SPC 1200Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 1228 AM CDT Sat Jun 21 2025 Valid 211200Z - 221200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF THE GREAT LAKES REGION... ...SUMMARY... Thunderstorms associated with marginally severe wind gusts and hail will be possible today from the northern Plains into the Upper Mississippi Valley and Great Lakes. ...Great Lakes... A mid-level ridge will move quickly eastward into the Northeast today, as trailing shortwave trough moves through the Great Lakes. An MCS with some severe potential will likely be ongoing at the start of the period in the vicinity of Lake Superior. Warm-advection-related convection is expected to develop along the southern edge of the decaying MCS from the Upper Peninsula of Michigan eastward into northern sections of the Lower Peninsula of Michigan. As low-level lapse rates steepen during the morning, isolated damaging wind gusts will be possible with short line segments. During the afternoon, the brunt of the convection is forecast to move across southwestern Ontario. The southern edge of this cluster may move back into the lower 48, affecting parts of New York during the late afternoon. In central New York, moderate instability is forecast along with northwest mid-level flow around 50 knots. This should be enough for an isolated wind-damage threat with the faster-moving short line segments. Hail will also be possible with the stronger cores. Further south into parts of northern Pennsylvania, moderate instability is forecast to develop by afternoon. Due to a lack of large-scale ascent, convective coverage is expected to be very isolated. Any cell that can initiate in the higher terrain could have potential for marginally severe wind gusts. ...Northern Plains/Upper Mississippi Valley... Anticyclonic southwesterly mid-level flow will be in place today across much of the north-central U.S. At the surface, a post-frontal airmass will be in place over much of the northern Plains. Behind the front, a moist airmass will remain over parts of the Dakotas, where surface heating should yield moderate instability by afternoon. Although large-scale ascent will remain weak across most of the region, pockets of low-level convergence are forecast to develop within this airmass. This should be enough for isolated convective initiation during the mid to late afternoon. Marginally severe wind gusts and hail will be the primary threats. Further west into the northern High Plains, scattered thunderstorms are expected to develop during the overnight period, ahead of a mid-level trough in the northwestern U.S. Although a relatively dry airmass will be in place, low to mid-level lapse rates will be steep, and could be enough for a marginal wind-damage threat. ..Broyles/Moore.. 06/21/2025