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Photos of a tornado near Elba, Colorado (south of Akron) on 30 May 1996. Our estimates put us at 2 to 3 miles from the tornado and a National Weather Service damage survey rated the tornado as F3.
As usual, a mixture of good and bad current and forecast data provided a mixed
signal for tornado possibilities.
At the surface, weak northerly flow existed
along the immediate Front Range. South of the Palmer Divide, winds were stronger
and southerly while northeast Colorado had southeast surface flow. Moisture
values indicated moderately high dewpoint temperatures in much of northeast CO
and the typically lower values on the Palmer Divide, but no dry-line had formed
in northeast CO as of 1 PM MDT. A relatively weak "Denver Cyclone" was present.
[Winds are southerly or southwesterly on and south of the Palmer Divide
while the winds turn to the southeast in northeast CO and then northeast as
they approach the I–25 region, then north or northwesterly right up against the
Foothills. This creates a counter-clockwise circulation at low-levels — the
same sense of wind direction in a Low pressure system (in the Northern Hemisphere),
hence the name "cyclone." These types of winds are referred to as cyclonic whereas
winds around a High pressure system are anti-cyclonic (clockwise).]
As the day progressed, surface evaporation contributed to increasing the low-level
moisture and the dewpoints reached 60°F by late in the PM in northeast CO.
At upper levels of the atmosphere winds were slightly weaker than optimal with
southwest flow at around 10–15 knots from 5,000 to 10,000 feet above ground.
This was indicated on the wind profilers at
Medicine Bow, WY and
Platteville, CO
in the morning hours. However, the
McCook, NE wind profiler had stronger winds
and we inferred that good flow existed between Platteville and McCook in the
northeast corner of CO. Also at upper levels,
the Denver sounding from 6 AM MDT
showed relatively cool mid-level air with temperatures of −14°C at 13,000 feet
above ground. This temperature profile was reasonably unstable and any storms
that could break through the stable layer near the ground, called the "cap," would
have ample buoyancy to produce strong thunderstorms. The Denver sounding and
wind profilers also showed reasonable amounts of wind shear (turning and quickening
of the wind with height) so the classic ingredients of supercell thunderstorms and
tornadoes, instability and shear, were present.
One additional data source to view before deciding whether supercells/tornadoes may happen is the numerical model predictions of future conditions. Just because all of the morning data indicate favorable conditions for supercells/tornadoes doesn't mean those conditions will be present by afternoon (when all of the surface heating makes thunderstorms happen). A look at the model data indicated that upper-level winds and temperatures would remain quasi-steady. Any dramatic increase in upper-level temperatures or decrease in wind shear would send up a caution flag (meaning conditions were becoming less favorable). Luckily, those conditions were not forecast to occur that afternoon. The model data did reveal that the relatively cool airmass in place over northeast CO for nearly a week would retreat northeastward throughout the day. Any storms that formed were likely to be on this border of relatively cool air and the warm air to the southwest.
Next, we had to decide on the most likely place for storm initiation. All of the
data indicated northeast CO would contain the right parameters for storms to
fire, but we had to narrow that region down to a multi-county region where the
first storms were likely to initiate; and decide which location(s) were likely
to contain supercells/tornadoes as opposed to weaker thunderstorms which might
cover the whole region. A key component to making that decision was satellite
and surface data.
The satellite imagery showed a region of low-level clouds
out towards the CO/NE/KS border but clear skies elsewhere. The cloudy-clear
boundary region is often the initiation point for thunderstorms. The surface data
indicated a weak convergence zone between Byers and Akron, CO. This zone
also corresponded to the cloud-clear boundary. From all the data we had, we
decided to make a triangle from Keenesburg to Limon to Akron, CO our target
zone for chasing. We left the office at 1:30 PM MDT to meet with other chasers
from Colorado State University at 2:00 PM.
A bunch of us met at I-25 and Highway 52 (Dacono Exit) including Jason Nachamkin, Jim Bresch, Ian Wittmeyer, Dave Blanchard, Matt Wolf, Jason Knievel, his sister, and Zach Eitzen. Jason N, Matt, and I went in my car, Jim and Dave went in Dave's car and the others piled into Ian's Ford Explorer. We drove out Hwy 52 to Hudson. At this time, towering cumulus clouds (TCu) had formed in 2 regions: near Fort Morgan and about 20 miles to the southeast of there. The one near Fort Morgan looked better at this time and we drove up I-76 to intercept it. Upon arriving in Brush, the storm directly ahead of us had excellent upward motion (A-bomb mushroom-like) but was quite narrow and its base was relatively high. From our vantage point, the storm to the south (about 20 miles away) was titled but had a wider base and appeared to be improving. Haze obscured our view of it but as we continued to view both of these storms, the southern one had more promise. Not long after deciding to leave the northern storm, all of us commented on how narrow and swiss-cheese-like the northern storm had become. Now, we're on an intercept mission for the southern storm. We decide to drive east on U.S. 34 until we overtake the storm's position, then we'll drop south to be directly ahead of it. Dave was worried about hail overtaking that planned route (and damaging his new Suburu) and decided to turn south first, then drive east and catch up with the storm.
