Severe Storm Outlook Narrative (AC)
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Issued by NWS
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733 ACUS01 KWNS 211302 SWODY1 SPC AC 211300 Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0800 AM CDT Tue May 21 2024 Valid 211300Z - 221200Z ...THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS OVER MUCH OF IA AND NEARBY PARTS OF SOUTHEASTERN MINNESOTA...WISCONSIN...ILLINOIS...AND MISSOURI... ...SUMMARY... An outbreak of severe thunderstorms, including the potential for strong tornadoes, is expected mainly this afternoon to early evening. The greatest threat is over Iowa and parts of adjacent states. ...Synopsis... In mid/upper levels, a persistent, slow-moving cyclone will meander over southern SK through the period, while a separate closed low digs southeastward from northern BC to WA. These will anchor a mean trough extending from the northern Rockies across southern CA and offshore from northern Baja. In the downstream southwesterlies, a shortwave trough was evident in moisture-channel imagery over the High Plains from western NE to northeastern NM. This perturbation should pivot northeastward to southern MN, central IA and northwestern MO by 00Z, then northeastward over western and northern parts of the upper Great Lakes by 12Z tomorrow. At the surface, 11Z analysis showed lows near SUX and CNK, along a cold front arching to southwestern KS and the Raton Mesa. The lows should consolidate today into a more-coherent cyclone center and move to southern MN by 00Z, with cold front to southeastern IA, central MO, and central OK, becoming quasistationary to warm across western OK and the northern TX Panhandle, to a low in southeastern CO. A dryline -- initially analyzed from a frontal intersection over west-central KS south-southwestward to between MAF-FST, should shift eastward today over southern/central OK, northwest/west- central TX, and the Edwards Plateau. By 12Z, the low should become occluded over the Boundary Waters region, with cold front across Lower MI, southern IL, southeastern MO, southern OK, northwest TX, and southeastern NM. ...Upper Mississippi to lower Missouri Valleys... An ongoing, organized band of thunderstorms, with a history of producing several measured severe gusts in central/eastern NE -- will sweep across parts of IA through the remainder of the morning. This activity will overtake isolated strong-severe convection near the outflow boundary, where relatively maximized low-level vorticity and instability will be most strongly juxtaposed. Severe gusts, sporadic large hail, and a tornado or two all are possible. See SPC Severe Thunderstorm Watch 275 and related mesoscale discussions for near-term guidance on this threat. That activity is a precursor to the main event, which is expected to sweep across much of the same areas (and portions of adjoining states) from midday through the evening. Scattered to numerous thunderstorms (including multiple fast-moving supercells) are expected to form over parts of eastern NE from midday into early afternoon, and over IA and northern/central MO this afternoon. This will occur as the cold front impinges on a retreating outflow boundary from the morning storms, and an airmass destabilizing rapidly on both sides of the boundary from a combination of diurnal heating and intense warm advection. Sufficient airmass recovery for a surface-based, all-hazards severe threat is expected across most of IA and probably into southern MN as well, spreading into parts of western/northern IL and WI ahead of the main convective arc. Several tornadoes -- some strong (EF2+ damage potential) are possible, along with large, damaging hail and severe downdrafts. Given potential for cell motions of 45-50 kt, potential exists for a few long-track mesocyclones/tornadoes. The approach of the shortwave trough will increase large-scale lift -- now evident with post-frontal convection over southwestern NE and northwestern KS that may be trackable across southern NE to the earliest substantial convection in the midday hours. The ascent should spread over the region, atop that from the front and surface heating, supporting the development of steep low/middle-level lapse rates. With surface dewpoints recovering into the upper 60s to around 70 F, a corridor of 2000-3000 J/kg MLCAPE should develop over much of IA, with slightly weaker (but still favorable) values clipping parts of southern MN as well. Activity should begin to gradually outrun the corridor of favorable surface-based buoyancy in the evening after crossing the Mississippi River, but may remain well-organized in terms of severe-wind threat across Lake Michigan. The largest low-level hodographs should be near and north of the remnant outflow boundary, with effective SRH exceeding 300 J/kg -- but will be favorable for potentially tornadic supercells farther south into southern IA and parts of MO as well. ...Ozarks to south Texas... Isolated to scattered thunderstorms are expected to form from midafternoon into evening near the front and dryline, offering large to very large hail and locally severe gusts. Hail at least 3 inches in diameter is possible. A tornado or two is not out of the question -- especially in northern areas (Ozarks vicinity) under the fringes of the gradient-flow/deep-shear influence from the ejecting shortwave trough -- and with high-CAPE storm-scale processes amid favorable deep shear elsewhere. Though capping from an EML will limit convective potential for much of the day, a combination of strong surface heating, rich boundary- layer moisture, and frontal/dryline lift will weaken MLCINH enough by mid/late afternoon to support deep convective development. Upper 60s to low 70s F surface dewpoints should become common beneath a high equilibrium level (deep troposphere) and steep midlevel lapse rates, supporting a corridor of 3000-4000 J/kg MLCAPE (locally higher). Sufficient veering of winds with height, and effective- shear magnitudes in the 45-60 kt range, will support supercell potential. Activity over TX my weaken within a few hours after sunset as nocturnal boundary-layer cooling re-establishes the strong capping beneath the EML. However, farther north across parts of AR/MO, a weaker EML influence may permit activity to persist later and farther east this evening, before weakening. ...Adirondacks region to northern New England... Widely scattered to scattered thunderstorms should develop over portions of the Adirondacks and White Mountains today, and perhaps also move into the area from nearby parts of extreme eastern ON and southernmost QC. Damaging gusts and isolated severe hail will be possible. A mid/upper-level shortwave trough and embedded MCV -- related to the significantly severe MCS over KS two days ago -- is apparent in satellite imagery over Lake Huron. This feature will move eastward toward northern NY/New England and times well with the diurnal- heating cycle there. Activity should form as large-scale DCVA/lift and enhancement of midlevel flow -- preceding the MCV/shortwave trough -- spread atop a destabilized, suitably moist boundary layer, heated to convective temperature over elevated terrain. These processes will erode MLCINH to negligible levels, fostering MLCAPE commonly in the 1000-1500 J/kg range, locally/briefly near 2000 J/kg. Deep shear should remain modest, with effective-shear magnitudes generally 25-35 kt, though enough veering with height in low levels may develop to shape somewhat enlarged hodographs. Multicell and transient supercell modes should predominate, with the severe threat diminishing markedly after dark. ..Edwards/Goss.. 05/21/2024 $$