Extended Forecast Discussion NWS Weather Prediction Center College Park MD 244 PM EDT Wed Oct 30 2024 Valid 12Z Sat Nov 02 2024 - 12Z Wed Nov 06 2024 ...Multi-day Heavy Rain/Flooding Threat from the Southern Plains to the Mid-Mississippi Valley this weekend into early next week... 19Z Update: The 12Z model guidance suite features good overall agreement across the Continental U.S. going into next weekend, given a more amplified upper flow pattern evolving with a deep Western U.S. trough and a building ridge axis east of the Mississippi River. This will result in a pronounced temperature dichotomy across the country, with highs running up to 20 degrees above early November averages across the Ohio Valley early next week, and 15 degrees below average for portions of the Intermountain West. With respect to the Day 4 and 5 excessive rainfall outlooks, no major changes were needed for this update, with Slight Risk areas maintained for the Central Plains. Going into Monday, more noticeable differences emerge with the next trough approaching the Pacific Northwest coast, with the ECMWF faster with the trough progression. Towards the end of the period, the models still have great agreement with the ridge near the East Coast, but vary more across the north- central U.S. as the main trough begins lifting out and the second one from the Pacific Northwest moves inland. The ensemble means were gradually increased to about 40% by next Wednesday. The previous forecast discussion is appended below for reference. /Hamrick ----------------------------- ...General Overview... An emerging excessive rainfall event is expected to develop in earnest by the weekend into early next week over the south- central U.S. with return Gulf of Mexico inflow in a deepening moisture channel between a East/Southeast upper ridge and slow approach of amplified upper trough/low energy and front from the West. Towards that end, aforementioned upstream upper trough energy will dig sharply across the Western U.S. this weekend, with moderate rains and high elevation snows from the Northwest southward to California. Activity should spread more modestly to the Southwest and the Intermountain West/Rockies by early next week with slow and amplified flow translation and energy reinforcement and closed low development at the base of the upper trough/fronts. ...Guidance/Predictability Assessment... Models and ensembles have trended toward a better clustered overall solution over the coming weekend in a pattern seemingly now with closer to average predictability. A composite model blend tends to mitigate still lingering individual system timing issues to offer a seemingly reasonable forecast starting point, generally in line with the National Blend of Models and threat messaging continuity. Forecast spread increases into longer time frames, so pivoted to a composite blend of best clustered and cycle to cycle consistent guidance from the 12 UTC ECMWF/Canadian and their ensemble means. ...Weather/Hazards Highlights... A heavy rainfall pattern seems likely on tap by the weekend with main focus from the southern Plains through the Mid-Mississippi Valley and vicinity. The WPC Day 4/Saturday and Day 5/Sunday Excessive Rainfall Outlooks (EROs) continue to offer Marginal and Slight Risk threat areas given consistent guidance signals, with a modest expansion of the Marginal Risk areas to account for the latest model guidance. Expect activity to expand and focus along a southwest to northeast axis ahead of additional frontal boundaries this weekend and into early next week as robust upper trough/closed low energy digs upstream into the West Coast to re- amplify the flow over the Lower 48 and vicinity. Multiple days of southerly, moist Gulf inflow look to increase the chances of more widespread, heavy rainfall with flooding concerns despite dry antecedent conditions, with a risk of repeat/training convection in a slow to dislodge and deepened moisture channel between the West/Southwest U.S. upper trough and East/Southeast U.S. warming upper ridge. If the heavy rains on Day 5 trend heavier, or appears likely to fall on the same areas that get impactful rain in the preceding days, an eventual Moderate Risk could be needed as the event gets closer in time. The multi- day threat will likely continue into early next week while spreading northeastward over the Midwest in this pattern with the deep moisture leading to some warm sector record high overnight temperatures. Precipitation with Pacific system digging will be locally enhanced by terrain as moderate long-fetch moisture is forecast to work southward over the West Coast this weekend, with high-elevation snow also reaching favored terrain of the northern Intermountain West/Rockies and enhanced rains reaching down into California. Regardless, the cold frontal surge and possible closed low development at the southern base of the main upper trough will bring in cooler temperatures to much of the West. This and slow and amplified flow translation will also act to spread more modest rains over the Southwest and into the south-central Great Basin and Rockies into early next week. Meanwhile, additional Pacific system energy upstream may also bring another round of moderate precipitation back into the Northwest. Schichtel/Hamrick 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 (QPF), excessive rainfall outlook (ERO), winter weather outlook (WWO) probabilities, heat indices, and Key Messages can be accessed from: 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