Extended Forecast Discussion NWS Weather Prediction Center College Park MD 226 PM EST Sun Dec 31 2023 Valid 12Z Wed Jan 03 2024 - 12Z Sun Jan 07 2024 ...Overview... The medium range period features a rather progressive flow pattern with systems of modest amplitude moving west to east across the Lower 48. A series of upper lows/shortwaves shifting from the Southwest and across the Southern U.S. should have the best moisture tap keeping the bulk of the precipitation during the period confined to the West Coast and the Gulf Coast/Southeast where locally heavy rain will be possible in some spots. In the northern stream, shortwaves should travel across the Upper Midwest/Great Lakes with possible eventual interactions with the southern stream systems over the East. It should be cooler than average in the West and Southeast while warmer than average temperatures remain for the northern tier of the country. ...Guidance/Predictability Assessment... There is good agreement with the forecast pattern, other than southern stream progression issues in the southern tier of the country and the East, where the 06z GFS was uncharacteristically slow, though not by a significant amount. Since its solution was within the ensemble spread and overall deterministic spread wasn't particularly large, saw no reason to discount it fully. Early on for the pressures, winds, QPF, and 500 hPa pattern, a compromise of the 06z GFS, 00z UKMET, 00z Canadian, and 00z ECMWF was used. Later on, up to 40% of the 00z ECMWF and 00z NAEFS means were included to help deal with cyclone depth issues near the East Coast. The biggest change to continuity was out East, where a stronger surface low now resides off the coast next Sunday morning. The remaining grids were mostly 13z NBM derived. ...Weather/Hazards Highlights... A system in the Southern Plains moves into the Southeast and is expected to tap Gulf of Mexico moisture and some instability to continue to fuel widespread showers and thunderstorms across parts of the central Gulf Coast and Southeast on Wednesday into early Thursday. A marginal risk remains in the forecast for the Day 4/Wednesday Excessive Rainfall Outlook but the latest guidance seems to be suggesting the heaviest rainfall and better instability may remain offshore so it could either be a more urbanized flood risk or even disappear in later cycles should the offshore trend continue...stay tuned. A cyclone is forecast to approach the West Coast by Wednesday and move into the Southwest by Thursday may bring precipitation to the West Coast and some areas farther inland, with relatively higher totals most likely over favored terrain in portions of California. This should bring heavy rainfall back into Texas, the Gulf Coast, and the East late week/next weekend. Due to the heavy rainfall expected earlier in the week, and recent heavy rainfall in the East, this following system will also need to be watched for increased flooding concerns. Models show increased support for East Coast cyclogenesis with this low next weekend, so trended stronger with the low moving through the Southeast and offshore the East coast, per the early morning guidance. A northern tier system may bring light precipitation to parts of the Great Lakes/Northeast periodically next week, with precipitation moving into the Pacific Northwest later in the week as well. The best chances for above normal temperature anomalies should focus from the Northern Plains into the Northeast, with reading generally 5-15 degrees above normal. Below average temperatures should be confined across the South and West. Roth/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