Severe Storm Outlook Narrative (AC)
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Issued by NWS
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997 ACUS01 KWNS 220556 SWODY1 SPC AC 220555 Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 1255 AM CDT Sat Jun 22 2024 Valid 221200Z - 231200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS LATE THIS AFTERNOON AND EVENING ACROSS PARTS OF CENTRAL AND EASTERN IOWA...NORTHERN MISSOURI...SOUTHERN WISCONSIN...NORTHERN ILLINOIS...CENTRAL AND SOUTHWESTERN LOWER MICHIGAN... ...SUMMARY... Severe thunderstorms are possible this afternoon and evening across parts of the Midwest and Great Lakes region. One or two clusters of storms may evolve and pose a risk for damaging wind gusts, but a couple of supercells posing a risk for tornadoes are possible as well. ...Synopsis... Models indicate that persistent mid/upper ridging will become increasingly suppressed across and east-southeast of the Upper Midwest later today through tonight. As flow trends broadly cyclonic across the Great Lakes and Ohio Valley into the Northeast and Mid Atlantic, the slowly weakening center of this ridging is forecast to shift from the Mid South into the southern Great Plains by 12Z Sunday. It appears that one notable, but relatively low-amplitude, short wave perturbation emerging from the Great Basin will accelerate east-northeastward, then eastward, across the Upper Midwest through the Great Lakes during this period, as a somewhat stronger upstream perturbation digs east-southeast of the Canadian Prairies toward the upper Great Lakes region. Although there is substantive spread among the various model output concerning the evolution, the lead impulse is forecast to support modest cyclogenesis along a surface front, generally across or east-northeast of Iowa through the lower Michigan vicinity by late tonight. As this occurs, a trailing cold front likely will advance southward through portions of the central Great Plains, lower Missouri and Ohio Valleys. The modifying western flank of a preceding intrusion of cooler air may be slow to shift northeast of the southern New England and Adirondacks vicinity, while a reinforcing intrusion of cool air begins to advance south of the international border into the northern U.S. Great Plains by this evening. Destabilization along and just south of all of these boundaries may provide a focus for strong to severe thunderstorm development this afternoon and evening. ...Great Lakes into lower Missouri Valley vicinity... Severe weather potential remains somewhat unclear for this period due to the model discrepancies concerning the surface cyclone evolution, and the stabilizing influence of potentially considerable lingering early period convective development across the Midwest into Great Lakes region. However, a belt of 30-50+ kt southwesterly to westerly flow in the 850-500 mb layer may overspread a destabilizing environment along and south of a left-over outflow boundary across parts of eastern Iowa through southern Wisconsin/northern Illinois, and the stalled frontal zone over central lower Michigan during the day. Seasonably high boundary-layer moisture content (including dew points near 70F) will become supportive of at least weak to moderate CAPE, in the presence of moderate to strong shear, including sizable clockwise-curved low-level hodographs. This environment may become supportive of a few supercell structures accompanied by a risk for tornadoes, before increasing thunderstorm development ahead of the southeastward advancing cold front poses primarily a risk for damaging surface gusts into this evening. ...Northern Mid Atlantic... Deep-layer wind fields and shear are likely to remain weak, but moderately large CAPE (in excess of 2000 J/kg) may once again develop within weak surface troughing, along and southwest of the remnant surface frontal zone. It appears that this will become supportive of scattered strong thunderstorms with potential to produce damaging wind gusts, as mid-level heights begin to slowly fall late this afternoon into this evening. ...Northern Great Plains... Stronger mid/upper forcing for ascent and mid-level cooling are likely to remain north of the international border beyond peak daytime heating, but an isolated strong storm or two may advect south of the international border by this evening, accompanied by a risk for marginally severe hail and wind. ..Kerr/Wendt.. 06/22/2024 $$