Extended Forecast Discussion NWS Weather Prediction Center College Park MD 354 PM EDT Fri Sep 25 2020 Valid 12Z Mon Sep 28 2020 - 12Z Fri Oct 02 2020 ...Guidance/Predictability Assessment... The models are in increasing agreement on the development of a highly amplified pattern across the CONUS next week, with ridging across the West and a deep upper trough in the Midwest/Mississippi Valley and wavy surface front in the east. However, there remains quite a bit of forecast spread with respect to the degree of flow separation within the main upper trough. Several 00z ECMWF ensemble members and the operational ECMWF run show development of closed 500 mb low in the lower MS Valley/TN Valley early-middle portions of next week. The majority of the members did not, and neither did the operational 00z UKMET/00z Canadian/00-06z GFS. Given the split in solutions, like in the previous forecast, I placed more weighting on the 00 UTC ECMWF ensemble mean,along with the GEFS Mean, to mitigate the differences in the trough amplitude/timing. Upstream, the mean upper ridge along the west coast persists due to warm advection downstream from a slow moving north Pacific cyclone. The ridge is slow moving and persistent, so the models have better than average agreement, leaning the situation to a consensus-based approach. ...Weather/Hazard Highlights... A slow moving cold front moving through the eastern U.S. through the period and provide focus for clusters of showers/storms. Amplification trends have slowed the eastward translation of a main lead front and reinforcement. Moderate to locally heavy rainfall totals may focus from the Appalachians eastward to the southeast and mid Atlantic for the first part of the workweek. Florida will also see some rain along the fronts as they try to push across the state and bring drier air. Cold air aloft provides instability for showers across the Midwest/Mid-South to Great Lakes region under the upper trough, with great lake enhancement likely. The upper ridge will return hot, dry conditions to the West, so fire danger is a concern. Much above normal high temperatures are forecast to build from the Desert Southwest and California/Nevada to the Pacific Northwest. Some record-breaking highs/warm lows look to be approached/exceeded especially over parts of California. Given the upper-level trough persisting in the central US, persistent below normal temperatures are forecast across the Plains across the MS Valley into the Ohio Valley. Daytime highs should be 5-15 degrees F below normal early to mid week. But by Thu/Fri, below average lows are also forecast for the Central/Northern Plains toward the upper MS Valley, and there is a chance for a first frost or freeze of the season in those areas. Petersen/Schichtel 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 Great Lakes, the Northeast, the Mid-Atlantic, the Appalachians, and the Southeast, Tue-Wed, Sep 29-Sep 30. - Flooding occurring or imminent across portions of the Southeast. - Much above normal temperatures across portions of California, the Great Basin, and the Pacific Northwest, Mon-Fri, Sep 28-Oct 2. - Much below normal temperatures across portions of the Central/Northern Plains, the Middle/Upper Mississippi Valley, and the Great Lakes, Thu-Fri, Oct 1-Oct 2. - Heavy precipitation across portions of mainland Alaska, Tue-Fri, Sep 29-Oct 2. - Heavy precipitation across portions of the Alaska Panhandle and mainland Alaska, Mon-Fri, Sep 28-Oct 2. - High winds across portions of mainland Alaska, Tue-Wed, Sep 29-Sep 30. 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