Extended Forecast Discussion NWS Weather Prediction Center College Park MD 146 AM EST Sat Dec 30 2023 Valid 12Z Tue Jan 02 2024 - 12Z Sat Jan 06 2024 ...Overview... The medium range period features a very progressive flow pattern with a couple of upper lows/shortwaves moving from California to the Southern Plains/Southeast, while in the northern stream weaker shortwaves maintain somewhat mean troughing across the Northeast as a result of strong ridging across western Canada. This type of pattern keeps the bulk of the precipitation confined to the West Coast and the Gulf Coast/Southeast with locally heavy rain in some spots. Expect milder temperatures across mainly the northern states with a tendency for more below average temps across the southern tier. ...Guidance/Predictability Assessment... The guidance continues to show very good agreement through the middle to latter part of next week regarding a departing trough off the East Coast, an upper low shifting from the Southwest to the Southeast Tuesday-Thursday, and a second upper low dropping down the West Coast into the Southwest Wednesday-Thursday. There are some detail differences, which does have implications for rainfall intensity and coverage across the Gulf Coast, but an overall blend of the deterministic guidance seemed to provide a good starting point. By Friday/Day 6, there are some timing differences with additional troughing into the West Coast, which looks to develop another closed upper low off Washington/Oregon by next weekend. There are also strength and timing questions with the second upper low as it moves into the Southern Plains/Gulf Coast Friday-Saturday, and lots of variability with northern stream energies through the northern Plains into the Great Lakes. Introduction of the ensemble means by days 6 and 7 seemed to help smooth out some of these differences for now, which do show a lot of run to run variations/uncertainties in the guidance. ...Weather/Hazards Highlights... An initial upper low/shortwave moving out of the Southwest should begin to tap Gulf of Mexico moisture and some instability to fuel widespread showers and thunderstorms across parts of the Southern Plains/Lower Mississippi Valley on Tuesday. There is some overlap between the heavier rainfall/QPF signal and the more sensitive areas due to a recent wet pattern near the Middle/Upper Texas coast, so a marginal risk was maintained on the new Day 4/Tuesday Excessive Rainfall Outlook covering much of southeast Texas and southwest Louisiana. This activity should shift east along the central Gulf Coast into the Southeast on Wednesday, with another marginal risk this time confined mainly along the coast from New Orleans to the Florida panhandle. 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 on Tuesday into early Wednesday across southeast TX and southwest LA, this following system will need to be watched for increased flooding concerns. 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 greatest temperature anomalies during the period should focus across the far northern tier near the Canadian border and into the Great Lakes/Northeast with some locations seeing multiple days with readings 10-20F or so above normal, with morning lows tending to be farther above normal than daytime highs. Most areas from California eastward across the southern tier, particularly the Southeast, should see near to below normal high temperatures during the period as a series of upper level systems move through. 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