Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 PM EDT Sun Mar 30 2025 Valid 12Z Wed Apr 02 2025 - 12Z Sun Apr 06 2025 ...Multi-day heavy rain/flooding and severe threat for Lower/Middle Mississippi, Tennessee, and Ohio Valleys this week... ...Overview... The medium range upper pattern will be amplified, blocky, and active through the period. A surface low emerging into the Plains by Wednesday will track towards the Great Lakes as a building upper ridge over the western Atlantic into the Gulf should keep the associated frontal boundary from making much progress on the southern side, especially later in the week. This sets the stage for a multi-day heavy rain and severe weather event from the Lower Mississippi into the Ohio Valley, and significant flooding is likely. Late-season snow is likely north of this system from the northern Plains to Great Lakes and Interior Northeast for midweek, and the Colorado Rockies are likely to see snow by late week. The West should stay relatively cool underneath persistent troughing, while above normal temperatures progress from the central U.S. and Midwest, eventually settling late week across the Southeast and into the Mid-Atlantic. ...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 relatively more spread, but the first shortwave/compact upper low and its associated surface low tracking from the Central U.S. to Great Lakes into Thursday continues to show agreement. 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. ECMWF runs in particular have been on the western side with this energy compared to the larger consensus of other models and even EC-based AI/ML models. The new 12Z ECMWF seems to be coming into better alignment with the majority in the energy and trough placement. Regardless there is still some spread in what becomes of this energy next weekend, perhaps producing a closed upper low within the trough becoming positively tilted while models are converging on showing an upper ridge axis over the Northwest. At the surface, a stationary frontal boundary will set up between the Southeast ridge and Western trough that persists for several days. There remain subtle differences still in the exact placement of this front, which has notable implications on the exact locations of heavy rainfall expected to accompany the front. Expect these to vary a bit in future forecasts, though the general idea of heavy rain leading to possibly significant flooding in parts of the east-central U.S. is pretty locked in. A northern stream trough may help to slowly nudge the stagnant east-central U.S. boundary eastward a bit next weekend, though some typical spread remains with the timing and depth of that trough. The WPC forecast was able to utilize a deterministic model blend through Day 4. By Day 5, began incorporating some ensemble mean guidance to help temper the detail differences, while reducing the proportion of operational models (especially the 00Z ECMWF) in favor of over half means for Days 6-7. This approach maintained relatively good agreement with the previous WPC forecast. ...Weather/Hazards Highlights... Overall pattern amplification and a leading compact closed low will support a surface low pressure system forming in the central U.S. by Wednesday with widespread thunderstorms and heavy rainfall across the east-central U.S. in its warm sector. Moist and unstable inflow into the associated surface fronts along with the strong upper level support will lead to widespread severe weather and flash flooding concerns. SPC shows severe probabilities from the south-central U.S. into the Midwest on Wednesday and Thursday. The greatest concern for flash flooding on Wednesday and Thursday will stretch across similar areas from the Ark-La-Tex northeast into the Ohio Valley, as the front anchors storms for significant training concerns. On Wednesday, precipitable water anomalies above the 95th percentile with ample instability will be in place. Models are indicating embedded rainfall totals of 5 to 8 inches during the 12Z Wednesday to 12Z Thursday period alone, with the GFS already explicitly forecasting hourly rain rates to 2", and certainly expect the CAMs to show higher rates than that once we get into their timeframe. Thus plan to upgrade the Day 4/Wednesday ERO to a Moderate Risk from northeast Arkansas across the Ohio River Valley to far southern Indiana. Much the same environmental pattern sets up on Day 5/Thursday, with the added concern of very wet antecedent conditions by then. The previously issued Moderate Risk for Day 5/Thursday was maintained, with a southwestward expansion per coordination with the Little Rock local forecast office and considering the higher instability there. Heavy rainfall is likely to continue into late week, with perhaps a bit of a westward shift into the southern Plains at times as the West trough pivots. Through midweek into the weekend, significant flood impacts are likely with the multiple rounds of heavy rain. 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. Through Wednesday, some April snow is likely on the backside of the initial low track, stretching across the Dakotas to Upper Midwest and Interior Northeast. Snow amounts have typical uncertainty, and an axis of freezing rain and/or sleet is possible as well. Meanwhile 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. Additionally, high winds are possible for the central Rockies on Wednesday. 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 be located across the south-central U.S. to Midwest on Wednesday, and settle across much of the southern and eastern U.S. Thursday and beyond. Daytime highs 10 to 20 degrees above normal are possible. 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 slightly east, and daytime highs across parts of the southern Rockies could be 10 to 20 degrees below normal next weekend. Tate/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