Alert archive / Illinois / Mesoscale Discussions in 2017

Mesoscale Discussion 754

May 17, 20177 counties in Illinois

The Storm Prediction Center issued this mesoscale discussion at May 17, 2017, 9:45 PM CDT for 7 counties in Illinois. It was in force for 60 min.

The area

The outline as issued.

The facts

Issued: May 17, 2017, 9:45 PM CDT (02:45 UTC)
Expired: May 17, 2017, 10:45 PM CDT
In force: 60 min
Issued by: the Storm Prediction Center
Chance of a watch: 40%

Counties DuPage County, IL, Grundy County, IL, Kane County, IL, Kendall County, IL, Lake County, IL, McHenry County, IL, Will County, IL

The radar

The Chicago radar (KLOT), 38 km from the middle of the area, began a volume at 02:46 UTC, the scan nearest the moment this went out.

Open the radar archive at this moment

The archived warnings, watches and discussions in force at that minute are drawn over the scan, this one among them.

The nearest sounding before it

Davenport, (KDVN), launched 00:00Z, 2.8 hours before the alert went out and 193 km from the middle of its area. This is the routine balloon launch nearest before it, the air the forecaster had in hand, not a measurement taken for the alert.

Skew-T log-p

Temperature in red, dew point in green, the mixed-layer parcel dashed.

Hodograph

The wind turning with height, colored by layer: ground to 1 km, 1 to 3 km, 3 to 6 km, 6 to 10 km.

Mixed-layer CAPE: 1161 J/kg
Mixed-layer CIN: -9 J/kg
Mixed-layer LCL: 1502 m above ground
0-6 km bulk shear: 53 kt
0-1 km storm-relative helicity: 211 m²/s²
0-3 km storm-relative helicity: 306 m²/s²
Significant tornado parameter: 1.0
Supercell composite: 7.2

Open this sounding in Storm Lab

The text

As transmitted.

857 
ACUS11 KWNS 180245
SWOMCD
SPC MCD 180245 
ILZ000-180345-

Mesoscale Discussion 0754
NWS Storm Prediction Center Norman OK
0945 PM CDT Wed May 17 2017

Areas affected...northeast IL

Concerning...Severe potential...Watch possible 

Valid 180245Z - 180345Z

Probability of Watch Issuance...40 percent

SUMMARY...The squall line over northwest IL will move into northeast
IL later this evening.  Isolated severe gusts are possible.

DISCUSSION...Radar mosaic shows a broken line of cells to the east
of the Quad Cities with a continuous squall line over parts of
northern IL into south-central WI.  The boundary layer has been slow
to cool this evening in part to considerable mixing with
temperatures slowly falling from the upper 70s into the middle 70s
degrees F and dewpoints are around 60 F.  The KLOT VAD has shown an
increase in 0.5-2.0 km flow and hodographs have become quite large. 
Although the tornado risk is thought to be low, a nonzero risk for a
brief/weak tornado may accompany any updraft with persistent
rotation.  Isolated gusts 50-65 mph are possible with the stronger
downdrafts which would result in wind damage.

..Smith/Hart.. 05/18/2017

...Please see www.spc.noaa.gov for graphic product...

ATTN...WFO...LOT...

LAT...LON   42448863 42458778 41788770 41348795 41148859 42448863

Nearby

Mesoscale Discussions in Illinois, 2017 Every mesoscale discussion in 2017

Sources

The product and its follow-ups from the Iowa Environmental Mesonet's archive of National Weather Service and Storm Prediction Center text; the sounding from the Mesonet's radiosonde archive; radar from the NEXRAD Level II archive.