SPC AC 241257

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0757 AM CDT Mon Jun 24 2019

   Valid 241300Z - 251200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PARTS OF
   NORTHERN ILLINOIS/SOUTHERN WISCONSIN AND VICINITY...OVER PARTS OF
   THE CENTRAL/SOUTHERN APPALACHIANS AND OHIO VALLEY...AND OVER SOME OF
   WEST-CENTRAL/SOUTHWEST TEXAS....

   ...SUMMARY...
   Scattered storms, some severe with hail or wind, are expected today
   over parts of northern Illinois/southern Wisconsin and vicinity,
   over parts of the central/southern Appalachians and Ohio Valley, and
   over some of west-central/southwest Texas.

   ...Synopsis...
   The upper-air pattern will become less-amplified over most of the
   CONUS through tomorrow morning.  A leading trough -- now located
   from the eastern Dakotas to the Arklatex -- will eject northeastward
   past the mid/upper Mississippi Valley by 00Z, then across the upper
   Great lakes overnight. A Pacific cyclone will dig southward from the
   eastern gulf of Alaska to well offshore from the Northwest.  Largely
   zonal flow will prevail in between, from the northern Pacific Coast
   across the central/northern Great Plains.

   At the surface, 11Z analysis showed an occluded low over the Apostle
   Islands area between DLH-IWD, with triple point over southwestern
   WI, and cold front across eastern IA, southwestern MO, eastern OK,
   and west-central TX.  The southern Plains portion of the front
   should stall and then retreat northward today, while a dryline to
   its south over northwestern Chihuahua develops northward over
   west-central/northwest TX.  The northern part of the front will move
   slowly eastward over the WI/northern IL region.

   An extensive arc of thunderstorms overnight has left an outflow
   boundary from middle TN across extreme eastern MS, southeaster LA,
   the upper TX Coast near GLS, between AUS-SAT, northwestward across
   the Hill Country to between SJT-COM.  Though virtually its entire
   arc still remains expansive (progressive) at this hour, ongoing and
   expected further weakening of supportive convection should slow the
   boundary and make the cold pool more shallow behind it, with
   stalling and northward retreat likely over central TX prior to the
   afternoon convective cycle.  The eastern part of the outflow arc
   still has a broad area of trailing precip and is oriented more
   orthogonally to the flow aloft, and should progress eastward across
   northern AL, KY, TN, and the lower Ohio Valley through at least
   early/mid afternoon.

   ...Ohio Valley/Appalachians...
   Scattered thunderstorms should develop well ahead of the surface
   cold front -- predominantly near the outflow arc and related
   corridor of relatively maximized ascent, but also in the warm sector
   near outflow/differential-heating boundaries.  Activity should move
   generally northeastward as clustered to linear multicells, with
   isolated supercells possible.  The main concern will be damaging to
   severe gusts, though isolated, marginally severe hail also may
   occur.

   The 12Z RAOBS ahead of the outflow arc sampled a richly moist
   boundary layer underlying variable mid/upper-level lapse rates that
   generally steepened with southward extent.  Surface dew points in
   the mid 60s to low 70s F should combine with favorable diurnal
   heating to enable MLCAPE 500-1500 J/kg to develop, locally near 2000
   J/kg across parts of AL/GA.  Deep shear generally will increase
   northward.  A broad area of convective/severe potential is apparent,
   through coverage is unlikely to be uniform within this swath before
   activity generally diminishes this evening.

   ...IL/WI...
   Scattered thunderstorms are expected to develop mainly during the
   afternoon in a weakly capped air mass, offering sporadic damaging
   winds and severe hail.  The threat should diminish with time this
   evening, and with eastward extent across Lake Michigan.

   Time series of forecast soundings reasonably suggest steepening of
   mid/upper-level lapse rates through the afternoon as the trough and
   its DCVA field approaches, conterminous with a rising tropopause and
   diurnal heating of the boundary layer.  These factors will combine
   with adequate low-level moisture (e.g., surface dew points 60s F) to
   produce MLCAPE commonly in the 1000-1500 J/kg range, locally briefly
   near 2000 J/kg.  Strong upper-level/ventilating winds will overlie
   meager and nearly unidirectional low/middle-level flow, leading to a
   vertical kinematic profile supporting organized multicells, small
   bows, and short-lived supercell structures.  Lift will be maximized
   near a weak front/trough in the low levels, moving eastward across
   the outlook area.

   ...West-central/southwest TX...
   Widely scattered thunderstorms are expected to develop this
   afternoon near the dryline, along and south of its intersection with
   the outflow boundary.  Damaging gusts and large hail are possible.

   In the absence of substantial upper-level perturbations, low-level
   thermodynamic processes and lift, followed by storm-scale cold-pool
   aggregation, will drive the convective processes today.  A wedge of
   relatively undisturbed moist sector, between the dryline and outflow
   arc, will be strongly heated this afternoon, removing most or all
   MLCINH such that only modest lift along the boundaries will be
   needed to generate convection.  With surface dewpoints commonly in
   the upper 60s to mid 70s F, and mid/upper-level lapse rates
   remaining relatively steep for this area and time of year, peak
   preconvective MLCAPE values of 4000-5000 J/kg are likely, locally
   higher.  Weak low/middle-level winds will limit vertical shear
   overall and keep hodographs small, contributing to predominantly
   multicellular storm organization.  Still, a tornado cannot be
   completely ruled out, given stretching potential attendant to the
   large/deep buoyancy, and residual low-level vorticity near the
   outflow boundary.  Some clustering of convection may occur for a few
   hours, which would boost local severe-wind potential.  Overall
   convective/severe threat should wane with time late this evening as
   diurnally generated convective outflows spread into a cooling
   boundary layer.

   ..Edwards/Mosier.. 06/24/2019

   CLICK TO GET WUUS01 PTSDY1 PRODUCT

   NOTE: THE NEXT DAY 1 OUTLOOK IS SCHEDULED BY 1630Z