sep 25 2023 - 12Z Fri Sep 29 2023 ...Overview... The pattern features an upper trough moving into the West and a downstream Omega Block evolving into a Rex-like Block across the eastern portion of North America and the adjacent northwest Atlantic. ...Guidance Evaluation/Predictability Assessment... A blocky pattern will start the forecast period with ridging present south of Hudson Bay while shortwave energy over the Central U.S. weakens as it moves south/east. Meanwhile, forecast guidance is in really good agreement with the approaching of a stronger, early season storm system / shortwave energy into the Pacific Northwest beginning late this weekend through next week, with a couple stronger embedded shortwaves expected to move onshore. The greatest model uncertainty was toward the end of the forecast period, where weak troughing over the Great Lakes and Eastern U.S. may eventually become cut-off from main upper flow pattern over the Mid-Atlantic region. A lingering trough and possible surface low development off the coast may bring an unsettled period of weather to the region late in the week into next weekend. There is lower confidence and predictability with the main synoptic features and any sensible weather impacts, but the trends from the 00Z to 12Z guidance suggest some sort of weather feature will likely be present. The WPC blend largely followed a near equal weight of the latest deterministic guidance initially then increasingly included the ECENS and GEFS means by day 6 to account for greater model uncertainty and lower confidence, particularly for the eastern U.S. ...Weather/Hazards Highlights... An early season storm system will bring unsettled and active weather to the Pacific Northwest beginning late this weekend and through early next week. A few rounds of heavier precipitation are likely, favored for the coastal regions and upslope areas of the Washington and Oregon Cascades into northern California/southwest Oregon where some localized flooding may be possible with repeating rounds of moderate rainfall, particular in burn areas/burn scars near the western border of CA/OR Monday into early Tuesday. A slow moving, fading front moving into the southern Plains and eventually the western Gulf Coast region may act as the focus for some areas of excessive rainfall at time, particularly across portions of south-central to southeast Texas and portions of the lower Mississippi Valley. Across the East Coast, particularly Mid-Atlantic region, there remains considerable uncertainty how the upper level pattern evolves and any associated surface features that are able to tap into Atlantic moisture. Compared to the previous cycle, there was an increase in the precipitation chances and QPF but there remains many questions and overall confidence is fairly low for any areas of heavy rainfall. Temperatures should be most above average across portions of the Plains/Midwest -- moreso on overnight lows -- and below average across portions of the Mid-Atlantic States and West, which will be most noticeable for daytime highs. Roth/Taylor 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 California, and the Pacific Northwest, Mon, Sep 25. - Heavy rain across portions of the Pacific Northwest, Mon-Tue, Sep 25-Sep 26. - Heavy rain across portions of the Mid-Atlantic and the Central Appalachians, Thu-Fri, Sep 28-Sep 29. WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation forecast, excessive rainfall outlook, winter weather outlook probabilities, heat indices and Key Messages 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/#page=ero https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml https://www.wpc.ncep.noaa.gov/#page=ovw