FXUS02 KWNH 010701 PREEPD Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Tue Apr 1 2025 Valid 12Z Fri Apr 04 2025 - 12Z Tue Apr 08 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 much of the period. A frontal boundary parked over the Ohio Valley to Southern Plains in between an amplified western trough and strong Atlantic to Gulf upper ridge, will continue to be the focus for an ongoing multi-day heavy rain and severe weather event from the Lower Mississippi Valley to the Ohio Valley, which starts during the short range period. Significant flooding and flash flooding is increasingly likely for portions of this area. A gradually amplifying Canada into northern U.S. upper trough nearing the East by Sunday- Monday may finally help to push out the frontal boundary. Most of the West should stay relatively cool underneath persistent troughing with some progression eastward early next week into the Central U.S., while upper ridging brings a warming trend to the Northwest during the weekend. The East will see above normal temperatures late this week and then a cooler trend that moves in from the north and west. ...Guidance/Predictability Assessment... The latest model guidance continues to agree fairly well on most aspects of the forecast for Friday and Saturday. There are differences still with some embedded lower-predictability details, which would have sensible weather impacts. A building and blocky ridge over the Atlantic to Gulf should help amplify troughing over the West. Shortwaves and energy perturbations exiting 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. Energy digging through the western trough on Friday should interact with a northern stream shortwave next weekend to eventually help push the wavy front and heavy rain threat eastward. This shortwave should amplify over the East by early next week. There are some differences in the weekend interactions between this shortwave and western U.S. energy and then timing of surface low pressure/cold front through the East next week. Any leftover troughing over the West should weaken and shift east as an upper ridge moves into the Northwest next Monday- Tuesday. WPC was able to use a general deterministic model compromise for about the first half of the period. Transitioned to more ensemble means, eventually comprising 60 percent of the blend, by Day 7. This helps mitigate the smaller scale differences and also maintains good agreement with the previous WPC forecast. ...Weather/Hazards Highlights... By Friday, a wavy frontal boundary will be 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 flash flooding concerns which should begin during the short range period and continue through about Saturday. 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). For Friday, the heaviest rainfall axis looks to be from eastern Oklahoma northeastward into the Ohio Valley, with some slight shift to the east by Saturday, but there has been some wobble in exactly where the heaviest rain sets up. Regardless, the potential for high impact and life-threatening flash flooding is there. As such, moderate risks are in place for this general region for both the Day 4/Friday and Day 5/Saturday ERO periods. High Risks are likely in future EROs as details become clearer, especially into the timeframe of the CAMs. 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 Tennessee Valley/Southeast, and rainfall farther north into the Mid-Atlantic and Northeast as well along the front. A possible upper low in the Four Corners region on Friday 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. Elsewhere by next Monday, the cold upper trough reaching the Great Lakes should lead to unsettled weather over the region with rain or snow depending on location and time of day, while the upper shortwave/front reaching the West Coast may bring some light activity to far western Washington/Oregon and northwestern California. Above normal temperatures will linger across much of the East through the weekend with daytime highs 10 to 20 degrees above normal. Best potential for daily records will be across the South/Southeast into 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 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. Parts of the Northwest could see highs reach up to 10 to 15 degrees above normal which eventually expands into the northern Plains next week. This shifts cooler temperatures eastward, and daytime highs across parts of the southern Rockies and High Plains could be 10 to 25 degrees below normal next weekend. Below normal temperatures also shift into the central U.S. and Midwest by next Monday, including highs as low as 10 to 15 degrees below normal over the Great Lakes/Ohio Valley. 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 $$