SPC AC 191258

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0758 AM CDT Sun May 19 2024

   Valid 191300Z - 201200Z

   ...THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS IN PORTIONS OF
   KANSAS...

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PARTS OF
   EAST-CENTRAL/SOUTHEASTERN FLORIDA...

   ...SUMMARY...
   An organized swath of severe thunderstorm wind (potential derecho)
   is expected mainly across parts of Kansas this afternoon and
   evening, possibly extending into northern Oklahoma.  Gusts of 75-100
   mph are possible, along with large hail and a few tornadoes.

   ...Synopsis...
   A broad fetch of westerly to southwesterly flow will cover most of
   the central CONUS.  This will persist downstream from a synoptic-
   scale trough over the Pacific Northwest that should amplify into the
   Great Basin region through the period.  The downstream flow field
   contains several embedded shortwaves, including a convectively
   generated perturbation now over parts of KS/NE.  This feature should
   eject northeastward over the upper Mississippi Valley this evening,
   and away from the main threat area.  However, an upstream shortwave
   trough -- apparent in moisture-channel imagery over the Four Corners
   region -- will move east-northeastward and influence convective
   potential over the central Plains this afternoon and evening.

   Elsewhere aloft, a persistent trough was evident across portions of
   VA, the Carolinas, GA, and AL, with cyclonic flow downstream over
   the southern Atlantic Coast and as far south as most of the FL
   Peninsula.  This trough should reach the NC/SC/GA coastlines and
   northwestern FL by 00Z, and south FL by 12Z tomorrow.

   The 11Z surface analysis showed a cold front from northeastern ON
   across Lower MI, western IL, west-central MO and east-central KS,
   where it is being overtaken by an outflow boundary arching
   southwestward across south-central KS to the OK Panhandle. The
   outflow boundary is expected to stall in the next few hours, then
   shift northward over southern and central KS through the day.  A
   dryline extended from eastern CO across the western TX Panhandle to
   near LBB, MAF and northern Coahuila.  The dryline should mix
   eastward slightly today, reaching near the KS/CO line and eastern TX
   Panhandle by mid/late afternoon.  Elsewhere, a weak cold front was
   drawn from a low offshore from NC southwestward over northern FL,
   and should move slowly southward into central FL today.

   ...Central Plains...
   Thunderstorms are expected to develop during mid/late afternoon
   behind the dryline over the eastern Raton Mesa region, as well as
   along the dryline itself, and move east-northeastward to eastward
   into increasing moisture and instability.  As they do, some of the
   activity should aggregate into clusters as smaller-scale outflows
   combine and enlarge.  With supportive internal feedback of an
   organizing cold pool, associated forced ascent at the front and
   rear-inflow jet, and supportive midlevel winds for downward momentum
   transfer, as well as the residual and diurnally destabilized outflow
   boundary, the overall meso-alpha-scale pattern appears quite
   suitable for an organized, potentially significant-severe
   convective-wind swath to result.  The most probable corridor for
   such growth still appears to be across KS in and near the "moderate
   risk" corridor, driven by wind probabilities.

   Initially separate dryline development over the northeastern TX
   Panhandle and northwestern OK may blend into (or even get undercut
   by outflow from) the broader event as well, after a few hours'
   discrete time to its south, with a threat of large hail, locally
   severe gusts and perhaps a tornado or two.  Other afternoon
   development, along the dryline, in the mountains of northern CO and
   southeastern WY, and over the Black Hills, may move east-
   northeastward across a narrow moist sector and produce severe gusts
   and hail into parts of the adjoining High Plains of western NE and
   the Dakotas.

   Near and behind the KS-to-Panhandles dryline segment, the storm-
   initiation environment will be characterized by increasing
   large-scale ascent and upper divergence ahead of the ejecting Four
   Corners shortwave trough, as well as strong diurnal heating, mixing
   and deepening of the boundary layer.  Superposition of these effects
   will lead to nearly 9 deg C/km lapse rates throughout the lower/
   middle troposphere, supporting 2000-3000 J/kg MLCAPE just east of
   the dryline.  A deep subcloud layer conducive to strong-severe
   downdrafts will be available to storms over western KS from the
   beginning, and should support growth/merger of resultant outflows
   before activity moves into greater moisture.  Forced ascent of the
   moist boundary layer should help to sustain the forward-propagating
   complex into strengthening nocturnal MLCINH at least across eastern
   KS, and perhaps into parts of the lower Missouri Valley, before it
   weakens tonight.

   ...FL...
   Scattered thunderstorms should develop this afternoon near the I-95
   corridor and roughly between MIA and the Space Coast, along and
   south of a weak cold front.  Organized multicells and a few
   supercells are possible, offering large hail and damaging to locally
   severe gusts before they move offshore.  An active Atlantic
   sea-breeze pattern will be augmented by mid/upper support preceding
   the trough aloft.  Strong diurnal heating, a deep troposphere and
   rich low-level moisture (with dewpoints commonly in the 70s F and
   16.5-18.5 g/kg mean mixing ratios) will support 2000-3000 J/kg peak
   MLCAPE.  Though weak near-surface winds will limit hodograph size,
   30-45 kt effective-shear magnitudes and potential boundary
   interactions will support storm organization, including supercell
   potential.

   ..Edwards/Goss.. 05/19/2024

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