Extended Forecast Discussion NWS Weather Prediction Center College Park MD 156 AM EST Sun Nov 26 2023 Valid 12Z Wed Nov 29 2023 - 12Z Sun Dec 03 2023 ...Overview... An upper low over/near Hudson Bay will allow for general troughing with some embedded shortwaves through Thursday across the Great Lakes and Northeast. This pattern would support some ongoing lake effect snows, locally enhanced in spots. An upper low off the California coast on Wednesday should shift inland across the Southwest with its shortwave energy eventually shifting east into the central U.S. bringing potential for widespread moderate to heavy rainfall across parts of the south-central U.S. into the Southeast. The next system moves into the West Thursday-Friday promoting amplified troughing shifting across the West next weekend and a building ridge downstream across the Southeast. ...Guidance Evaluation/Predictability Assessment... Overall, the guidance continues to show good agreement on the synoptic scale pattern through much of the period. There are some lingering differences in the details pertaining to individual shortwaves through the Great Lakes/Northeast and the initial shortwave shifting across the Midwest late in the week, but for the most part, a blend of the deterministic guidance gave a good starting point for days 3-4/Wednesday-Thursday. By Friday, there are some more notable differences with timing and strength of the next shortwave into the Pacific Northwest and a lot of lingering uncertainty in the pattern going into next weekend. The GFS has been fairly consistent in showing a less amplified shortwave that progresses more easterly across the Northwest/Northern Rockies. Meanwhile, the CMC and ECMWF show much more digging of this trough into California and the Southwest and more amplification of the pattern next weekend as the trough progresses into the central U.S., which is more widely supported by the ensemble guidance as well. Beyond day 4, the WPC blend incorporated increasing amounts of the ensemble guidance through next weekend, with still some modest ECMWF and CMC for a little bit of added system definition. ...Weather/Hazards Highlights... Favorable westerly cold flow over the still relatively warm Great Lakes will continue to promote some locally enhanced lake effect snows through Thursday especially downwind of Lakes Erie and Ontario, but not nearly as heavy as is expected in the short range period. Elsewhere, shortwave energy into the mid-south and a warm front lifting through the Gulf of Mexico will allow for increasing moisture and instability across the Gulf Coast/Southeast region, with increasing confidence in some widespread moderate to heavy rainfall beginning Thursday and lasting into next weekend. Amounts and timing remain quite uncertain still, but there was enough support for a marginal risk on the Day 5 (Thursday) Excessive Rainfall Outlook from eastern Texas into the Lower Mississippi Valley. Out West, precipitation will increase Thursday into next weekend as a shortwave or two move through the region, with some precipitation reaching southward into California and the Southwest. The heaviest precipitation (coastal rain and mountain snow) will be across the Washington/Oregon coast and Cascades next weekend. The East Coast should start out with below normal temperatures on Wednesday, but should moderate back towards normal by Thursday. Mean troughing across the interior West should allow for cooler than normal temperatures lasting into next weekend. Elsewhere, there is signal for a period of above normal temperatures across the Northern Plains on Wednesday, with daytime highs 10-15 degrees above average, but as ridging builds into the Midwest and Eastern U.S. later this week, above normal temperatures should expand with time with much of the Northern Plains to Midwest and Southeast above normal by next weekend. Santorelli 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