SPC AC 271958

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0258 PM CDT Thu Aug 27 2020

   Valid 272000Z - 281200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
   THE NORTHEAST/MID-ATLANTIC AND THE NORTHERN PLAINS...

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
   THE LOWER MS VALLEY...

   ...SUMMARY...
   A few tornadoes along with significant severe wind and hail will
   remain possible in the Northeast through this evening. A few
   tornadoes along with significant severe wind and hail will be
   possible across the northern Great Plains during the late afternoon
   into tonight. A few brief tornadoes may also occur in association
   with tropical cyclone Laura across the Lower Mississippi Valley.

   ...20Z Update...
   Have compressed the northern extent of severe probabilities across
   NY and southern New England, as additional development to the north
   of ongoing severe storms appears unlikely. Main threats with this
   activity will continue to be scattered damaging wind gusts, large
   hail, and perhaps a few tornadoes in close proximity to the warm
   front across southern NY into CT.

   A rather conditional tornado threat continues in association with
   Tropical Storm Laura, with a general lack of robust convective
   development in MS so far this afternoon. There is still some severe
   potential across this area through the rest of this afternoon, as
   the low-level wind field remains favorable for transient
   circulations. For more information on this isolated tornado threat
   across the lower MS Valley, see Mesoscale Discussion 1598. Have also
   expanded the Marginal Risk for damaging winds a bit eastward into
   parts of central/northern AL to account for an ongoing band of
   storms moving northward.

   No changes have been made to severe probabilities across the
   northern Rockies/Plains. For more information on the near-term
   severe threat across parts of south-central/southeastern MT and
   northern/central WY, see Mesoscale Discussion 1597.

   ..Gleason.. 08/27/2020

   .PREV DISCUSSION... /ISSUED 1124 AM CDT Thu Aug 27 2020/

   ...Northeast...
   No change made to prior categorical outlook. Scattered thunderstorms
   are expected to develop in a roughly west/east-oriented corridor
   across southern NY before spreading into southern New England. 12Z
   regional soundings sampled very strong 700-500 mb west-northwest
   flow, the bulk of which will become centered on southern New England
   later this afternoon. A rather moist boundary layer sampled in the
   12Z Buffalo sounding characterized by surface dew points into the
   low 70s is expected to yield a moderately unstable air mass south of
   the warm front with MLCAPE from 1000-2000 J/kg. Elongated hodographs
   with some low-level curvature should prove favorable for initially
   splitting supercells with potential for golf to tennis ball-size
   hail and a few tornadoes. Aggregation of early convection into a
   couple of fast-moving clusters with enhanced wind damage is still
   possible.  

   ...Northern Great Plains...
   Primary changes are to add a broad significant severe wind area
   within/near the cat 3/ENH risk and expand cat 2/SLGT risk northwest
   across eastern MT into far western ND where at least a few
   supercells evolving into small bowing clusters are possible. This is
   the anticipated area for initial severe development during the late
   afternoon as convection develops off the higher terrain in
   association with a minor shortwave impulse currently over western
   MT. An additional couple supercells should form separately over the
   Black Hills. Very large hail of 2-3 inches and a couple tornadoes
   will be possible during the initial supercell phase and especially
   near the MT/SD/WY border area across western SD. 

   With a moderately buoyant and steep mid-level lapse rate environment
   expected to persist across most of SD this evening into tonight, one
   or more accelerating clusters should ultimately develop with
   eastward extent along a stalled surface front. Convectively enhanced
   mid-level westerlies should support potential for a bowing MCS to
   develop with a risk of significant severe wind gusts up to 80-100
   mph and brief QLCS tornadoes, persisting east into at least
   southwest MN overnight. 

   ...Lower MS Valley...
   Tropical cyclone Laura will continue to gradually decay as it tracks
   north from northern LA towards central AR. The peak risk for brief
   tornadoes should be focused this afternoon into early evening to the
   east and southeast of the center from southeast LA north into the
   Ark-La-Miss where mid to upper 70s surface dew points can overlap
   with weakening but initially intense low-level wind fields. Overall
   tornado threat should nocturnally decrease and become marginal
   tonight as both instability and 0-1 km SRH subside.

   CLICK TO GET WUUS01 PTSDY1 PRODUCT

   NOTE: THE NEXT DAY 1 OUTLOOK IS SCHEDULED BY 0100Z