A serial derecho has evolved from a strong MCV on the southeastern edge of a belt of strong mid & upper level wind fields over Iowa.
This derecho will outrun the best wind fields this afternoon, but now it is literally "off to the races" given the heat, humidity & the pocket of enhanced shear & winds with the MCV.
Ahead of it is a very hot, highly-unstable airmass with deep instability throughout the column with particularly high instability concentrated in the low levels as temperatures rise to 94-98 in Illinois with dew points of 77-82. This is dangerous, extreme heat & humidity for this derecho to feed upon.
There is a small branch of a bit higher mid- & upper flow that branches off from this stronger flow in Iowa that noses into Illinois.
There is also a nose of dry air with the hot upper ridge at mid-levels enhancing the wind, as well. This is shown in the temperature drops with the derecho in Iowa with temperatures crashing from 92 to 62.
A gust of 97 mph was just measured northeast of Cedar Rapids, Iowa & gusts have been as high as 106 mph. Tens of thousands of acres of corn are laid flat.
The worst of this derecho looks to pass through northern Illinois to Chicago area to far northwestern Indiana & southeastern Michigan.
Gusts +80 mph are possible.
Farther south, we are looking at the bowing line passing in our viewing area in the 6-10 p.m. time frame northwest to southeast.
For Greater Lafayette, it will tend to be around 7 p.m. to around 9 p.m.
Widespread damaging winds are the main threat +58 mph. Gusts exceeding 75 mph look much more isolated that areas northwest & north of our area where they have been widespread.
Any hail looks small & sub-severe. The risk of an isolated tornado is highest closer to comma head of the derecho, which would be in northern Newton, Jasper, Pulaski counties.
Even though the storms are moving quickly, the rainfall rates will be very high, given such high dew points in the 70s to near 80 here.
Once this passes, a few storms may actually fire in the core of the MCV or meso-low in the 11 p.m. to 2 a.m. time frame with the risk of locally-heavy rainfall.
Some patchy fog may follow after 4 a.m. with lows in the 60s to around 70.
Tuesday highs should run 82-91 north to south with a northeast wind at 5-11 mph. It still looks rather humid, but not as bad as today. Heat indices will run 84-100 north to south.
Front will tend to get hung up in our southern counties, so kept 25% storm coverage in for tomorrow.
Patchy fog with lows in the 60s are likely tomorrow night.