Extended Forecast Discussion NWS Weather Prediction Center College Park MD 148 PM EST Sun Dec 03 2023 Valid 12Z Wed Dec 06 2023 - 12Z Sun Dec 10 2023 ...Pattern Overview... It remains the case tha 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 now more rapidly eject eastward across the Plains and spur surface cyclogenesis into the weekend across the central to east central U.S.. This will likely result in a well organized storm system with increasing and enhancing rain and storm chances across the broad region with frontal system advance/focus, with wrapping wintry precipitation northwest of the main surface low track. ...Guidance Evaluation/Predictability Assessment... Model and ensemble solutions seem overall reasonably well clustered into mid-late week in a pattern with seemingly above normal predictability. Accordingly, the WPC medium range product suite was primarily derived from a composite of the best clustered 06 UTC GFS and 00 UTC ECMWF/UKMET/Canadian models along with the compatible 13 UTC National Blend of Models (NBM) and WPC continuity for best detail. Newer 12 UTC guidance remains mainly in line and multi-center ensemble support remains very good in this period. Forecast spread increases steadily heading into next weekend, leaning a switch in preference to a blend of best compatible 06 UTC GEFS/00 UTC ECMWF ensemble means in an effort to mitigate variance consistent with predictability. WPC product continuity has trended toward progression across the lower 48 and vicinity at these long time frames. ...Weather/Hazards Highlights... There continues to be 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. A swinging of a now moderately amplified western U.S. trough by the end of the week will bring moisture across the Intermountain West and the Rockies, with moderate to heavy snow for particularily the higher elevations and possible spilling a bit into the adjacent High Plains with system progression. 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. This activity looks to become quite expansive over next weekend over the east-central U.S. and could result in runoff issues over time. 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/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