Chicago and Areas-West Under-the-Gun this Afternoon
Posted by JAC on 6/18/2010, 12:06 pm
Clear skies ahead of a developing MCS currently in Iowa that is moving west will rapidily destabilize the air.

Helicity and shear is not too bad, so not a big risk for tornadoes.

The MCS could bow as it approaches Chicago - perhaps a Derecho.














MESOSCALE DISCUSSION 1005
  NWS STORM PREDICTION CENTER NORMAN OK
  1041 AM CDT FRI JUN 18 2010
 
  AREAS AFFECTED...EXTREME NRN MO...ERN IA...NRN IL INTO SRN WI
 
  CONCERNING...OUTLOOK UPGRADE
 
  VALID 181541Z - 181745Z
 
  THE MODERATE RISK ISSUED AT 13Z WILL BE ADJUSTED EAST AND NORTH TO
  INCLUDE ALL OF NRN IL AND SRN WI IN ADDITION TO ERN IA AN EXTREME
  NRN MO. DAMAGING WIND AND LARGE HAIL WILL BE THE MAIN
  THREATS...THOUGH ISOLATED TORNADOES WILL ALSO BE POSSIBLE.
 
  STORMS ARE EXPECTED TO CONTINUE DEVELOPING IN ASSOCIATION WITH
  ONGOING MCS AS IT ADVANCES EWD THROUGH ERN IA...NRN IL AND SRN WI.
  DEEP LAYER SHEAR WILL STRENGTHEN TO 40+ KT IN VICINITY OF MCS AS
  MID-LEVEL JET ROTATES THROUGH BASE OF PROGRESSIVE UPPER TROUGH
  CURRENTLY MOVING THROUGH THE UPPER MS VALLEY. THE ATMOSPHERE WILL
  DESTABILIZE INTO WI THIS AFTERNOON AS SWLY LOW LEVEL JET ADVECTS
  HIGHER THETA-E AIR NEWD WITH TIME. EMBEDDED ORGANIZED STORMS
  INCLUDING BOW ECHOES AND SUPERCELLS WILL BE POSSIBLE AS ACTIVITY
  DEVELOPS EWD INTO THE DESTABILIZING BOUNDARY LAYER.
 
  ..DIAL.. 06/18/2010



DAY 1 CONVECTIVE OUTLOOK  
  NWS STORM PREDICTION CENTER NORMAN OK
  0759 AM CDT FRI JUN 18 2010
 
  VALID 181300Z - 191200Z
 
  ...THERE IS A MDT RISK OF SVR TSTMS OVER PARTS OF IA...MO...AND
  IL...
 
  ...THERE IS A SLGT RISK OF SVR TSTMS SURROUNDING THE MDT RISK AREA
  FROM THE LWR MO AND MID MS VLYS INTO THE UPR GRT LKS...
 
  ...SYNOPSIS...
  ACTIVE PATTERN WILL PERSIST THIS PERIOD.  SRN MB UPR LOW WILL
  CONTINUE ENE INTO SRN ONT AS BROAD UPSTREAM TROUGH/LOW LINGERS OVER
  THE WEST AND FLAT RIDGE PERSISTS OVER THE S CNTRL/SERN U.S.  IN WAKE
  OF SHORTWAVE IMPULSE ASSOCIATED WITH MB SYSTEM NOW MOVING NRN
  MN...SATELLITE AND MODEL DATA SUGGEST THAT IMPULSE NOW OVER THE WRN
  DAKOTAS WILL SWEEP ENE LATER TODAY...REACHING ERN MN/NRN WI BY EARLY
  EVE.
 
  AT THE SFC...LEAD COLD FRONT ASSOCIATED WITH MB LOW SHOULD CONTINUE
  NE ACROSS THE UPR GRT LKS AND WEAKEN AS A NEW COLD FRONT EVOLVES
  TODAY OVER THE ERN DAKOTAS/MN.  THE NEW FRONT SHOULD ACCELERATE E/SE
  ACROSS THE NRN HALF OF THE PLNS AND UPR MS VLY TONIGHT/EARLY
  SAT...SERVING AS A FOCUS FOR STORMS.  FARTHER S...OUTFLOW BOUNDARY
  FROM OVERNIGHT MCS IN THE MID/UPR MS VLY...AND REMNANT WARM FRONTAL
  SEGMENT WILL HELP INITIATE STORMS OVER PARTS OF THE LWR MO VLY/MID
  MS VLYS.
 
