Extended Forecast Discussion NWS Weather Prediction Center College Park MD 134 AM EDT Thu Apr 29 2021 Valid 12Z Sun May 02 2021 - 12Z Thu May 06 2021 ...Multiple rounds of heavy rainfall possible from Central Gulf Coast to Tennessee Valley... ...Guidance/Predictability Assessment and Preferences... Models and ensembles continue to come into better agreement regarding an initial closed low/shortwave energy forecast to weaken as it lifts from the southern Plains on Sunday into the Northeast by Tuesday. As has been the case for a few days now, the GFS/GEFS continue to be just a hair faster than the rest of the guidance, however a general model compromise for this system presents a good starting point. The next system drops through the Western U.S. Sunday-Monday and should continue east/northeastward through the Plains into the Eastern U.S. Tuesday-Thursday. Models show good consensus the first half of the period, but begin to diverge on the details and timing by mid week. The ECMWF/CMC are more amplified both with this trough and western U.S. ridging, while the GFS/GEFS favor amplification of the trough a day or so later as additional northern stream energy drops in from the northern Rockies. WPC favored a blend more towards the ECMWF/CMC/ECENS which also fits well with WPC continuity. By day 6-7, a deep closed low should move into the northeast Pacific sending a cyclone towards the Pacific Northwest coast. Models generally agree on the presence of such a system, but disagree on evolution/timing at later time frames. As such, prefer a blend towards the ensemble means by the end of the forecast period. ...Weather/Hazards Highlights... Gulf of Mexico moisture and instability ahead of the initial trough/closed low over the Southern Plains on Sunday will fuel numerous showers and thunderstorms across parts of the central Gulf Coast states and into the Tennessee Valley Sunday-Monday. Confidence is increasing in heavy rainfall potential in this region, some of which has already seen above normal precipitation over the past few weeks. This would likely increase the flooding threat as well. The system is expected to shear northeastward into early/mid next week with a swath of enhanced rainfall possible farther northern into the Ohio Valley and the Northeast. The next system into the Western U.S. should support mountain snows across the Colorado Rockies and organized rain into the central High Plains Sunday-Monday. As the troughing amplifies across the Plains on Tuesday, moderate to heavy rainfall should develop along and ahead of a cold front. Again, the threat seems to be centered over much of the same region as the previous system -- central Gulf Coast/Tennessee Valley/lower Ohio Valley Tuesday into Wednesday. The Eastern U.S. should stay above normal in terms of temperatures much of the period ahead and south of a wavy frontal boundary. The Western U.S. system will spread below to much below normal temperatures from the Rockies into the Plains Monday and Tuesday, moderating as the system moves eastward. Meanwhile, much of the Western Coast to Desert Southwest will see a warming trend Monday-Thursday as upper level ridging builds aloft. 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, 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