Extended Forecast Discussion NWS Weather Prediction Center College Park MD 310 PM 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 latest run of model guidance continues to depict the GFS faster in ejecting an upper low/Southwest energy along with a much more amplified/faster shortwave energy through the West on Wednesday. This solution favors the development of a robust surface low pressure system over the Central Plains that tracks a snowstorm across the Northern Rockies and Northern Plains. There was better clustering with the ECWMF, CMC, UKMET and the ensemble means when compared to the GFS and given continuity utilized a combination of these solutions, maintained a similar approach for this forecast. This continues to be a low confidence forecast and likely will take until the short range period to fully resolve details. Increased uncertainty remains in regards to the next shortwave across the Northwest. The ECMWF and CMC persist in showing a closed low in 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... Remnant moisture from what is currently Tropical Storm Norma along with highly anomalous moisture interacting with a stationary frontal boundary will lead to another wet period across the Southern Plains, increasing the risk for heavy to excessive rainfall. The inherited Day 4 Slight Risk area was expanded west/north across Texas, Oklahoma and into south Kansas and eastward across central Texas. The Marginal Risk area also was expanded westward across parts of West Texas and the Texas Panhandle. A Heavy rainfall is possible farther north along another cold front moving through the north-central U.S., but with more uncertainty on amounts/location and is covered by the northern extend of the Marginal Risk area. On Day 5/Thursday, rainfall should trend lighter across the Southern Plains but may continue across the Upper Midwest/Great Lakes region. The Marginal Risk for Day 5 was maintained along with an extension along the south/southwest bounds into western Missouri/eastern Kansas. 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. Total accumulations and exact location of the highest amounts remain uncertain as it will be 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. Campbell/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