COLORADO MINI-SURPRISE by Greg Stumpf

Every June since I left Colorado in 1989, I've travelled to the Front Range of Colorado to visit friends, do some fishing, and perhaps, if lucky, catch a 'spout or two in the Denver Cyclone area. Last June 22, 1995, I was invited by the National Weather Service Forecast Office (NWSFO) at Pueblo, Colorado to give a talk on High Plains convection. Paul Wolyn (PUB SOO) said it perfectly, "Greg gave the lecture in the morning, and then we 'had lab' in the afternoon!".

After my visit to the NWSFO, I was to drive up I-25 to Fort Collins to visit friends and spend a long weekend. I was curious though, and did check the data at the NWSFO to see if my drive "needed diverting" to the east for some storm action. Well, not to my surprise, the upper air data was not very conducive to supercell storms, or even landspouts for that matter, as the upper level winds were rather weak and the CAPEs only marginal for severe hail. Therefore, I decided to drive straight up. Well, on may way through El Paso County (Colorado Springs), a small storm developed over I-25. As it slowly moved east (about 15 kts), the updraft base began to detach itself from the rain core. So I decided to go after it anyway in hopes to see some hail. Over the next 30 minutes, I watch in amazement this sequence of events.

The updraft base could not have been more than 3 miles in diameter, and had LP characteristics (circular base with smooth striated lower edges from the S-E-N-NW sides). I got on the updraft base near Black Forest in El Paso County (yes, in a forest of conifers), and then I turned south from there and out of the trees. The base was about 4 miles to my east, and then within 5 minutes, a miniature RFD began to slice into the back of the circular base (Fig 1). I began to see an enhanced area of rotation under the north half of this (north of the clear slot), and then a funnel began to condense. In about 2 minutes, it condensed to 40% down when I noticed the debris cloud. All this time, I was driving east toward it. In the first 2-3 minutes, for a brief time (about 15 seconds) condensation connected with the ground, but for the most part and the strongest portion of the tornado, it was about 60% with the classic "up-down" motion apparent on both sides of the condensation funnel (Fig 2). Well, I soon realized that the debris cloud was very close, so I drove to about 1 mile from it, as it blew apart several barns, and aircraft hanger, and the roof of a house (lots of chunky debris flew into the air) (Fig 3). The tornado dissipated rather rapidly, and I shot about 10 pix of it (mainly from my car as I was trying to get close). While outside watching it, I heard no sound. I surveyed the damage and found 6 structures with damage, all F1. Mostly roof damage and lots of corrugated sheet metal (those were the "chunks").

What makes this storm so intriguing was that it developed out what began as a rather benign looking updraft base, and that surface and upper air conditions did NOT support supercells. I did observe a weak north wind at the surface while 1W of the tornado, and after the tornado dissipated, some light rain (big drops) and 1/2" hail fell for about 2 minutes (a small hook). Therefore, I would classify this as a Classic-LP hybrid, and definitely a mini-supercell (but NOT low-topped). The radar images from KPUX also confirmed this, as the core of the supercell was about 1/3 the sizei of a typical Oklahoma supercell.

I followed the updraft for 2 more hours (moving east about 15 kts) before a new storm to its SW seeded it and started its demise. All this time, the updraft would pulse with more RFD's cutting into the back side, but the rotation never spun up as fast as just before the tornado. The updraft had a classic horseshoe shape though, just much smaller than what we're used to in typical supercells (Fig. 4). FYI, this was my first supercell tornado in Colorado. Also, the tornado occurred at 6500' elevation! And all this on a day when not even expecting landspouts, and another tornado during a vacation!


Figure 1. Developing rotating updraft, © Greg Stumpf.

Figure 2. Tornado, © Greg Stumpf.

Figure 3. Funnel with debris cloud, © Greg Stumpf.

Figure 4. Dissipating rotating updraft, © Greg Stumpf.


Figure 5. Falcon, Colorado tornado and rotating updraft, looing ENE, June 22, 1995, © Greg Stumpf.