Extended Forecast Discussion NWS Weather Prediction Center College Park MD 337 PM EDT Wed Apr 28 2021 Valid 12Z Sat May 01 2021 - 12Z Wed May 05 2021 ...Weekend heavy rainfall/convection threat from southern Texas to western/central Gulf Coast... ...North-central to east-central heavy rainfall/convection threat early next week... ...Guidance/Predictability Assessment and Preferences... Models and ensembles have come into agreement since yesterday regarding an upper low/trough forecast to exit northern Mexico into the southern Plains during the weekend. The GFS/GEFS exhibited the most notable change by significantly pulling back the eastward progression of this system in much better agreement with the ECMWF and CMC, which raised confidence in a heavy rain/thunderstorm threat from southern Texas to western/central Gulf Coast through this weekend. Meanwhile, confidence is also increasing for an area of enhanced precipitation across the northern to central High Plains Sunday and Monday with the arrival of an upper trough from the Pacific Northwest. Farther north, there continues to be a general model trend for a more progressive northern stream flow across the northern tier states. This will tend to sharpen a wavy front which will likely linger across the Midwest to the Northeast early next week as a ridge will tend to remain in place over the Southeast. Some of the moisture from the western Gulf Coast region will likely be drawn northeastward to merge with the sharp front in the North. 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. Recent GFS/GEFS runs trended in this direction, but were more progressive overall until finally the latest 00 UTC runs. ...Weather/Hazards Highlights... The medium-range period will begin with subsiding strong winds over the Northeast as a deep low moves further away into the Canadian Maritimes. Meanwhile, confidence is increasing for slow-moving southern stream troughing to support heavy rainfall and strong convection from Texas to the lower MS Valley this weekend--fueled by deepening Gulf of Mexico moisture and instability. The system is expected to shear northeastward into early next week but a swath of enhanced rainfall is possible across the east-central and then eastern U.S. with lead return flow toward a sharp and wavy front across the Midwest to the Northeast. Much above normal temperatures will overspread much of the central Rockies to the northern and central Plains early this weekend. Upstream, an approaching Pacific trough aloft and leading cold front will start a pronounced cooling trend by later in the weekend and focus generally modest precipitation from the Pacific Northwest to north-central Rockies, with a threat of terrain enhanced spring snows. Steady system progression and lead impulse ejections may combine with north-central Plains cyclogenesis/frontogenesis and TX dryline formation to favor an emerging pattern for ample Plains convection by early next week that should shift eastward along the sharp and wavy front from the east-central U.S. to the Northeast. Expect above normal warm sector temperatures for the East and the South as cooler air envelops the northern to central Plains. Meanwhile, a warming trend is expected for the West Coast into the Desert Southwest. Kong/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 Hazards: - Heavy rain across portions of the Lower Mississippi Valley and the Southern Plains, Sat-Sun, May 1-May 2. - Heavy rain across portions of the Central Rockies and the Central Plains, Sun-Mon, May 2-May 3. - Heavy rain across portions of the Lower Mississippi Valley, the Tennessee Valley, the Middle Mississippi Valley, the Southern Appalachians, the Southeast, the Great Lakes, and the Ohio Valley, Mon-Tue, May 3-May 4. - Flooding possible across portions of the Central Plains, the Lower Mississippi Valley, the Tennessee Valley, the Middle Mississippi Valley, the Southern Plains, and the Ohio Valley. - Flooding occurring or imminent across portions of the Lower Mississippi Valley. - Flooding likely across portions of the Lower Mississippi Valley. - Much above normal temperatures across portions of the Central Plains, the Central Great Basin, the Northern Plains, the Northern Great Basin, the Central Rockies, the Middle Mississippi Valley, California, the Upper Mississippi Valley, and the Southwest, Sat, May 1. 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