  ...UPR MS VLY/UPR GRT LKS THROUGH LATE TONIGHT...
  STRONG WIND FIELD WITH VERTICAL SHEAR FAVORABLE FOR SUPERCELLS WILL
  DEVELOP OVER WI/UPR MI LATER TODAY AS DAKOTAS UPR IMPULSE APPROACHES
  REGION.  SUFFICIENT LOW LVL DESTABILIZATION SHOULD OCCUR AHEAD OF
  DEVELOPING COLD FRONT TO SUPPORT LATE DAY STORM DEVELOPMENT FROM NE
  IA NEWD THROUGH CNTRL WI INTO UPR MI.  STRENGTH OF WIND FIELD /40-50
  KTS AT 700 MB....WITH 50-60 KTS AT 500 MB/ SUGGESTS A RISK FOR
  STRONG TORNADOES GIVEN LIKELY ENLARGED LOW-LVL HODOGRAPHS.  IN
  ADDITION...SETUP EVENTUALLY SHOULD SUPPORT BROKEN LINE SEGMENTS OR
  QLCS STRUCTURES...WITH HIGH WIND FROM EMBEDDED SUPERCELLS/BOWS.  THE
  THREAT SHOULD DIMINISH LATER TONIGHT AS THE STORMS/FORCING OUTRUN
  LOW LVL INSTABILITY AXIS IN LWR MI.
 
  ...LWR MO VLY TO MID MS VLY THROUGH LATE TONIGHT...
  MCS NOW DEVELOPING OVER ERN NEB MAY PERSIST AND POSSIBLY
  STRENGTHEN...MOVING ESEWD THROUGH MIDDAY AS SFC HEATING DESTABILIZES
  REGION PARTIALLY AFFECTED BY YESTERDAY'S STORMS.  OUTFLOW BOUNDARY
  FROM THIS ACTIVITY AND/OR EXISTING OUTFLOW BOUNDARY/WARM FRONT
  SHOULD SERVE AS FOCI FOR ADDITIONAL...INTENSE STORM DEVELOPMENT THIS
  AFTN AS SBCAPE INCREASES TO AROUND 4000 J/KG OVER PARTS OF IA/NRN
  MO/NE KS.  ALTHOUGH DEEP SHEAR WILL BE MODEST RELATIVE TO THAT
  FARTHER N...GIVEN DEGREE OF INSTABILITY ON EDGE OF EML AND RICH
  MOISTURE AVAILABILITY /SFC DEWPOINTS AROUND 70 F/ WILL BE AMPLE FOR
  SUSTAINED STORMS AND A FEW SUPERCELLS BEFORE ACTIVITY CONGEALS INTO
  ONE OR MORE LARGE CLUSTERS.  EMBEDDED FORWARD-PROPAGATING SEGMENTS
  WILL BE CAPABLE OF DMGG WIND/HAIL E/SE INTO IL.  SOME VERY LARGE
  HAIL COULD OCCUR OVER WRN PORTIONS OF THIS REGION DURING THE EARLY
  STAGES OF STORM DEVELOPMENT.
 
  ...SRN NEB/NRN KS LATE IN PERIOD...
  INCREASING WAA/MOISTURE INFLOW ATOP SHALLOW FRONTAL ZONE MAY SUPPORT
  DEVELOPMENT OF SCTD ELEVATED STORMS WITH HAIL LATE TONIGHT/EARLY SAT
  ALONG NERN FRINGE OF EML IN NRN KS/SRN NEB.
 
  ..CORFIDI/GARNER.. 06/18/2010




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Chicago and Areas-West Under-the-Gun this Afternoon - JAC, 6/18/2010, 12:06 pm
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