Extended Forecast Discussion NWS Weather Prediction Center College Park MD 345 PM EDT Fri Sep 17 2021 Valid 12Z Mon Sep 20 2021 - 12Z Fri Sep 24 2021 ...Weather Pattern Overview... A strong trough pushing onto the plains Sunday night continues to sweep across the rest of the CONUS, bringing below normal temperatures and fallesque conditions for a day or two behind the cold front. The next system enters the Pacific Northwest in the middle of next week. ...Overview and Guidance Evaluation/Preferences... The deterministic guidance remains in above average agreement through early next week, with the 06Z and to a lesser extent 12Z GFS still showing a faster solution by Tuesday night with the trough/cold front crossing the Great Lakes. This remains more apparent with the second trough that approaches the Pacific Northwest Wednesday which is still ahead of its own ensemble mean. By Thursday, model spread becomes substantial across much of the northern tier states with timing of the initial low/front over the northeastern US and the progression of the next system which stalls over the West Coast in the 12Z ECMWF, over the Intermountain West in the 12Z CMC, with the 12Z GFS progressing it east. The WPC front/pressures forecast was primarily based on a nearly equal blend of the 00Z/06Z deterministic guidance through Tuesday/Day 4, begins a transition to ensemble means on Wednesday/Day 5, then heavily relies on ensemble means, Thursday and Friday. QPF with the main system from the Great Lakes and east was timed fairly well in the 00Z CMC/UKMET which were between the faster GFS and slower ECMWF. ...Weather and Potential Hazards... Widespread showers and scattered thunderstorms will develop along and ahead of the cold front from the Central Plains to the Great Lakes Tuesday, shifting east to the Mid-Atlantic Wednesday before pushing off the Northeast around Thursday. Plenty of Gulf-sourced moisture will spread north in advance of the cold front and heavy rainfall is possible. Some strong to severe storms will also be possible where favorable parameters of wind shear and instability exist, although location specifics remain uncertain. Snow in the northern Rockies will taper off Monday as the low moves onto the Plains. Temperatures more like late October will spread east across the Plains behind the cold front Monday, crossing the eastern Seaboard by Thursday. Temperatures will generally be 5 to 10 degrees below normal behind the cold front which will be a refreshing break for many, particularly in the south who have not had a break in summer heat yet. Depending on the progression of the next wave, the Southwest would heat up as a ridge develops in its wake, or it is cool if the low stalls near the Southwest. Jackson 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, 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