Severe Storm Outlook Narrative (AC)
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Issued by NWS
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829 ACUS01 KWNS 161632 SWODY1 SPC AC 161630 Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 1130 AM CDT Sun Jun 16 2024 Valid 161630Z - 171200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF THE NORTHERN/CENTRAL PLAINS INTO THE UPPER MISSISSIPPI VALLEY AND MIDWEST... ...SUMMARY... Scattered severe storms appear possible across parts of the upper Mississippi Valley late this afternoon, with additional storms likely tonight across the north-central Plains to upper Mississippi Valley. Large hail and damaging winds are the primary risks. A small bowing complex, with mainly a severe/damaging wind threat, may also develop across parts of northern Illinois and vicinity this afternoon. ...Northern/Central Plains to the Upper Midwest/Upper Mississippi Valley... A strong upper trough/low will continue advancing east-northeastward across the northern Plains and Canadian Prairie provinces today. While the primary surface low is expected to remain in central Canada, a weak secondary low should develop generally eastward across MN through this evening in tandem with a cold front. Warm mid-level temperatures associated with a cap should tend to inhibit robust convective development for at least a few more hours into the early afternoon. But, most guidance now does show thunderstorm initiation occurring along the front in northern MN by 20-22Z. There are still concerns/uncertainty with how many intense thunderstorms will be able to form given fairly prevalent cloudiness that should temper daytime heating to some extent. Still, strong deep-layer shear and steepening mid-level lapse rates atop a gradually destabilizing low-level airmass should support a threat for supercells and large hail. Some guidance suggests a fairly quick progression to a MCS, which seems reasonable given the linear forcing of the cold front. If this occurs, then a severe/damaging wind threat may persist through the evening into parts of northern WI and the U.P. of MI. Enough low-level shear is also forecast to support low-level updraft rotation and a tornado or two across this region. A separate area of intense thunderstorm development is possible late this afternoon and early evening along the trailing portion of the cold front in NE/northwest KS and vicinity. If convection can initiate along this boundary, strong instability and modest deep-layer shear would support a threat for mainly multicells posing a risk for severe winds given the well mixed boundary layer forecast. Some hail could also occur with marginal supercell structures. The Slight Risk has been expanded southward across the central Plains to account for this scenario. Otherwise, scattered to numerous elevated thunderstorms are still expected to develop this evening and overnight generally north of the front from eastern WY into NE/SD and southern MN. A strengthening low-level jet in this time frame across these regions and ample deep-layer shear/MUCAPE will easily support supercells and a hail threat with initial development. But, convective mode may quickly become messy, with one or more bows/clusters possible as thunderstorms move eastward across the upper MS Valley overnight. Even though this activity should tend to remain elevated, an isolated severe/damaging wind threat may persist given the strong MUCAPE forecast. ...Northern Illinois/Southern Wisconsin... A well defined MCV remains evident on visible satellite and radar imagery late this morning over northeast MO and southern IA. This feature should continue to track northeastward this afternoon and evening in the modest southwesterly mid-level flow regime across the mid MS Valley/Midwest. Ample daytime heating of a moist low-level airmass should support around 1500-2500 J/kg of MLCAPE by mid afternoon across northern IL and vicinity. Ascent associated with the MCV and its associated mid-level vorticity maximum will likely encourage the development of isolated to widely scattered thunderstorms by 20-22Z. Modest but sufficient deep-layer shear for updraft organization is indicated in various RAP forecast soundings across this region. Scattered severe/damaging winds, along with isolated hail, could occur with a small bowing complex that may develop and move east-northeastward across the Chicago metro area late this afternoon and early evening. Enough low-level shear is also forecast to support a low chance for a tornado or two. Given recent guidance and observational trends, a small Slight Risk has been added across northern IL/southern WI to account for this scenario. ...Southern High Plains... Isolated/widely spaced thunderstorms should form once again this afternoon along the length of a surface lee trough/dryline extending southward across the southern High Plains. Although deep-layer shear is forecast to remain weak, moderate instability and a well-mixed boundary layer should support an isolated threat for severe downdraft winds. Occasional hail may also occur with the more robust cores before convection weakens after sunset. ..Gleason/Bentley.. 06/16/2024 $$