Mesoscale Discussion 1581

Issued

Aug 23, 2021, 2:18 PM CDT (19:18 UTC)

Expired

Aug 23, 2021, 4:15 PM CDT

In force

117 min

Issued by

the Storm Prediction Center

Concerning

Severe potential...watch unlikely

Chance of a watch

20%

Counties Bond County, IL, Fayette County, IL, Greene County, IL, Jersey County, IL, Macoupin County, IL, Madison County, IL, Montgomery County, IL, Saint Clair County, IL

826 
ACUS11 KWNS 231918
SWOMCD
SPC MCD 231918 
ILZ000-MOZ000-232115-

Mesoscale Discussion 1581
NWS Storm Prediction Center Norman OK
0218 PM CDT Mon Aug 23 2021

Areas affected...Parts of west central into southwestern Illinois
and adjacent portions of east central Missouri

Concerning...Severe potential...Watch unlikely 

Valid 231918Z - 232115Z

Probability of Watch Issuance...20 percent

SUMMARY...There appears at least some possibility for the evolution
of an increasingly organized cluster of thunderstorms, which could
pose a risk for potentially damaging wind gusts near or just east of
the Greater St. Louis metropolitan area by 4-6 PM CDT.  It is not
clear that a severe weather watch will be needed, but trends are
being monitored.

DISCUSSION...Thunderstorm activity has recently intensified near the
intersection of southward advancing convective outflow and a remnant
low-level baroclinic zone extending across west central Illinois. 
This likely has been aided by inflow of very moist boundary layer
air (with surface dew points in the mid/upper 70s F), which appears
characterized by large CAPE up to around 3000 J/kg.  In the presence
of modest shear, largely due to veering of ambient winds from
southwesterly in low-levels to westerly in mid-levels, southerly
low-level system-relative inflow appears moderately strong, on the
order of 15-20 kts.  It appears that this may contribute to
continuing southward propagation toward at least eastern portions of
the Greater St. Louis metro area during the next few hours.  

Latest objective instability analysis suggests that the southward
propagation will coincide with the axis of largest mixed-layer CAPE,
along the eastern periphery of warmer and more strongly capping
elevated mixed-layer air emanating from the Great Plains.  Given the
 degree of CAPE, coupled with additional steepening of already
modestly steep low-level lapse rates, there appears at least some
potential for further intensification and upscale growth of the
cluster of thunderstorms, which could be accompanied by increasing
risk to produce potentially damaging surface gusts.

..Kerr/Dial.. 08/23/2021

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

ATTN...WFO...ILX...LSX...

LAT...LON   39708948 38968873 38388992 39409075 39619018 39708948

Mesoscale Discussions in Illinois, 2021Every mesoscale discussion in 2021

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.