SPC AC 301255

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0755 AM CDT Sun Jun 30 2024

   Valid 301300Z - 011200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS OVER PARTS OF
   THE I-95/NEAR-COASTAL CORRIDOR FROM THE DELAWARE VALLEY TO
   SOUTHEASTERN NEW ENGLAND...INCLUDING THE NEW YORK CITY METRO...

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS SURROUNDING THE
   ENHANCED RISK FROM NORTHERN NORTH CAROLINA TO NEW ENGLAND...AND ALSO
   OVER PARTS OF THE NORTHERN HIGH PLAINS...

   ...SUMMARY...
   Scattered damaging winds will be possible across parts of the
   Northeast and Mid-Atlantic States this afternoon/evening.  Hail and
   damaging winds may also occur from eastern Montana into western
   North Dakota.

   ...Synopsis...
   In mid/upper levels, the synoptic pattern over the CONUS will remain
   dominated by progressive troughs in the Northwest to northern
   Rockies, and in the Great Lakes to Northeast.  An anticyclone aloft,
   attached to a more extensive, zonal, subtropical ridge, will remain
   anchored over the southern Plains.  What will become the main
   shortwave perturbation associated with the Northeast trough is
   apparent over portions of the Upper Great Lakes and MI, and will
   amplify while moving over southern ON around 00Z.  A sharp shortwave
   trough should result overnight, extending from northern VT across
   eastern NY to northern WV by 12Z tomorrow.

   Meanwhile, a series of minor/low-amplitude shortwaves and vorticity
   lobes will eject northeastward ahead of the Northwest trough, across
   the central/northern Rockies.  A weak, convectively induced
   perturbation now near the southern AZ border should move slowly
   northwestward around the subtropical ridging.  The northern part of
   another trough in the mid/upper easterlies is evident in moisture-
   channel imagery from the MX coastline northwest of Mazatlan,
   northward to at least southern Chihuahua, moving northwestward.

   The 11Z surface analysis showed a low over south-central QC, with
   cold front southwestward over Lake Ontario, southwestern OH,
   northern/western KY, north-central OK, to east-central NM.  By 00Z
   the cold front should reach parts of ME, southeastern NY, northern
   VA, central AL, and southern OK, becoming a quasistationary to warm
   front over the TX Panhandle to northeastern NM.  By the end of the
   period, this boundary should move offshore north of the Outer Banks
   of NC, and extend over central GA, southern AL/MS, the Red River
   Valley of TX/OK, and southeastern CO.  Separate cyclogenesis should
   occur today over south-central/southeastern MT, with cold front
   southwestward across parts of the Great Basin. 

   ...Northern/middle Atlantic Seaboard...
   Scattered thunderstorms -- mainly in lines but with some discrete
   activity -- are expected to move mostly eastward across this
   corridor from late morning through early evening in episodes, with
   damaging winds likely, a few embedded severe gusts possible, and
   isolated potential for large hail or a tornado.

   Low/middle-level destabilization will occur in the preconvective/
   prefrontal environment throughout the day, essentially removing
   already weak MLCINH.  This will occur via diurnal heating of the
   surface and large-scale lift ahead of the approaching Great Lakes
   trough aloft.  Steepening boundary-layer lapse rates amid rich
   low-level moisture, with dewpoints commonly in the upper 60s to low
   70s F, will offset modest mid/upper-level lapse rates enough to
   foster peak/preconvective MLCAPE in the 1500-2500 J/kg range. 
   Greatest buoyancy is possible over the coastal plain between the
   Delmarva peninsula and eastern MA.  Wind profiles will be largely
   unidirectional from the southwest or west-southwest, limiting
   low-level hodograph size and overall vertical shear.  However,
   strengthening flow aloft with northward extent will support
   increasing potential for a few supercells northward from the
   northern Mid-Atlantic into New England, and especially across ME.  A
   counterbalancing effect, however, will be diminishing buoyancy with
   northward extent over New England.  Farther south over much of VA
   and NC, weak shear will support predominantly multicellular storm
   modes, with damaging/isolated severe gusts possible in water-loaded
   downdrafts.

   ...Northern Plains to central Rockies/High Plains...
   Scattered to isolated thunderstorms should develop over the central
   Rockies and higher terrain near the Four Corners and move
   northeastward to eastward this afternoon -- some of it out onto the
   central High Plains.  Activity should produce locally strong-severe
   gusts and isolated large hail from downdrafts penetrating deep,
   well-mixed subcloud layers, though vertical shear this far south of
   the main flow belt should remain too weak to support well-organized
   severe potential.

   Farther north, however, a better-organized severe threat may occur
   this evening with later-developing convection in the northern
   Rockies and adjacent High Plains.  Increasing large-scale ascent
   ahead of the Northwest trough will support this activity, along with
   backed low-level flow and strengthening convergence positioned
   poleward of the deepening surface low.  Early-stage convection may
   include a few supercells with large hail and damaging gusts, with
   some convection potentially expanding upscale into one or more
   clusters as it encounters greater low-level moisture content across
   central/eastern MT.  Large hail will remain possible -- especially
   with embedded supercell(s), and the severe-gust threat should
   increase with potential for mesobeta-scale cold-pool development,
   especially considering the moderately high LCL.  A narrow corridor
   of mid 40s to mid 50s F surface dewpoints, and steep low/middle-
   level lapse rates, should support MLCAPE in the 1000-1500 J/kg range
   (locally/briefly higher).  Veering winds with height will contribute
   to effective-shear magnitudes in the 45-55-kt range in support of
   storm organization.

   ...Central/southeastern AZ...
   Cloud debris and mostly light lingering precip are evident over
   southern AZ, related to the first perturbation, with warming IR
   cloud tops and overall thinning of clouds noted in recent satellite
   trends.  This should continue and shift northwestward with the
   initial perturbation through midday into early afternoon, allowing
   for diurnal heating/destabilization of the boundary layer over
   southeastern AZ by mid/late afternoon.  This, along with favorable
   low-level moisture, orographic forcing, and weak large-scale ascent
   preceding the next perturbation aloft, will support widely scattered
   to scattered afternoon thunderstorms, some of which may produce
   severe gusts.

   Valley-floor dewpoints in the 50s F largely should be maintained
   through the heating/mixing cycle, beneath very steep low-level lapse
   rates and a well-mixed subcloud layer.  A deep southeasterly wind
   profile in low and middle levels will support northwestward motion
   of activity, which may aggregate their outflows to encourage both
   reinforcing development and localized channels of strong-severe
   wind.  The somewhat delayed nature of the destabilization process,
   and uncertainties about storm coverage/aggregation, preclude greater
   unconditional probabilities at this time.

   ..Edwards/Kerr.. 06/30/2024

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