Ramblings of a Graduate Student

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.

Originally I planned to blog tonight about VORTEX 2′s deployment and successful intercept of a supercell and tornado in northeast Colorado. (Ironically, this successful intercept occurred near Last Chance, Colorado!). However, details of how successful the deployment was are still a little sparse. I do know that the tornado was observed by many pieces of the armada, however it will be a few days before we know just how good the data collection actually was. The armada went a long way today – starting in Chadron, Nebraska and going as far south as several miles south of Last Chance, Colorado – so I’m extremely happy for them.

Since I don’t have any good images of the VORTEX 2 deployment, I thought I’d display a meteogram from Beaver, Oklahoma. A meteogram displays a time series of meteorological data for a given station for multiple fields. This meteogram displays temperature (red) and dewpoint (green) in the top panel; sustained wind speed (dark blue), maximum wind gust (light blue), and wind direction (diamonds) in the second panel; surface pressure (brown) in the third panel; accumulated rainful (light green) in the fourth panel; and incoming solar radiation (orange) and approximate maximum possible incoming radiation (grey) in the bottom panel.

This meteogram is special in that it captured a rare atmospheric phenomenon known as a heat burst. In a heat burst the temperature increases rapidly, the air becomes really dry (dewpoint falls), wind speed quickly increases, and often times the pressure changes. All of this occurred at Beaver, OK this evening when an apparent heat burst affected the area. Oklahoma is very lucky to have the Oklahoma Mesonet so that we can record such awesome atmospheric phenomena!

VORTEX 2 operated in standby mode today before eventually calling things off early this evening without targeting a storm. Ironically, after dark, a tornado was reported 20 miles west of where they armada waited all day. Who says Mother Nature doesn’t have a sense of humor!

In any event, as VORTEX 2 heads into the final days, the crew is hanging its hopes on a storm currently moving on shore (circled in yellow) along the west coast. This storm has a very strong jet (circled in blue below) which should help to provide decent shear over the central plains in the coming days. As you can imagine the forecast is for several good chase days coming up for the VORTEX 2 crews. Let’s hope the field campaign of VORTEX 2 goes out with a bang!

I’m sure I’ll have more to say about this in the coming days.

The VORTEX 2 armada once again intercepted a tornado from a storm moving into, through, and then out of Goshen county, Wyoming. For those of you who don’t remember, this is the county where VORTEX 2 intercepted it’s first (and only) tornado of 2009. I don’t have all the details of the deployment because it was very late, and the armada is still traveling, but what I do know was the several members of the armada saw at least a brief touchdown on the east side of Scottsbluff, Nebraska. The Doppler On Wheels (DOW) trucks were able to collect data on the tornado, and actually saw what appears to have been an additional tornado. Granted, no one “saw” this tornado, we’re just inferring it’s presence from the radar data. Hopefully I’ll have more information in the next few days.

As exciting as this intercept was, the image tonight is of a storm just east of the storm(s) the armada targeted. The above radar image was taken shortly before 10 PM CDT (03 UTC) and clearly shows a very strong supercell thunderstorm (complete with “hook”) beginning to wrap up. It’s a shame this storm didn’t develop earlier in the evening (before dark) so that VORTEX 2 could have targeted it. I can only imagine the dataset a storm like this would provide! Oh well. There is always tomorrow…

Last night I blogged about AccuWeather’s seasonal tornado forecast. If you haven’t read it, the gist was that their forecast of increased tornadoes across the Great Lakes region and a decrease over traditional Tornado Alley for April through June appeared to be incorrect. So, naturally, a tornado outbreak would occur over the Great Lakes region today! (As a public service announcement I’m compelled to state that there are several tornadoes ongoing as of my writing. If you live in the Great Lakes region, please sleep with a NOAA Weather Radio on tonight.) As tonight’s title implies, Alanis Morissette must be getting ready to sing here hit Ironic.

VORTEX 2 attempted to blast east from central Nebraska to be able to target this evening’s tornado outbreak. Unfortunately it was just too far of a travel for the armada to get in position. Instead, the armada stopped in central Iowa and targeted thunderstorms developing along the cold front. At least one member of the armada did observe a brief tornado, but it was so brief that VORTEX 2 was unable to collect data on it.

