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Storm Chase: April 7, 2008

I woke up this morning to a rather optimistic 12Z NAM forecast sounding for KLAW for 00Z.

You probably notice the nice veering winds with height, so naturally, the hodograph looked like this.

This was the most classic setup for severe weather so far this season - except 1 thing......moisture! Also, the upper-level dynamics were slightly out of phase, with the main trough forecast to swing through towards the evening. At 12Z, the 500 mb SPC objective analysis showed this trough over Nevada, with weaker winds over Oklahoma.

There was no strong low-level jet, because the mid-level height falls associated with the trough was not progged to enter the region until the late afternoon/early evening. However, there were southeasterlies at 850 mb. Unlike our last chase, directional shear would not be a problem today! Speed shear could have been better though.

With the weaker forcing aloft, the setup vaguely reminded me of what happened on March 30th, with storms not being as widespread, but rapidly acquiring rotation once they develop. Because of this, we were optimistic about the discrete mode of the storms that were expected to develop. Near the surface at 925 mb, the atmosphere was trying hard with moisture recovery, which was the only major fly in the ointment today.

KOUN launched a special 18Z sounding, and again, shear looked nice, but surface moisture left much to be desired. It was a race against time for the atmosphere to advect in the moisture before dark.

Looking at the 9:05 am CDT mesonet obs, one would not have guessed that it could have been a chase day - northeasterly winds at the surface with dewpoints in the....30's!! However, there was a good moisture gradient setting up in southwest Oklahoma already, which kept everyone optimistic.

However, by 2:00 pm CDT, the story was a lot different. Moisture return had commenced in earnest, and in fact, there was a tongue of 60's dewpoints creeping into the region. The good news was that winds were veering from the morning's obs, but still backed enough to provide some decent shear!

At 2000 Z, the 850-500 mb crossovers showed nice directional shear.

The 0-1 km SRH at 2000 Z was promising, with a second maxima in north central Texas. However, there appeared to be a local minima over southwest Oklahoma.

Moisture convergence fields at that same time showed a double maxima patter, which would hit at possible convective initiation over SW Oklahoma or north-central Texas.

However, MLCAPE was better to the south.

At 3:05 pm CDT, the SPC issued a PDS tornado watch that covered most of southwest Oklahoma and a small chunk of north-central Texas. Of course, this got everyone excited. Buckey, Craig, and I were at the Lawton gas station at the time.

At 2045 Z, storms initiated just south of the Red River in the area of interest.

All we could see in Lawton were stratocumulus boundary layer rolls. There was no Cu field in Oklahoma yet. However, we decided to stay put, expecting storms to form farther north. At 3:19 pm CDT, the KFDR radar began to pick up reflectivity from our storms to the south.

There was some major OU chaser convergence at the gas station in Lawton. We pretty much took over the entire parking lot. Dr. Howie Bluestein was there as well, along with scores of other grad students and undergrads. We were all being very pessimistic about the existing storms, so we just remained at that location for some time. At 4:15 pm CDT, the mesonet meteogram from the Medicine Park, OK station showed some major moisture return throughout the day, as evidenced from the stratocu rolls overhead.

As we sat around, Craig ate one of those amazing subs that that particular gas station was known for. And of course, just to spite all the meteorologists ther at the time, the southwesternmost storm (out of the three cells that had formed) decided to spin into a monstrous supercell! Dr. Bluestein rounded corner and said, "There's a TVS on it now..." I nodded, then ran back to my car to look at the radar more closely. At 4:34 pm CDT, the hook on the reflectivity display was unmistakeable.

It is usually a bad idea to move signifcantly away from the target area during a chase, but we were faced with the serious question of whether or not to ditch our current plan and head south. At that time, Lawton was still under the stratocu rolls, with no Cu field in sight except for one sad excuse for a towering Cu to the south.

After much deliberation, we decided to head south and west toward Frederick to try and intercept the storm from behind. At 5:23 pm CDT, the supercell had become even more tornadic on radar.



