Mesoscale Discussion 1455
NWS Storm Prediction Center Norman OK
0136 PM CDT Mon Aug 10 2020
Areas affected...Portions of far eastern IA into southern WI and
northern IL...including the Chicago metro
Concerning...Severe Thunderstorm Watch 426...
Valid 101836Z - 102030Z
The severe weather threat for Severe Thunderstorm Watch 426
SUMMARY...The potential for widespread and destructive damaging wind
gusts of 70-100+ mph and perhaps a tornado or two will continue as a
line of storms moves quickly eastward across northern Illinois. This
line will likely impact the Chicago metro area around 2000-2030Z (3
to 3:30 PM CDT).
DISCUSSION...The bow echo over eastern IA has developed a comma head
structure on its northern flank and very large rear-inflow jet, with
measured wind gusts of 97 and 112 mph recently reported in the
vicinity of Cedar Rapids IA. The 17Z sounding from DVN observed
steep 700-500 mb lapse rates of 8.5 C/km. These lapse rates combined
with strong heating of a very moist low-level airmass is supporting
3000-3500 J/kg of MLCAPE downstream of the bow. The 25 kt of
effective bulk shear also observed on the 17Z sounding is somewhat
marginal for storm organization. Regardless, with the bow echo
already very well developed/organized, confidence remains high that
a swath of widespread, destructive damaging winds of 70-100+ mph
will sweep eastward across northern IL and vicinity over the next
hour or two. If the current fast eastward motion of the bow (around
60 kt) holds steady, then the widespread damaging wind threat will
increase across the Chicago metro area by 2000-2030Z (3 to 3:30 PM
CDT). In addition to the substantial straight-line wind threat, some
increase in the tornado threat may be developing on the northern
flank of the bow, where mid-level rotation has recently been noted.
Other cells have also formed ahead of the line across far southern
WI and northern IL along a weak warm front, and this convection will
pose a threat for isolated large hail, damaging winds, and perhaps a
tornado or two.
...Please see www.spc.noaa.gov for graphic product...
LAT...LON 41069105 42299097 42949014 42878775 42438773 42058765
41708754 41008757 40938905 41069105