September 14, 2017 Southwest Colorado Chase

by John Farley

I decided SPC's slight risk for the Four Corners area for this day was too close to home to ignore, so headed west on 160 to see what I could see, leaving Pagosa shortly after 1 p.m. The day's setup combined monsoon moisture with an approaching closed low in southern Nevada, which would be accompanied by a 50-60 knot upper jet that would dig into the Four Corners area. The moderate available moisture, combined with very cold upper air temperatures, decent surface heating, and forcing from the approaching storm system would make for plenty of thunderstorms, and the approaching jet would both help with forcing for storms and with giving them enough shear for some at least transient supercell structures. SPC had much of northern Arizona and southern Utah, southwestern CO, and northwestern NM in a slight risk, with all but the easternmost part of the slight risk area also under a 2% tornado outlook. Also a consideration: although the Four Corners area does not get a lot of tornadoes, the ones that do happen often are in September. So for this area, the season was right.

The first storm I encountered was moving into the eastern part of the Durango area, from the SSW to the NNE. The storm was not severe, but did produce quite a bit of lightning, heavy rain, and probably some small hail. I caught this bolt in the edge of the frame, looking west toward Durango from U.S. 160 around 2:10 p.m.:

After this storm passed on to the NE of Durango, I stopped briefly and checked data. The strongest storms were still well to the west in Utah and Arizona, probably farther than I could get, but a closer line of storms was developing on a line from NNE to SSW crossing through the Mancos area, about 45 miles to my west. Also, a second SPC mesoscale discission of the afternoon for the Four Corners area indicated, as had the first one, that greatest short-term potential for severe weather was farther west than my present location. So, I continued west on U.S. 160 to Mancos, noticing both visually and on radar a strong storm that was located just NW of Mancos, part of the aforementioned broken line of storms through the Mancos area. It displayed a nice shelf cloud, but by the time I got to Mancos it was well to the north and with the rapid movement of storms today (40 mph, sometimes even faster) I figured I could not catch up.

After getting as far as Mancos I noticed a nice storm on radar just west of Farmington, NM and figured I could catch it by doubling back to Hesperus and heading south on CO route 140 as the storm moved to the NNE, tracking just west of 140. The storm seemed to hand off its energy to a new, stronger storm that formed just to its north and appeared to take on supercell characteristics for a short time as it passed between Hesperus and Mancos. These pictures were taken from a couple miles SW of Breen, CO looking northwest between 3:55 and 4:10 p.m.:

As I was losing the storm in the rain after a while, I headed back to Hesperus and west on 160 to re-intercept. But the fast moving storm disappeared into the mountains, and all I could catch was this blurry picture through the windshield of the now well-developed wall cloud:

Just before I got this picture, there was a pronounced bulbous lowering on the north (right, from my viewpoint) end of the wall cloud. But it quickly disappeared behind the mountain; you can barely see the top of it on the right side of this picture. There weren't any severe reports with this storm, though I did see small limbs down east of Mancos. However, the strongest part of the storm may have been in thinly populated mountainous areas. There were several wind reports from other storms, some near hurricane force, well to my west and north from Lake Powell to around the Grand Junction area. Seems like the most intense storms were centered in an area from the northern part of the slight risk northeastward into NW Colorado.

That was pretty much it for the chase, as I was now behind the fast-moving storms, which were in any case becoming more multi-cellular and linear and less intense. I was behind them all the way back to Pagosa, never really catching up to them, though getting in to the back-edge precipitation at times. I did have intermittent rainbows all the way back, and got this nice rainbow from my driveway just after getting home around 6:20:

Total chase distance: 215 miles.

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