Ramblings of a Graduate Student

During the evening of 23 July 2010, Vivian, South Dakota experienced a severe thunderstorm no homeowner would enjoy; the thunderstorm packed winds estimated to be in the 110-120 miles per hour range. If that wasn’t problem enough, the thunderstorm was also producing hail that measured over 8″ in diameter and 18.5″ in circumference. (Anecdotal reports indicate that the hail was closer to 11″ in diameter when it fell!) Photographs from the region show holes in roofs. Not damaged roofs, but actual holes greater than 5″ in diameter! At least one hailstone was preserved and it officially weighed in at 1.9 pounds — even after substantial melting! You can read more about this thunderstorm, and see pictures of the hailstone(s) here! (Note, I would have blogged about this sooner, but I was waiting for official word that this hailstone is the world record holder, however that will take a while longer.)

If this hail stone is verified as legitimate, and I have no reason to believe it won’t, it will smash the previous record for largest hail stone. That hail stone measured 7″ in diameter and weighed closer to 1.6 pounds. It fell in Aurora, Nebraska on 22 June 2003. Prior to the Aurora, NE hailstone, the world record hailstone was from Coffeyville, Kansas.

Our current understanding of thunderstorms indicates that in order for a thunderstorm to generate a hailstone of this size, several things must come together. First, we need to have a lot of atmospheric instability. This allows for thunderstorms to develop very strong updrafts, which are necessary to suspend the hailstones aloft. Next, we need the freezing level to be relatively close to the ground. When the freezing level is close to the ground, more of the thunderstorm is below freezing. This means that most of the thunderstorm’s moisture is located in a part of the atmosphere that is conducive for ice formation. Lastly, we need the thunderstorm to rotate. This is because a rotating thunderstorm actually acts to make the updraft stronger than it would be if the updraft was not rotating. This allows for the thunderstorm updraft to suspend hailstones even longer, allowing them to become larger.

The radar images above clearly depicts a very strong thunderstorm, known as a supercell, shortly before the town of Vivian, SD experienced the potentially record hailstone. (Vivian is located in the “hook-like” appendage in the bottom-middle of the thunderstorm.) The two images above are both evaluating a height of 15,000 feet above the radar. At this height we can clearly see in the right image that this supercell thunderstorm had very strong rotation!

If we look at around 27,000 feet, we can see what is known as a Bounded Weak Echo Region (BWER) signature in the reflectivity (above/below left). (It looks like a donut-hole. If you are unfamiliar with this signature, the image below is annotated.) This signature is indicative of an extreme updraft. If we examine the velocity image, we see that the thunderstorm is still rotating, albeit, it is more difficult to discern.

The last set of images are a bit different from what I tend to show. Normal radar images depict a quasi-horizontal cross-section through a thunderstorm. These images are actually a vertical cross-section through the thunderstorm (beneath the vertical cross section is a typical radar image!). I’ve annotated a couple of important parts of the thunderstorm in the image below. This thunderstorm had a very strong forward-flank downdraft (which is where most of the heavy rain falls), a very strong rear-flank downdraft (strong winds, moderate rain, and hail¹ are typically found here), and an extremely strong updraft (the largest hail fails around the updraft). At the time of the cross section, Vivian, SD was located almost directly beneath the strong updraft, which helps explain why the largest hail fell there.

Those of us involved in VORTEX 2 often joked about how the atmosphere would produce out ideal storm the day after the project ended. Well, we were right.

Tonight in South Dakota, a complex of supercells has developed and is nearly stationary. These supercell thunderstorms have produced what appear to be large tornadoes one right after the other. A chaser in the area has reported seeing at least 13 tornadoes today, 3 of them very large, and up-to 5 at the same time. That is just amazing. The towns of Dupree, SD and Green Grass, SD appear to be the two areas that have experienced the worst weather so far this evening, with Dupree, SD experiencing the worst. The town had numerous tornadoes develop in and around the area, prolonged period of 70-80 mph winds, and frequent bursts of very large hail. I’d be very surprised to hear there is a single building in Dupree, SD that has not been damaged.

The image above depicts what appears to be a tornado moving through the southern portion of Green Grass, SD. Tomorrow, I’ll be interested to hear what kind of damage reports come from this part of the country. This is certainly a very rare, very high impact event. The exact kind of data VORTEX 2 was looking for…

VORTEX 2 attempted to operate in the Texas panhandle this afternoon on a thunderstorm that produced a short-lived tornado and 6″ diameter hail. The hail was so large that it smashed through the vehicle’s windshield. Yikes! Unfortunately (or maybe in this case fortunately) for VORTEX 2 the thunderstorm weakened substantially before they were able to deploy. Additionally the storm began to turn into a huge thunderstorm complex resulting in a lot of heavy rain.

Several hours later, the heavy rain threat is continuing to increase as the number of thunderstorms increase across the central United States. These thunderstorms are tapping into an extremely moist airmass across the southern United States, so they will be efficient rain makers! This is part of the heavy rain threat forecast by the Hydrometeorological Prediction Center (HPC)discussed briefly in last night’s post.

I awoke this morning thinking that VORTEX 2 would take a down day or do some light traveling in preparation for tomorrow. I figured that they must be extremely tired from the day-after-day grind. Boy was I mistaken! The principal investigators decided to head west, across the entire state of Nebraska, and target storms coming out of Wyoming. I have to tip my hat to their dedication!

