Extended Forecast Discussion NWS Weather Prediction Center College Park MD 310 AM 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... ...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 into an unsettled West this weekend, with moderate rains and elevation snows from the Northwest southward to California. Activity should spread more modestly to the Southwest and the Intermoutain West/Rockies by early next week with slow and amplified flow translation and energy reinforcement and closed low development at the base of the quite cooling 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) offer Marginal and Slight Risk threat areas given consistent guidance signals. 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. 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, aditional Pacific system energy upstream may also bring another round of moderate precipitation back into the Northwest. Schichtel 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