Extended Forecast Discussion NWS Weather Prediction Center College Park MD 203 AM 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. ...Guidance/Predictability Assessment... Despite some minor timing/detail differences, there is good agreement the first half of the period regarding amplified troughing across the East and the first of two upper lows sliding down the West Coast into the Southwest. By next weekend, the greatest forecast uncertainty surrounds this upper low shifting into the Southern Plains/Southeast with some potential for East Coast low pressure development next Sunday/Day 7. There is increasing support for a low pressure system lifting off the East Coast in the latest models and ensembles, but a lot of uncertainty on timing and precipitation type. The GFS and CMC are the quickest to lift the system off the coast, while the ECMWF is noticeably slower. The ensemble means are a good middle ground timing wise, but are noticeably weaker than their deterministic counterparts suggesting a lot of spread and variability still. The WPC forecast for tonight used a blend of the latest deterministic solutions the first half of the period, with more emphasis towards the ensemble means late period, though with still modest inclusion of the GFS and ECMWF for some added system definition. ...Weather/Hazards Highlights... An initial upper low/shortwave through the Southern Plains into the Southeast should 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. 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 could be a more urbanized flood risk. The second low forecast to approach the West Coast by Wednesday and move into the Southwest by Thursday may bring organized 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 also eventually bring heavy rainfall back into Texas and the Gulf Coast late week/next weekend. Due to the heavy rainfall expected earlier in the week across southeast TX and southwest LA, 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, with some snowfall potential on the north side of the low, but highly dependent on the exact track of the system determining where and how much snow. Elsewhere, northern tier systems 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. Moderately below normal temperatures should be confined across the Southern tier towards the end of this week, especially across the Southeast, but increase in coverage over the West next weekend as amplified troughing moves into the region. 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