Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 AM EST Sat Dec 2 2023 Valid 12Z Tue Dec 5 2023 - 12Z Sat Dec 9 2023 ...Heavy rain expected for the Pacific Northwest with an atmospheric river event Tuesday and Wednesday... ...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 an atmospheric river event, 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 is forecast to be in place from the Great Lakes to the East Coast, with a departing storm system followed by a clipper type system 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, followed by a potentially organized storm system over the west-central U.S. by next Saturday. ...Guidance Evaluation/Predictability Assessment... The 00Z model guidance suite is in good overall agreement on Tuesday, and a general model compromise works well as a starting point in the forecast process. This holds true going into Wednesday as well, with only some modest timing differences regarding the arrival of frontal boundaries and shortwaves across the central and western states. With the upper flow pattern evolving more zonal by Thursday, the guidance differs more with the timing of the western U.S. trough crossing the Rockies, with the UKMET appearing more progressive than the model consensus. The GFS is stronger with a shortwave trough over the Northeast U.S. during this time, but close enough to the consensus elsewhere to remain part of the forecast blend. By the end of the forecast period next Saturday, the models are trending stronger with an organized storm system over the Intermountain West that will eventually emerge over the central/southern Plains, with the GFS/ECMWF/CMC indicating a potential southern stream upper trough developing. It is interesting to note the JMA model is not nearly as strong, so there is still some uncertainty regarding how this will evolve. Ensemble means accounted for up to 50% of the forecast by next Saturday. ...Weather/Hazards Highlights... Out West, precipitation will be ongoing on Tuesday, 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, a Slight Risk area remains valid for western slopes of the Washington Cascades on Tuesday, and also for the Oregon Coastal Ranges. 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. 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