Extended Forecast Discussion NWS Weather Prediction Center College Park MD 253 PM EDT Sat May 01 2021 Valid 12Z Tue May 04 2021 - 12Z Sat May 08 2021 ...Heavy rainfall threat across the South into Tuesday/Wednesday... ...Weather Pattern and Model Guidance... Guidance remains in above average agreement days 3-7 and offer good run to run continuity, bolstering forecast confidence. Broad cyclonic flow over much of the lower 48 states early next week will feature numerous embedded shortwaves in two streams. The active pattern will then significantly amplify as upper troughs work into the West Coast and the East to sandwich a warming west-central U.S. upper ridge. Variance remains with the timing and strength of smaller scale systems. Composites tend to mitigate these differences consistent with predictability. Accordingly, the WPC medium range product suite was primarily derived from a blend of well clustered guidance of the 06 UTC GFS/GEFS mean, the 12 UTC ECMWF/Canadian/ECMWF ensemble mean and the 13 UTC National Blend of Models. Latest 12 UTC models and ensembles remain in line. ...Weather/Hazards Highlights... A threat of widespread moderate to heavy rain and strong thunderstorms remains mainly into Tue/Wed for portions of the South, especially from eastern TX and the lower MS/TN Valleys to the southern Appalachians. This may cause some flooding, particularly where convective training develops or across areas that will have already received noteworthy prior rains. Depending on the downstream evolution and track of developing low pressure off the East Coast later next week as the flow amplifies, moderate rain can be expected for portions of the Mid-Atlantic and the Northeast. Trough amplification into the West Coast may meanwhile support enhanced precipitation over the Pacific Northwest that seem slated to work inland to the northern Rockies next weekend. In terms of temperatures, highs are expected to be about 5-15 degrees above average for much of the eastern U.S. early next week. This will be followed by a return to below normal temperatures to close out the week. 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 CA and southern AZ. Hamrick/Schichtel 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