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Potential Severe Weather Mesoscale Discussion

The National Storm Prediction Center has issued a Mesoscale Discussion covering the potential severe weather development over Illinois (escalloped area on the headlined map) late this Thursday afternoon and evening. Following is that discussion…

SUMMARY...At least some increase in severe weather potential is
   expected  with developing thunderstorm activity across parts of
   northern Illinois by the 4-6 PM CDT time frame.  It is possible that
   this threat may remain low enough that a watch is not needed, but
   trends will be monitored.

   DISCUSSION...Near/just ahead of the lower/mid tropospheric cold
   front, forcing for ascent appears to be contributing to the ongoing
   increase in thunderstorm activity across northwestern Illinois into
   southwestern Wisconsin.  This activity is largely rooted above a
   near-surface stable layer to the north of the surface warm front,
   with relatively warm mid-levels and weak mid/high level lapse rates
   allowing for only weak CAPE.  

   The band of convection is expected to gradually spread eastward
   during the few hours, with southward development into the warm
   sector of the surface cyclone also possible by 21-23Z.   Even within
   the warm sector, relatively low surface dew points across parts of
   eastern Missouri into west central Illinois are currently resulting
   in negligible boundary layer CAPE.  However, modest low-level
   moisture (reflected by mid 50s+ surface dew points) appears to be
   returning northward/northwestward in a corridor to the
   east/northeast through north of Springfield, into the vicinity of
   the warm front.

   A combination of at least weak boundary-layer destabilization along
   the warm front, coupled with increasing lift ahead of the occluding
   surface cyclone triple point (and associated eastward advancing band
   of mid-level forcing for ascent), may allow for at least isolated
   boundary-layer based storm development.  This seems most likely
   north/northeast of the Peoria vicinity, and may include one or two
   supercells accompanied by at least some risk for severe hail and
   localized strong surface gusts.  Although low-level thermodynamic
   profiles appear rather marginal, given the synoptic environment, and
   large low-level hodographs near/beneath 50+ kt southerly 850 mb
   flow, an isolated tornado may not be out of the question.
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