Why One Cell and not the Other?

A follower asked me during last night’s Facebook storm coverage, “What do you see on the “one to watch” in comp to the top one? I see clouds that are really pretty.” While the satellite view confirmed my suspicion, there was a lot of data that went into that short statement.

It was kind of fun to run storm coverage last night from home rather than being out in the field. I had a number of reasons for staying home yesterday, most not related to the weather. During the coverage I was asked in relation to the satellite photo I posted above, “What do you see on the “one to watch” in comp to the top one? I see clouds that are really pretty.”

There were a lot of factors going into the seven-word statement “this complex is the one to watch.” The single biggest factor is the other note on that image: “tops getting blown off.”

Here’s what the radar looked like about half an hour before that satellite photo:

Precipitation, lowest radar tilt, 6:31pm

Vertically-Integrated Liquid (VIL – a proxy for hail), 6:27pm

At about 6:30, here is what I was analyzing on the radar views:

  • A weak warm front sitting just south of I-70 at Hays (67 degrees) and just north of Russell (68 degrees). They aren’t displayed in the graphic, but there were temperature readings from the Stockton and Osborne areas a few degrees cooler, while Great Bend was still in the warm air at 77-82 degrees, as I recall. In addition the dew point at Hays was 49, while 30 miles east in Russell, it was 55. So my thinking was the front was still slightly south of Hays, hence the drier air, and just north of Russell, hence the slightly moister air.
  • The strong cold front, as I recall, stretched from about WaKeeney to east of Dodge City. Temperatures were dropping quickly at frontal passage, in some places 10-15 degrees over as many minutes. Strong southerly wind was hitting the cold front, in the order of 30-40mph as I recall, and right as the front passed wind immediately switched to the north or northwest and picked up by 5-10mph.
  • The center of the low was right in the Hays/WaKeeney/Ness City area at that time, as nearly as I could tell.

All that combined to set up what’s known as a “triple point” — the place where the warm front, the cold front, and the occluded front meet, somewhere near to east of the deepest low pressure. (see https://www.theweatherprediction.com/habyhints2/530/ for more details)

While there were a lot of good factors in place for storm development yesterday, there were also several negatives — low dew points (65 at Russell would have been a completely different story than 55 was, for example), and the depth of the cooler air north of the warm front (fairly shallow, but enough for storms to ride on top and still have room to grow, leading to what are known as “elevated” storms).

The triple point is really the only place in the system that had much of a chance of letting storms tap into what low-level moisture there was.

The satellite shot was actually the last frame of a two-hour loop.

It’s also zoomed in on Kansas, and some of what I was using in this decision was just out of the frame in Nebraska. But the big thing that stood out to me was that the northern cell (the one near Osborne) appeared to be having the tops of the cloud blown significantly northeast. In this case, Concordia was already in the shade from a storm southwest of Osborne, about 70 miles away. I really put the arrow a little too close to the base of that storm. It kind of looks like there’s a “chute” of some type that extends all the way back into the WaKeeney area, but there’s a subtle difference right at the updraft of the storm just west of the point of the arrow. You can see it because it shadows the clouds between it and the arrow point. That tiny spot is the storm’s core, where the mesocyclone has overshot the cloud tops.

Meanwhile, the southern cell (which had been generating some radar returns in the Russell vicinity for almost 45 minutes by now) stands out from the clouds around it. Here’s a zoom in on that:

You’ll have to kind of imagine this, as I had the advantage of watching it on a loop and you’re seeing a zoomed-in screen shot. But just west of the arrow, see how the clouds have a more bubbly appearance, where the clouds a few miles further west are more ripply? Imagine it’s a pot of water right at the boiling point…the clouds to the west are the water rolling and wavy, while the ones to the east are that little area where the first bubbles break through as the water breaches the boiling point. The dynamics are the same with severe weather.

That bubbling area was more compact, more circular, and appears from the shadows to be a bit higher up in the atmosphere than the core of the storm to its north. The northern storm was more oblong. At the time, its updraft was overshooting a bit more than the southern storm, but again, the tops were being blown significantly downstream. The southern storm’s tops were not being blown as far.

This video (it’s silent) explains how the updraft in a storm forms. It also shows the updraft in its most-efficient storm mode…a slight tilt that puts the rain-producing area of the storm slightly northeast (usually) of the area where the moisture is feeding into the base of the storm.

When the updraft has a specific tilt, the storm can be a very efficient engine for converting the atmospheric energy to rain, hail and tornadoes. Tip it over too far and the storm loses strength because the updraft is getting stretched. Stand it up too much and the rain falls right back into the inflow, cooling it down and robbing it of energy, so the storm collapses on itself and dies.

What is “enough” and what is “too much?” I’m sure there are calculations based on a number of factors, but I’m not a meteorologist. This is where experience comes into the game. I’ve seen enough of these over the years that my intuition “pings” when it appears the updraft is at a decent angle.

As it turned out, the cell I identified did become the storm of the day, generating warnings along its path from where it is in the photo above until well after it exited the state into Nebraska. But just southwest of Concordia the storm rode up over the warm front and lost its connection to the ground. From there it was all a matter of the upper-air dynamics continuing to feed the mesocyclone (the updraft/downdraft couple that makes the core of the storm spin). That enabled it to continue being a hail generator for a couple more hours, but the chance for significant severe weather diminished pretty rapidly after it rode over the warm front.

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