Extended Forecast Discussion NWS Weather Prediction Center College Park MD 302 AM EDT Mon Mar 31 2025 Valid 12Z Thu Apr 03 2025 - 12Z Mon Apr 07 2025 ...Multi-day and significant heavy rain/flooding and severe threat likely for Lower/Middle Mississippi, Tennessee, and Ohio Valleys through next Saturday... ...Overview... The medium range upper pattern will be amplified, blocky, and active through the period. A frontal boundary will remain parked over the Ohio Valley to Southern Plains in between an amplified western Trough and strong Atlantic to Gulf upper ridge. This pattern sets the stage for a multi-day heavy rain and severe weather event which should be ongoing by the start of the period on Thursday from the Lower Mississippi into the Ohio Valley, with significant flooding increasingly likely. An upper trough into the East next weekend may finally help push out the frontal boundary. The West should stay relatively cool underneath persistent troughing with some progression eastward early next week into the Central U.S., while above normal temperatures remain late week for much of the East before moderating. ...Guidance/Predictability Assessment... Model guidance is relatively agreeable on the large scale pattern midweek into next weekend, but still uncertain with some of the details, which could have sensible weather impacts. A building and blocky ridge over the Atlantic to Gulf should help amplify troughing over the West through the period. Shortwaves within that trough show more spread, and may play a role in exactly where the heaviest rainfall occurs. Either way, models have been extremely consistent in showing a potentially significant flooding event from the lower Mississippi Valley into the Ohio Valley. By Friday and into the weekend, there is somewhat more uncertainty in the details of another shortwave digging through the western side of the trough and how much of this eventually ejects eastward next weekend. The main model outlier here is the GFS which is a little faster with this scenario, while the ECMWF and CMC (and ensemble means) hold back the main energy a little longer across the West. A northern stream shortwave shows some timing/depth issues, but otherwise guidance agrees this should amplify over the Great Lakes and move into the Northeast early next week, and finally help push the wavy surface front eastward. The WPC forecast was able to utilize a deterministic model blend through Day 5. Afterwards, incorporated some ensemble mean guidance to help temper the detail differences, while reducing the proportion of operational models (especially the 18z/Mar 30 GFS) in favor of over half means by day 7. This approach maintained relatively good agreement with the previous WPC forecast. ...Weather/Hazards Highlights... A surface low exiting the Great Lakes on Thursday will leave behind a wavy frontal boundary that essentially becomes stuck between a strong upper ridge to the east and amplified troughing over the West. Moist and unstable inflow pooling along the front will lead to widespread severe weather and flash flooding concerns which should begin during the short range period and continue for several days. SPC shows severe probabilities from the lower Mississippi Valley into the Ohio Valley. The greatest concern for flash flooding for Thursday will stretch across a similar area, as the front anchors storms for significant training concerns. For several days now, models have been indicating several inch rainfall totals, especially across Arkansas into western Kentucky where 24-hour totals in excess of 5-8 inches could be realized in some spots (and this is on top of several inches of rainfall in the short range as well). As such, a moderate risk is in place for this region for the Day 4/Thursday ERO. The heavy rainfall axis should remain over the same areas into Friday as well, with a slight shift to the south and west. Another moderate risk is in effect on the Day 5 ERO from northeast Texas to southeast Missouri. High Risks are not out of the question in future EROs as the event draws nearer and it becomes clearer how the details could evolve, especially into the timeframe of the CAMs. Heavy rainfall is likely to continue into Saturday as well for roughly the same areas, leading to the potential for high impact and life- threatening flooding and flash flooding over the course of several days. By Sunday, the heavy rain threat should finally begin to shift eastward with moderate to heavy rains possible across the Gulf Coast states into the Southeast, and more widespread rain also moving into the East. As troughing sets up over the West with multiple shortwaves moving through, rounds of precipitation are possible from the West Coast into the Intermountain West and Rockies. Generally lower elevation rain and higher elevation snow are expected, with mainly light to modest amounts. Into late week, a possible upper low in the Four Corners region could lead to enhanced precipitation totals across the southern and central Rockies and Plains. Snow is likely in higher elevations of the Rockies in Colorado and northern New Mexico, with uncertainty at this point in if/how much snow could spill into lower elevations of the High Plains. Above normal temperatures will linger across much of the East through the weekend with daytime highs 10 to 20 degrees above normal, especially for portions of the Southeast and southern Mid- Atlantic. Temperatures should moderate early next week as the upper ridge shifts eastward and troughing moves into the East. Meanwhile, the West should trend and stay cooler underneath upper troughing through Friday. By next weekend, temperatures may moderate somewhat along the West Coast while eastern Pacific upper ridging moves in. This shifts cooler temperatures eastward, and daytime highs across parts of the southern Rockies could be 10 to 20 degrees below normal next weekend. Below normal temperatures also shift into the central U.S. and Midwest by next Monday. 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 (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