SPC AC 021247

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0747 AM CDT Sun May 02 2021

   Valid 021300Z - 031200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER THE LOWER
   MISSISSIPPI VALLEY AND CENTRAL GULF COAST REGION...

   ...SUMMARY...
   Damaging gusts, marginally severe hail and a few tornadoes are
   possible today across much of the lower Mississippi Valley and
   central Gulf Coast.  Severe thunderstorms also are possible over
   parts of the central High Plains, and isolated strong-severe
   thunderstorms may affect the upper Mississippi Valley.

   ...Synopsis...
   The transition to a more-progressive upper-air pattern is underway,
   as a preciously cut-off low causing split flow devolves into an open
   wave over the southern Plains.  The associated 500-mb low and
   vorticity max -- initially apparent in moisture-channel imagery over
   north-central TX -- should move northeastward across the Arklatex
   region today, reaching central AR by 00Z.  By the end of the period,
   the trough should reach IL and western KY, with the embedded remnant
   low either diffuse or dissipated.

   Meanwhile, low-amplitude synoptic-scale troughing now over the
   western CONUS will strengthen somewhat through tomorrow morning, as
   a series of small shortwaves and vorticity lobes traverse the
   associated cyclonic flow. By 12Z, a trough should extend from the
   Dakotas across the Four Corners to the Gulf of California, with
   distinct northern- and southern-stream speed maxima over the Dakotas
   and northwestern MX, respectively.  Downstream from the northern
   speed max, a small perturbation now over eastern SD will move
   east-northeastward across the upper Mississippi Valley today, to the
   upper Great Lakes by 00Z.  Northeast of the southern speed max, a
   shortwave trough currently over UT will cross CO through the day.

   At the surface, 11Z analysis showed a quasistationary front across
   northern FL, becoming a slow-moving warm front over coastal MS to
   central LA and north-central TX.  The TX portion was rather diffuse,
   in an air mass processed by earlier convection.  This boundary is
   expected to shift northward/inland through the day, possibly
   reaching western AL, northern MS and southern AR by 00Z.  A decaying
   cold front and surface trough was drawn from the DFW Metroplex to
   east of AUS and near LRD, and this boundary should continue to lose
   definition throughout the day.  

   A surface low initially over southeastern CO should remain in that
   area through much of the day, with a dryline southward across the
   OK/TX Panhandles, until being overtaken by a cold front now drawn
   across central/southwestern NE and northeastern CO.  That boundary
   extended northeastward to a frontal-wave low over southwestern MN,
   becoming a stationary to slow-moving warm front eastward over
   south-central WI to central Lower MI.  This boundary should move
   little over the upper Mississippi Valley region before sagging
   southward slightly after 00Z. 

   ...Delta/Mid-South region...
   Episodes of scattered thunderstorms are expected to move
   northeastward across the outlook area through this evening,
   including at least a few supercells.  Damaging gusts, sporadic large
   hail, and a few tornadoes are all possible.

   The air mass across the region will be very moist -- especially near
   and south of the warm front -- with surface dew points commonly in
   the mid 60s to low 70s F and PW commonly around 1.75 inches.  This
   will support areas of 500-1500 J/kg MLCAPE along and south of the
   warm front, and away from convection and precip.  The greatest
   buoyancy is possible over western areas experiencing the largest
   low- and mid-level lapse rates closer to the mid/upper low and
   trough.  Environmental shear generally will be favorable, with
   effective-shear magnitudes commonly 40-50 kt.  SRH should be
   maximized near the front, but will be at least marginally favorable
   for supercells in the free warm sector as well. 

   Initial severe potential may arise from the northeastward shift of
   ongoing convection over east TX (east of a mid/upper dry slot) and
   southeastern LA, each of which may encounter a favorably
   destabilizing inland boundary layer through the remainder of the
   morning.  Considerable uncertainties otherwise remain regarding
   mesoscale foci in such a messy, broad regime with subtle low-level
   forcing.  Additional development is also possible this afternoon
   into early evening behind the morning convection, mainly over LA/MS,
   as the drying aloft encourages stronger diabatic boundary-layer
   heating.  As mesoscale details develop, a better-focused area of
   greater unconditional probabilities may be needed in one of the
   day-shift outlooks. 

   ...Central High Plains...
   Scattered thunderstorms should form this afternoon along and north
   of the front, as far west as the foothills/I-25 corridor.  Activity
   will move generally eastward to east-southeastward across the
   outlook area, offering large hail and damaging wind.  The wind
   threat may persist well into the evening if activity can aggregate a
   cold pool sufficient to force forward propagation across
   northwestern/north-central KS and near the NE border, as some
   convection-allowing progs indicate.  A conditional, marginal tornado
   threat also may develop, strongly dependent on duration of favorable
   mode (be it early multicell stages for "landspout"/nonsupercell
   processes, especially near the boundary, or short-lasting/
   surface-based supercells subsequently).  

   This activity will be supported in the boundary layer by a corridor
   of convergent, upslope flow. A substantial easterly component is
   possible, especially along and just east of an inverted trough
   connecting low and front over extreme eastern CO or along the KS
   line.  The backed winds north of the front and near the trough will
   perform 3 favorable functions:
   1.  Moisture advection/transport into the area,
   2.  Relative maximization of storm-relative winds in the inflow
   layer, 
   3.  Locally enlarged low-level hodographs and shear vectors,
   especially along the front where low-level vorticity already is
   maximized natively.  

   Steep midlevel lapse rates, overlying surface dew points in the 40s
   to low 50s and a well-mixed subcloud layer, will support MLCAPE
   ranging from 500-1000 J/kg within about 100 nm east of the Front
   Range to 1000-1500 J/kg over northwestern KS, amidst 25-40-kt
   effective-shear magnitudes.  Isolated, high-based convection also
   may form farther south across the southwestern KS/OK Panhandle
   region, near the dryline, in an environment of strong diurnal
   heating and deep boundary-layer mixing.  Any sustained cell off the
   dryline will be capable of severe hail and isolated strong-severe
   gusts, with the severe threat declining markedly after sunset. 

   ...Upper Mississippi Valley...
   Scattered thunderstorms should develop on the front this afternoon,
   offering marginally severe hail and sporadic gusts approaching
   severe limits.  Isolated 50+ kt gusts may occur in the most intense
   downdrafts. 

   Warm-sector surface flow will be modest near the front (generally
   5-10 kt), but may back subtly as the SD perturbation approaches,
   resulting in a relative max in frontal convergence timed coincident
   with strongest diurnal warmth in the boundary layer.  Effects of the
   SD shortwave are uncertain because of its timing (passing the area
   around mid-late afternoon and weakening thereafter) -- but should be
   neutral to weakly supportive.  The juxtaposed elements of lift
   otherwise will support convective development/growth, atop a well-
   mixed boundary layer with dew points generally in the 50s to near 60
   F.  This will enable MLCAPE near 1000 J/kg.  

   Modest vertical shear in low levels, and through much of the
   midlevels as well, will limit organization, with effective-shear
   magnitudes mostly 20-30 kt. Still, multicellular cold-pool
   aggregation may foster a loosely organized band of convection
   evolving from the frontal activity and moving southeastward.  This
   would continue the threat for isolated severe for a few hours,
   followed by weakening late this evening as the activity encounters
   more-stable boundary-layer air.

   ..Edwards/Jewell.. 05/02/2021

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