SPC AC 171620

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1120 AM CDT Wed May 17 2017

   Valid 171630Z - 181200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS ACROSS MUCH OF
   IA...SOUTHEAST MN...SOUTHWEST WI...AND SMALL PARTS OF NORTHWEST IL
   AND NORTHERN MO...

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS SURROUNDING THE
   ENH RISK AREA...

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS SURROUNDING THE
   ENH AND SLGT RISK AREAS...AND EXTENDING SOUTHWARD ACROSS THE OZARKS
   AREA...

   ...SUMMARY...
   Severe storms are expected to develop this afternoon from the Mid
   Missouri Valley into Wisconsin. A few tornadoes, large hail, and
   strong wind gusts are possible. Farther south, at least a marginal
   severe risk will extend south across southern Missouri and possibly
   into Arkansas.

   ...Synopsis...
   While ridging aloft persists across the eastern U.S. through the
   period, a large/long-wave trough will reside over the western and
   central portions of the country.  This trough will remain comprised
   of two main pieces -- a closed low digging southeast across the
   Intermountain west, and second/weaker lead low and associated
   negatively tilted short-wave trough, moving quickly northeastward
   out of the central Plains toward the upper Mississippi Valley area.

   At the surface, a low -- nearly co-located with the center of upper
   circulation currently crossing central Kansas per WV loop -- is
   expected to move northeast across the mid-Missouri Valley into IA
   this afternoon, and then into the upper Great Lakes region
   overnight.  Trailing south from the low, a weakening baroclinic zone
   will shift across Kansas into Iowa/Missouri through the afternoon,
   with bands of strong/severe thunderstorms developing starting early
   this afternoon in advance of this system.


   ...Mid-Missouri Valley into the Upper Great Lakes, and southward
   across the Ozarks...
   The back edge of high-level warm-conveyor cloud cover continues
   spreading quickly northeastward into southwest IA/western MO late
   this morning, while an overall thinning of the high clouds is also
   occurring over the IA/MO vicinity.  As this trend continues,
   resulting diabatic heating combined with mid-level cooling spreading
   across the area as the upper low/trough shifts steadily
   northeastward will result in substantial destabilization, with 1000
   to 2000 J/kg mixed-layer CAPE by mid-afternoon centered over the
   southern WI/IA/MO area and vicinity.  

   While storm development southward toward the Ozarks remains less
   certain with ascent shifting northeastward toward the upper
   Mississippi Valley with time, storm development appears to be
   essentially a certainty -- likely by early afternoon -- farther
   north, with the area of maximum diurnal heating within the dry slot
   over the southeast NE/northeast KS/northwest MO/southwest IA area. 
   Other development should -- either simultaneously or shortly
   thereafter -- occur northeastward across IA and into southern MN and
   WI.

   Aforementioned uncertainty regarding degree of convective
   development southward across central and southern MO and into AR
   suggests maintenance of only MRGL risk across this area.  Still,
   with sufficient destabilization expected and very strong flow aloft
   to reside atop the area, any storms which do form would likely pose
   a severe risk -- including potential for large hail and locally
   damaging winds.  

   Farther north in the higher-risk area, multiple bands of storms --
   both cellular/mixed-mode and possibly bowing line segments -- are
   eventually expected to evolve after initial cellular development. 
   Very strong -- but roughly unidirectional/south-southwesterly --
   flow aloft will yield deep-layer shear supportive of
   well-organized/rotating storms, with large hail and damaging winds
   likely.  Risk for a few tornadoes is also evident, especially across
   the IA area roughly along the projected path of the surface low,
   where surface flow backed to an east-of-south direction will enhance
   low-level shear.  

   A rather rapid eastward/northeastward shift in storm activity/severe
   risk is expected, with the strongest convection spreading largely
   out of IA/MN and across WI/upper MI/northern IL through the evening
   before convection weakens/diminishes late this evening/overnight.

   ..Goss/Coniglio.. 05/17/2017

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