We received rumors from David Bodine on the phone that a multiple-vortex tornado was sighted. We believed this at the time, because the radar signature was certainly impressive. However, we later found out that that report may have been unfounded.

At 2300 Z, the surface winds everywhere were nicely backed, with decent dewpoints in north-central Texas.

We finally caught up with the storm, but it was weakening steadily. Once we got into position to view it, we saw the updraft base, but the entire cloud was tilted. Based on that, we thought that was the death sentence for the storm.

We started to ponder other storms to target. After looking around and scanning the skies, we noticed two cells to our southwest that were developing, but another to our north back in Oklahoma was as well. This was the visible satellite image at 2325 Z.

At 6:37 pm CDT, we could see our storms of interest on radar as well as our dying storm.

The storms to our southwest resembled the initial three-celled pattern of the storms we missed earlier, and the cell in southwest Comanche County visually looked like it was being tilted, so we decided to head south and west to intercept the storms. Our initial target was to go after the cell west of Archer City, TX, but the cell to the northwest had just split, which was a sign that perhaps the right-split would be worthwhile to follow.

Sure enough, as I peered out of my passenger side window, I noticed an organizing updraft base on the right-split, so we went after it!

We got just south of the updraft of the right-split, and as it removed itself from its left-split, that storm, as well as the one farther south, became rather intense supercells.



Looking to the north, we saw this spectacular rotating disk of the mesocyclone drift eastward across the road. It was an amazing sight, especially with the blue sky behind it!

Looking directly above me, I saw several horizontal funnels that were rotating clockwise looking to the north (positive vorticity along a north-south oriented vortex line with vorticity vector pointing to the north). I wondered if perhaps this was visual evidence of streamwise vorticity being ingested into the storm?

Just to give a perspective on the size and proximity of the mesocyclone, this was me standing in front of it with some other chase vehicles in the background.

And at one point, there was a nice rainbow to our east associated with the FFD of the supercell to our south.

Soon, we were forced to move north and east to get out of the path of the southernmost supercell that was beginning to encroach upon us. Plus, the updraft base on our storm was lowering, so we were hoping to catch tornadogenesis. It came very close! There were some funnels emanating from the base, and at one point, I was convinced that the storm was about to drop a tornado. However, the rotation was not as fast as I would have liked. The following photo was taken at the closest point to tornadogenesis, looking to the northeast.

This is a time-lapse of the storm around that time.

This supercell never developed a tornado, which was probably a good thing considering it was heading straight for Wichita Falls, TX. At this time, we decided to head south to follow the second supercell to our south. A tornado was no longer an imminent possibility, and we thought the inflow would be choked off by the southern storm soon. We were right. As we drove south, I turned around to snap this wide-angle shot of our storm at sunset.

Up ahead, to our south-southeast, was the back side of the supercell we were now persuing, sporting an extremely healthy updraft column and backsheared anvil.

We approached the storm from the west, and at 8:29 pm CDT, we sat and watched the rotating monolith from just southeast of Archer City, TX.

The radar signature was not necessarily tornadic, but there was a healthy mesocyclone with a high base. Again, if only the dewpoints were a bit higher!

From this angle, we saw the back side of the supercell nicely lit by the waning sunlight. The entire storm was slowly drifting away from us. Rather than pursuing it, we decided to stay back, relax, and soak in the rare scenery.

From the field, there was no sound but the crickets and the wind rolling through the grass. The supercell put on quite a lightning show, but the thunder never reached us. It was like a natural fireworks display - one that humans can neither create nor destroy.

Looking up, we could see the stars of the night sky. Look back, we could see the moon and the faint glow of twilight. And yet, there was a supercell directly ahead of us, minding its own business, dominating the plains in the solitude of the night.

Although we didn't see any tornadoes today, that was overshadowed by the fact that we still witnessed the atmosphere at its best, with two spectacular supercells. With our atypical approach from the south and west, we were able to observe the storms from unique vantage points with different lighting effects. The only major ingredient missing today was moisture, but it is still early in the season.

On our way back to Norman, we satiated our hunger with a trip to Whataburger. Denny's came to mind for an instant, but we missed the exit!