Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 PM EDT Thu Apr 29 2021 Valid 12Z Sun May 02 2021 - 12Z Thu May 06 2021 ...Heavy rainfall possible from eastern Texas to the lower Mississippi Valley on Sunday and then across the Deep South Tuesday to Wednesday... ...Guidance/Predictability Assessment and Preferences... Models and ensembles continue the theme of broad upper-level troughing sliding across the northern tier states as the southern stream directs a generally southwesterly flow across the mid-section of the country toward the Northeast. Models again exhibit a trend for a faster/more progressive northern stream flow, which helps sharpening a slow-moving front that extends from the Mid-Mississippi Valley to New England. In addition, models continue to struggle with predicting the multiple wave interactions along the northern stream, lending to the greatest amount of forecast uncertainty across the northern tier states into the Northeast for the longer range period. Across the western Gulf Coast states, an upper-level trough/low will lift northeastward across the southern Plains late this weekend with a threat of heavy rain mainly from the lower Mississippi Valley westward to eastern Texas. Meanwhile, models agree well regarding the next upper-level trough forecast to exit the central Rockies early next week with a period of enhanced rainfall for the High Plains and some significant wet snow possible along the Front Range of Colorado. This wave will likely play a key role in setting up a possible heavy rain event across the Deep South ahead of a cold front by Tuesday and into Wednesday. Remaining moisture will likely move into the East Coast mid-week before a general drying trend sets in as the upper trough is forecast to edge toward the coast. Meanwhile, moisture from the next Pacific cyclone is forecast to reach the Pacific Northwest toward the latter part of next week. The WPC product suite was derived from a composite blend of the 00 UTC ECMWF/EC mean, 06Z GFS/GEFS and some contribution from the 00Z Canadian/CMC mean, with increasing usage of the ensemble means toward Day 7. ...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 north 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 over 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 West Coast to Desert Southwest will see a warming trend Monday-Thursday as upper level ridging builds aloft. Kong/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