Extended Forecast Discussion NWS Weather Prediction Center College Park MD 157 AM EST Fri Dec 1 2023 Valid 12Z Mon Dec 4 2023 - 12Z Fri Dec 8 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... The 00Z model guidance suite is in relatively good model agreement to start the week, although the 12Z run of the CMC was out of alignment with the model consensus across the eastern U.S. on Monday and thus it was not used for the Day 3 fronts (00Z run now with the consensus). The guidance has also 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, with the GFS more progressive with the next shortwave moving inland across the western U.S. and becoming stronger with the trough over the Great Lakes region by the end of the week, so the GFS was gradually phased out by Friday. Ensemble means were gradually increased to about 50% by the end of the forecast period. ...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. 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 shifting south across Oregon on Tuesday, a Marginal Risk is warranted for the Day 5 outlook for western Oregon and the southern portions of the Washington Cascades and Olympics. Farther inland across northern portions of the Intermountain West and Rockies, some rounds of moderate to locally snow are expected into next midweek with the passage of the next organized system. Snow is also expected for New England on Monday with the coastal low nearby, with Maine likely to have the greatest impacts. 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 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