238 FXUS02 KWBC 241957 PMDEPD Extended Forecast Discussion NWS Weather Prediction Center College Park MD 357 PM EDT Sun Aug 24 2025 Valid 12Z Wed Aug 27 2025 - 12Z Sun Aug 31 2025 ...Heavy rainfall in store for parts of the southern and central High Plains and the Middle and Lower Mississippi Valleys... ...Overview... An amplified synoptic pattern continues through the medium range period, featuring troughing along both coasts and varying degrees of ridging in between. Monsoonal showers and storms across parts of the Intermountain West will last through the end of the week before drying out some over the weekend. A growing signal for heavy rain remains in place from the central/southern High Plains to parts of the Middle and Lower Mississippi Valleys for the latter half of the week. Near to below normal temperatures will persist across most of the Lower 48 to close out the month of August. ...Guidance/Predictability Assessment... Model guidance remains in generally good agreement with its depiction of the large scale pattern through the medium range period. The prevailing 500mb flow is expected to remain amplified and rather blocky with troughing flanking both sides of a persistent ridge across the Central U.S. Uncertainty is mostly found in the smaller scale details of shortwaves beyond Day 5, of which there are many. These waves seem to shift both spatially and temporally with each run, affecting the overall evolution of the coastal troughing and their interactions with the Central U.S. ridge late this week into the weekend. Given the above, the WPC forecast was made using a blend of the 00Z and 06Z deterministic guidance through Day 4. For Day 5 and beyond, the GFS nudged the trough across the Eastern U.S. too far east compared to consensus (again due to timing and interactions of shortwaves), so its contribution was removed. Similarly, the CMC was too deep with troughing in the East, and too fast and close to the coast in the Western U.S. by Day 7, so its contribution was also removed. To build more stability and reduce the influence of differences in shortwave timing and placement in the latter half of the forecast period, a greater emphasis was placed on the EC and GEFS ensemble means, which made up 40% of the blend by Day 6 and 60% by Day 7. ...Weather/Hazards Highlights... Flash flooding concerns from the short range period spill over into the medium range time frame for portions of the High Plains and Mid/Lower Mississippi Valleys through much of the week. Shortwave energy will round the western side of the trough and provide ample lift from the jet stream in a broadly moist and unstable environment (precipitable water values of 1.75-2" and MUCAPE towards 1000 J/kg) in the vicinity of a meandering front. A Slight Risk remains in place for Day 4/Wednesday for Kansas and vicinity which shifts east towards parts of the Mid/Lower Mississippi Valleys on Day 5/Thursday with portions of CO and NM also gaining a Slight Risk area due to expected rainfall and anticipated soil saturation between now and then. Heavy rain concerns lessen but aren't zero for similar areas Friday heading into the weekend. Monsoonal moisture will remain in place across much of the Intermountain West around the meandering ridge to the east of approaching upper level troughing. The moisture combined with diurnal heating will likely trigger storms capable of producing heavier rainfall, which could cause localized flash flooding especially over more sensitive areas (steep terrain, in/around burn scars, dry washes/arroyos, urban areas). Broad Marginal Risks remain highlighted for much of the Great Basin, Southwest, and Rockies/Plains for both Wednesday and Thursday for the isolated flash flood potential. The West should start begin to dry out across portions of CA, OR, and NV on Thursday and be mostly dry by Saturday outside of NM and southeast AZ. Within the fall-like cold sector, overall pleasant weather is in store across the Midwest, Ohio Valley, and the Mid-Atlantic states, with plenty of sun, mild temperatures, and low humidity during the day, and comfortably cool overnight temperatures at night. Lake effect/enhanced showers, with thunderstorms possible at times, are expected from the short range period into Friday within the post- frontal cool air mass near the Great Lakes before relaxing by this weekend. Down south, the front stalling over Florida may provide a focus for locally heavy rain there, a risk that appears to gradually increase Thursday through Saturday across the Southern Peninsula. In the vicinity of the Slight Risk ERO areas across the Plains and Mid/Lower Mississippi Valleys, it should be especially mild/cool for high temperatures (15-25F below normal, only reaching the 60s and 70s) within the moderate to heavy rain shield poleward of the front; record cool high temperatures are possible in some locations. Below average temperatures are likely through most of the country from the Rockies eastward. Record low maximum and minimum temperatures are also possible in the Midwest and Ohio Valley Wednesday, and for widely scattered locations in the Southeast, southern Appalachians, and Mid-Atlantic states on Thursday. The excessive heat footprint/magnitude should be fading early on across the Northwest, as moderation of the heat wave in the short range period is anticipated Wednesday onward as upper lows/troughing and fronts approach from the Pacific. Miller/Roth 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 $$