Severe Storm Outlook Narrative (AC)
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Issued by NWS
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168 ACUS01 KWNS 131945 SWODY1 SPC AC 131944 Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0244 PM CDT Thu Jun 13 2024 Valid 132000Z - 141200Z ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS IN SOUTHERN IA...NORTHERN MO...AND NORTHERN/CENTRAL IL... ...SUMMARY... Severe thunderstorms are expected late this afternoon and early evening across parts of southern Iowa into northern Missouri and northern/central Illinois. Very large hail, destructive wind gusts, and a couple of tornadoes all appear possible. A broader corridor of severe hail/wind potential will extend from portions of the southern/central High Plains to the Great Lakes. ...20Z Update... Overall prior forecast appears reasonably on-track. Only minor changes were made based on latest observational trends. This was largely to the west/north side of ongoing convection/frontal placement from MI to IA and in south FL. For additional short-term forecast information, please see MCD 1248 regarding the south-central High Plains and MCD 1249 for the Mid-MS to Lower MO Valleys. ..Grams.. 06/13/2024 .PREV DISCUSSION... /ISSUED 1128 AM CDT Thu Jun 13 2024/ ...Great Lakes... A mid-level shortwave trough with associated westerly mid/upper-level jet will move eastward across the Upper Midwest and Great Lakes today. An attendant surface cold front will likewise sweep eastward across these regions through this evening, with the primary surface cyclone forecast to remain in Canada. Partly cloudy skies this morning should give way to better opportunity for diurnal heating this afternoon. Increasing low-level convergence along the front, along with ascent associated with shortwave trough/mid-level jet, should encourage convective development along/near the front by mid afternoon across parts of Lower MI into northern IL/IN and vicinity. Sufficient instability and deep-layer shear are forecast to support robust thunderstorms with a threat for scattered damaging winds and severe hail. The magnitude of the severe threat will be dependent on how much instability can develop, with differences remaining in high-resolution guidance regarding both how much convection develops, and how intense it will become. Regardless, some severe threat remains apparent, and the Marginal/Slight Risks have been maintained across this region with minimal changes for this update. ...Mid Mississippi Valley... A surface cold front is progged to sag more slowly south-southeastward across the mid MS Valley today, with better forcing aloft generally remaining to the north across the Upper Midwest and Great Lakes. A rather moist low-level airmass, with surface dewpoints generally in the upper 60s to low 70s, will remain along/south of this boundary across the mid MS Valley. Robust daytime heating of this airmass, along with steep mid-level lapse rates, will promote moderate to strong instability (MLCAPE 2500-3500 J/kg) by mid/late afternoon from southeast NE/northeast KS into southern IA/northern MO and vicinity. Warm mid-level temperatures noted on area 12Z soundings (700 mb temperatures of 13+ C) will likely inhibit robust convective development for much of the day, until surface temperatures reach at least the low 90s and finally erode lingering MLCIN. Explosive thunderstorm development is likely by 20-22Z along/near the front, as strong instability combines with enhanced deep-layer shear to support an threat for multiple intense supercells. Very large hail of at least 2-3 inches in diameter, and perhaps a couple tornadoes, will be a threat with these initially discrete supercells. With mid-level westerly flow largely parallel to the surface boundary, a quick transition/upscale growth into one or more bowing clusters seems likely by early evening. This activity should tend to have a greater threat for severe/damaging winds, including the potential for some significant gusts of 75-80 mph. The severe wind/hail threat should continue in a relatively narrow corridor this evening across the mid MS Valley, before increasing MLCIN and nocturnal cooling gradually reduce the overall severe risk tonight. ...Kansas into the Southern/Central High Plains... A hot and deeply mixed airmass is anticipated along/near the front across KS into the southern/central High Plains. This region will generally be displaced to the south of stronger forcing aloft, and warm mid-level temperatures may inhibit convection for much of the day. Still, most guidance suggests isolated to scattered thunderstorms will develop with a very well mixed boundary layer and steepened low/mid-level lapse rates. Severe/damaging downdraft winds appear to be the main threat with this activity, although some hail may also occur with any marginal supercell structures that can form. Based on latest guidance trends, the Slight Risk has been expanded westward to include more of western KS and eastern CO. Isolated significant severe wind gusts (75+ mph) may occur with any loosely organized clusters across KS. But, weaker deep-layer shear compared with locations farther east suggests the overall severe threat will probably remain fairly isolated. ...South Florida... A loosely organized band of convection is ongoing across parts of south FL. While mid-level lapse rates are expected to remain poor, sufficient instability and deep-layer shear may support occasional strong/gusty winds with the more robust activity this afternoon and early evening. A brief tornado or two may also occur given modestly enhanced low-level flow. $$