Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 PM EST Mon Dec 04 2023 Valid 12Z Thu Dec 07 2023 - 12Z Mon Dec 11 2023 ...Pattern Overview... The ridge axis that will be in place across the north-central U.S. mid-week is expected to flatten out by Thursday while a ridge then builds across the eastern U.S. to close out the week. Pacific shortwave energy reaching the West Coast Thursday night will help build an upper trough across the western U.S. Friday and into Saturday, and this will then 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 tracking northeast towards the Ohio Valley and Great Lakes, with increasing rain and storm chances ahead of the cold front, and wintry precipitation northwest of the surface low. ...Guidance Evaluation/Predictability Assessment... The latest model guidance continues to be in good agreement with the large scale pattern, allowing for a general model compromise of the deterministic solutions as a starting point for the days 3-5 forecast. After Day 5/Saturday, there are some minor timing and strength differences in the trough as it progresses through the Midwest and East. The ECMWF has been most consistent in wanting to close off an upper low over the Great Lakes by next Monday. There are some differences with individual shortwaves rouding the upper ridge as well, and the 00z CMC is faster and becomes out of phase with the GFS/ECMWF concerning a shortwave into the West on Saturday. To mitigate these differences, incorporated 40 percent of the ensemble means into the blend along with the GFS/ECMWF days 6-7. Overall, this maintains good continuity with the previous WPC forecast as well. ...Weather/Hazards Highlights... The next round of rainfall for the Pacific Northwest is expected to be considerably lighter on Thursday compared to the ongoing atmospheric river event in the short range forecast period, and although some portions of the Oregon coastal ranges may get an additional inch of rain, it does not appear the intensity nor the overall totals are currently high enough to merit any excessive rainfall concerns, although local rivers may still be running high. Even less precipitation is expected here on Friday, before the next round of Pacific moisture advects inland next weekend. The developing western U.S. trough by the end of the week is expected to result in moisture advecting inland across the Intermountain West and the Rockies, with widespread moderate to locally heavy snow for the higher elevations. There may also be some snow for adjacent portions of the western High Plains as the low exits the Rockies. Farther south in the warm sector of the developing surface 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. Heavy rainfall is looking increasingly likely particularly across parts of the mid-South on Saturday, with the Storm Prediction Center also highlighting parts of this region for a severe thunderstorm threat. This storm system should then reach the East Coast by late Sunday into Monday with widespread rain from the Mid-Atlantic to New England, with some snow behind the system across the central and northern Appalachians. Pleasantly mild conditions for early December are expected from the central Plains to the Dakotas and Montana on Thursday, with temperatures running 15 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. Santorelli/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