SPC 1200Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 1249 AM CDT Sun Sep 18 2022 Valid 181200Z - 191200Z ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF THE MID MISSISSIPPI VALLEY... ...SUMMARY... Scattered severe thunderstorms should develop across the Mid Mississippi Valley late Sunday afternoon. Hail, wind, and some threat for a few tornadoes will spread toward central Illinois during the evening hours. ...Mid Mississippi Valley... Late-evening model guidance suggests upper ridge will build across the southern Plains and strengthen during the day1 period. This feature will force a notable short-wave trough currently located over WY to top the ridge over eastern SD/NE around 18z before it turns southeast and digs toward the OH Valley by 19/12z. As a result, broad height rises will be noted across much of the western/central US during the first half of the period. Early this morning, thunderstorm clusters continue across southern IA with more isolated activity into eastern KS. This activity is likely being sustained by a focused LLJ that should move little over the next 36hr, aside from veering toward central IL Sunday evening. Remnants of this convection are expected to be ongoing at the beginning of the period from southeast IA into central IL. Latest guidance suggests weakening is likely after sunrise, though it may not completely dissipate as it propagates southeast. While marginally severe hail/wind could occur with this early-period convection, the primary concern for more significant severe will occur later in the afternoon/evening as influence of the aforementioned short wave approaches. Strong boundary-layer heating is forecast once again across KS where surface temperatures may approach 100F in the central part of the state. Readings into the low-mid 90s are possible across northwest MO. If this occurs convective temperatures may be breached around 22z. There is some concern that isolated convection could develop along the boundary shortly after peak heating but large-scale forcing will not be particularly focused before sunset. However, as the short wave digs southeast, scattered convection will likely develop near the boundary over central IA. Forecast soundings favor supercells, which should mature and dig east-southeast toward a very unstable air mass with MUCAPE in excess of 4000 J/kg. Very steep lapse rates and favorably forced/sheared environment suggest very large hail with the stronger storms. Additionally, while these updrafts may be slightly elevated, low-level shear appears favorable for some risk of tornadoes. Latest HREF guidance supports this scenario with a cluster of supercells evolving over IA and growing upscale as they spread toward central IL during the late evening. Damaging winds may also occur, especially if storm mergers and bow-type features evolve. ..Darrow/Squitieri.. 09/18/2022
