SPC AC 291258

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0758 AM CDT Sat Aug 29 2020

   Valid 291300Z - 301200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PORTIONS OF
   THE EASTERN SEABOARD...OZARKS AND VICINITY...AS WELL AS THE
   SOUTH-CENTRAL HIGH PLAINS...

   ...SUMMARY...
   Severe storms are possible for parts of eastern Oklahoma into
   Arkansas this afternoon and evening, with damaging winds and large
   hail the main threats. A few strong to severe storms will also be
   possible from portions of the Carolinas into the Mid Atlantic and
   New England during the day, with damaging winds and perhaps a
   tornado possible.  Severe wind will be the main threat with
   thunderstorms over the south-central High Plains.

   ...Synopsis...
   In mid/upper levels, a more-amplified northern-stream belt will take
   shape across the northern CONUS and adjacent Canada, while the
   longstanding southwestern high weakens and is shunted southward to
   northern MX.  A compact cyclone was apparent in moisture-channel
   imagery over northwestern ON, with trough southward across Lakes
   Superior and Michigan.  The perturbation as a whole should pivot
   southeastward then eastward through the period, reaching southern
   QC, western New England and the coastal Mid-Atlantic by 12Z
   tomorrow.  Meanwhile, south of another compact cyclone crossing
   northern AB/SK, a strong shortwave trough will amplify and move
   east-southeastward from southern BC across the northern Rockies.

   Elsewhere, a shortwave trough -- representing the mid/upper remnants
   of former Hurricane Laura -- will move eastward from the central/
   southern Appalachians offshore from the Carolinas and Delmarva by
   00Z.  A shortwave trough and embedded MCV will move east-
   southeastward across central/southern KS to the western Ozarks
   today, then proceed over the western TN/KY region tonight.

   At the surface, 11Z analysis showed a low over Lake Ontario, with
   col front southwestward across southwestern OH, southern portions of
   IN/IL, to southeastern KS, where it is being overtaken by a
   convective complex and related outflow.  An extensive, strong field
   of convective outflow from that and prior activity farther southwest
   has overwhelmed the synoptic front across the southern Plains and
   essentially taken over as the effective baroclinic zone.  This
   boundary was moving eastward/southeastward across southern KS,
   central/southwestern OK and portions of northwest TX, and southward
   over the TX South Plains and eastern NM.  The boundary should
   decelerate and perhaps stall across parts of central/southern/
   eastern OK and the southern High Plains today, on either side a low
   over southern OK near the Red River.  The Northeast CONUS frontal
   segment should proceed eastward across the Mid-Atlantic, NY and New
   England through the period, as the low deepens and moves roughly
   down the St. Lawrence Valley. 

   ...Mid-Atlantic/Northeast...
   A very messy convective/precip pattern is forecast along/ahead of
   the cold front today, with the most coherent focus for low-level
   lift being the front demarcating the rear rim of the substantial
   convective potential.  To its east, thunderstorms may develop
   virtually anywhere in the warm sector, where only subtle forcing
   will be needed.  Occasional damaging wind and a few severe gusts are
   possible over a broad area, with isolated severe hail possible in
   northern fringes.  A tornado cannot be ruled out, but the threat
   appears rather unorganized and dispersed diffusely over a large
   corridor.

   The prefrontal air mass will be characterized by rich low-level
   moisture and very weak MLCINH, but also, mostly modest midlevel
   lapse rates related in part to trajectories within (and emanating
   from) the remnants of Laura.  Somewhat greater lapse rates will
   develop amidst cooler air aloft across the northern Mid/Atlantic,
   New York/New England area.  Moving pockets of relatively maximized
   diabatic heating, behind ongoing areas of precip/convection, may
   enhance boundary-layer lift locally in support of convection
   initiation.  Under such conditions, buoyancy fields will be highly
   variable, fragmented and fluidly evolving, but with general 500-1500
   J/kg MLCAPE prevailing and values near 2000 J/kg locally possible. 
   Expect greater mid/upper winds and deep shear with northward extent,
   while somewhat larger low-level hodographs are expected near the
   coast and farther south, over and just inland from the
   Tidewater/Delmarva regions. 

   ...Eastern OK to Mid-South...
   Scattered thunderstorms in multicell clusters should develop in and
   near western parts of the outlook area today and move east-
   southeastward to southeastward, while intensifying, expanding and
   merging.  Large hail and severe gusts are possible from relatively
   discrete convection and small clusters initially, with the threat
   transitioning to wind-dominant as upscale growth and cold-pool
   amalgamation proceed.  

   The severe potential may ramp up this morning from a re-intensifying
   version of the convective cluster now crossing southeastern KS, as
   it impinges upon a favorably moist and intensely diabatically heated
   boundary layer.  See SPC mesoscale discussion 1625 for near-term
   guidance.  Additional development is expected along/ahead of the
   combined frontal/outflow baroclinic zone, also moving into the area
   of large buoyancy.  High near-surface theta-e, a deep troposphere, 
   and steep, EML-related lapse rates in low/midlevels above a slight
   inversion will yield peak preconvective MLCAPE in the 3500-4500 J/kg
   range across eastern OK and much of AR.  Although deep-layer winds
   will be modest, and the area resides well south of the strongest
   mid/upper westerlies, strong directional shear (veering with height)
   will aid in storm organization.

   ...Central/southern High Plains...
   Isolated to scattered thunderstorms are forecast to form
   discontinuously this afternoon, along higher terrain from the
   eastern WY area southward to the Sangre de Cristos and Raton Mesa
   region.  At least isolated severe gusts/hail are possible, and a
   more-concentrated area of thunderstorms with wind threat is fairly
   consistently (and reasonably) progged across and out of the higher
   terrain either side of the CO/NM border, where greatest convective
   coverage is expected.  Though low-level flow is not forecast to be
   intense, the moist, easterly, post-frontal component will contribute
   to 1) upslope lift, 2) modest but supportive deep shear, with
   30-40-kt effective-shear magnitudes possible, and 3) favorable
   storm-relative near-surface vectors.  Around 800-1500 J/kg MLCAPE
   should be available to this activity, though heating will be
   tempered by areas of cloud cover for much of the day.  Some activity
   potentially will persist as strong/locally severe clusters/small
   complexes into late evening across the TX/OK Panhandles, before
   weakening tonight.

   ..Edwards/Kerr.. 08/29/2020

   CLICK TO GET WUUS01 PTSDY1 PRODUCT

   NOTE: THE NEXT DAY 1 OUTLOOK IS SCHEDULED BY 1630Z