Extended Forecast Discussion NWS Weather Prediction Center College Park MD 301 PM 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, as well as 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... Two distinct cut-off lows will dominate the synoptic pattern across the Continental U.S. through mid to late week, and the latest deterministic and ensemble guidance are in good overall agreement with the depiction of their evolutions. With that being said, the evolution of smaller scale details, particularly with regards to embedded vort maxes and shorter wavelength troughs, becomes increasingly uncertain beyond Day 5, which will likely have an impact on the spatial and temporal distribution of QPF and the placement of frontal systems and features. Given the above decided to roll with a general model blend featuring the 00Z ECMWF, 00Z CMC, 00Z ECENS, 00Z GEFS, and the 06Z GFS. Unlike the overnight blend, the 00Z UKMET was the outlier with this cycle, mainly with how it handled the evolution of a northern stream shortwave digging across the northern Plains in the beginning of the period. Also unlike the previous blend, the 00Z CMC was much more usable this go around. In fact, the 00Z CMC was very close to the 00Z EC AIFS and the 06Z GFS, which added confidence for its inclusion. By the end of the period, the EC and GEFS ensemble means offered stability and favorably matched continuity despite smaller scale differences from deterministic guidance. ...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 for anomalous moisture to continuously pool near a boundary in the vicinity. Several rounds of moisture-laden thunderstorms are likely to result in a multi-day heavy rainfall event across the Central and Southern Plains and into the lower Mississippi Valley Monday through 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. For Tuesday, a growing signal with high confidence of widespread 2-5"+ rainfall amounts across portions of the ArkLaTex and Lower Mississippi Valley warranted a broader expansion of the slight risk area. Higher end probabilities of excessive rainfall are focused across northeast Texas, extreme southeast Oklahoma, southern Arkansas, west central Mississippi and northern Louisiana. Given relatively good spatial and temporal model agreement and amounts trending higher, would not be surprised to see an embedded upgrade to a moderate risk in future forecast updates. 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. Miller/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