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    Day 2 Outlook >
May 21, 2024 1300 UTC Day 1 Convective Outlook
Click to see valid 1Z - 12Z Day 1 Convective Outlook
Updated: Tue May 21 13:00:53 UTC 2024 (Print Version | 20240521 1300Z Day 1 shapefile | 20240521 1300Z Day 1 KML)
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 Forecast Discussion
   SPC AC 211300

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0800 AM CDT Tue May 21 2024

   Valid 211300Z - 221200Z


   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

   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

   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



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