Extended Forecast Discussion NWS Weather Prediction Center College Park MD 255 AM EDT Sun Oct 22 2023 Valid 12Z Wed Oct 25 2023 - 12Z Sun Oct 29 2023 ...A heavy to excessive rainfall threat continuing into Wednesday for parts of the southern and central U.S.... ...Overview... The medium range period begins Wednesday with an upper low near the Southwest/northern Mexico which should weaken and lift northeastward late in the week in response to energy dropping through the Northwest, which guidance continues to show significant uncertainty on, particularly as it reaches the Plains/Rockies Thursday-Friday. Regardless, remnant energy from what is currently Tropical Storm Norma looks to interact with Gulf of Mexico moisture and the Southwest upper low to spread heavy to excessive rainfall across parts of the southern and central Plains. To the north, accumulating snows are likely in the Northern Rockies on Wednesday, with parts of the High Plains/North Dakota possibly seeing their first accumulating snow event of the season too by Thursday. Another shortwave is forecast to drop into the Northwest next Friday, with mean amplified troughing likely across the West next weekend, as a somewhat blocky ridge sets up over the Southeast. ...Guidance Evaluation/Predictability Assessment... The GFS based solutions (including ensembles) have been persistent the past few days in showing a much quicker ejection of Southwest U.S. energy/upper low and much more amplified/faster shortwave energy through the West on Wednesday. This eventually spins up a rather strong surface low pressure system across the central U.S. resulting in a decent snowstorm for the Northern Rockies/Plains. WPC forecasts have sided more with the better consensus of a slower/weaker evolution closest to the ECMWF, CMC, and UKMET and there is no reason to go against continuity at this point, but it is interesting how consistent GFS solutions have been. This continues to be a low confidence forecast and likely will take until the short range period to fully resolve details. There is also a lot of uncertainty later in the week with the next shortwave moving into the Northwest. The ECMWF and CMC both show a closed low into the region, while the GFS is much weaker/more open (likely in response to the stronger system downstream). Both the ECMWF and CMC are showing a decent surface low across the northern U.S. next weekend while the GFS and ensemble means are less enthusiastic. The WPC forecast used a ECMWF based blend for days 3-5, with smaller amounts of the deterministic models. Trended quickly towards a vast majority ensemble mean based for days 6 and 7 given the great uncertainty. Some minor ECMWF was continued through day 7 though just for some added system definition. ...Weather/Hazards Highlights... Another day of heavy rains are likely Wednesday associated with Norma remnants and highly anomalous moisture interacting with a stationary boundary across the southern Plains. Given the set up, and heavy rainfall likely on Tuesday as well, a sizable slight risk was added to the new Day 4/Wednesday Excessive Rainfall Outlook spanning across central Texas into Oklahoma. Heavy rainfall is possible farther north along another cold front moving through the north-central U.S., but with more uncertainty on amounts/location. A marginal risk was maintained into the Upper Midwest for Day 4. On Day 5/Thursday, rainfall should trend lighter across the Southern Plains but may continue across the Upper Midwest/Great Lakes region, so a marginal risk was included on tonights Day 5 ERO. Meanwhile, meaningful snow appears likely across the Northern Rockies on Wednesday, with increasing potential for accumulating snows even intending into the lower elevations and the Northern Plains. Exact amounts and resulting impacts are still very uncertain and highly dependent on the eventual upper level and surface pattern evolution. On Wednesday/Thursday, precipitation is likely across the Northwest as the next shortwave moves towards the region, and this moisture should spread east with time. Additional rounds of at least locally heavy rains are possible across the Central U.S. again by next weekend with snow potential well north still very uncertain but worth watching. Well above normal temperatures will spread from the Midwest/Southern U.S. into the East Wednesday-Friday and beyond underneath a building ridge. Meanwhile, troughing across the West will continue a prolonged period of below normal temperatures, with the greatest daytime high anomalies (20-30 degrees below normal) expanding from the Northern Rockies into the Northern Plains 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