Extended Forecast Discussion NWS Weather Prediction Center College Park MD 144 PM EST Fri Dec 01 2023 Valid 12Z Mon Dec 04 2023 - 12Z Fri Dec 08 2023 ...Heavy rain expected for the Pacific Northwest with multiple atmospheric river events next week... ...Synoptic Overview... The main theme going into the first half of next week will be a strong Pacific jet stream that will be accompanied by a series of atmospheric river events, with heavy rainfall expected for the Pacific Northwest. A building upper ridge axis is expected across the Intermountain West and then across the Plains going into the middle of the week with well above average temperatures expected. Meanwhile, a broad upper trough will likely be in place from the Great Lakes to the East Coast, with a departing storm system on Monday, followed by a Saskatchewan screamer (a variant of the Alberta clipper) crossing the Upper Midwest and reaching the Ohio Valley through Wednesday with some snow showers accompanying it. A more zonal flow pattern aloft likely resumes after this system exits the East Coast. ...Guidance Evaluation/Predictability Assessment... Latest model guidance remains in relatively good model agreement at mid-larger scales to start next week and has good ensemble support, although the 00 UTC UKMET seemed a bit out of alignment with model consensus. A model composite tends to mitigate smaller scale variances consistent with predictability. This generally seems in line with the National Blend of Models guidance and WPC product continuity. Guidance is still solidified on the building ridge over the central U.S. through midweek, before transitioning more to a zonal flow type pattern by Thursday and Friday. Model uncertainties are more apparent by the end of the week, and of the models and for more detail, the 00 UTC ECMWF remained most in line with GEFS/ECMWF ensemble means into these longer time frames. Accordingly opted to blend this ECMWF with the GEFS/ECMWF ensemble mean during the less certain pattern transition. ...Weather/Hazards Highlights... Out West, precipitation will be ongoing on Monday, with a series of atmospheric rivers into the Northwest lasting into the middle of the week, with rising creeks and river levels expected. Given enhanced rainfall during the weekend, the soils will be more vulnerable and thus the potential for greater run-off and flooding. Snow levels are initially expected to be rather high, and therefore western facing slopes of the Cascades may be subject to an additional risk of flooding due to snow melt. Accordingly, coordinated the issuance of a Slight Risk area for western slopes of the Washington Cascades for Monday, shifting a bit southward for Tuesday. Also, given a stronger model signal for terrain enhanced 3-6 inch rainfall totals on Monday-Monday night across the Olympic Peninsula, a Slight Risk is warranted for the Day 4 excessive rainfall outlook for this area. With the atmospheric river focus most likely shifting a bit more south into Oregon on Tuesday, a Marginal Risk was also warranted for the Day 5 outlook for western Oregon into the Cascades and Olympics. Farther inland across northern portions of the Intermountain West and Rockies, some moderately heavy snow is expected into next midweek with the passage of the next organized system. Heavy snow is also expected for New England into Monday with the organizing coastal low nearby, with Maine likely to have the greatest impacts with wrap-back flow and upper support. Some upslope snow showers are also likely for the higher elevations of the central Appalachians going into midweek as the next low pressure system passes by. In terms of temperatures, much of the central and northern Plains will enjoy late season warmth with highs possibly 20-30 degrees above early December averages from northeast Colorado to the Canadian border on Wednesday and into Thursday, with some record highs within the realm of possibility. The same is expected to hold true with overnight lows as well across much of the central and western U.S., with the greatest positive anomalies across Montana and the Dakotas. Conditions should be close to seasonal averages for the eastern U.S. for much of the week with no major arctic air intrusions expected. Hamrick/Schichtel 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 forecast, excessive rainfall outlook, winter weather outlook probabilities, heat indices and Key Messages 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/#page=ero https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml https://www.wpc.ncep.noaa.gov/#page=ovw