06-12-13 - What happened?

[chrisbray]

Hi everyone.

Yesterday was a really discouraging chase for me personally. I reached my target area of Dixon Illinois around 2pm. Based on the models I figured the best option was to continue northwest from there and I reached Lanark, Illinois as the cells were firing up. There were a handful of cells around the Northwest Illinois, Eastern Iowa area and as they developed everything looked good, they looked sheared and started to take on a classic supercell look. I followed the cell that went between Lanark and Freeport, and a it passed I saw nothing other than rain and some lightning. No evidence of organization, no low cloud base, no rotation. Very confusing since the forecast and environment seemed to indicate explosive storm development and supercells.

Shortly after that my storm, as well as the other cells in that area, disappated. The only exception was the storm just west of Dixon, which of course was my INITIAL TARGET. That cell eventually became tornado warned, though I have yet to see any reports of tornados or photos/videos either. From there, that cell moved east/northeast and others fromed with it, eventually turning into a squall line. Meanwhile, nothing happened west of there. You had to go all the way into Iowa to find anything until very late evening when some cells approached the far northwest corner if Illinois. Downstream across Indiana and Ohio, seems like several cells formed ahead of the action, and way upstream, in Central Iowa there were several tornados near the low pressure center.

If you look at the storm reports for yesterday, I essentially chased the huge gap between the Mississippi River and where the hail/wind reports start in north-central Illinois. In both the breif/weak cells I chased, as well as the back end of broken line of storms across central Illinois, I never saw anything remotely like low level circulation, wall clouds, lowerings, or even supercell structure. Just lots of scud, some shelf clouds,and heavy rain with occaisional small hall. Cloud bases, especially in the evening storms, seemed a bit high.

I guess my question is, what lead to this? Was it the timing of the shortwave? It seemed like parameters were at least in place for supercells, but that first wave of cells never even seemed to get to a mature stage. Just a frustrated chase looking to learn, so if anyone had a similar experience or insights into the weather yesterday, feel free to chime in.

 

[GPhillips]

Looks like you were very close to a very nice supercell. The one that moved southeast from Hanover through Mt Carroll had great rotation on radar, a Tornado Debris Signature, and quite a few reports of tornadoes on SpotterNetwork. Of course that may be too close to the MS River for fun chasing, but it was a very nice supercell on radar.

 

[chrisbray]

GPhillips,

That was a cell that moved in from Iowa to Illinois very late in the evening. There is a 3-4 hour gap from when I was in that general vicinity to when that cell moved in to Illinois. Unfortunately, I had already started moving eastward with the first wave of storms because of the huge gap of nothingness behind it and the Iowa storms, and also I needed to be bnack home in far eastern Illinois at a reasonable time, so I could not hang out on the Iowa border forever. Believe me, I was keeping an eye on that as the evening went on, but it was the exception of the day, not the rule

 

[Rob H]

You had unidirectional shear which isn't very conducive to large, persistent mesos because you end up with lots of splitting and collision. The timeframe for tornadoes was also extremely small because of this - pretty much between 21z and 0z. In that timeframe there were a ton of supercells, probably 7-8 of them that looked capable of producing a tornado, and some of them did. The high risk was for wind, not for tornadoes. Still they put down a 15% hatch for tornadoes which I didn't really understand, but I'm just a software engineer and not a meteorologist.

I'm curious why storms did so well at the back of the line near I35 in IA. Shear profiles didn't look that great last time I looked.

 

[chrisbray]

I don't know, observed soudnings from Davenport and Lincoln at 1800z showed curved hodos, not sure where the unidirectional shear would come from. I too am curious what happened in central Iowa though

 

[Skip Talbot]

The atmosphere is lumpy, and there are lots of small details that the models can't resolve or that fall in the gaps between the few points where observations are taken. Perhaps there was a pocket of warm air aloft suppressing updrafts, or the storms were in between upper level impulses so there wasn't much lift. I really didn't look at the hodos closely on this setup, but I did noticed that both the surface winds and 850's were badly veered south of the warm front. You can still get curved hodos of course, and I believe the helicity was higher across central IL because the low level jet was stronger down there, despite it being out of the southwest. The Dubuque storm that was tornado warned for a long time and produced a couple tornadoes on the IL side of the river had a real interesting motion. It was moving southeast, which was deviant from many of the other storms, and enhanced the storm relative shear. It almost appeared to be following the river for awhile, perhaps influenced by the river valley where elevations are lower and the moisture is pooling.

