Weather X-Files: The July 20, 2009 Arvada/Wheat Ridge Storm Revisited (for fun):
On the night of July 20th, a strong cell made it’s way into the Denver Metro area off the mountains bringing with it a ton of lightning, inches of hail, very strong winds, and possibly a tornado. I kind of wanted to revisit this chase for many reasons. Not only was this my biggest surprise chase (yes, it actually beats the 3 tornadoes I saw on June 15) of the year, but also because I feel the NWS did an unsatisfactory follow up investigation that was hardly thorough enough to come to the conclusion that they did. Here are the initial storm reports from the SPC that day:
Red dots in CO
The next day, the NWS in Boulder did not make a trip to the scene to see the damage themselves or to conduct a survey. They said they didn’t detect any rotation on the storm, therefore it could not have been a tornado. Also, I believe they said winds only reached a maximum of 61 mph. Many Colorado chasers were on this storm and I thought it would be cool to post their initial write-up’s in this article:
It think it would be safe to assume that based on all of the above write-up’s and first person experience, a tornado is a very likely possibility. You can see on all of the radar scans that there was for sure a weak couplet embedded in the cell right over Wheat Ridge. Yet the NWS says there was no rotation in the storm? Then why was a tornado warning ever issued? And doesn’t a storm that caused 50,000 people to lose power and $350 million in damages merit at least a visit to the area? Apparently not.
My friend Carl Wells was up camping that night on Kingston Peak (directly W of Blackhawk/Central City) and noticed the cell starting to form. Not even realizing what the cell would do once it got down to the Metro, he still took some shots which he has kindly let me use for this piece. So here is the storm in it’s infancy in the mountains:
Notice the anvil, and the yellow is reflecting city lights...
Huge thanks again to Carl Wells for letting me use these!
Anyway, it’s always fun to recall that night because it was so odd in nature. And it always seems to come up whenever us local chasers get together. In fact, it was talked about at Chasercon last weekend as well. The smell of the downed tree’s and the hail fog are still the things I remember the most. I know the utterly lazy follow up from the NWS Boulder made a lot of us angry but unfortunately we will probably never get a solid answer about what really happened that night. I want to believe! (see song 1 in the music player)
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Scott Hammel: Semi-successful storm chaser from Englewood, Colorado. Chasing since 2006.
Dann Cianca
17 Feb, 2010
I disagree with you that the damage on the ground exhibits that of a tornado. To me, the damage was very obviously straight-line wind damage consistent with what would in this case be the downburst of the rear-flank downdraft. The Weather Service did issue a tornado warning and I believe it was possible that there may have been a small, weak spin-up, but from what I recall of the velocity data, there was only a broad circulation and never a tight couplet. Also, the resultant damage from the downburst would have very likely erased any ground evidence of tornado damage. All the trees blown over in same direction and all the damage to the same sides of the houses is consistent with straight-line damage. I bisected the damage path a couple of times and found no evidence to contradict this.
Scott Hammel
17 Feb, 2010
I agree that all the trees blown over in the same direction does suggest that some of the damage was caused by a downburst but I don’t buy the argument that this would have magically washed away all the evidence of a tornado. This is why I believe it was critical for the NWS to actually come to the scene and perform a thorough damage survey instead of posting a lazy, 2 sentence paragraph about the event.