Mesoscale Discussion – Possible Watch for portions of Chicago area

Mesoscale Discussion 0638
   NWS Storm Prediction Center Norman OK
   1209 PM CDT Thu May 16 2019

   Areas affected...Parts of northern and 
   central Illinois and Indiana

   Concerning...Severe potential...Watch possible 

   Valid 161709Z - 161815Z

   Probability of Watch Issuance...40 percent



   SUMMARY...Vigorous thunderstorm development is currently being
   maintained near a southward advancing cold pool, to the southwest of
   the Chicago area.  It is not certain how long this will persist this
   afternoon, but there is at least some potential for further
   intensification and organization which could require a severe
   weather watch.

   DISCUSSION...Warm and capping elevated mixed-layer air is in the
   process of advecting across the mid/lower Missouri Valley, toward
   portions of the Upper Midwest, around the northern periphery of
   amplified ridging within the mid-latitude westerlies.  Along the
   leading edge of this air mass, a zone of enhanced lower/mid
   tropospheric warm advection is maintaining vigorous thunderstorm
   development, which may be also increasingly forced by lift above an
   associated surface cold pool.  This appears to be occurring in the
   presence of modest (30+ kt) west-northwesterly deep layer mean flow.

   The influence of a 30-40 kt west-southwesterly 850 mb speed maximum
   nosing east of the Mississippi River may contribute to more of a
   southward propagation near and west of the Illinois/Indiana state
   border area through 18-20Z.  While this track will be increasingly
   into weak mid-level height rises, mid/upper ridging and weaker flow
   aloft, the boundary layer ahead of the southwestern flank of the
   outflow continues to gradually moisten across much of central
   Illinois.  Additional boundary layer warming and moistening may
   contribute to weakening inhibition for boundary layer parcels with
   CAPE increasing in excess of 2000 J/kg.

   Thus, while it remains unclear if synoptic forcing for ascent will
   maintain ongoing convection this afternoon, it appears at least
   possible that mesoscale processes will, with subsequent further
   intensification and organization possible.  This probably would be
   accompanied by increasing potential for strong winds at least
   approaching severe limits along a strengthening gust front.
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