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Colorado as low-level winds back and increase in response to approach of a jet streak over Western Colorado. Surface-based lifting index between negative 8 and negative 11 from Eastern Wyoming/Western Nebraska southward into extreme Northeastern Colorado. Weather Watch expected to be required within the next hour or so as energy from the approaching shortwave increases UVV (upper vertical velocity) fields over upslope area." By 3 p.m. MDT a tornado watch box had been issued for much of Eastern Wyoming and adjacent South Dakota and the Nebraska panhandle. Charlie and I were not interested in driving any farther north or east than was necessary. We had a nice little thundershower to watch, it was easy to follow, and it had good potential. It was isolated and moving into an environment more favorable for strengthening.
Still at Byers we visited with a team from NCAR that was watching the "storm" for a hail study. Shortly thereafter they were told to leave the storm and to head back west to their base. (I never could quite figure the reasoning there!) Our storm, meanwhile, was developing, and we headed east on U.S. 36. From about 3 p.m. to 7 p.m. the cell intensified and plodded ss..ll..oo..ww..ll..yy eastward, just north of U.S. 36 the entire time. Charlie and I were transformed from storm chasers to storm perusers (not pursuers---perusers!). Just east of Byers we sat in the truck, right underneath the storm's updraft base and amidst long cloud-to-ground lightning bolts, relishing each crash of thunder. (Believe me, that is a real treat in itself for us California boys!) We viewed a little landspout which spun up below the southern edge of the updraft base. We drove south a mile or so for a better overall view of the storm. We drove east a few miles down U.S. 36 through dusty outflow winds. Again we went a little south, then back to U.S. 36 and east again to keep up. The updraft base was quite ragged, and the precipitation area to its north was getting darker. About seven miles west of Last Chance I set the camcorder up on the tripod. The base of the storm was getting lower, and a wall cloud was trying to organize. Air Global Weather Central put the Plains of Eastern Colorado under a tornado watch at 4:37 p.m. MDT.
Finally, near Last Chance in southwest Washington County, the storm developed to a point beyond mere mediocrity. From 5 to 7 p.m. the storm drifted from northwest of Last Chance to northeast of Last Chance. Around 6 p.m. several gustnadoes (or, perhaps more precisely, "funnel-less debris-whirl tornadoes") spun up simultaneously beneath a rather ill-defined wall cloud. At the same time, a "clear slot" just northwest of this area marked the storm's rear-flank downdraft. (Also at this time, a fellow riding his bike (from east to west) across the country stopped by to marvel---good timing, dude!) Heavy precipitation fell north and northeast of the updraft, and greenish/aqua-marinish tinges suggested big hail. This was becoming a textbook storm. It was moving into an ever-moister and unstable airmass. It was isolated with a capital I--free to ingest as much juicy surface air to its east and southeast as it deemed necessary. Several other storm chasers converged around Last Chance to see what this storm could do. The National Weather Service in Denver had southwest Washington County under a tornado warning shortly after 5 p.m. MDT.
(The 00Z (6 p.m. MDT) weather map showed the warm front through the corner of Northeast Colorado: from the CO/WY/NE triple-point to Brush to Burlington. This put it about 20 miles east of Last Chance. Limon, southwest of the front, had south-southeast winds at 20 knots and a temp/dew point of 83/44. Goodland, northeast of the front, had a southeast wind at 10 knots and a temp/dew point of 76/68. Akron, Colorado, about 30 miles NNE of Last Chance and just east of the front, had southeast winds at 20 knots and a temp/dew point of 75/64.)
Near 7 p.m. a very-impressive-looking updraft was rotating north of Lindon, and it continued to issue funnel clouds and brief debris-whirl tornados. The sides of this saucer-shaped updraft were fabulously smooth and curved! At 7:20 p.m. several chase teams watched from Roads S and 15, three miles north of Lindon. We had two additional cells to watch by then, a small one to the northwest and a new one to the southwest. The primary cell, drifting off to our northeast, was still a monster, but it was having a lot of difficulty maintaining its wall cloud. Charlie and I noticed that low clouds overhead were streaming towards the activity to our southwest. Though we did not grasp the significance at the time, the storm we had been following for over five hours was now feeding an offspring to its southwest. We were reluctant to abandon our primary cell, so we left the chase party and drove a few more miles north and east. We stopped to look around in light rain near Roads W and 17. Our primary storm was even more disorganized, and that little bell-shaped base to the northwest was fizzling fast, too. Low clouds continued to race towards the cell back towards Last Chance!
Lightning in this new cell became more and more frequent, and its very low base was issuing some very intriguing and constantly changing shapes. Though it was about 15 to 20 miles away, we had a decent view of this base to our southwest: intervening precipitation was light, backlighting was excellent. Finally, it became obvious that we should forsake the dying for this burgeoning tail-end Charlie cell. The time was 7:45 p.m. We had one hour of daylight left in our storm-chase season. This was our last chance chase.
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