Extended Forecast Discussion NWS Weather Prediction Center College Park MD 155 AM EDT Tue Sep 22 2020 Valid 12Z Fri Sep 25 2020 - 12Z Tue Sep 29 2020 ...Overview... Upper pattern will transition toward much more amplification with a strong western ridge/upper high and eastern trough/low. This will spell another warm-up (perhaps near record highs in CA/AZ) for much of the West and mid-fall temperatures for the East along with a heavy rain threat next week. ...Guidance/Predictability Assessment... Models and ensembles remained in good agreement overall and a multi-model/ensemble approach sufficed to start, with a few exceptions. The first was with the remnants of Beta combined with a shortwave exiting the Mid-Atlantic Saturday. The 12Z ECMWF was perhaps too aggressive in maintaining this feature (as a surface low) riding up the coast into New England Monday. Preferred a much weaker representation. By Sunday in the West, the GFS remained flatter than the other models with the height field into WA/OR due to upstream troughing. Trend has been toward stronger ridging so the GFS was downplayed. Incorporated increasing weight of especially the ECMWF ensemble mean by next Mon/Tue with uncertainty in the strength/evolution of the eastern upper trough/low. Despite the GFS favoring a northern stream shortwave through Canada this weekend, all models end up with a robust system in the Great Lakes/Ontario early next week. ...Weather/Hazard Highlights... Remnants of tropical storm Beta combined with an upper-level trough will continue to bring rain, perhaps locally heavy, to northern Georgia/southern Appalachians/upstate South Carolina Friday. Rainfall should diminish into the weekend as the feature moves offshore. Approaching cold front out of the High Plains will promote an expanding area of rain initially over the Upper Midwest Saturday through the Great Lakes Sunday and Northeast Monday into Tuesday. As the upper pattern turns more north-south oriented, Atlantic moisture will stream northward along the coast which may enhance rainfall from the Mid-Atlantic to the Northeast. Tail-end of the front through the Southeast may have more limited Gulf moisture in the wake of Beta. In the West, any rainfall at the end of this week will diminish into next week as ridging will favor dry conditions for areas west of the Plains. Temperatures initially cooler than normal in the Pacific Northwest Friday will moderate into next week as upper ridging builds in. Max temperatures are forecast to be 5-15 degrees warmer than normal over much of California with offshore flow and an increased risk of fire conditions. Record highs may again be approached/exceeded especially over parts of CA and AZ. The East will trend cooler in time after an initial warm-up ahead of the cold front. By next week, temperatures may be below normal for many areas east of the Rockies and especially around the Great Lakes. 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 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