SPC AC 291256

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0756 AM CDT Fri Sep 29 2017

   Valid 291300Z - 301200Z

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS NEAR THE
   SOUTHEASTERN SHORE OF LAKE ERIE...

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS OVER PARTS OF
   WESTERN NEW MEXICO AND SOUTHEASTERN ARIZONA...

   ...SUMMARY...
   Hail and gusts near severe levels may occur with a few thunderstorms
   this afternoon and early evening near Lake Erie, and over the
   western New Mexico/southeastern Arizona region.

   ...Synopsis...
   In mid/upper levels, a progressive and somewhat amplified
   northern-stream pattern will lead to breakdown of the short-lived
   Rex configuration now over the northern Rockies and Intermountain
   West.  A strong shortwave trough -- now evident in moisture-channel
   imagery just offshore the Pacific Northwest -- will move inland
   during the next several hours, followed by weaker shortwaves and a
   speed max to its south and southwest.  By the end of the period, the
   net effect should be negatively tilted synoptic-scale troughing from
   the interior Northwest to the lower Colorado River region.  That
   will contribute to weakening/ejection of the vortex now centered
   over southwestern WY, with a residual/open-wave trough over the
   northern High Plains by 12Z.  Downstream, ridging will shift
   eastward across the central/northern Plains toward the mid/upper
   Mississippi Valley.  An intense, northwest-flow shortwave trough --
   currently crossing the upper Great Lakes, should reach southeastern
   ON, Lake Erie and OH by 00Z.  By 12Z, the trough should extend from
   portions of New England and the interior Mid-Atlantic to parts of
   the central/southern Appalachians.

   At the surface, a cold front extended from offshore New England
   southwestward over the Carolinas and southern AL, to the LA coastal
   area and northeastern Mexico.  This boundary should move slowly
   southward/southeastward over the northern Gulf and northern FL. 
   Meanwhile, a surface low related to the leading northern-stream
   shortwave trough was analyzed over Lake Huron, and should move
   southeastward to near NYC by 12Z, with a cold front trailing to its
   southwest.

   ...Lake Erie vicinity...
   Showers and widely scattered thunderstorms should move rapidly off
   Lake Erie mid/late this afternoon, with origins over the lake and
   perhaps in more moisture-starved areas upstream in Lower MI and ON. 
   Some hail and damaging gusts may occur.  This peculiar scenario
   resembles others responsible for early-winter, convectively intense,
   lake-effect thundersnow events -- but earlier in the cool season,
   with wet thunderstorms instead, and thermodynamic profiles shifted
   to the right (warmer) on sounding diagrams.

   Strong large-scale/deep-layer lift and intensifying midlevel flow
   will precede the approaching mid/upper trough.  This will include
   favorable low-level convergence near the surface low and trailing
   cold front.  Steepening low/middle-level lapse rates over the lake
   will yield enough buoyancy for strong convection including
   thunderstorms.  Modified forecast soundings yield MLCAPE values
   400-700 J/kg over the lake and near the immediate shoreline. 
   Similarly to winter snow events ahead of strong shortwave troughs,
   forecast buoyancy profiles are relatively shallow.  As such, only
   small adjustments to surface conditions on and near the lake yield
   large changes not only in specific MLCAPE values, but also, great
   variation in effective shear.  The latter is related to the
   effective calculations' dependence on storm depth, and resultant
   sampling (or not) of the base layer of the most intense midlevel
   westerlies.  Regardless, cloud-layer shear will be strong, amidst
   nearly unidirectional wind profiles featuring 50-70-kt flow near a
   roughly 500-mb equilibrium level.  These profiles should support
   organized convection over water, some of which may reach land with a
   risk of marginal-severe wind/hail.

   ...AZ/NM...
   Widely scattered thunderstorms should develop this afternoon into
   early evening, offering the potential for isolated severe hail and
   damaging gusts.  Surface dew points upper 40s F to near 60 will
   combine with diabatic surface heating to remove MLCINH and boost
   peak MLCAPE into the 1000-1500 J/kg range over much of the outlook
   area, with lifting aided by localized orographic processes. 
   Well-mixed subcloud layers should contribute to the potential for
   maintenance of gusts/hail to the surface.  Though the parent
   mid/upper trough will be weakening/ejecting away from the area,
   enough flow aloft should remain, in concert with favorable
   directional shear, to yield 30-40 kt effective-shear magnitudes in
   support of marginal storm organization.

   ..Edwards/Dial.. 09/29/2017

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