July 22 Monticello, Iowa Severe Wind Event

[OceanStWx]

I figured I would break this out of the severe weather thread, to avoid dueling conversations with a couple potentially active days upcoming.

So as I had mentioned in last week's severe/heat threads, I was making a trip back to Iowa for the Great Jones County Fair. Held in Monticello, just on the north side of town, the fair routinely draws many thousands of people for the large music acts that the fair hosts. On Friday evening (July 22 or 00z July 23) Lady Antebellum was the concert, and a sold out crowd was there. So you can see the problem for DVN, considering that number of people exposed outdoors. Interesting side note, it was Lady Antebellum that was playing the night that a foot of rain fell along Highway 20 and the Lake Delhi dam failed two days later as a result of the stress.

The previous night Jason Aldean featured some vivid lightning and torrential downpours at times, so I was clued into the threat we had discussed in the heat thread. A lingering outflow boundary the next day would likely be the focus for at least nocturnal convection. For this reason I was paying close attention to my phone for radar. At 0011z the KDVN 88D showed this image with DMX having just issued a SVR, with reports of 60-70 mph winds in Parkersburg in the SVS.

A nice wind signature of around 55 kt (at 15kft however) is clearly apparent.

Around a half hour later (0041z) there was a weaker wind signature, but now there was a large number of warm air advection storms popping up and another strong cell to the southwest of the original cell. I believe DMX still maintained a SVR for the storm near Waterloo. At this point I was becoming concerned with storm/cold pool interaction. Of our large group at the concert I told anyone who didn't want to get stormed on to head back sooner rather than later.

Another half hour later (0116z) there was a warm air advection wing of convection evident from south of Waterloo east southeastward towards Center Point, while the formerly warned northern cell was developing more a bookend vortex look to it. It was also clear that the two storms that were once separate are definitely interacting now.

About 20 minutes later (0137z) notice how little the northern portion has moved through Buchanan County, while the southern flank is beginning to accelerate east southeastward. In hindsight I think the bookend vortex in central Buchanan helped to foster the development of a rear inflow jet that accelerated the cold pool towards Central City.

Now just 10 minutes later (0149z) the leading edge of the cold pool shows up on the KDVN 88D, in advance of the reflectivity gradient, indicating gusty but sub severe wind. Of more concern at this point would be where the gust front is coincident with the reflectivity gradient. Shortly before this DVN issued a SVR, correctly anticipating the storm evolution that I was concerned with.

Another 10 minutes pass (0158z) and my phone is now locked in a Ziplock bag because elevated showers and started to intermittently pour on us. So I do not see that cold pool merger is about complete, and it is quickly accelerating through Center Point and catching up quickly with the original gust front evident from from Marengo through Cedar Rapids (which recorded a 31 kt peak gust with this feature). So at this point I'm expecting to get soaked and see a good puff of wind but nothing out of the ordinary.

Around a half hour later (0219z) the cold pool has about reached its peak, with 70 mph wind gusts both north and east of the Cedar Rapids metro area. The KDVN 88D now is sampling up to 75 kt winds at 4500 ft. And I have no clue it is on its way, other than Lady Antebellum seemed to be playing an awful lot of "hits" as opposed to mixing in new stuff. Definite red flag for me, since I knew at least a strong storm was to the northwest.

You can see above that the warm air advection wing had developed almost right on top of the fair. At this time I was sitting next to a former coworker and noticing a strange line in the clouds overhead illuminated by the stage lights. We were trying to figure out exactly what it was (I assumed elevated shelf cloud) when we noticed thousands of points of light appear simultaneously. After a split second of we realized it was raindrops being lit up by the stage lights and the downpour started all at once rather than gradually. I had never experienced such a thing before, but it was one of the coolest sights I've ever seen with the combination of lighting and the heavens opening up.

Shortly thereafter, in the midst of their biggest single to date, Lady Antebellum announced "there is some weather moving in so we're going to let you go." Not seconds after they left the stage did the wind gust up (to roughly 30 mph). After a minute or so passed, a low hanging shelf shot over our heads from behind, moving roughly west southwest to east northeast. I announced we should probably get moving. Not sooner did I say that that the wind started to roar a little louder. One of the concert screens facing into the wind toppled over, and the large stacks of concert speakers began to rock back and forth by a few feet. By the time we hit the fairground exit at North Maple Street, the wind was easily hitting 50 mph and I remarked that this had to be a severe storm (still unaware there was a warning). A large majority of fair goers ran for the trees along the left side of the road (walking north) to escape the wind driven rain. I began to yell for everyone to get to the opposite side of the street where cars were parked in case limbs or whole trees began coming down. I was positive we were seeing 60 mph gusts now. And I still had a 1/2 mile walk ahead of me to get to the (relative) safety of the camper. This image was about 5 minutes before (0248z) the 52G59 mph at KMXO at 0255z.

For the next 10 minutes or so I walked through 60 mph gusts and sustained winds above 40 than threatened to knock me off my feet at times. The rain felt like hail being driven into your skin, and lightning nearly continuous. At least 3 to 4 inches of water was freely flowing down the roads too. We managed to get the remaining younger kids into a car, and that left a few of us behind to trek the last 1/4 mile. The most intense part for me (I rather enjoyed the walk in the wind) was the 500 yard dash across a wide open field in continuous CGs with 6 inches of standing water on the ground. I didn't need a whole lot of motivation to reach the opposite side of the field. The next goal was to dash hard right to the bathhouse, which required cutting through a double row of campers. It was dark save for the lightning and I prayed I wasn't going to get clotheslined by a, well, clothesline or tripped by an electrical cord coming from the campers. It was 10:15 when we finally made it to the camper, with winds still gusting up over 50 mph. As an example of the radar sampling issues we can sometimes face, here is the 0305z volume scan sampling little in the way of wind at Monticello yet I was worried about trees coming down in 60 mph gusts.

The issue here being that the 88D is sampling wind towards and away from the radar. So if winds are moving neither towards nor away from (but perpendicular to) the radar the velocities will be underrepresented. As I mentioned the shelf cloud blew through the north side of Monticello in a WSW/ENE fashion, indicating the wind direction was similar. So while KMXO was at 270, we were more like 290/300 which is perpendicular to KDVN's radials at that location. So while the wind appears to be nothing to write home about on the radar, on the ground it was wild.

Looking back at it a few days later, we realized that Monticello was sitting right on the instability gradient (always beware the gradients!). Here as a proxy I used dewpoints, but you can clearly see both the sharp increase to the south of the line, as well as the convergence along it (with east northeast winds at KMXO and southeast at KCID). Back towards KALO the air is already contaminated by these storms, so it likely was oriented more west northwest/east southeast as they formed, and the cold pool accelerated right down that line. That realization likely allowed the warning forecaster at DVN to anticipate storm evolution and get long lead time, that from my phone I could only guess at. Luck also played a part that 70 mph winds were not realized at Monticello but rather to the south and west, because there is frankly no good place for tens of thousands of fair goers to take cover.

In addition another year of training storms on the Highway 20 corridor produced radar estimated totals around 5 to 6 inches overnight, very similar to the year before (almost to the day). As a result the Maquoketa River jumped 5 or so feet in a day and eventually forced us to move the camper out early on Sunday as water crept up to the back door through a back flooding storm drain into an adjacent field.

So I thought this was a nice example of merging cold pools, and how they accelerate and propagate down the instability gradient for a short lived but intense wind event.

[oldlogin]

Very nice. Text book quality with personal insights .. Nice touch, and a good read.....Thanks

[Hoosier]

Good read Ocean. Thanks.