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    Day 2 Outlook >
May 21, 2024 2000 UTC Day 1 Convective Outlook
Updated: Tue May 21 19:57:58 UTC 2024 (Print Version | 20240521 2000Z Day 1 shapefile | 20240521 2000Z Day 1 KML)
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 Forecast Discussion
   SPC AC 211957

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0257 PM CDT Tue May 21 2024

   Valid 212000Z - 221200Z


   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 Iowa,
   Missouri to northern Illinois, southeast Minnesota, and southern

   ...20Z Update...
   A broken line of storms with supercells within continues to get
   better organized over southwest IA near the surface low, with a
   possible meso low developing near Red Oak IA as of 1930Z. Low-level
   shear is very favorable for intense tornadic supercells, with 0-500m
   SRH over 200 m2/s2 noted on the DMX radar. Pressure falls continue
   into this region, along with clearing and further destabilization.
   As such, tornadoes appear imminent across much of IA over the next
   few hours.

   Farther south into eastern KS and northern MO, a very unstable air
   mass has developed here as well, with an impressive 18Z TOP
   sounding. Steep midlevel lapse rates over 8 C/km with strong
   deep-layer shear including 50-60 kt speeds around 700 mb will favor
   downstream supercells producing very large hail with tornado risk.

   ..Jewell.. 05/21/2024

   .PREV DISCUSSION... /ISSUED 1140 AM CDT Tue May 21 2024/

   ...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



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