Extended Forecast Discussion NWS Weather Prediction Center College Park MD 144 AM EDT Sat Sep 23 2023 Valid 12Z Tue Sep 26 2023 - 12Z Sat Sep 30 2023 ...Overview... The pattern features an amplifying upper trough moving into the West and a downstream Rex-like Block across the eastern portion of North America and the adjacent northwest Atlantic. ...Guidance Evaluation/Predictability Assessment... The guidance showed reasonable agreement from Tuesday into Thursday. Thereafter, both the 12z ECMWF and 12z Canadian significantly diverged with the expected amplified trough seen in the 18z GFS/12z NAEFS mean/12z ECMWF ensemble mean. The ECMWF was also amplified, just farther west, while the Canadian simply flattened out the mid-level flow. The initial blend for the pressures, 500 hPa heights, fronts, QPF, and winds was a compromise of the 12z ECMWF, 12z UKMET, 12z Canadian, and 18z GFS. After Thursday morning, the blend simplified to a 12z NAEFS mean/12z ECMWF ensemble mean/18z GFS compromise. The remainder of the grids were more heavily weighted on the 01z NBM. ...Weather/Hazards Highlights... An early season storm system will bring unsettled and active weather to the Pacific Northwest 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. An amplifying trough moving through the Southeast will reinforce the polar front across FL and the Gulf of Mexico. The degree and coverage of heavy rainfall in and near the Southeast near and ahead of the upper trough remains a question mark in the guidance, which precluded any excessive risk areas for mid next week at this time. This can be reevaluated on future shifts. 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 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