Extended Forecast Discussion NWS Weather Prediction Center College Park MD 125 AM EST Tue Nov 3 2020 Valid 12Z Fri Nov 6 2020 - 12Z Tue Nov 10 2020 ***Major pattern change for the Western U.S. with snow and colder weather, and pleasant fall weather for the eastern U.S.*** ...Overall Synopsis... A major pattern change will be arriving for the western U.S. to close out the work week after an extended period of dry weather and above normal temperatures. A rather pronounced upper level trough will build in across the Intermountain West for next weekend along with a powerful cold front, and the opposite will hold true for the eastern half of the nation as a strong upper level ridge axis builds near the East Coast, in response to the amplified western U.S. trough. The mid-upper level high will likely be strong enough to result in a slower than usual progression of the trough axis, with a potential blocking pattern setting up. ...Guidance Evaluation/Preferences... The deterministic and ensemble guidance is in excellent agreement through much of the forecast period across the continental U.S., largely owing to the amplified pattern that will be in place. The first noticeable difference is by Sunday across the Dakotas and south-central Canada regarding the speed of the low, with both the 18Z and 00Z runs of the GFS indicating a more progressive solution, whilst the non-NCEP deterministic guidance is slower. By early next week, the CMC and ECMWF are both stronger with the next Pacific Northwest system compared to the less amplified GFS, but these differences are not that major for a day 7 forecast. The region with greatest uncertainty exists across the Gulf of Mexico and the Caribbean regarding the eventual track of tropical cyclone Eta. This is dependent to some degree on the level of land interaction across Central America, and also how the broad monsoon gyre evolves. It appears likely that the low center will emerge over open water again, and then model spread increases significantly and limits forecast confidence by next weekend. ...Weather/Hazard Highlights... The building western U.S. trough will herald the arrival of much colder temperatures from the Rockies to the West Coast by this weekend, with readings falling at least 25 degrees for many areas compared to highs on Friday. There is a good chance for a significant precipitation event across portions of the Northern Rockies, where the latest winter weather probabilities for exceeding 4 inches of snow exceeds 70 percent this weekend across portions of Idaho and Montana. Maximum temperatures will likely be 10-20 degrees below normal across much of the Intermountain West and the Rockies by Sunday, and lasting into Tuesday. A warming trend is forecast to commence across the central and eastern U.S. ahead of this system, with temperatures that will likely be 10-20F above normal as mid-level high pressure builds across the eastern states. As the trough axis slowly advances eastward over the western High Plains by early next week, moisture interacting with the front could bring locally heavy rainfall to some locations across the central Plains and Midwest. Elsewhere, heavy rainfall is possible across south Florida as moisture interacts with persistent easterly post-frontal flow well to the north of tropical cyclone Eta. Hamrick 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