Extended Forecast Discussion NWS Weather Prediction Center College Park MD 316 AM EDT Mon Aug 25 2025 Valid 12Z Thu Aug 28 2025 - 12Z Mon Sep 01 2025 ...Heavy rainfall in store for parts of the Southern/Central High Plains and the Middle/Lower Mississippi Valleys... ...Overview... The upper-level pattern will remain relatively stagnant across the lower 48 late this week, with troughing in the East Pacific and East Coast as well as a blocking high over the West/Central U.S.. Shortwave energy rotating within and around the periphery of the upper ridge will likely generate thunderstorm activity over parts of the Central and Southern U.S. late this week. ...Guidance/Predictability Assessment... The evolution of troughing in the East Pac. and shortwave activity over and within the West/Central U.S. ridge are the most uncertain aspects of the guidance for late this week through next weekend. A general model blend consisting of the EC/GFS/UKMET/CMC was used on day 3 with higher weighting toward the GFS and Euro. Euro and Canadian ensemble means are introduced to the blend on day 4 to account for growing variance in the deterministic guidance related to troughing in the East Pac.. A Euro/Canadian-centric blend is used through the rest of the period due, in part, to notable clustering of their deterministic and ensemble mean solutions in the West. Machine learning models like the AIFS, Aurora and Pangu agree with the Euro and Canadian solutions in the E. Pac. as well. The GFS, GraphCast and FourCast models identify a deeper and more phased trough pattern over the E. Pac on day 5. The GEFS mean has significantly more spread than the ECE and CMCE beyond day 5. ...Weather/Hazards Highlights... Troughing in the Eastern Pacific as well as shortwave energy embedded within an upper ridge over the West should promote monsoon storms over much of the interior West later this week. A slight risk (at least 15% chance) of excessive rainfall is in effect for portions of eastern Colorado into northeastern New Mexico on Thursday due to antecedent rainfall from monsoon moisture interacting with a surface front and area of low pressure over the High Plains. Monsoon moisture will likely interact with the downstream portion of the aforementioned front and produce heavy rainfall over the Central Plains on Wednesday before spreading into the Lower Mississippi Valley on Thursday. Therefore, another slight risk of excessive rainfall is in effect for portions of the Central/Southern Plains into the Middle/Lower Mississippi Valley. By Friday, the signal for heavy rainfall becomes much more diffuse, due, in part, to weakening upper-level support. Isolated instances of flash flooding may still occur from storms that develop along the stationary front draped from the High Plains, Lower Mississippi Valley and Southeast. Surface high pressure from Canada will descend on much of the Eastern U.S. this week. This high pressure will bring a cool and dry continental airmass to the Midwest and East Coast. Cloudy and stormy conditions over the Plains will likely contribute to well below average temperatures beginning Thursday. Kebede 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