Extended Forecast Discussion NWS Weather Prediction Center College Park MD 237 AM EDT Sat May 1 2021 Valid 12Z Tue May 4 2021 - 12Z Sat May 8 2021 ...Heavy rainfall threat across the South for Tuesday/Wednesday... ...Weather Pattern and Model Guidance... Broad cyclonic flow across the northern tier of the U.S. on Tuesday is expected to remain in place across the Great Lakes and the Northeast through the end of the week. The western Atlantic ridge near the East Coast will initially result in warm conditions ahead of the cold front. Out West, an upper level ridge axis builds in ahead of the next Pacific storm system as the overall upper level flow pattern becomes more amplified for the middle to end of the week. The deterministic guidance remains in above average agreement on the synoptic scale features across the nation, especially through Thursday. One of the main differences noted was a more amplified and slower solution with the 12Z CMC across the western U.S. for the second half of the medium range period, and is an outlier with the strength of the ridge across the Rockies. The 00Z CMC was more in line with the consensus here. The GFS is considerably stronger with the potential nor'easter off the East Coast by Friday, and would be an impactful system if that solution comes to pass. The WPC forecast was primarily derived from a multi-deterministic blend through Thursday, and then a GFS/ECMWF blend along with some of their respective ensemble means for Friday and Saturday, while depicting a more modest low near the Northeast for now. ...Weather/Hazards Highlights... Widespread moderate to heavy rain remains likely across portions of the Deep South and southern Appalachians during the Tuesday-Wednesday time period. The region that appears most favored for excessive rainfall is eastern Louisiana, southern Mississippi, and into central Alabama. This region will be in the warm sector ahead of a slow moving cold front, and some strong thunderstorms are also likely across this same general area. This may be enough to cause some flooding, particularly where convective training develops or across areas that will have already received noteworthy prior rains. Depending on the evolution and track of the developing low pressure off the East Coast late in the week, light to moderate rain can be expected across portions of the Mid-Atlantic region and the Northeast states. In terms of temperatures, highs are expected to be about 5-15 degrees above average for much of the eastern U.S. ahead of the front for early next week, with some 90 degree readings likely across portions of the Virginia and North Carolina Piedmont on Tuesday. This will be followed by a return to slightly below normal temperatures to close out the week behind the cold front with highs in the 50s and 60s, along with increased cloud cover. Across the western U.S., heat is expected to build in response to the upper level ridge, with highs 10-20 degrees above normal possible for the middle to end of the week for portions of the Intermountain West and the Desert Southwest. This will equate to some 100+ degree high temperatures for the lower elevations of eastern California and southern Arizona. Hamrick 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, winter weather outlook probabilities and heat indices are at: 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/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml