SPC AC 141953

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0253 PM CDT Fri May 14 2021

   Valid 142000Z - 151200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS THIS AFTERNOON AND
   EVENING ACROSS PARTS OF EASTERN COLORADO INTO SOUTHWESTERN
   NEBRASKA...WESTERN KANSAS AND THE TEXAS/OKLAHOMA PANHANDLE
   VICINITY...

   ...SUMMARY...
   Strong to severe thunderstorm development is expected this afternoon
   and evening, east of the southern Rockies into parts of the central
   and southern Great Plains. Some of these storms may pose a risk for
   large hail and strong wind gusts.

   ...Synopsis...
   No changes have been made to the ongoing forecast. Thunderstorms
   continue to slowly develop along the higher terrain from
   southeastern Wyoming into New Mexico. Some of these storms are
   expected to intensify as they move east/southeast and encounter
   greater low-level moisture.

   ..Wendt.. 05/14/2021

   .PREV DISCUSSION... /ISSUED 1125 AM CDT Fri May 14 2021/

   ...Synopsis...
   Much of southern Canada and the U.S. remains under the influence of
   split branches of west-northwesterly mid/upper flow, and models
   indicate that a more prominent split will continue to evolve inland
   of the Pacific coast through this period.  To the east of building
   ridging over the eastern Pacific, a short wave trough (in the
   southern branch) is forecast to amplify and dig toward the
   California coast, to the south of ridging (in the northern branch)
   building inland across British Columbia.

   Downstream of the digging trough, broad mid-level ridging will be
   maintained across the eastern Great Basin into the central Great
   Plains.  However, mid-level troughing within a branch of westerlies
   emanating from the subtropical eastern Pacific is forecast to
   progress across Baja, northwestern Mexico and adjacent portions of
   the Southwest, preceded by a more subtle perturbation or two across
   the southern Rockies and southwestern Texas.

   Downstream, flow remains broadly confluent, but models indicate that
   the most pronounced confluence will begin to shift east of the
   Atlantic Seaboard.  As this occurs, the center of expansive surface
   ridging is forecast to  continue to slowly weaken while shifting
   east of the Mississippi Valley, into the vicinity of the
   Appalachians by late tonight.  However, potentially cold/dry air
   associated with this feature will continue to spread southeast of
   the northern/eastern Gulf of Mexico, and through much of the
   remainder of the Florida peninsula by the end of the period.

   In response to the evolving upstream mid/upper flow, surface
   troughing is forecast to deepen to the immediate lee of the southern
   Rockies through tonight.  Various model output suggests that
   strongest pressure falls may become focused either side of the Raton
   Mesa vicinity, where a low may develop by late tonight.  In response
   to associated strengthening southerly low-level flow across the
   southern high plains, seasonably modest moisture return is expected
   to continue to gradually develop northward from the lower Rio Grande
   Valley.  This may include surface dew point increases into the
   mid/upper 50s as far north as the western Kansas vicinity by late
   this afternoon.

   Beneath steep lower/mid tropospheric lapse rates associated with
   elevated mixed-layer air advecting east of the southern Rockies, the
   moisture return probably will become sufficient to contribute to
   thermodynamic profiles supportive thunderstorms capable of producing
   large hail and strong surface gusts.

   ...Southern Rockies into adjacent Great Plains...
   A combination of moistening upslope flow, broad low-level
   convergence and low/mid-level warm advection appear likely to focus
   the most widespread thunderstorm development from the Front Range
   vicinity into the adjacent central Great Plains late this afternoon
   into tonight.

   This may be aided by a number of smaller-scale perturbations
   progressing around the broad mid-level ridging.  A couple of fairly 
   prominent impulses are already progressing east of the middle
   Missouri Valley, with another couple shifting east of the northern
   Rockies/Front Range.  In response to these developments, there may
   be some southward shift of the leading (northeastern) edge of the
   plume of elevated mixed-layer air emanating from the southern
   Rockies/Plateau region.  A developing zone of stronger differential
   surface heating beneath this feature may provide one, if not the
   primary, focus for upscale growing convection by this evening, from
   parts of northeastern Colorado into northwest and west central
   Kansas.

   Despite the seasonably modest, though improving, low-level moisture
   return, the steep lapse rates may support mixed-layer CAPE on the
   order of 1000-1500 J/kg by late afternoon.  Aided by favorable
   vertical shear beneath 30-40+ kt northwesterly flow around 500 mb,
   the environment probably will become conducive to widely scattered
   to scattered storms capable of producing severe hail and locally
   strong surface gusts.  Congealing surface cold pools may contribute
   to upscale growing, southeastward propagating storm clusters, with
   strong wind gusts becoming the primary hazard before diminishing
   around mid evening with the loss of daytime heating.

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