June 6, 2020 Southwest Colorado Storm Chase

by John Farley


An unusual severe weather setup for the Colorado and Wyoming Rockies occurred on this day, resulting in a rare-for-this-area derecho that began in southeast Utah and southwest Colorado and extended all the way to southwest North Dakota. I had been watching the setup for this day for a couple days, and it became increasingly evident to me that severe-weather parameters were going to be unusually high for this area. A negatively tilted trough, associated with surface low pressure that would move from the Four Corners area rapidly northeastward, would produce an unusual combination of moisture, instability, and wind shear. A couple days ahead, the NWS in Grand Junction, CO was concerned with the potential for flash flooding with this system, due to precipitable water values forecast to be around 1 inch, which is 2-3 standard deviations above the mean. Normally, late May and early June are one of the driest times of year in the Four Corners area, so this would be unusual. But I also noticed a couple days ahead that the combination of instability and wind shear would be pretty high, so I thought severe weather was also a possibility.

By the day before the event, the NAM was showing CAPE in excess of 1000 j/kg, high for this mountainous region. Also the 0-3 km energy-helicity index (EHI), a measure of combined instatility and shear, was forecast to be above 3 around mid-day on the 6th across southwest Colorado from around Cortez eastward through Pagosa Springs - pretty much everywhere in Colorado southwest of the San Juans. This was with some directional shear and strong speed shear, with strong mid and upper level winds. The mid-level winds would also mean rapid storm motion. By the morning of the event, thunderstorms were scattered across southwest Colorado, and a fair amount of rain had fallen overnight. By now the models were even more aggressive, with the RAP showing CAPE of over 2000 in southwest Colorado and both the RAP and the NAM still showing similar EHI levels above 3. And they were also both showing more low-level directional shear than the NAM had the day before. Two questions with the setup were evident. First, would the sun get out for a while behind the overnight and early morning storms, allowing the atmosphere to destabilize as forecast? The Grand Junction NWS said they were not sure, but the models were very consistent in forecasting the instability. As it turns out, it did partially clear, so that there was some heating before the main line of storms moved in behind the earlier one, though I don't know exactly what level of instability was achieved. The second question was storm mode - while the storms would likely start as supercells, they would evolve into a linear structure over time due to the strong forcing. How quickly this would occur was uncertain. As it worked out, the evolution occurred fairly quickly, but I was able to observe one supercell that formed near a break in the line. We'll get to that shortly.

By 9:00 a.m., a few supercells had formed in southeast Utah. They had hail markers of up to 3 inches on radar, and produced both large hail and high wind as they moved rapidly to the NNE. The hail did not turn out to be quite that big, but there were several reports of golfball hail and one report of 75 mph wind with these cells. I watched the evolution of the storms for a while, finally deciding around 11 a.m. that it was time to head out. By then, the easternmost strong storm was near Hesperus, 15 miles or so west of Durango, and moving rapidly almost due north. Scattered storms were forming farther south, but the storms did not seem to intensify until they got near the U.S. 160 corridor. I thought that given the large northward component of the storm movement, that I could head toward Durango and watch the storms cross ahead of me looking west from somewhere near the Archuleta-La Plata County line or maybe as far west as Bayfield. Given the poor chase terrain and rapid movement of the storms (more on both of those later), that would be about the best I could hope for.

Starting out I realized I had less gas than I thought, so I did stop for gas before going into chase mode. Now, you hear a lot about the difficulties of chasing in places like Dixie Alley and Missouri, but I assure you the Rockies can hold their own in that regard with either of those. Basically what I was dealing with was trees and mountains obstructing any distant views except occasionally looking down a valley, storms moving 50-65 miles per hour (a speed that makes chasing a challenge even on flat, treeless terrain), and no data anywhere but in towns. In other words, no data anywhere between Pagosa Springs and Bayfield. But hey, with a setup like this, you gotta give it a go, right?

As I headed west on 160, I couldn't see a lot, but I could see that storms were trying to go up in lots of places ahead of me and off to my left (south). What I could not tell without radar was whether these storms were part of the main line or scattered stuff ahead of it. With the strongest storm near Hesperus when I started out, I kind of figured that the main line, with the northward movement, could not yet be east of Durango, so I kept going. By the time I got to the county line, I was increasingly thinking the convection I was seeing ahead of me and to the left might be part of the main line, but the best way to be sure would be to get to Bayfield where I could get radar, so I kept going. And a few miles before Bayfield, hit intense downpours and gusty wind, but no hail. This continued right into Bayfield, but it was looking more and more like it was clear behind these storms, meaning this was the main line. Radar confirmed this, and after briefly heading to the southeast edge of town to look at a cell I saw just south of town on radar (which turned out to be not much visually), I started to blast back east to try to get back ahead of the line. The heaviest storms were now near Navajo Dam to my south, extending in a line southward into New Mexico. There was now more of an eastward component to the movement of the storms in the southern part of the line, perhaps reflecting the increasing negative tilt of the system. It did still look like the strong storms near Navajo Dam might pass pretty near to Pagosa, so I thought that if I could get back ahead of them, I might still be able to see something.

