SPC AC 261957

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0257 PM CDT Wed May 26 2021

   Valid 262000Z - 271200Z

   ...THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
   THE CENTRAL PLAINS...

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OUTSIDE OF THE
   MODERATE RISK AREA ACROSS PARTS OF THE GREAT PLAINS AND LOWER TO MID
   MISSOURI VALLEY...AS WELL AS IN PARTS OF THE NORTHEAST...

   ...SUMMARY...
   A regional outbreak of severe thunderstorms is expected across the
   central Great Plains this afternoon through tonight. The greatest
   tornado threat, including the potential for strong tornadoes, is
   expected between 5 to 11 PM CDT. Intense damaging winds and very
   large hail are expected as well.

   ...DISCUSSION...
   The only change to the outlook for this issuance is to adjust the
   forecast across parts of the southern High Plains. The slight and
   marginal risk areas have been realigned with the current position of
   the dryline and distribution of instability according to the RAP.
   The corridor with the greatest severe threat across west Texas is
   expected to remain relatively narrow, oriented from south-southwest
   to north-northeast along the western edge of the strongest
   instability.

   ..Broyles.. 05/26/2021

   .PREV DISCUSSION... /ISSUED 1112 AM CDT Wed May 26 2021/

   ...Central Plains...
   Several severe storms are expected later today over parts of NE/KS
   and extreme northeast CO. Morning surface analysis shows a weak
   boundary extending across northern KS.  A small cluster of
   hail-producing thunderstorms has formed near this boundary this
   morning (reference MCD #744).  This boundary is expected to weaken
   and lift slowly northward through the day, with ample low-level
   moisture and CAPE becoming established across much of
   western/central KS and parts of southern/western NE.  Model guidance
   is consistent in bringing a weak shortwave trough out of the central
   Rockies into the High Plains this afternoon/evening, aiding in the
   rapid development of intense thunderstorms by late afternoon.  

   Storms are expected to form along the dryline near the CO/KS border,
   as well as other subtle surface boundaries over the region. 
   Forecast soundings show a combination of very steep mid-level lapse
   rates and CAPE for intense updrafts, favorable deep-layer shear for
   supercells, and increasingly strong low-level winds that veer with
   height for the potential of tornadoes - including strong tornadoes. 
   A consensus of morning CAM solutions indicate that confidence in
   convective initiation diminishes with southward extent over southern
   KS, but isolated supercells will be possible as far south as the
   eastern TX Panhandle.  Activity will persist through much of the
   evening.  Very large hail will also be possible, along with an
   increasing risk of wind damage as storms congeal after dark over
   central and eastern NE/KS.

   ...Northeastern States...
   No substantial changes have been made to the forecast in this area.
   Morning water vapor loop shows an upper trough extending from the
   Great Lakes region into the lower OH Valley.  Subtle midlevel height
   falls and strengthening winds aloft ahead of the system are
   overspreading much of the northeastern states.  Meanwhile, a surface
   cold front is tracking eastward across MI/IL with a pre-frontal band
   of showers moving into OH.  Heating ahead of the band of showers
   will yield afternoon MLCAPE values around 1000 J/kg and the
   intensification of scattered thunderstorms.  Vertical shear profiles
   values are not particularly strong, but sufficient southwesterly
   flow in the 2-6km layer may aid in downward momentum transfer as
   low-level lapse rates steepen.  Initial activity may be rather
   isolated, but as storms organize and congeal through the afternoon,
   the risk of locally damaging wind gusts will increase.  Storms are
   expected to remain strong/severe into the early evening until storms
   move offshore.

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