Severe weather expected to develop this evening- west and south of the Chicago Metro area
Mesoscale Discussion 1717
NWS Storm Prediction Center Norman OK
0346 PM CDT Mon Aug 12 2019
Areas affected...Central/southeast IA...Northern IL...Far northeast
Concerning...Severe potential...Tornado Watch likely
Valid 122046Z - 122215Z
Probability of Watch Issuance...80 percent
SUMMARY...Thunderstorms capable of primarily damaging winds and a
few tornadoes are possible late this afternoon into the evening.
Tornado watch issuance is likely prior to 23Z.
DISCUSSION...An MCV is currently moving through the upper
Mississippi Valley, with a weakening area of primarily stratiform
rain noted across northeast IA into northern IL. In the wake of the
MCV, clearing has been noted across central into southeast IA, with
cumulus gradually increasing along an outflow/differential heating
boundary from west-central IL into southeast IA. Lapse rates are
generally weak across this region, but continued heating of a very
moist low-level airmass will result in the development of moderate
to locally strong buoyancy (MLCAPE of 1500-3000 J/kg) from southeast
IA into northwest/central IL.
Forcing for ascent will be somewhat nebulous in the wake of the
leading MCV, but scattered thunderstorm development is eventually
expected in the vicinity of the surface boundary and a weak surface
low across central IA. Area VWPs and the 20Z ILX sounding depict
sufficient deep-layer shear for organized convection, and initial
development will include the potential for a few discrete
supercells. Some increase in low-level flow/shear is expected with
time into the evening, and a tornado threat will evolve with any
supercell, particularly in the vicinity of the surface boundary
which may move slowly northward over the next 2-3 hours. A strong
tornado will be possible if any supercell can become sustained in
the zone of enhanced low-level helicity near and just north of the
boundary. Otherwise, locally severe wind gusts and marginally severe
hail will be possible with any discrete convection. Most HREF
members depict some sort of upscale growth this evening into
portions of central IL, which would pose an increasing risk of more
widespread damaging winds and potentially QLCS tornadoes.
Tornado watch issuance by 23Z is likely in order to cover these
threats, though earlier watch issuance is possible if deep
convective initiation appears imminent.