SPC AC 051953

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0253 PM CDT Wed Aug 05 2020

   Valid 052000Z - 061200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FOR PORTIONS OF
   THE CENTRAL HIGH PLAINS...

   ...SUMMARY...
   Severe thunderstorms are possible this afternoon and evening from
   the Front Range of the Rockies into adjacent portions of the central
   Great Plains.

   ...Discussion...
   Only two minor changes were made to the previous outlook.  
   1) Expanded the Slight Risk into southeastern CO.  Forecast
   soundings show 35-kt effective shear and moderate instability to the
   east of developing storms over the Palmer Divide and the Sangre De
   Cristos.
   2) Removed 5-percent wind probabilities to the north of
   outflow/southeastward-moving storms over southeast TX.

   ..Smith.. 08/05/2020

   .PREV DISCUSSION... /ISSUED 1130 AM CDT Wed Aug 05 2020/

   ...Synopsis...
   A fairly significant mid-level low to the northeast of the Great
   Lakes region appears likely to accelerate east-northeastward across
   Quebec through the remainder of today and tonight.  In its wake, a
   broadly confluent mid-level flow regime will prevail east of the
   Mississippi Valley, and broadly anticyclonic across much of the
   Southeast, on the western periphery of prominent subtropical ridging
   over the western Atlantic.  Weak troughing may persist west of the
   southern Appalachians, west-southwestward into the north central
   Gulf of Mexico, between the Atlantic ridging and another prominent
   subtropical high shifting east of the southern Rockies.  A notable
   short wave trough is forecast to continue digging from the eastern
   Dakotas toward the lower Ohio Valley, downstream of a broad
   mid-level ridge axis gradually shifting across and east of the
   northern Rockies.  Farther upstream, models indicate that
   significant mid-level troughing within the mid-latitude westerlies
   will pivot into the British Columbia and Pacific Northwest coast,
   gradually coming in phase with weak troughing slowly migrating
   into/across California through late tonight.

   Despite the transition toward a less amplified mid/upper flow
   regime, in the wake of days of larger-scale troughing across the
   east, seasonably high moisture content air generally remains focused
   along and east of the middle and southern Atlantic Seaboard, and a
   corridor across the southeastern Great Plains into the western Gulf
   coast.  Steeper lower/mid tropospheric lapse rates associated with
   warm elevated mixed-layer air are largely confined to the higher
   plains.

   ...Front Range into adjacent Great Plains...
   In conjunction with the steep lower/mid tropospheric lapse rates,
   and modest low-level moisture, moderate mixed-layer CAPE may again
   develop within a sheared regime beneath modest northwesterly
   mid-level flow.  In conjunction with orgographic forcing, it appears
   that a subtle short wave impulse may focus the most widespread
   thunderstorm development near and south of the Cheyenne Ridge
   vicinity.  This probably will initiate across the higher terrain by
   20-22Z, before intensifying while propagating southeastward.  A
   couple of supercells are possible initially, before perhaps growing
   upscale into an organizing cluster of storms which may pose a risk
   for severe wind, in addition to severe hail.

   ...Central Great PLains...
   As the short wave trough generally digs to the east of the Missouri
   Valley, forcing to support thunderstorm development along a
   southwestward trailing wind shift across parts of southern South
   Dakota and northern Nebraska is unclear.  Moderately large
   mixed-layer CAPE may develop along this axis by late this afternoon,
   but it may not be until later tonight when a developing zone of
   low-level warm advection near/east of the Sioux Falls/Sioux City
   area provides supports for appreciable attempts at sustained
   thunderstorm activity.  If this occurs, a few strong surface gusts
   might not be out of the question.

   ...Southern Appalachians and Piedmont...
   South-southwesterly deep-layer mean flow is generally weak, and in
   the process of weakening.  However, downstream of weak mid-level
   troughing, moderate boundary-layer destabilization appears possible
   across the Piedmont into the higher terrain, where orography may
   provide the forcing and focus for thunderstorm development this
   afternoon.  Northwestward advancing outflow from ongoing convection
   across the coastal plain may provide another focus for thunderstorm
   development.  Given the instability, the environment may become
   conducive to locally strong, potentially damaging surface gusts
   through early evening.

   ...Piney Woods into upper Texas/Louisiana Gulf coast...
   A weak mid-level short wave impulse, and perhaps a convectively
   generated impulse and associated outflow, may provide the support
   for vigorous new thunderstorm activity while continuing to progress
   southeast of the Red River Valley.  Given the seasonably moist
   environment, which may also contribute to sizable CAPE in excess of
   2000 J/kg, this convection may become accompanied by the potential
   to produce strong surface gusts.

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