Funnel Clouds Spotted in De Kalb and Ogle Counties Friday Afternoon
Several reports of funnel clouds were received from De Kalb and Ogle counties Friday afternoon. While atmospheric conditions were not conduscive to thunderstorm or tornado development, the combination of low level wind convergence and moist air beneath the updraft region of developing cumulus clouds was able to develop very clearly defined funnels. These funnels rarely reach all of the way to the ground, though these appear to come close! The following picture was supplied by Brad Hruza, of De Kalb, and was taken by his daughter Tayler:
Photo by Tayler Sexton
Below is a visible satellite image which depicts the area of clouds developing along the weak boundary from Kane county, west across De Kalb and Lee counties, and another area of cummulus clouds was seen along a boundary from Winnebago into Ogle county. Winds were generally light, less than 10 knots at the surface, but were slightly convergent where the cumulus fields are noted.
Visible satellite imagery at 2215 UTC (515 pm CDT). Note cumulus clouds near De Kalb.
A temperature and wind profile was obtained from an aircraft departing O'Hare International Airport, about 50 miles east of De Kalb. This "sounding" is shown below. Modifying the sounding for the surface temperature and dew point conditions at De Kalb, we see that the thermal profile shows fairly strong instability of over 1850 J/kg, though the wind field is quite weak. There is some vertical wind shear present however, as indicated by the change in direction with height. This can allow an updraft to acqiure weak rotation as it develops within and below a cumulus cloud, which can be further enhanced by stretching as the updraft ascends. This results in the development of the funnel cloud.
AMDAR ascent sounding from ORD. Note positive area left of the pink curve, which represents available instability of over 1000 J/kg
The corresponding reflectivity image from the NWS Chicago WSR-88D radar in Romeoville is shown below. Note that there are only some weak returns from the cumulus clouds, with no strong returns indicating precipitation. In fact, no shower or thundertorm developed there. The funnels were associated only with quick developing cumulus clouds.
Radar reflectivity from KLOT WSR-88D at 2215 (515 pm CDT)