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Mesoscale Outlook – potential severe weather watch issuance for northern Illinois later this afternoon

The National Storm Prediction Center has issued a Mesoscale Outlook describing about a 20% chance of a severe weather watch issuance later this afternoon for portions of northern Illinois, eastern Iowa and southern Wisconsin (escalloped area depicted on the headlined map). A technical discussion follows below…

Mesoscale Discussion 0890
   NWS Storm Prediction Center Norman OK
   0131 PM CDT Tue Jun 26 2018

   Areas affected...Northwest Illinois...Eastern Iowa...and Southern
   Wisconsin

   Concerning...Severe potential...Watch unlikely 

   Valid 261831Z - 262000Z

   Probability of Watch Issuance...20 percent

   SUMMARY...A tornado or two is possible this afternoon. The best
   tornado threat resides near the warm front in northern Illinois,
   southern Wisconsin, and far eastern Iowa.

   DISCUSSION...Moderate surface heating beneath the broken cirrus
   shield has led to moderate surface-based destabilization (1500 to
   2000 SBCAPE) in northern Illinois and eastern Iowa. Mid-level lapse
   rates are only marginally supportive for overall severe weather,
   however, low-level flow is backed near the warm front. While 1-km
   flow is currently relatively weak (~15 kts at KLOT and KMKX VWP),
   KDVN is currently showing 30 knots of southwesterly flow at 1-km. A
   few storms have shown some weak rotation early this afternoon, and a
   modest increase in the tornado threat is possible in the next hour
   or two as this stronger flow at 1 km overspreads the warm frontal
   zone and backed low-level flow. A tornado watch is not expected due
   to the marginal nature of the tornado threat, and the lack of a more
   widespread large hail and damaging straight-line wind threat.
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