Extended Forecast Discussion NWS Weather Prediction Center College Park MD 153 PM EST Tue Dec 3 2024 Valid 12Z Fri Dec 6 2024 - 12Z Tue Dec 10 2024 ...Return flow may fuel emerging rainfall from the west-central Gulf Coast to the Appalachians this weekend to early next week... 19Z Update: The models are initially in good agreement across the domain for Friday with the trough over the Eastern U.S., the cut- off low over the Southwest, and the ridge over the Pacific Northwest. Going into the weekend, the UKMET becomes notably faster with the next shortwave trough moving inland across the Pacific Northwest and quickly loses ensemble support, so a non-UKMET consensus best represents the overall weather pattern by Saturday. The 12Z CMC has trended more in line with the consensus compared to its faster 00Z run for the West Coast region. Looking ahead to early next week, there is good overall agreement with a deep trough developing over the Plains and back to the southern Rockies, but greater differences across the eastern Pacific and also with extent of return flow and QPF across the Gulf Coast region, so the ensemble means were gradually increased to about 40% by the end of the forecast period next Tuesday. The previous forecast discussion is appended below. /Hamrick ---------------------------- ...Guidance/Predictability Assessment... Leaned on recent ECMWF/UKMET and trends from GEFS/ECMWF ensembles that hold main rainfall development closer to the west-central Gulf coastal regions late week until later weekend/early next week breakout up through the Appalachians. Inland QPF development is delayed compared to recent GFS/Canadian runs and the NBM. Across the nation, it is notable that recent deterministic model guidance forecast spread and cycle to cycle continuity issues arise earlier than normal into the medium range time scales, lowering forecast confidence from the weekend into next week despite some common transition of the larger scale pattern. Accordingly, opted to pivot broadly toward a more compatible and run stable GEFS/ECMWF ensemble mean blend along with the National Blend of Models to maintain feasible WPC continuity amid uncertainty. ...Weather/Hazards Highlights... Arctic air that has persisted over the east-central U.S. will moderate this weekend as amplified upper troughing finally lifts out after a prolonged time period. Nevertheless, lake effect snow is likely to be ongoing on Friday and into early Saturday in the wake of a strong clipper-type system. By early next week, temperatures should be near to even a little above average across much of the Eastern U.S. Meanwhile, moist return flow from the Gulf of Mexico with retreating high pressure to the east will support increasing coverage of showers and thunderstorms, some of which could be heavy, from the west-central Gulf Coast states up through the southern Appalachians this weekend into early next week, albeit with uncertainty on the timing and inland extent of the rain. In terms of the excessive rainfall outlook, Day 4 (Friday) will be kept void of any risk areas, although some areas of moderate rainfall are possible across Texas by this time. The latest GFS and CMC models, along with the biased corrected QPF guidance, suggest the potential for heavier swaths of rain and embedded storms to develop during the Day 5 (Saturday) period, and therefore a Marginal Risk area is now planned for portions of eastern Texas to western Louisiana to account for this. Additional heavy rainfall is expected beyond the Day 5 period across the Deep South as strong moisture flux from the central Gulf continues ahead of an approaching cold front. Upstream, moderate terrain enhanced precipitation is expected to develop this weekend over the Pacific Northwest with energetic upper trough passage and lead moisture feed. Terrain enhanced snows will likely spread inland across the Northwest to the north- central Rockies with cold frontal passage over the West into early next week. 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 (QPF), excessive rainfall outlook (ERO), winter weather outlook (WWO) probabilities, heat indices, and Key Messages can be accessed from: 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