I'm most interested in that Iowa storm that produced (and two at the same time even). It was west of the surface low, and then there were lots of warned cells in eastern Iowa ahead of it that did not produce.

 

[Jeff House]

Hi everyone.
..Just a frustrated chase looking to learn, so if anyone had a similar experience or insights into the weather yesterday, feel free to chime in.

We've all been there and done that. Picking the wrong cell happens. Sometimes it's hard to know. Go after the first one or be patient? Tie breaker usually found by looking for intersecting boundaries, surface winds, and other details. It's much easier if you have data in the field. I was nowcasting from home for a Chicago friend, so here is what I remember. He had targeted Amboy which is basically in agreement with your Dixon.

Surface winds were veered pretty bad in northwest Illinois. Relative to NW IL, central IL winds were less veered and also stronger. Central IL winds were better for creating an inflow environment for storms. Sounds like you already keyed in on the outflow boundary which likely contributed to the tornadoes west of Sandwich. It's not on SPC but they happened with the southern supercell. Believe the Storm Riders documented both brief tornadoes.

We were concerned that the lake breeze front might create enhanced low level shear in the suburbs. Fortunately things started to line out and reduce the tornado threat in town. Boundaries are a double sided coin. Wind shear is enhanced, but the cool side is stable. Thankfully stable won out for the public in the suburbs and Chicago.

 

[Dan Rupnow]

Cloud bases, especially in the evening storms, seemed a bit high.

The evening storms were higher based because air in the lower levels was cooled and stabilized by rain and outflow from earlier storms that had already moved through. This cooled air no longer wants to rise, so the updraft is forced to source it's inflow from more bouyant air located above the rain cooled stable near-surface air, hence higher cloud bases.

 

[Dan Melby]

Like Skip said, I am hoping someone more knowledgeable than me can chime in on how the cell in NC Iowa produced. I ended up in Illinois on the cell NW of Mount Carroll, but am still scratching my head on how that happened and how to predict it next time so I don't have to leave my backyard.

 

[Kevin Walters]

The storms in NC Iowa moved along the cool side of the warm front where surface winds were backed. SRH was high for any storm that stayed on the boundary.

 

[Jeff Duda]

The atmosphere is lumpy, and there are lots of small details that the models can't resolve or that fall in the gaps between the few points where observations are taken. Perhaps there was a pocket of warm air aloft suppressing updrafts, or the storms were in between upper level impulses so there wasn't much lift. I really didn't look at the hodos closely on this setup, but I did noticed that both the surface winds and 850's were badly veered south of the warm front. You can still get curved hodos of course, and I believe the helicity was higher across central IL because the low level jet was stronger down there, despite it being out of the southwest. The Dubuque storm that was tornado warned for a long time and produced a couple tornadoes on the IL side of the river had a real interesting motion. It was moving southeast, which was deviant from many of the other storms, and enhanced the storm relative shear. It almost appeared to be following the river for awhile, perhaps influenced by the river valley where elevations are lower and the moisture is pooling.

I'm most interested in that Iowa storm that produced (and two at the same time even). It was west of the surface low, and then there were lots of warned cells in eastern Iowa ahead of it that did not produce.

I agree. There's still a lot we don't know about influences on storm-scale behavior. We just don't have the observations to make any definitive conclusions one way or the other. The RAP mesoanalyses on the SPC site are error-prone. I was comparing the mesoanalyses to the 18Z DVN and ILX soundings and noting errors in the temperature profile at 700 and 500 mb of on the order of 1 degree or more! There were also errors in the structure of the PBL (the RAP tends to efficiently mix out the PBL, whereas the observed soundings didn't show quite so good of mixing). So unfortunately, even a close look at that will not necessarily tell you the true reason.

I also wondered what was going on in NC IA for that one storm to produce. The nearby VWPs on the 88Ds that surrounded that area (DMX, MPX, ARX, DVN) did not show much of any impressive shear in the low levels, and I also noticed the westerly flow behind the front/low center. Bizarre, really. I did notice that even behind the front in western IA, much CAPE and some shear remained. That's not uncommon - if fronts are weak enough such that there isn't much cooling or drying behind it, sometimes post-frontal storms can end up being very interesting. I can only presume that there was some very local-scale variation in the wind field that was not sampled by the VWPs and was not well resolved by the observations so that it wasn't assimilated in the RAP analysis, so we didn't see it in the output products. It could also have been one of those situations where the storm has a strong enough updraft to alter it's inflow environment just right to produce the tornadoes it did.