Extended Forecast Discussion NWS Weather Prediction Center College Park MD 320 PM EDT Mon Apr 26 2021 Valid 12Z Thu Apr 29 2021 - 12Z Mon May 03 2021 ...Heavy rain threat Lower Mississippi/Tennessee Valleys and Central Gulf Coast Mid-Week... ...Guidance/Predictability Assessment and Preferences... The evolution of the large scale pattern from a more amplified ridge west/trough east is forecast to move to more zonal/fast flow with some hints of a western trough late in the forecast period. This change results in lower predictability at first with the phasing or lack of between the southern/northern streams early on (day 3/4). There continues to be some timing differences with the southern stream shortwave progression, where the GFS is on the faster side while the ECMWF is slower. The UKMET was noticeably deeper and digs the trough further into the Southeast. As the system lifts northeast, it phases with northern stream energy and this should favor a faster progression as depicted by the GFS. Given uncertainty and with an eye toward best possible WPC product continuity, opted to take a middle of the road approach with a blend of the ECMWF/GFS/CMC early on and then favored the ECENS/GEFS means day 5-7 to mitigate forecast spread and maintain continuity from the previous forecast. ...Weather/Hazards Highlights... Along and ahead of a slow moving frontal boundary, moisture and instability pooling will contribute to locally heavy rainfall Thursday from portions of eastern Texas to the Mid-Mississippi River Valley and portions of the Lower Ohio Valley. This threat will spread toward the TN Valley and Southern Appalachians late Thursday into Friday. In the wake of the frontal passage, more benign sensible weather is forecast for the CONUS. The aforementioned front will settle/stall over Florida and could be the focus for unsettled weather. Meanwhile, as the western U.S. ridge breaks down late in the forecast period, another storm system will spread precipitation across portions of the Pacific Northwest into the northern Rockies. The strong upper ridge building into the West past midweek will spread much above normal temperatures with 15-20+F above normal anomalies Thursday into Saturday across much of the West and High Plains. Some locations may reach daily records for highs/warm lows. The approaching Pacific trough aloft and leading cold front will start a pronounced cooling trend back toward climatology by next weekend. Taylor 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 Southern Plains, the Lower/Middle Mississippi Valley, the Tennessee Valley, the Ohio Valley, the Southern/Central Appalachians, and the Southeast, Thu, Apr 29. - Flooding possible across portions of the Central/Southern Plains and the Middle/Lower Mississippi 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 California, the Pacific Northwest, the Great Basin, the Northern Rockies, and the Southwest, Thu-Fri, Apr 29-Apr 30. - Much above normal temperatures across portions of California, the Great Basin, the Northern/Central Rockies, the Northern/Central Plains, and the Southwest, Fri-Sat, Apr 30-May 1. - Much above normal temperatures across portions of the Central Great Basin, the Central/Southern Rockies, the Southwest, the Plains, and the Upper/Middle Mississippi Valley, Sat-Sun, May 1-May 2. 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