And for the record, I still stand by last night’s blog post. A single event does not make (or break) a seasonal forecast.

Today was day 26 of VORTEX II and the armada operated in the suburbs of Denver in northeast Colorado. Before the armada could even leave the hotel, a very strong thunderstorm developed in the vicinity of Denver International Airport and produced a brief tornado. The armada scrambled southeast toward the storm as quickly as they could and were able to get an extended deployment on this very slow moving thunderstorm. Unfortunately for VORTEX II (yet fortunately for the residents of northeast Colorado) the thunderstorm appears to have been unable to produce a second tornado. In any event, this will once again be an interesting dataset to analyze!

As previously mentioned, the targeted thunderstorm developed very near the Denver International Airport. The image above is a infrared satellite image of the thunderstorm taken very near the time of the tornado. Without going into all the details, an infrared satellite image is essentially displaying the temperature of the cloud top. In the image above the dark reds indicate the coldest temperatures and the grays indicate the warmer temperatures. A colder temperature means the cloud is higher in the atmosphere than a warmer temperature.

Overlaid on the satellite image are surface reports from various official weather stations in northeast Colorado. The observation that is important is the one overlaid on the thunderstorm as it is the Denver International Airport observation. The symbols to the immediate left of the “KDEN” text are the observed “weather” ocuring at the time the observation was made. One of the two weather symbols is what appears to be a funny looking “R”. This is the symbol for thunderstorm. The observation on the far left – the one that looks like “)(” is pretty unique – it is the weather symbol for “funnel cloud”. So in other words, at the time of this report, the weather observer was able to see a funnel cloud with the thunderstorm near the airport. Amazing!

That would make for an interesting blog post in and of itself, but there is something else unique in this image as well. A “thermal couplet” is displayed. In the image below, I’ve removed the observation to give you a clear look at the infrared image. Can you identify anything that might be considered a “thermal couplet”?

In case you were unable to identify the thermal couplet above, I’ve circled it below. In words, the thermal couplet in infrared satellite imagery is identified as an area of colder cloud tops located next to warmer cloud tops. In this particular set of images, the dark reds located to the southwest of the oranges.

So what is going on here, and why is it important? In simplest form, the answer is “warm air rises and cold air sinks”. The longer answer is a bit more complex. To understand, we need to briefly discuss the physics of thunderstorms.

The thermal couplet is the direct result of one of the two key components of a thunderstorm – the updraft. In the updraft warm, moist air from the surface is rapidly transported upward. As the air in the updraft rises, it expands and cools slightly, however, as long as the air in the updraft is warmer than the air immediately surrounding the updraft, the updraft will continue to rise. At some point the air surrounding the updraft will become warmer than the updraft itself causing the air in the updraft to fall back toward the ground. However, so much momentum will have built up in the updraft that it cannot stop immediately. Part of the updraft will continue to rise into the colder surrounding air before falling back into the main portion of the thunderstorm. This is known as an “overshooting top”. As the air falls back toward the main portion of the thunderstorm it warms slightly.

A good way to visualize this process is to put a water hose under the surface of a pool. If you point the hose toward the surface of the pool and then turn it on you’ll notice that bubbling effect of the surface of the pool. Namely, a hill in the water tends to develop on the surface in the direction of where the hose is pointed. This hill develops because the water streaming out of the hose can’t stop when it hits the surface of the pool. It rises out of the pool and then falls back into the water. This is essentially what is happening in an updraft.

So how does this tie into the image satellite image and create a thermal couplet?

The dark colored area circled above is the cold air associated with the overshooting top. When these dark spots begin appearing in the infrared satellite imagery, a meteorologists knows that an extremely strong updraft is present. This is a necessary ingredient for extremely strong thunderstorms.

The lighter colored area circled below is the relatively warmer air caused by the air falling back to the main part of the thunderstorm. It isn’t as concentrated as the updraft because as the air falls back toward the thunderstorm it is blown downstream by the strong winds located in the upper troposphere and lower stratosphere.

A thermal couplet is an indicator that a thunderstorm is intensifying or already extremely intense. The report of a funnel cloud by the weather observer at the Denver International Airport at the same time as this image just acts to confirm what we were able determine from looking only at satellite imagery!