I had to refuel in Akron, CO but after that brief stop we were on our way. We drove through light rain (of very large drops) in Akron that was falling out of the storm's anvil. At this time the supercell appearance began to take shape and a thin veil of dust was being kicked-up and lifted all the way to the base of the cloud. The storm was moving relatively slowly and we turned south on County Road LL to intercept the storm, stay ahead of it, and stay safe. While driving south on LL, we stopped for a photo with a windmill in the foreground. We continued south and the storm continued to intensify. Then, we turned west on County Road 30 and the storm began to show striations and was obviously rotating (the entire updraft portion of the storm). Not long after turning west, the storm produced a small cone shaped condensation funnel that lowered and soon after a swirl of dirt appeared at the ground. Then we came upon the only grove of trees for miles and it obscured our view for 20 seconds after which the tornado had dissipated. We continued west for another couple of minutes and decided to stop and wait for a new tornado (hopefully). We chose to position ourselves about 2 miles from the next tornados initial touchdown point because of 2 reasons: 1) twister/storm should move toward us based on mean upper-level winds, and 2) formation of the tornado looked likely and we wanted to capture its entire life-cycle (not be driving during its formation and have crummy video).
Expecting the storm to move toward our location, I turned the car to face east to allow for a quick get-away if needed. After only a couple of minutes, a new funnel began to form at the cloud's base. It slowly became a cone and lowered little by little. Then, a small condensation funnel appeared at the surface and spun around in some dust and met up with the cone shaped funnel above. This tornado slowly grew in size/width and moved relatively slowly to the northeast (perhaps 5–10 mph). The tornado continued to mature and appeared to move almost due north when it crossed the road we were on (Road 30) about 2 miles to our west. Jim and Dave were on the opposite side of the tornado about 4–5 miles from it and then drove east toward it after it had dissapated. They discovered downed power lines (snapped off about 5 feet above ground) for about 1.2 miles along Road 30 during a brief damage survey. We felt perfectly safe considering our distance from the tornado but of much greater concern was the prolific lightning that was occurring just over a mile to our north. We stayed low to the ground the entire time and actually got back into the car at one point because of fear of getting struck by lightning. [Lightning is much more dangerous to chasers than are tornados, but driving is the activity which I'd say is most dangerous to chasers.]
The tornado lasted nearly 20 minutes and by its end was hard to distinguish from
the 1–2 mile wide dust/debris column that it had kicked up. We didn't document the
entire dissipation stage of the tornado because another column of dust had
begun forming only ¾ mile to our south-southwest. It never formed a tornado,
but it was enough to make us move east (otherwise we were directly in its path).
The radar reflectivity image seen here shows the
classic tornado "hook echo" and the typical velocity
signature associated with a rapidly rotating mesocyclone. However, there are
many times when a NWS doppler radar will show both features are well developed,
however a tornado is not a certainty.
The parent storm that formed the big tornado was generally moving northward and the region of heaviest precipitation (called the "core") was intensifying and dominating our view. We continued to stay with this storm and another tornado or 2 appeared but was difficult to view because of all the precipitation nearby. Meanwhile, during this time, the storm we'd left earlier (now to our northwest) was producing 2 landspouts simultaneously that a local news station video'd from helicopter. These 2 landspouts were typical weak and high-based tornadoes but one did do significant damage to a barn. The much larger tornado we watched was over open farmland and did not damage any structures that we know of. We followed the storm for another couple of hours and eventually ended up on U.S. 34 at the Washington/Yuma County border and decided to break off the chase. We drove to the town of Otis and stopped and picked up the large hail left behind by the core. Ian had a tennis ball with him and a few hailstones identically matched it in size. Hail of that size was only in a swath about 2 miles wide with smaller hail (marble) for many miles on either side. Two helicopters were hovering nearby documenting this hail. We ended with a group photo with hail in our hands.
Book: Tornado Alley. Monster Storms of the Great Plains
Book: The Tornado: Nature's Ultimate Windstorm
Magazine: Weatherwise
Web: Storm Track
Web: TESSA Weather Bulletin
Web: Storm
Chasing with Safety, Courtesy, and Responsibility
Web: SPC Tornado FAQ