This dedication paid off with two deployments on two separate tornado warned thunderstorms. Although neither storm appears to have produced a tornado, they did drop extremely large hail! I heard at least one report from a probe of numerous vehicles with all the windows smashed out from hail. As was mentioned to me, “hail + high wind + glass = $$$”. How strong was the wind? At least one probe measured a 72 miles per hour wind gust. Just east of this report, the National Weather Service had a report of 80 miles per hour wind gusts. As I mentioned, these storms meant business.

The image above shows what has happened to the thunderstorms since VORTEX 2 ended operations for the evening. The multiple thunderstorms in the area have all grown together (this is known as “growing upscale”) and have become what meteorologists call a “Mesoscale Convective System”, or MCS for short. MCSs tend to last longer than an individual thunderstorm and can travel hundreds of miles fairly quickly, or they can remain nearly stationary. This was is moving at a decent clip to the south and east. It will be interesting to see how far south it makes it before dissipating. Will I wake up to rain in the morning? I’ll see in about 6 hours!

Today was day 18 of VORTEX II’s second year, and boy was it a day to remember. A single supercell thunderstorm developed in far western Texas panhandle and slowly tracked toward Oklahoma. This storm is still ongoing at this late hour.

The image above is when the storm was about to produce one of it’s many tornadoes of the evening. This tornado developed to the north and west of the armada, and was briefly visible by several members before becoming wrapped in rain. The image below captures the rotation a few minutes later. This rotation was probably the best rotation during data collection by the mobile mesonets, but was too far away from most of the good road network to be sampled. Alas, Mother Nature teased the armada once again.

Although there were several tornadoes today, the big story was the hail. Extremely large hail once again fell on many members of the armada resulting in lost windows. Part of the problem today was that there was only one main west-east road and the storm tracked right down this road. This might not have been a problem in and of itself, but a numerous chasers were also present. I am not blaming chasers for being there, they have as much right to be there as VORTEX II. However, the combination of one main road, the VORTEX II armada, and numerous chasers resulted in extremely slow going for vehicles which, in turn, allowed the slow moving storm to catch up to those stuck and unload giant balls of ice.

It will be interesting to see what happens tomorrow. Tomorrow’s target area appears to require quite a drive for VORTEX II to be in place for operations. This means that all the vehicles with damage will need to be repairs ASAP or will have to miss out on operations. We’ll know more in the morning…

Mother Nature must have it in for the residents of central Oklahoma. Mere days after a violent tornado outbreak affected residents of central Oklahoma, a very powerful thunderstorm tore through the Oklahoma City, OK metropolitan area this afternoon. In the wake of this storm area residents are left to deal with damaged roofs and broken windshields.

The storm (shown above) was a classic supercell with a rotating updraft, heavy rain, and a lot of hail. I’ve seen at least one report of softball sized hail (4.75″ diameter) falling across portions of the area. Several people reported picking up hail stones measuring nearly 2″ in diameter almost 2 hours after they fell! The hailfall was so intense that photographs after the fact looked as if they were taken during the Christmas Eve blizzard last winter.

So what did I do during all this? I slept on the couch. The last 20 days of preparing for and then execution of VORTEX II activities left me exhausted. Since today was a VORTEX II travel day, I did not have to worry about V2 operations. Thus, I came home early, saw the hail storm would miss us to the north, and promptly took a nap. Tomorrow starts the Experimental Forecast Program at the National Severe Storms Laboratory and Storm Prediction Center. So in addition to worry about VORTEX II operations, I’ll also be working this program as well.

It’s going to be a long five weeks…

The past two days (Day 109 and Day 110) I’ve discussed my forecast for today on my blog – with Day 109 (Monday) being the day where I really laid out my forecast.  I thought tonight I’d be forthright and see how I did.

The picture above is of a Tornado Watch that was issued this afternoon for portions of western Texas and the southern Texas Panhandle.  Below is a picture of baseball sized hail that fell this afternoon east, northeast of Lubbock, Texas.  The hail photographs were taken by good friend, Kiel Ortega.  When it comes to anything hail related – including finding it falling from the sky – he is “the man”!  You can view Kiel’s Twitter site to see more of his storm chasing expeditions over the next few days.

The relevant portion of the Day 109 blog post is below:

The previous  images (first four) were all valid at 18 UTC (1 PM CDT) on Wednesday.  This last image is valid at 00 UTC Thursday (7 PM CDT Wednesday).  It displays the forecast Convective Available Potential Energy (CAPE) late Wednesday.  This is a measure of how much energy a thunderstorm would have to work with should one develop. Forecast values in the image above in far southwest Oklahoma and northwest Texas are more than sufficient for large updrafts to develop – a key ingredient of severe thunderstorms this time of year.  Couple this large amount of instability with a potential warm front in the area, Wednesday afternoon just might turn out to be “The Day Before The Day Before”.

So it turns out that my 48 hour forecast was slightly too far east of where the thunderstorms actually developed today.  Also, as of writing this, no tornadoes were reported with the thunderstorms.  (Although I didn’t explicitly forecast tornadoes to occur!)  All in all, not too bad for the little time I put into the forecast – and it being 48 hours into the future.  I’m pleased with the forecast, but I (and the science) still have a lot of room for improvement.