May 21 Colorado Chase: Aurora to Last Chance
Big HP Supercell with Brief Tornadoes and Lots of Hail

by John Farley

The setup:

A back-door cold front had stalled in a NW-SE allignment just east of the Rockies on Colorado's front range. Southeast winds northeast of the front were pumping in moisture, while a jet streak approached from the southwest, providing for good directional shear. The Colorado plains would be over the right exit region of the jet streak, with upper divergence that would likely enhance the potential for strong thunderstorms. There would be decent instability, with CAPE likely somewhere between 1000 and 2000 j/kg.

Already the previous day, a severe supercell had formed just west of Denver, tracking across the city and going all the way to the Kansas state line. Today, the dynamics would be stronger, creating an even higher potential for severe weather. In addition, a Denver cyclone would be setting up, further enhancing a situation favorable for severe weather. This also meant that storms might develop closer to Denver than I had previously anticipated. About the only factors I could see that would limit the day as a chase day were 1) weak winds at the 500 mb level, which concerned some chasers but also would mean slower-moving storms, and 2) most of the directional shear would be fairly high above the ground, with southeasterly surface winds extending up a ways before the wind direction began to veer to the S and SW. This might limit tornado potential, but there would still likely be big, severe supercell storms.

The night before, I had planned to go to Limon to begin my chase, but by the morning of the 21st, the short-term models indicated that storms would develop farther west than that, from around Denver northward to the Wyoming state line. Accordingly, I set an initial target of somewhere north of Bennett, figuring the storms would initiate just to my west and come to me. Starting from Walsenburg where I had overnighted, I headed to Bennett, planing to have lunch there, get data, and refine my target at that time.

The chase:

Before lunch at McDonalds in Bennett, I can already see storms going up over the mountains W of Denver . Perhaps these storms will come off the mountains and become the stoms to be chased.

1:00 - I'm back on the road, heading W from Bennett on U.S. 36. The tail-end storm is looking good visually, and radar shows it is in the foothills southwest of Denver. The first SVR warning of the day is issued at 1:00 for the SW Denver suburbs.

1:35 - A new storm goes up and rapidly intensifies over Denver and Aurora, between the original storm and DIA. I am on Jewel Rd. just W of E470, just east of where subdivisions begin at the east edge of the Aurora area and at the south edge of Buckley AFB. This new storm gets a SVR warning at 1:36.

1:40 - I begin to see a large wall cloud in the darkness under the updraft base. The storm is already taking on the appearance of an HP (high-precipitation) supercell.

1:50 - Sirens are blowing as the wall cloud moves over Aurora, big and low. There is another wall cloud on the next cell southwest also. A video capture of the wall cloud around this time with some lightning can be seen here. I am still in the same location.

1:56 - The storm gets its first TOR warning, based on radar-indicated rotation over Commerce City. I video the wall cloud from Jewell Rd. just east of E470.

2:05 - Wall cloud is approaching or crossing over I-70, looks like just west of the E470 interchange. Tornadoes reported in W Aurora (2:05) and 3 SW of Aurora (2:10). Some doubt has subsequently been placed on the validity of these particular tornado reports by meteorologists and some chasers. The report in W. Aurora could be associated with this meso; the SW one is probably from the second, trailing meso. The trailing one seemed to be producing occasional wall clouds and funnels, but was too far away for me to tell. I photograph the wall cloud from Powhattan Road.

2:10-2:15 - I'm stopped at the north end of the overpass of Powhattan Rd over I-70, parked behind Illinois storm chaser Chris Heater. The meso is to my W near or just N of I-70, and is now becoming quite rain-wrapped. A partially rain-wrapped rotation approaches, just north of I-70 by 2:15. A video capture around this time can be seen here.

2:23 - 2:32 - I video and photograph a rotating wall cloud, partially hidden by rain, looking W or NW from a couple locations near Watkins. The location of the rotation is just south of DIA. Chasers in the inflow notch observe a brief tornado, and a tornado is reported to the NWS 4 miles NE of Aurora - those are probably the same one. I couldn't see it from my location due to rain, but it was within the rain-wrapped circulation I photographed. TOR warning is re-issued due to rotation over Aurora. Here is a picture looking in the direction of the rain-wrapped tornado:

2:35-2:38 - I video a lowering that appeared to have weak rotation just to my north from somewhere near, probably just east of Watkins. No evidence of ground contact. This was ahead of the meso associated with the tornado reports.

2:42-2:43 - I video a large, low, rotating wall cloud looking west from between Watkins and Bennett, but now I am likely closer to Bennett. Some rain is wrapping in and blocking my view. I suspect the possibility of a rain-wrapped tornado back in there. Sirens blow as I get hit by RFD wind for the first time of the day. A tornado is reported near Watkins at 2:45. New TOR warning is issued based on rotation over NE Aurora. Here are two slightly contrast-enhanced video captures of the rotating wall cloud associated with this possible tornado:

I am not completely sure about the validity of the tornado report associated with this, but based on a TV news report of dust being lifted into the wall cloud, along with some rather shaky TV-station video and other chaser reports, I think it is more likely than not that there was a tornado north or northeast of Watkins around 2:45. The time stamps on my video are from 2:42-2:43 and 2:46-2:48. Depending on the duration of the tornado, if the report is valid, I may or may not have had my camera on the circulation at the time of the tornado, but if not, I sure was very close in time and did capture the tornadic circulation. I could not tell at the time and cannot tell from my video whether or not it was a tornado, as there was low contrast and some rain wrapping around the circulation near the ground.

Video of the circulation can be seen here.

2:46-2:48 - Just west of Bennett, I look NW at the rotating wall cloud/possible tornado, the base of which is still partially obscured by rain.

