Extended Forecast Discussion NWS Weather Prediction Center College Park MD 211 AM EDT Wed Sep 23 2020 Valid 12Z Sat Sep 26 2020 - 12Z Wed Sep 30 2020 ...Overview... Models and ensembles indicate a high-confidence upper pattern transition toward much more amplification with a strong western ridge/upper high and eastern trough/low. However, smaller-scale details of the embedded systems remain more uncertain. The amplified pattern will bring another warm-up for much of the West (and perhaps near record highs in California/Arizona) versus mid-fall temperatures for the East along with an exapanding area of rain next week. ...Guidance/Predictability Assessment... Through the 12Z/18Z cycles, the deterministic models offered reasonable clustering to start the period. Differences lied over the East with the exiting shortwave Saturday (preference for a middle-ground solution) and then Sun-Mon out of Canada as the flow amplifies. The Canadian quickly became slower/deeper over the Upper Midwest vs the other models and was discounted. Thereafter, preference drifted toward the 18Z GFS and 12Z ECMWF, though the latter was likely too strong over the western Great Lakes by Tuesday with low pressure vs the ensembles. Incorporated more ensemble mean weighting (18Z GEFS mean and 12Z ECMWF ensemble mean) to balance the stronger deterministic solutions. Given the shifts in evolution/track, this seemed prudent. Multiple frontal boundaries may eventually merge/coalesce into one or two more defined entities by the middle of the week as the trough reaches peak depth. ...Weather/Hazard Highlights... Aside from coastal Washington, precipitation will be rather scant through the weekend into early next week. As the front(s) move out of the Plains, Gulf/Atlantic moisture will stream northward along the coast and may enhance rainfall from the Midwest/Mid-Atlantic to the Northeast next Tue/Wed. In the West, Pacific Northwest/extreme northern Rockies rainfall should gradually diminish/retreat northward upper ridging strengthens. The upper ridge will also favor dry conditions for areas west of the Plains through the period. Temperatures initially cooler than normal (by 3-8F or so) in the Pacific Northwest Saturday will moderate into next week as upper ridging builds in. Coverage of plus 5-15F anomalies for highs will increase with time west of the Rockies. Some locations in California and the southwest corner of Oregon will see the best potential for max temperatures on the higher end of that range with offshore flow and an increased risk of fire conditions. Also record highs/warm lows may be approached/exceeded especially over parts of California and Arizona. Highs into the low 100s will be common over the lower deserts with some areas near 110F. The East will trend cooler in time after an initial warm-up ahead of the leading cold front. Greatest anomalies for warm sector daytime highs this weekend (at least plus 10F) should be from the southern/central Plains into the Great Lakes/Northeast. By next Tuesday and especially Wednesday, expect moderately below normal highs (5-10F below normal) from the Upper Midwest into the Great Lakes and Ohio/Tennessee Valleys. Many areas may stay below 60F 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