This was a chase that I was not that excited about, moisture was lacking big time and shear was marginal at best. But this chase proved how meso and even micro-scale environment changes can allow a storm to really increase in intensity. This was also a great example of storm collision creating an even stronger storm. One of the most interesting chases so far for myself.
|Forecasted Target||Ottumwa, IA|
|Intercepted Target||Middle of Nowhere, SE IA|
|Largest Hail||None Encountered|
|Maximum Wind Speed||>50 mph estimated|
This was a chase that I was not all excited about the set-up. I did not think there was enough moisture or instability to make much happen. But, it was a local chase and Jesse talked me into. Most of the attention of the day was tuned to an MCV which moved across Missouri and southern Illinois during the morning hours. There was no chasing that derecho, it was moving about 75 mph with gusts upward of 100 mph.
Back to our chase. We headed north from Macomb pretty late, around 3:30 or so. Crossing over into Iowa through Burlington I decided to measure the dew point with my kestrel. Across the entire area the dew points were in the low 50′s, but within 3 or 4 miles of the river I was recording dew points pushing the lower 60′s. Me and Jesse had a short discussion about a river playing into a micro-scale environment. Little did we know that this very feature would come into play later in the day.
Convection was on-going near Iowa City back to Des Moines, but most of this was struggling to mature. Two, more discrete cells developed just before 5pm. The first near Ottumwa, IA and the second on the Missouri/Iowa border north of Kirksville, MO. We decided that the cell near Ottumwa had the best environment to work with (Around 1000 j/kg CAPE, and 0-3 SRH around 250).
When we first pulled up to the storm near Ottumwa, it had a nice wall cloud clearly pulling in scud but quickly transitioned to an outflow dominate storm. Trying to keep ahead of the storm was not easy, with a weak cold front quickly undercutting the storm and creating storm motion around 60mph. Normally this would be the end of the chase, other than racing to the east trying to keep ahead of the storm trying to get shelf cloud photo’s, but this storm went through a series of events which quickly transitioned it to a tornadic super cell.
The following is the storm relative velocity and reflectivity respectively in chronological order. The write-up to these GR-3 screen grabs is written below.
The cells that developed along to the south of our cell moved to the northeast, but our cell continued to move east-southeast which put them on a collision course just to the west of Burlington. As the two storms merged, our storm almost immediately started to develop a wall cloud and weak rotation. I do not think this alone would have produced the eventually tornado (public reported in Little York, IL). But the merger happened close to the area of 60 degree + dew points, this I believe was the key to the storm creating a very large wall cloud with strong inflow.
While all of these events were occurring, we had a major problem on our hands. Turns out you can’t just jump over the Mississippi river. So while this storm looked like it was going to produce at almost any minute, we had to drop 11 miles due South and try to catch back up with the storm on the other side. A maneuver which would cost us our sunlight, about an hour to catch back up with the storm and most importantly a tornado. Near the town of Little York, there was a reported tornado that was on the ground for a few minutes.
We caught back up with the storm near Galesburg, where it was still showing decent gate to gate velocity but with the loss of the sun it was very difficult to pick out many features and let the storm go shortly before it completely crapped out.
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