Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Sat May 3 2025 Valid 12Z Tue May 06 2025 - 12Z Sat May 10 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 should still be in place by the beginning of the medium range period on Tuesday. The dominant features at the very beginning of the period will be two closed upper lows, one over the West/Southwest and one over the East, that will both slowly drift and weaken by 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. The eastern upper low should interact with a northern stream trough by Wednesday 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, weakens, and becomes absorbed into a new redeveloping trough over the East which may hang out into next weekend. ...Guidance/Predictability Assessment... Two distinct cut-off lows will dominate the synoptic pattern across the Continental U.S. through about mid week, and the latest deterministic and ensemble guidance remain in good overall agreement with the depiction of their evolutions. However, the 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. The GFS and UKMET are on the slightly faster side as the initial low exits the Northeast by Thursday. The interaction of a weakening western low and a new shortwave into the Northeast becomes increasingly uncertain later in the week. The guidance generally agrees that another upper low may form within the flow as general broad troughing extends across the East, but lots of uncertainty in the exact placement. The ECMWF actually shows this upper low meandering over the Ohio Valley again, similar to the early week pattern, but this is more of an outlier solution. Out West, an upper low should meander across the northeast Pacific with uncertain shortwave timing into the West during the week. The WPC forecast was able to utilize a general model blend for the first couple of days, but quickly transitioned towards majority ensemble means by Day 6 and 7. ...Weather/Hazards Highlights... 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. Into Tuesday, the system should be more or less 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 through Wednesday. Portions of the Plains into the Lower Mississippi Valley 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). For Tuesday, there is a growing signal with high confidence of widespread 2-5"+ rainfall amounts across portions of the ArkLaTex and Lower Mississippi Valley warranting a moderate risk upgrade for far eastern Texas into Louisiana, with a broad slight risk extending eastward from here, as well as back into Oklahoma. On Wednesday, the activity shifts more towards the coast, though some guidance indicates potential for additional very heavy rainfall. A lot of uncertainty in the amounts and placement though, so just a slight risk is indicated on the Day 5 ERO across Louisiana into southern Mississippi, some of which overlaps with the activity on Day 4 as well. Activity may linger across the Southeast into late week as the frontal boundary slows. Moderate snowfall is expected on Tuesday as well for parts of the central Rockies. 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. 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 parts of the Mid- Atlantic and Northeast. The upper low over the Southwest early in the week will result in below to well below normal temperatures for the Southwest/Plains where daytime highs could be 15-20 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 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