Extended Forecast Discussion NWS Weather Prediction Center College Park MD 210 PM EDT Thu Sep 24 2020 Valid 12Z Sun Sep 27 2020 - 12Z Thu Oct 01 2020 ...Overview... A highly amplified pattern is expected to develop across the CONUS next week, with ridging across the West and a deep upper trough/low over the central and eastern U.S. This will lead to warm to hot temperatures once again in the West, and cool temperatures more reminiscent of mid- to late fall farther east. Rainfall should focus in the Great Lakes toward the Appalachians and eastward next week. ...Guidance/Predictability Assessment... Model guidance has been persistent with troughing located across the north-central U.S./south-central Canada on Sun, and energy coming southeastward through the western side of the trough serving to deepen it as the week progresses. The overall troughing in the East/ridging in the West pattern is highly confident, but the more uncertain parts of the forecast include the exact trough axis/tilt at a given time, as well as where the closed upper low develops within the trough. Used a mainly deterministic multi-model blend for Sun/Mon as the major models all seemed reasonable. The 00Z ECMWF seemed to be a bit of an outlier with its development of a closed low south of other guidance over the Mid-South around Tue, but the 00Z UKMET also had hints of this solution. Did incorporate small weights of these into the model blend of mass fields to show the potential, but leaned more toward the 06Z (new FV3-based) GEFS and 00Z EC ensemble means on Tue onward while keeping the 00Z and 06Z GFS runs in the mix as well. The pattern does not progress much after midweek as the upper low spins. ...Weather/Hazard Highlights... The potent upper trough/low will push cold fronts across the central and eastern U.S. through the period and provide focus for rain. The initial cold front tracking through Sun-Tue trended a little slower than the previous forecast, allowing for more time for rain to impact the East before pushing into the Atlantic. Modest to locally heavy rainfall totals may be possible from the Appalachians eastward for the first part of the workweek, with some uncertainty due to the trough axis differences (which could affect how much moisture inflow there is) along with the frontal timing. Florida could also see some rain along the fronts as they try to push across the state and bring drier air. Showery conditions are expected to persist across the Midwest to Great Lakes region. Given the upper-level pattern and the cold fronts, below normal temperatures are forecast to push across the Plains toward the Eastern Seaboard, especially in terms of daytime highs, which should be 5-15 degrees F below normal. But by Thu, below average lows are also forecast for the Central/Northern Plains toward the Midwest, and there is a chance for a first frost or freeze of the season in portions of those areas. Any remaining rainfall should push out of the Pacific Northwest Sun as the upper ridge begins to strengthen and bring hot, dry conditions to the West. Fire danger could be a threat--see SPC for official fire weather forecasts. Much above normal high temperatures are forecast from the Desert Southwest into California and the Pacific Northwest. Record-breaking highs/warm lows look to be approached/exceeded especially over parts of California and Arizona, where highs of 100 to 110F will be common over the lower deserts. Tate 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 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