3:00-3:05 - I photograph the meso looking NW from the S. edge of Bennett:

Shortly after taking the above picture, I video very low clouds and hail shafts wrapping around it, appearing to pass just north of Bennett.

Video of the low, fast-moving clouds near Bennett can be seen here. I think that these clouds were either outflow features related to the wet RFD coming in from the left in the video, or ones that formed on the far outer edge of the circulation, as I cannot see any real rotation in them. Very menacing, though, and likely indicative of some strong wind just north of Bennett. Also, copious amounts of golfball hail fell back in the core, north of Bennett.

3:15-3:22 - I am now just outside Strasburg. A new TOR warning issued. It is hard to see much back in the rain now.

3:45-3:55 - I am a few miles north and perhaps a bit east of Byers. The storm is showing signs of a new hook echo on radar. The storm may be cycling, with a new meso forming east of the earlier one. I have a pleasant encounter with veteran storm chaser David Hoadley. We discuss the storm and our chase thus far for a few minutes. He decides to re-position east; I decide to watch here a little longer. Here is a picture showing the colors of the storm around this time, with the front-lit areas taking on a pinkish color while the darker areas took on the blue-green that often occurs with big hail-producing storms:

Through much of the life of this storm, there were parts of it that very distinctly had the aforementioned blue-green color - probably about as pronounced as I have ever seen.

4:00 - Strong cloud-base rotation develops to my northwest, probably north of Strasburg. But I can't stay long as the hook is again wrapping around directly toward me

Video of this rotation can be seen here.

Since there is no east option except U.S. 36 from this point for a while, I return south to 36.

4:08 - I am stuck in a line of chase vehicles trying to turn left into massive chaser traffic on U.S. 36. I nearly get cored by the rain and hail in the hook/wet RFD before I finally get into the traffic and head east, with occasional slowdowns due to the heavy traffic. Here is a picture of the traffic, with the precipitation in the hook rapidly approaching in the background:

4:17 - I stop east of Byers on 36, somewhere near Cabin Creek, and video dust getting lofted into the air by wind that seems to be at juncture of inflow and RFD SE of the meso. Fast moving low clouds, too, but I see only broad circulation around the meso. A video capture showing the dust being lofted into the air can be seen here.

4:27 - I video a rain-wrapped circulation and also some lowerings ahead of it showing signs of rotation closer to me, from a bit farther east. A tornado is reported NE of Byers, and this is a legitimate report as chaser Verne Carlson got it on video; it lasted about 5 minutes. Also reports of trees and powerlines down in that area, but at least some of that may have been from RFD. A couple of the lowerings in my video look a little like what Verne's video capture shows, but I think I may have been too far east and what I saw was more likely something else. I can see no dust under the lowering I videoed here, and I think it is more likely that the tornado was back in the rain as I thought at the time. Also, Verne and other chasers who saw the tornado were very near the wrapping RFD rain, and by now I had gotten more ahead of it.

This video capture was taken around the time of the tornado. The tornado could have been associated with one of the visible lowerings in the center and right of the picture, but I think it is much more likely that it was back in the dark part at the left of the picture, hidden by rain from my viewpoint.

4:40 - I get video of hail shafts and chaser traffic on 36 east of Cabin Creek.

4:55 - I get video of a rain-wrapped lowering and a large chase tour group (Roger Hill?) farther east. A tornado is reported a little SE of leader at 5:05, so again there may have been something back in there.

5:15-5:30 - Now the storms are lining out, as a line of thunderstorms rapidly forms extending east then southeast from the original supercell. I get caught in a lot of small hail from a new cell that formed almost over me along the line SE from the original storm. This was at the Washington Co. line, west of Last Chance. After the hail passes, I stay for a while and video/photograph the hail shafts and lightning to my NE:

5:45 - 6 - North of Last Chance along 71 in the notch between original supercell and lined-out storms to its SE. I watch lightning and a few low but scuddy lowerings for a while. When I finally decided to leave, I nearly got stuck in mud turning around to return south at the end of chase

Some post-chase thoughts:

This HP supercell was quite unusual for Colorado. This one big storm was pretty much all that happened in central and northern Colorado, until the line went up extending southeast. It was a slow-moving storm, generally 10-20 mph for its entire life. Generally HP supercellls are hard to chase because you have to be directly in the path of a tornado, in the inflow notch of the storm, to see it. But a slow-moving one like this allows you to do that more safely, if you have a good road network. In this case, the road network was OK if not great near Denver, but it got worse as you went east. For this reason (and because I initially positioned a bit too far south, for the initial storm before the Aurora one developed), I avoided the notch most of the time. By much east of Byers, U.S. 36 was the only east option, and that, combined with the large number of chasers all on one storm, is why there was such a chaser jam on that road.

Although all of the tornadoes, which generally wrapped up quickly in rain, were short-lived and did no serious damage, the hail the storm produced caused a lot of problems. Although some was at least 2 inches in diameter, most of it was quite a bit smaller, but it was copious, accumulating to 6 inches in some areas. Planes were damaged at DIA, and houses in Green Valley Ranch were severely damaged by wind-blown hail. Both the siding and the windows of houses in this area suffered considerable damage. I think the relatively weak 500 mb winds, cited by some chasers as a negative for the day's setup, were a major reason for the HP mode, and perhaps the fact that the best directional shear was a bit high is why the tornadoes were all relatively weak and short-lived. Still, a VERY impressive storm and an excellent chase day, other than the massive chaser jam on U.S. 36. And it was certainly nice to have a chance encounter with David Hoadley!

Total chase distance, including travel to and from the target area - 930 miles.

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