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000 ACUS01 KWNS 280602 SWODY1 SPC AC 280600 Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0100 AM CDT Sat Mar 28 2020 Valid 281200Z - 291200Z ...THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS FROM EASTERN IOWA INTO CENTRAL AND NORTHERN ILLINOIS... ...SUMMARY... A significant severe weather outbreak is expected for portions of the Midwest this afternoon and evening. Threats include tornadoes, a few which may be significant, large to very large hail, and severe wind gusts. ...Synopsis... Upper trough now situated over the southern Rockies will eject northeast today, reaching the middle MS Valley and Midwest region later this afternoon. Surface low in southwest KS will deepen as it develops northeast within exit region of a strong upper jet, and should reach southwest IA by early this afternoon. A warm front will extend eastward from the low through northern IL into northern portions of the OH Valley. Trailing cold front initially from eastern KS through eastern TX will advance through the MS, TN and OH Valleys. By the end of the period this front should extend from a surface low in WI southeast through the OH Valley, TN Valley and central portion of the Gulf Coast states. ...Midwest through Ohio Valley region... Areas of elevated thunderstorms will be ongoing north of warm front from northern MO, IA, IL and eastward into the OH Valley and Great Lakes. This activity will gradually shift northward during the day with a gradual destablization of the boundary layer promoted by the northward advection of mid 60s F dewpoints through the warm sector. Widespread multi-layer clouds will slow the destabibliztion process somewhat. However, back edge of cirrus associated with subtropical moisture plume should shift east of this region and partial cloud breaks will contribute to at least modest diabatic warming of the surface layer. MLCAPE from 1500-2000 J/kg is expected across the Midwest by middle to late afternoon. The process of destabilization and deeper forcing for ascent within the upper jet exit region will likely contribute to development of storms along warm conveyor belt from northeast MO, eastern IA into western IL. Vertical wind profiles with 50+ kt effective bulk shear will support supercells. These storms will move northeast through central and northern IL during the late afternoon and evening within an environment characterized by a strengthening low-level jet which will promote large 0-1 km hodographs (300-500 m2/s2 storm-relative helicity). This parameter space along with low LCLs will support potential for supercells capable of producing long-track, strong tornadoes and very large hail. Assuming sufficient destabilization occurs, other supercells might develop in an arcing band within zone of ascent accompaying the primary vorticity maximum and just east of the surface low across central IA. These storms will also pose a risk for large hail and tornadoes. An upgrade to high risk will be considered in later day 1 updates for a portion of this region. Elsewhere, additional storms will develop east through the OH Valley and southern Great Lakes during the evening with all severe hazards possible. ...Lower Mississippi through Tennessee Valley region... Storms will likely develop along and ahead of an eastward-advancing cold front and will likely gradually intensify as the boundary layer destabilizes during the day. While low-level hodographs will not be as large as farther north, deep-layer shear in excess of 50 kt and moderate instability will be more than sufficient for organized storms including a few bowing segments and supercells capable of damaging wind, large hail and a few tornadoes. Activity will continue east into the TN Valley area into the overnight with a continued severe threat. ..Dial/Dean.. 03/28/2020 $$