4/6/2010
This would be our 3rd day in a row chasing. That's pretty unusual for us, especially so early in the season. Kind of nice to have chases so close to home, and also pretty much in the same area.
After cap busting in northwest Missouri the day before, we stayed in Bethany Missouri overnight. We decided central Iowa looked best for this day, as a deepening surface low would move along a warm front into that area by mid-afternoon. Respectable instability, with surface CAPE values over 2,500j/kg would be juxtaposed with very impressive wind shear. Also, a shortwave was due to move through this region later in the day. Dry surface air was pushing in behind a dry line, and when combined with the warm front, surface low, and cold front would make for some nice triple point potential east of the surface low.
We crossed back north into Iowa by late morning, and headed for our target near Des Moines. We settled a little northeast of Des Moines, near the town of Bondurant around 2pm. At that point the surface low was west of Des Moines, with a very stout warm front just north of Des Moines over towards Cedar Rapids. Where we were located the temperatures were in the lower 80s, with mid 60 dewpoints. Just to the north of us, on the other side of the boundary the temperatures quickly dropped to the upper 50s just north of Marshalltown. Dry air was quickly approaching from the southwest, with dewpoints in southwest Iowa only in the mid 40s. Cumulus clouds were building nicely in the warm sunshine, with a fresh south breeze gusting over 25mph at times. It definitely felt like we were in the right location.
By mid-afternoon the cumulus clouds began to grow into towering cumulus. We knew storm initiation was imminent.
The above towers quickly grew into thunderheads as they crossed over the boundary, which was now slowly settling south as a cold front. Soon there were some very nice looking storms to our north, but since they were into the cold air, we knew tornado prospects were about nil with those. At that point there was still no sign of convection in the warm sector.
After sitting in the same spot for about an hour or so, we decided to push east to stay ahead of the dry line. We stayed just to the south of the boundary in the warm air, as we figured that'd be where the best shear for tornadoes would be.
It wasn't much longer when we noticed new storms going up very quickly to our southwest, down towards Pella Iowa. They quickly grew and became severe within a half hour's time. We pushed south through Grinnell, and met up with a cluster of growing thunderstorms pushing north-northeast right at us.
This innocent looking rain-shaft would go on to become the dominant supercell that did significant damage to the city of Grinnell about a half hour after this picture was taken.
In the above picture you can see the dense rain shaft on the left side of the picture (south of the smaller rain shaft). That was a cell just to the south of the developing one, and was already producing large hail. It would weaken shortly after this picture though, as the quickly developing northern cell took over and became dominant.
It was amazing how rapidly this northern cell developed. This picture was taken only ten minutes or so after the above picture. Look how quickly the rain shaft grew, and more importantly, how quickly it developed a lowering on the southwest side of the exploding storm.
The storm continued to intensify quickly. The wall cloud grew and took shape literally right before our eyes. It was quite fascinating watching this all develop from what was once just a small rain shaft. In this picture you can even see a little funnel begin to form.
Here's a black and white version. There's something about black and white photos that give it a sinister/eerie feel.
Soon after the above pictures were taken we were slammed by the dying rain/hail core that came in from the south. This completely blocked our view of the funnel. Unfortunately there wasn't a road option to the north there, so we had to backtrack a few miles back to the east, and then head north in an attempt to catch back up to it. We drove through some hail that approached golf ball size near the town of Lynnville as we attempted to catch back up to the northern storm. We finally cleared the southern cell, but by that time the northern storm was a good 5-10 miles away. Storm speeds were ridiculously quick this day, so catching back up to the storm would be very difficult. As we drove back north through Grinnell, we saw some damage that had just been delivered by the northern cell. We immediately thought a weak tornado had hit, as we saw sheet metal scattered around, along with lots of tree damage. Some roof damage and street flooding was evident as well. The NWS in Des Moines did a damage survey the following day, and found that it was just straight-line winds that hit Grinnell. Apparently the funnel must not have touched town after we lost sight of it as we had feared happened after we saw the damage in Grinnell.
We had briefly attempted to catch the northern cell after we passed back through Grinnell, but it just kept pulling away. Since it was moving northeast, and we had no northeasterly road options, zig-zagging north and east to catch back up to it was literally impossible.
Storms were lining up along the dry line at this point, so we knew the tornado threat was basically over. At that point we called it a day and made our way back down I-80 towards home. |
