FXUS02 KWNH 020650 PREEPD Extended Forecast Discussion NWS Weather Prediction Center College Park MD 248 AM EDT Fri May 2 2025 Valid 12Z Mon May 05 2025 - 12Z Fri May 09 2025 ...Blocky pattern with closed lows over the West and the East with areas of heavy rainfall potential early to mid next week... ...Overview... A blocky pattern over the lower 48 will be in place by the beginning of the medium range period on Monday. The dominant features will be two closed upper lows, one over the West/Southwest and one over the East, that will both slowly drift around weakening through Wednesday/Thursday. These systems are expected to produce areas of significant rainfall over portions of the Central and Southern Plains, Northeast, and Mid- Atlantic and high elevation snows in the West. Eventually, the eastern upper low should interact with a northern stream trough that will swing across southern Canada, resulting in the low being swept up towards the Canadian Maritimes later next week. The Western low will likely remain isolated as it drifts east over the Plains and weakens. The remnants of the Western low may eventually combine with another trough developing over the East Coast late Thursday into Friday. By Wednesday, another trough and frontal system will approach the West Coast. ...Guidance/Predictability Assessment... Models and ensembles remain in good agreement on the presence of a blocky pattern to start next week, though subtle differences in exactly where the upper lows set up over the Southwest and the Ohio Valley will directly impact the distribution and locations of heaviest rainfall. In the East, the GFS was displaced slightly east and this resulted in a quicker push off the East Coast mid next week. Meanwhile, a northern stream shortwave swinging into the Great Lakes was much more pronounced in the CMC with another closed low over the East, which was very much an outlier solution at this point. The ECMWF (with the ensemble means and UKMET early in the period) was preferred for this system, but a little faster exit like depicted in the GFS is not out of the question. Out West, there remains some notable differences in energy rotating around the upper low early in the week, but general agreement that this low should drift slowly eastward as it weakens into the Central Plains and eventually gets absorbed into some broader troughing over the East late next week. A general model compromise, with the ensemble means later in the period worked well as a starting point, outside of the CMC due to the outlier upper low over the East late next week. ...Weather/Hazards Highlights... Early next week, two occluded frontal systems are expected to bring moderate to locally heavy rainfall to portions of the West and East. In the West, anomalous moisture accompanying the upper low will support heightened precipitation from the Great Basin to the Central and Southern Rockies and Plains. By monday, the system will become stationary over the Plains, allowing moisture to continuously pool over the Plains, which will likely result in a multi-day heavy rainfall event across the Central and Southern Plains and into the lower Mississippi Valley Monday-Wednesday. Portions of the Plains states will be especially vulnerable to heavy rain/flooding due to recent heavy rains and saturated soils. This activity is highlighted in the Day 4-5 Excessive Rainfall Outlooks (EROs) with a slight risk area highlighted across the Texas and Oklahoma panhandles on Monday, and another slight risk area on Tuesday for parts of the Lower Mississippi Valley/ArkLaTex vicinity. Marginal risks surround these areas. Under the eastern upper low, the frontal system will very slowly push eastward early next week, resulting in a stream of onshore moisture over the Mid-Atlantic and Northeast and persistent precipitation across these regions through Tuesday. Initially, rainfall could be beneficial and bring some relief to drought conditions, but repeated rounds of rainfall could produce locally heavy totals and may result in localized flooding in urban and poor drainage areas. The Day 4-5 EROs show a Marginal Risk area across these regions for Monday and Tuesday. By Wednesday, the frontal system should accelerate to the east as the upper low interacts with the northern stream, shifting precipitation farther into the Northeast. The upper lows over the East and Southwest early in the week will result in below to well below normal temperatures, especially the Southwest where daytime highs could be 15-20 degrees below normal. This should moderate with time as the East returns to near normal by Tuesday and the Southwest area slowly shifts eastward. Above normal temperatures to begin the week over the Northern Plains may expand into the Upper Great Lakes. Upper ridging building over the west as the upper low shifts east will allow for expanding coverage of above normal temperatures next week. 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 $$