Extended Forecast Discussion NWS Weather Prediction Center College Park MD 234 AM EDT Sat Mar 29 2025 Valid 12Z Tue Apr 01 2025 - 12Z Sat Apr 05 2025 ...Emerging heavy rain and severe threat for Lower Mississippi, Tennessee, and Ohio Valleys for midweek and beyond... ...Overview... The medium range upper pattern should trend more amplified mid to late next week and increasingly active. A system into the West around Tuesday should ultimately lead to a surface low emerging into the Plains around midweek and tracking towards the Great Lakes. A building upper ridge over the western Atlantic into the Gulf should keep the associated frontal boundary from making much progress, especially later in the week. This sets the stage for a multi-day heavy rain and severe weather threat from the Lower Mississippi into the Ohio Valley along this boundary. Some late- season snow may be possible north of this system from the northern Plains to Great Lakes. The West should stay relatively cool underneath of 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 next week into the weekend, but still very uncertain with some of the details, which could have sensible weather impacts. A shortwave exiting the East on Tuesday will be replaced by a building ridge over the western Atlantic to the Gulf that lasts into next weekend. This should help amplify and reload troughing over the West early to mid week. The guidance shows better consistency in this evolution compared to yesterday, but still considerable uncertainty in the details of multiple pieces of energy within the overall trough. A leading shortwave may briefly close off a compact low that induces surface cyclogenesis across the Central U.S. that tracks into the Great Lakes. Energy will allow for the Western U.S. trough to reload late week with a stationary surface boundary setting up between the Southeast ridge and Western trough. There are subtle differences in the exact placement of this front though, which has significant implications on the axis of heavy rainfall expected to accompany this. The 18z/Mar 28 GFS was a bit farther east with the overall evolution, but the new 00z run (available after forecast generation time) was more in line with consensus. Upper ridging upstream from this may sneak back into the Northwest next weekend. The WPC forecast was able to utilize a general deterministic model blend featuring the GFS, ECMWF, CMC, and UKMET through Day 4. By Day 5, began incorporating some ensemble mean guidance to help temper the detail differences. Trended days 6 and 7 to a 60/40 ensemble mean/deterministic guidance split. This approach maintained relatively good agreement with the previous WPC forecast (through Day 6). ...Weather/Hazards Highlights... Expect a series of shortwaves to bring rain and higher elevation snow to the West next week. An atmospheric river should be sinking southward along the California coast by Tuesday, but some lingering precipitation along the northwest Coast may bring at least locally moderate to heavy rainfall. Opted to not include a marginal risk on the Day 4 ERO tonight given the better dynamics and moisture anomalies were south. Precipitation will spread through the Intermountain West and Rockies into Tuesday, while the pattern may also support high winds focused from the central Great Basin into the central Rockies. Moderate to heavy snow may fall in parts of the Sierra Nevada. Overall pattern amplification and a leading shortwave will support a surface low pressure system forming in the central U.S. by Wednesday with widespread thunderstorms across the east- central U.S. in its warm sector. SPC indicates another round of severe weather from the south-central U.S. into the Mid- South and Ohio Valley on Wednesday (and possibly beyond). Potential for a multi- day heavy rainfall event is increasing as a wavy boundary sits over the Ohio Valley to Lower Mississippi Valley into next weekend. Ample moisture and instability along and ahead of the front should support training thunderstorms and high rain rates. At this point, the Day 5/Wednesday ERO indicates a slight risk from Arkansas into southern Indiana and southwest Ohio, surrounded by a broad marginal risk to account for uncertainty. Given the pattern, it's entirely possible a moderate risk may eventually be needed in subsequent updates, but there is still some uncertainty in exactly where the surface boundary sets up. Additionally, some April snow is likely on the backside of the low across the northern Plains and Upper Midwest Tuesday- Wednesday and spreading into the Interior Northeast Wednesday- Thursday. Some modest rain and snow could also continue across the West throughout next week, though with rapidly increasing uncertainty in the details after early next week. Above normal temperatures will shift from the Central/Southern Plains on Tuesday into the Midwest on Wednesday, and settle across much of the 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 and shift slightly east as eastern Pacific upper ridging moves in towards the West Coast. 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