Extended Forecast Discussion NWS Weather Prediction Center College Park MD 156 AM EST Sun Dec 3 2023 Valid 12Z Wed Dec 6 2023 - 12Z Sun Dec 10 2023 ...Pattern Overview... An amplified upper ridge is expected to be in place across the central U.S. for the middle of the week, along with a departing upper trough along the East Coast. The ridge axis is then expected to flatten out with a quasi-zonal flow pattern commencing for a brief time across the central and western U.S. on Thursday. Pacific shortwave energy reaching the West Coast Thursday night will help build an upper trough across the western U.S. to close out the work week, and this will eventually eject eastward across the Plains and spur surface cyclogenesis over the weekend across the central Plains. This will likely result in a well organized storm system with increasing rain and storm chances, and wintry precipitation northwest of the surface low. ...Guidance Evaluation/Predictability Assessment... The 00Z model guidance suite is in good overall agreement on Wednesday, and a general model compromise works well as a starting point in the forecast process. This holds true going into Thursday as well, with only some modest timing differences regarding the arrival of frontal boundaries and shortwaves across the central and western states, mainly a slower trough passage with the CMC. By the end of the forecast period next weekend, the model guidance is struggling with the timing of the big western U.S. emerging over the Plains, with the latest 00Z guidance indicating a more progressive trend compared to the 12Z guidance. This is apparent when compared to the 12Z ECENS, which holds the trough farther back to the west by Saturday night into Sunday. The GFS is the most progressive solution, although the CMC and ECMWF have trended in that general direction also. In any case, there will likely be a sub-1000mb surface low tracking southwest to northeast across the eastern U.S. on Sunday. Ensemble means accounted for up to 50% of the forecast by next Sunday. ...Weather/Hazards Highlights... There is some good news to report across the Pacific Northwest with an abatement in the atmospheric river and lesser rainfall totals going into Wednesday and through the remainder of the week. There is still a Marginal Risk area of excessive rainfall from extreme northwest California to the Oregon coastal ranges and the western Oregon Cascades on day 4 (Wednesday) with locally 1 to 2 inches of rainfall expected, but this is a noteworthy improvement compared to the much heavier rainfall expected during the short term forecast period. Even though rainfall totals are expected to be lighter, the grounds will be quite saturated by this time with elevated creeks and rivers, so there may still be some instances of minor flooding for those areas. Looking ahead to Day 5 (Thursday), further improvement is expected for this region and therefore no risk areas are currently warranted at this time. The developing western U.S. trough by the end of the week is expected to result in plenty of moisture advecting inland across the Intermountain West and the Rockies, with widespread moderate to heavy snow for the higher elevations. Snowfall chances are increasing for portions of the western High Plains by next weekend, and the location of the main snow axis will be dependent on the track of the developing surface low and the deformation band to the northwest of it. Farther south in the warm sector of the low, increasing coverage of showers and thunderstorms can be expected from the central Gulf Coast to the Ohio Valley, along with breezy to windy conditions. Pleasantly mild conditions for early December are expected from the central Plains to the Dakotas and Montana on Wednesday and into Thursday, with temperatures running 20 to 30 degrees above average in many cases. The mild conditions then reach the Midwest by Friday and much of the eastern U.S. for the weekend with widespread 10-20 degree above average temperatures likely, with gradually cooling temperatures across the Plains. Colder weather envelops much of the interior West and Rockies by the weekend in response to the upper trough/low and increased cloud cover and precipitation. 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