Severe Storm Outlook Narrative (AC)
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Issued by NWS
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811 ACUS01 KWNS 201956 SWODY1 SPC AC 201954 Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0254 PM CDT Fri Sep 20 2024 Valid 202000Z - 211200Z ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS THE MIDWEST...CENTRAL PLAINS...AND NORTHERN HIGH PLAINS... ...SUMMARY... Isolated strong to severe thunderstorms are possible during the late afternoon over parts of the Midwest including southwest Lower Michigan, Illinois, and Indiana. Isolated strong to severe storms are also possible across parts of the south-central Plains and the northern High Plains. ...20Z Update... No changes are needed to the ongoing forecast. See the previous discussion for additional details. ..Wendt.. 09/20/2024 .PREV DISCUSSION... /ISSUED 1136 AM CDT Fri Sep 20 2024/ ...Central Plains... Low-level moisture will advect north-northwestward throughout the day associated with a warm front. As a result, low to mid 60s dewpoints will likely be in place across south-central/southeast KS during the late afternoon. Dewpoints will likely be in upper 50s farther west in southwest KS, with notably drier conditions across the adjacent TX/OK Panhandles where downsloping southwesterly surface winds are likely. Southwesterly flow aloft ahead of the approaching upper low will advect steep mid-level lapse rates northeastward through the southern High Plains into western portions of the central Plains. Modest buoyancy is anticipated during the late afternoon where these lapse rates overlay the more favorable low-level moisture across southern KS. Limited large-scale ascent and low-level confluence near the warm front are anticipated over this region as well, with isolated to widely scattered high-based thunderstorms anticipated. Vertical shear will be modest, but still strong enough for persistent/organized storm structures, and the potential for some strong gusts with the more robust storms. Some isolated hail is also possible over south-central KS where buoyancy is maximized. Low-level moisture will continue advection northward during the evening and overnight, with a strengthening low-level jet anticipated as well. Consequently, additional thunderstorm development is expected after 04Z over northeast KS and adjacent far southeast NE and northwest MO. Isolated hail is possible with the strongest storms. ...Midwest including Illinois/Indiana to Lower Michigan... Upper 60s to low 70s dewpoints are expected from southwest Lower MI southwestward across western IN, southern IL, and southeast MO. Daytime heating of this moist low-level air mass will yield a moderately unstable and uncapped air mass ahead of the approaching front, and thunderstorm development is anticipated as the front interacts with this airmass. Moderate vertical shear will be in place, but poor lapse rates and notable mid-level dry air could make updraft maintenance difficult. A multicellular storm mode is anticipated. Highest storm coverage is expected over southwest Lower MI and western IN, with warm mid-level temperatures tending to limit coverage farther southwest. Locally strong wind gusts will be the primary severe risk, with isolated, marginally severe hail possible as well. ...Northern Montana/northwest North Dakota... At least widely scattered low-topped thunderstorms should develop across northern Montana and/or move into the region from southern Alberta as a cold front moves southeastward. This will be influenced by a southeastward-digging shortwave trough and a related strengthening of cyclonic westerlies within its base. Even with modest overall buoyancy, a diurnal steepening of low-level lapse rates and strengthening low/mid-tropospheric westerlies could potentially yield severe-caliber convectively enhanced wind gusts late this afternoon through around sunset. $$