ILX with a rare descriptive mid-morning update.
.UPDATE...
Issued at 1041 AM CDT Fri Jul 9 2021
A pair of observations from the 12z HREF: (1) impactful
convection is trending later into the evening, and (2) picking
a favorite hi-res model is not going to help nail down the
forecast; there is simply too much spread and uncertainty among
them.
To gain a better understanding of the convective potential, it is
best to take a step back into the synoptic space. Current GOES WV
imagery depicts a shortwave trough digging southeastward across
central Iowa, coinciding with a 50+ kt speed max at 500-mb. At the
surface, a warm front is draped from roughly Sioux City to
Nashville. This front and upper-level forcing will provide the
focus for convective initiation this afternoon across portions of
southern Iowa and northern Missouri as the capping inversion
erodes.
Storm relative hodographs suggest any discrete convection that
develops late this afternoon and early evening west of the
Mississippi River Valley will become linear as it approaches
central Illinois and interacts with the LLJ. The current
expectation is for an MCS to initially propagate along the warm
front/MUCAPE gradient, which runs roughly parallel to a Galesburg
to Effingham line. But, as the cold pool becomes more mature late
this evening, the MCS should show a tendency to follow the 0-3 km
shear vectors into portions of central and southern Illinois.
Depending on the strength of the cold pool (will need to assess
theta-e differentials from 00z ILX sounding), there could be some
bowing segments along the leading edge of the MCS. This will lead
to a damaging straight-line wind threat late this evening with
pockets of 70+ MPH gusts possible. A few brief tornadoes will also
be possible within these bowing segments wherever the
3-ingredients method (QLCS mesovortex system) comes into
alignment.
As mentioned previously, timing all of this out is perhaps our
biggest challenge. But for now, we`ll peg the peak severe weather
threat between 9pm - 3am.
Perhaps the bigger and understated threat this evening is hydro.
While the HREF doesn`t offer much continuity with convective mode
and timing, we do believe it offers a helpful solution on QPF;
namely the LPMM, which suggests a widespread 0.5"-1.0" with
localized streaks of 3.0"-5.0" wherever the heaviest convection
occurs.