Extended Forecast Discussion NWS Weather Prediction Center College Park MD 306 PM EDT Sun May 4 2025 Valid 12Z Wed May 07 2025 - 12Z Sun May 11 2025 ...Mid to late week heavy rainfall potential for the central Gulf Coast into the Southeast... ...Overview... A blocky pattern in the short range should finally begin to break down by Wednesday as a pair of upper lows over the East and West weaken and shift east. A stationary or slow moving frontal boundary draped across the Southeast may continue to support heavy rainfall across the region, and especially along the Gulf Coast. A shortwave dropping in from east-central Canada may briefly cut off another upper low over the Northeast on Friday, while the western low shifts east and eventually settles over the Southeast next weekend. This may continue the threat for showers and storms across the Southeast through much of the period. Upper ridging over the West Coast will build east into the Rockies/Central U.S. as a Pacific trough nears the West Coast next Sunday. ...Guidance/Predictability Assessment... While model guidance agrees in principle on the larger scale features described above, uncertainty in the details still produce sensible weather differences. Models show the cutoff upper low moving slowly from the southwestern to south-central U.S. as the week progresses. By Thursday-Friday model differences with this feature include the new 12Z GFS farther south and west with energy at the low's base near Texas and the Lower Mississippi Valley, pushing the front offshore into the Gulf for less rain onshore in the central Gulf Coast area but more QPF filtering into much of Texas. Did not prefer this 12Z GFS solution as it was against consensus. Meanwhile farther north, the initially closed low over the Northeast should quickly track northeast as additional energy dives in from Canada. This second round of troughing shows a lot more spread in placement into later week as it may form a closed low. Through the 00/06Z model cycle, on Saturday for example the 00Z GFS and ECMWF were reasonably well aligned with the upper and surface low position and generally in line with the ensemble means, so generally favored that cluster, which also jived well with continuity. The 06Z GFS was farther southwest and the 00Z CMC was much farther northeast. Many of the AI/ML models were farther northeast like the CMC so did not want to totally rule that out. However, now the newer 12Z model guidance is tending to favor slower/farther southwest positions for that feature. So this forecast continues to be in flux and further changes may be needed. In the West, upper ridging should build in the Great Basin and Rockies behind the upper low. By late week into the weekend greater model differences start to occur to its west with troughing and a possible embedded upper low in the eastern Pacific. The 00Z CMC looks like an outlier bringing a closed low into British Columbia and the Northwest. 12Z models continue to show some spread in the trough's timing moving toward the Northwest. The WPC forecast used a multi-model blend of deterministic guidance favoring the ECMWF and GFS early in the period. Used the ensemble means and increased their proportion to over half by Days 6-7 as model spread increased. ...Weather/Hazards Highlights... By Wednesday, the start of the period, the heaviest rains are likely to focus near the central Gulf Coast pooling along a frontal boundary. Heavy rain rates are likely along the cusp of the instability gradient and training storms are possible generally in southern Louisiana and vicinity. However, there are some competing factors for flooding to be too widespread Wednesday, including model spread with some focusing the front and thus the rainfall offshore. The farther south axis of heaviest rain compared to the short range puts the highest QPF atop drier antecedent conditions, along with higher flash flood guidance in swamplands. Thus maintained the Slight Risk across much of the Lower Mississippi Valley rather than any upgrades at this time and will continue to monitor trends. Urban areas could still be vulnerable to flash flooding. Heavy rain could persist into Thursday along the central Gulf Coast and into parts of the Southeast. Again, model spread and higher flash flood guidance limits flooding potential for now and will maintain a Marginal Risk for these areas on Day 5/Thursday. Rain and thunderstorms are likely to persist across the Southeast late week into the weekend. Meanwhile farther west and north, precipitation including higher elevation snow could continue into Wednesday for the south-central Rockies. Areas of rain are also likely across the central Plains to Middle Mississippi Valley Wednesday-Thursday and lingering into Friday in the Lower Mississippi Valley. Areas of rain are also possible in the northeastern U.S. at times later this week into the weekend. On Wednesday, some wrapback moisture and instability on the backside of the associated surface low could allow for modest rain in Interior Northeast, but with several model and QPF cycles showing this rain staying well under an inch, removed the Marginal Risk as any flooding chances should be below 5 percent. The upper low over the Southwest early in the period will result in lingering below normal temperatures for the Southwest and south- central Plains where daytime highs could be 10-20 degrees below normal. This should moderate with time but may still remain modestly below normal into late week for the southern Plains. Upper ridging building over the West as the upper low shifts east will allow for expanding coverage of above normal temperatures there as the week progresses, including 100 degree temperatures in the Desert Southwest. Above average temperatures will stretch into the northern tier as well, with highs reaching the 70s and 80s in the northern Plains to Upper Midwest. 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