SPC AC 291249

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0749 AM CDT Fri May 29 2020

   Valid 291300Z - 301200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS IN PARTS OF THE
   NORTHEAST...

   ...SUMMARY...
   Scattered thunderstorms will pose a threat for severe wind and hail
   today in parts of the Northeast.

   ...Synopsis...
   The large-scale pattern will be dominated by troughing over eastern
   North America and the extreme northeastern Pacific near the coast,
   with ridging from northwestern MX and the southern Rockies across
   western Canada to the North Slope of AK.  A strong shortwave trough
   initially over the upper Mississippi Valley will pivot eastward
   across Lake Michigan and Lower MI today, to southern QC and
   northeastern NY by 12Z tomorrow.  As this occurs, another shortwave
   trough -- evident in moisture-channel imagery over western/middle
   TN, and adjoining parts of KY/MS, will eject northeastward to the
   interior Mid-Atlantic by 00Z, weakening considerably.

   To the west, very low-amplitude perturbations will crest the mean
   ridge across the northern Rockies throughout the period.  An
   initially cutoff low -- located about 550 nm west of northern Baja
   near 30N -- will eject northeastward, approaching the CA coastline
   near MRY by 12Z.  A complex set of vorticity lobes near the
   lower/middle Rio Grande Valley of TX will, in the net, retrograde
   south-southwestward over northern MX.

   At the surface, 11Z analysis showed a cold front across eastern Lake
   Huron, central IN, to near a MEM-GLS-CRP line, diffused farther
   southwest by extensive convective outflow.  This front should move
   eastward across the upper Ohio Valley, north-central Appalachians
   and Tennessee Valley through this evening.  By 12Z, the front should
   reach eastern New England, NJ, the western Carolinas, southern MS
   and the northwestern Gulf.

   ...Northeast...
   Scattered thunderstorms in broken to solid bands are expected to
   move across the "slight" outlook area from late morning into this
   evening, offering damaging to severe gusts and sporadic hail.  A
   tornado cannot be ruled out as well.

   Activity should occur in multiple episodes, initially associated
   with:
   1.  The frontal zone in western parts of the outlook area
   2.  A convergence zone off the southeast side of the lower Great
   Lakes, and
   3.  A low-level trough/convergence line from the Adirondacks region
   to northern VA.
   Convection will form/intensify as these boundaries encounter a
   diurnally destabilizing air mass amidst weak MLCINH, and large-scale
   ascent increases ahead of the approaching shortwave troughs.

   Rich low-level moisture has spread poleward over the region --
   especially east of the Appalachians -- in the wake of former T.S.
   Bertha and its remnants.  Surface dew points are in the mid/upper
   60s across much of NY and New England away from the coast, and 70s
   from southeastern PA southward over the eastern Carolinas.  In
   concert with continuing diurnal heating, this will boost peak MLCAPE
   to the 1500-2000 J/kg range (locally higher) across the corridor
   east of the Appalachians, with a somewhat separate axis of mid-60s F
   dew points and about 1500 J/kg MLCAPE ahead of the front, over the
   upper Ohio Valley region.

   Deep-layer shear should increase from west-east through the day, as
   the mid/upper height gradient tightens in response to the
   approaching perturbations.  A broad area of 40-50-kt effective-shear
   vectors over the warm sector, oriented slightly rightward of the
   likely forcing boundaries, is possible by midafternoon, from
   northern NY across much of PA to western MD and northern parts of
   WV/VA.  This will aid in convective organization, with a blend of
   discrete, clustered and mostly quasi-linear storm modes expected. 
   Most of the activity should diminish after 00Z.  However, given the
   strengthening forcing aloft and mass response over rich moisture
   that will characterize the lower elevations of eastern PA/NJ/
   southern NY, strong-severe convection may persist into this evening
   over that area.

   ...Interior Northwest, northern Rockies...
   Isolated to widely scattered thunderstorms should develop this
   afternoon within this arc, moving mostly with the ambient
   anticyclonic flow into an environment favoring isolated severe
   gusts/hail.  The greatest convective concentration may be from the
   Bitterroots area toward the Bighorns.  Convection should develop
   preferentially/initially over areas of well-heated higher terrain,
   moving into lower elevations characterized by marginal yet adequate
   near-surface moisture, and strongly mixed subcloud layers. 
   Convective coverage becomes more uncertain with westward extent into
   OR; however, a favorable environment suggests isolated severe gusts
   and/or hail are possible with any sustained convection that can
   occur.  Pockets of favorable low-level moisture and strong surface
   heating will lead to steep low/middle-level lapse rates with a deep,
   well-mixed boundary layer in this part of the outlook as well.

   ..Edwards/Gleason.. 05/29/2020

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