Severe Storm Outlook Narrative (AC)
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Issued by NWS
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463 ACUS01 KWNS 211642 SWODY1 SPC AC 211640 Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 1140 AM CDT Tue May 21 2024 Valid 211630Z - 221200Z ...THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS ACROSS IOWA...NORTHERN MISSOURI...NORTHWEST ILLINOIS...SOUTHEAST MINNESOTA...AND SOUTHERN WISCONSIN... ...SUMMARY... An outbreak of severe thunderstorms, including the potential for strong tornadoes, large hail, and widespread damaging winds, is expected this afternoon through evening, especially across eastern Nebraska, Iowa, and Missouri to northern Illinois, southeast Minnesota, and southern Wisconsin. ...Middle/Lower Missouri Valley and Midwest... As an outbreak precursor, a well-organized cluster of storms across south-central/east-central Iowa mid-late this morning has had a history of producing at least isolated/episodic severe weather since the predawn hours. These storms are focused along/north of an outflow-reinforced warm front that will lift northward through late today into tonight. This will be heavily influenced by a highly dynamic mass response to a northeast-moving mid-level shortwave trough that is readily evident in water vapor imagery over the central High Plains this morning. That said, some short-term uncertainties exist as of 15z observational data, with gusty post-convective northeasterly winds still present across south-central Iowa, which relates to some uncertainty with the northward extent of the higher-magnitude severe risk across Minnesota/Wisconsin later today, although latest 16z data features a recent abatement of those northerly winds. Regardless, intense storm development is anticipated into the afternoon across a broad north/south corridor. A synoptically evident and regional outbreak-favorable setup exists, especially by mid/late May standards, highlighted by a coupled upper jet structure over the Upper Midwest, strengthening deep-layer wind fields, and steady cyclogenesis from the Nebraska/Kansas border vicinity this morning northeastward toward the Minnesota/Wisconsin border tonight. Low-level winds will remain strong diurnally ahead of the primary cyclone, but a noteworthy strengthening of mid-level winds 3-7 km AGL (reference 700mb/500mb) will rapidly occur through the afternoon/evening, which will likely have direct outbreak-relevant influences on initial supercell intensity, QLCS evolution and overall fast east-northeastward storm motions later today. Steepening mid-level lapse rates, noted in upstream 12z observed soundings such as Topeka/Dodge City KS and Springfield MO, will overspread an increasingly moist airmass generally characterized by mid/upper 60s F surface dewpoints, contributing to warm-sector MLCAPE to upwards of 2500-3000 J/kg this afternoon. Scattered to numerous thunderstorms, including scattered fast-moving supercells, are expected to initially form over parts of eastern Nebraska by late morning/midday, and into Iowa and northern/central Missouri 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. Additional, initially semi-discrete, supercellular development is also plausible farther east across Iowa this afternoon in vicinity of modifying outflow/warm front. Sufficient airmass recovery for a surface-based, all-hazards severe threat is expected across most of Iowa and potentially into southern Minnesota as well, spreading into parts of western/northern Illinois and Wisconsin. Several tornadoes, including some strong with EF2+ potential, are expected, along with large, damaging hail and severe downdrafts. This includes the possibility of long-track supercells/tornadoes and swaths of locally intense wind damage. Later tonight, storms 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 vicinity. ...Ozarks/Oklahoma to south Texas... Isolated to scattered thunderstorms are expected to form from mid afternoon 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. Renewed storm development, potentially severe, could also occur in the predawn hours of Wednesday across Oklahoma near the front. 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 mid-level 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 Texas 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 Arkansas/Missouri, 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 Ontario and southernmost Quebec. 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 Kansas two days ago -- is apparent in satellite imagery over Lake Huron. This feature will move eastward toward northern New York/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 sunset. ..Guyer/Wendt.. 05/21/2024 $$