SPC AC 251250

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0750 AM CDT Sat Jul 25 2020

   Valid 251300Z - 261200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS WITH A FEW
   TORNADOES TODAY AND TONIGHT OVER PARTS OF SOUTH TEXAS...

   ...SUMMARY...
   Hanna may spawn a few tornadoes today and tonight over parts of
   south Texas.  Isolated severe gusts and hail are possible over parts
   of the northern Plains and Upper Midwest.

   ...Synopsis...
   The upper-air pattern will feature a wavy, progressive northern-
   stream belt near the Canadian border, a loosely defined area of
   quasistationary troughing from central CA westward over the
   adjoining Pacific, and a ridge from the southern High Plains across
   the middle Mississippi Valley to the southern Appalachians.  A
   strong, positively tilted trough was evident in moisture-channel
   imagery from eastern AB across northwestern MT to central OR.  This
   perturbation should move east-northeastward across the Prairie
   Provinces through the period.  Its trailing portion will become more
   positively tilted as it moves more slowly across MT/ID.

   A monsoonal plume, characterized by relatively maximized deep-layer
   moisture and temperatures aloft, will continue from the southern
   Rockies across the central High Plains to MN.  Embedded/weak
   vorticity maxima and related small shortwaves will move northward to
   northeastward, exerting subtle influence on the convective
   environment through weak large-scale lift.

   At the surface, the 11Z analysis showed a cold front from south-
   central SK southwestward across central/southwestern MT, associated
   with the strong mid/upper wave.  The front is forecast to move
   southeastward to central ND, western SD and central WY by 00Z.  By
   12Z this front should reach northeastern/central MN, southeastern
   SD, ad the NE Panhandle.

   ...South TX...
   As of this writing, NHC forecasts the center of Hurricane Hanna to
   cross a remote stretch of Padre Island later today.  The center then
   is forecast to reach parts of the lower Rio Grande Valley overnight.
   See NHC advisories for latest tropical watches/warnings and
   specifics on track and intensity, and WPC and local NWS bulletins
   for more on the substantial threats of heavy rainfall/flooding.

   As for tornadoes, the most-favored sector of Hanna should be the
   eastern semicircle, which will be largely downshear with respect to
   the ambient/deep-tropospheric shear vectors influencing the system. 
   See SPC mesoscale discussion 1335 for details on marginal but slowly
   increasing near-term potential this morning along portions of the
   middle/upper TX Coast.  As that sector spreads westward/inland
   throughout the period, associated deeper/persistent convective
   structures in the middle/outer parts of Hanna's circulation could
   include supercells.  That part of the cyclone will be characterized
   by an area of enlarging low-level hodographs/SRH, relatively
   maximized boundary-layer theta-e, and at least marginal buoyancy. 
   The accompanying threat for a few tornadoes likewise should increase
   through the day and spread westward across the northern and middle
   parts of the outlook area.  Once the center moves inland toward the
   Valley, the tornado potential will spread southward down the coast,
   as the trailing part of the eastern sector spreads overhead.

   ...Northern Plains/Upper Midwest...
   Widely scattered to scattered thunderstorms with isolated severe
   gust/hail potential are possible in two primary regimes:

   1.  EASTERN:  Modest lapse rates aloft, associated with the
   monsoonal plume, as well as lingering clouds from morning convection
   and precip, will limit low/middle-level buoyancy through midday. 
   However, a deep troposphere, weak capping due to the lack of an EML,
   and very rich low-level moisture will be in place (e.g., dew points
   commonly upper 60s to mid 70s F).  In that setting, delayed heating
   still should support surface-based convection initiation across much
   of the region, beginning in early afternoon.  Outflow and
   differential-heating zones from earlier activity should be the
   primary foci within this otherwise nebulously defined, warm-sector
   corridor.  A more-concentrated area of wind probabilities, in
   particular, may develop. However, mesoscale-forcing uncertainties
   are too large to attempt to focus a 15% area at this stage.

   2.  WESTERN:  Being west of the monsoonal plume, with the
   progressive northern-stream trough to the northwest, this area near
   the front will be under colder air aloft, greater deep shear,
   stronger surface heating, and larger low/middle-level lapse rates
   and deep shear.  With vigorous boundary-layer mixing, more-modest
   low-level moisture will limit MLCAPE to around 1000-1500 J/kg near
   the cold front this afternoon.  Large DCAPE should be present to
   support isolated severe gusts, and marginally supercell-favorable
   deep shear indicates some hail potential as well.  Surviving
   convection developing in the afternoon near the front should
   encounter greater moisture/buoyancy over the astern Dakotas, with
   one or two clusters potentially growing upscale and reaching
   western/northwestern MN this evening.

   A spatial gap may exist between the earlier severe threats from the
   eastern regime and later potential with western one, especially with
   northward extent across MN.  However, mesoscale and convective-
   mode/longevity uncertainties preclude carving such a gap into the
   outlook area this soon.

   ..Edwards/Marsh.. 07/25/2020

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