I did eventually manage to get back ahead of the line, but by then I was back at the edge of Pagosa. I decided to go to Veterans Memoral Park, one of my favorite local spotting locations owing to its broad and relatively treeless view to the west. At that time, there was a heavy cell, probably a continuation of the Navajo Dam storm, coming up over Chimney Rock National Monument, and it had something of a supercell appearance on the radar. It formed in something of a gap in the line of the storms, a little behind the leading edge of the more linear storms to its NNW and SSE. This was likely around 12:40 or 12:45 p.m. Here is a view of that storm when I first saw it:

After getting a few still pictures, I switched to video. It was not very electrified at first, but then the lightning increased, including one burst of lightning over a second or so that included both CG and CC bolts. In the video I got part of the bolts in several different frames, so decided to try putting together a stack of the frames with the lightning. It didn't turn out great, but you get the idea:

By the time I was watching this storm, NWS Grand Junction had issued a significant weather advisory for the entire part of the line of storms that was in Archuleta county and a couple counties northwestward, for 50-55 mph wind gusts, with the storms moving northeast at 65 mph(!). It was clear on radar that the storm I was watching was the strongest one, and the only somewhat isolated one, in this part of the line, and it did have something of a supercell appearance on radar. Then, after a few minutes, it gave me some confirmation of that by forming what appeared to be a wall cloud. I was looking at the feature through some rain, and could not tell if it was rotating, but it did extend nearly halfway to the ground for a time, and also seemed to be under the main updraft of the cell. Here is a picture, captured from my video:

By now, the storm was also becoming more electrified, with an increasing frequency of CG lightning. Images of CG bolts in the cell's core can be seen here and here. The first one is a little out of focus, as it occurred just when I was starting video and the camera was not quite properly in focus yet. The second was a few minutes later; note that the wall cloud feature is visible in both pictures. With the storm racing to the northeast at 65 mph, it would not be long before it was out of my viewing range, so I decided to head home before the line of storms to its south moved over our house. Even though we were in a relatively weak part of the line, we got some heavy rain, a little small hail, and gusts I would estimate in the 40-50 mph range. I am sure the cell I had been watching was much stronger from its appearance on radar, and would guess it produced some larger hail out to the west or northwest of Pagosa Springs, perhaps in relatively uninhabited areas. As the storm neared the Continental Divide, NWS Pueblo issued a SVR warning, and a 61 mph gust was recorded at Wolf Creek on the Divide. It appeared from radar that the storm cycled as it neared the Divide, with the Wolf Creek gust coming from a new core that formed just to the southeast of the original one. Back at our house, we ended up with a much-needed half-inch of rain from this storm and the earlier ones.

As I mentioned earlier, these storms rather quickly evolved into linear structures, despite the presence of some supercell structures in southeast Utah and southwest Colorado. They organized into a derecho, that is a long-lived line of storms producing widespread wind damage. This occurred fairly quickly, and I feel happy to have gotten a little glimpse of some supercell structure. I think the rapid transition to a linear structure, despite the presence of some directional shear, was the result of very strong forcing from the strong upper winds and the rapid northeastward movement of the negatively-tilted system, including both the surface low that passed northwest of my location and the trailing dryline/pacific front/trough axis that extended to its southeast. I am not sure what the proper term for this synoptic feature really would be, but there was a sharp demarcation between the warmer and more humid air ahead of it and the drier, somewhat cooler air behind it. And very strong winds also behind the front/dryline, with numerous reports of 45-50 mph non-thunderstorm gusts in southwest Colorado and even 65 mph in northwest Colorado in the clear after the storms had passed. As to the derecho itself, take a look at this map of storm reports from SPC:

You can see that the severe storm reports extend from southeast Utah and southwest Colorado all the way across Colorado and Wyoming and western Nebraska and South Dakota into southwest North Dakota. According to SPC, this system more than met the criteria for a derecho - something that is very rare in the Rocky Mountain region. In fact, an article in the Denver Post the next day indicated that this is the first derecho ever to cover a large swath of Colorado. Once before one clipped one corner of the state, but never anywhere as much of the state as this one affected. There were more than 440 severe thunderstorm wind reports nationwide, though not all of them were with this system. About one in ten of the wind reports were for wind of 75 mph or more - hurricane force! And all of those wind reports WERE from this system. The strongest wind reports came from the higher elevations of the Rockies, with a 110 mph gust at the top of the Winter Park ski resort. But there were a couple other reports of 90+ mph winds in the Colorado Rockies, too, as well as numerous reports of 75-85 mph winds on the eastern plains of Colorado. There were also several reports of 75-90 mph winds in South Dakota, Nebraska, and Wyoming. Only two tornadoes were reported with this storm system, both in South Dakota. This was likely kept down by the mainly linear and strongly-forced nature of the storms, but the straight-line winds were unusually intense over an unusually large area, and in an area that does not usually get derecho-type storm systems.

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