Question about tomorrows (7-12-14) setup in IA, IL

[Andrew Clope]

So tomorrow looked like a potentially decent day for a little weather anyways across IA and northern IL. Really close for me, and on a Saturday, so I figured I may go poke around just in case. But I had a couple questions.

It looks like, based off of the 12Z 4K NAM from today that there's decent CAPE, good low level shear, and good mid level shear. However, what I'm confused about, is looking at the sfc winds, it looks like a low pressure system will be sitting right in the southern/south western portion of IA. And because of the counterclockwise spin of the low, it seems like it is disorienting the sfc winds. So I guess I'm wondering how that low sitting there would influence a chase day? It looks like the best area would be to the NE of the low (depending on boundaries I suppose), but the winds, instead of having backed winds out of the SE, they are coming in from the north around the low.

I would imagine that this would really mess with the helicity and shear, correct? I would also assume that if you were chasing right in front of a low, you'd want to stay to the immediate Eastern side of it, for various reasons, but also because the winds likely would still be coming out of the S, SE at the sfc?

Ultimately I'm just curious how chasing in front of a low like that would effect a chase day?

Thanks!

 

[Andy Wehrle]

Based on the 12Z NAM it looks like the place to be (if indeed there is one) will be somewhere along the WI/IL border or just south. It's actually depicting a fairly respectable setup (at least to my inexperienced eyes) and likely to be the last such opportunity for some time once the seasonally anomalous polar trough sets up afterward. I will probably go if it holds up since it's only 1-2 hours away, even though I haven't slept in the past 36 hours, and have to work 2AM-9AM tomorrow morning and again 3-10 Sunday morning.

The surface winds DO look a little wonky on that particular model on this particular run, with the way they are out of the SSE where you'd expect them to be, east of the low positioned near the Quad Cities area, but then still out of the southwest (as opposed to east or northeast) north of where you'd expect the warm front to be.

However, with the 500MB winds out of the west-northwest there is still reasonable veering with height, at least for this part of the country.

 

[Andrew Clope]

Andy,

I did notice this on the recent run, it almost looked to me as well that just south of the WI/IL boarder looked most promising. I'm still trying to get used to all of the different models/different runs/different model fields, etc... I'm also amazed at how quickly the models can and usually do change. I'm glad that it seems to be pushing its way more eastward seeing as how I'm in central IL. Tired of Iowa/Neb this year!

 

[Skip Talbot]

instead of having backed winds out of the SE, they are coming in from the north around the low.

I would imagine that this would really mess with the helicity and shear, correct?

So when we say the surface winds are backing, we mean that they are turning counterclockwise over time or distance. They are turning from south to east to north during the course of the day or as you approach a frontal boundary. So your north wind could be backing. Your backed winds don't have to be out of the south or southeast. Yes, this greatly influences the helicity, which is a measure of directional wind shear. A north surface wind can greatly increase your directional shear, resulting in large curving hodographs. There is more turning with height when your winds back more, and your hodograph curves more, more directional shear. Directional shear is how the direction of the wind is changing with height. With a south wind you've got about 90 degrees of turning to get to those westerlies aloft. If your winds are backing to the north, your winds have to turn 270 degrees to get to west. That's a lot more directional shear. Your highest helicity values are often north of a warm front where the surface winds are northeast. So a north wind in itself is not a bad thing. It's actually great for your shear values.

you'd want to stay to the immediate Eastern side of it, for various reasons, but also because the winds likely would still be coming out of the S, SE at the sfc?

What are the various reasons we'd want to be the east of the low? We need four ingredients for supercells, shear, lift, moisture, and instability. We just talked about how you can get great directional shear where there are north surface winds north of the low. Those sharp changes in surface winds actually mark where the frontal boundaries are, and where your second ingredient, lift, is coming from. On the warm side of the boundary we find our moisture and instability, most often where we've got a southerly surface wind. So if your winds are backing or veering sharply and you've crossed the cool side of the boundary, you tend to lose your moisture and instability. So we don't target east of the low because there is a south wind, but but because we've got the necessary moisture and instability. Also, we generally don't target north of the low, because we're on the cool side of the boundary where we're missing our CAPE. Our directional shear may be awesome up there, but with inadequate moisture and instability, it doesn't matter. East of the low, is where you often find that warm front, where we've got a favorable mix of everything. Chasers often comment that they've got nice south or southeast winds. Don't focus too much on those specific directions, as they're not necessarily the best. An east is often even better (provided it doesn't mean you're on the stable side of the boundary). Chasers generally prefer southeast winds though, because they're often dealing instead with a southwest wind which means less directional shear that often carries drier air from the desert southwest. This matters less for us in the Midwest this time of year, where a southwest wind can still carry ample moisture since it's not coming right off the desert southwest like it is in TX and OK. When you've got northwest flow aloft, southwest surface winds can still have great directional shear as well, as was the case on July 6 in Iowa.

There are exception to everything of course. Occasionally you can still have enough moisture and instability with a north surface winds. This happens on the northwest side of surface lows in Colorado sometimes, or in the plains when there is really cold air aloft.

As for tomorrow. Those northerly winds represent a cold front coming in from the north. The elongated surface low is not ideal since the winds don't back across a broad area providing a big warm sector with favorable directional shear. Instead, tomorrow you may find a southwest wind in the warm sector that abruptly shifts to north, north of the boundary. Cold fronts have a tendancy to undercut storms, and southwest winds in the warm sector generally mean less directional shear and more linear storm modes. However, I looked at a few hodographs and there's still nice veering with height in the warm sector ahead of that elongated surface low. So storms coming off that elongated low might still have a decent shot at being supercells.

I'm glad that it seems to be pushing its way more eastward seeing as how I'm in central IL. Tired of Iowa/Neb this year!

Woah, be glad it wasn't a Texas panhandle year!

 

[Andrew Clope]

Skip, thanks a ton for that post. It answered pretty much every question that I had in my original post (and then some). At times, it seems overwhelming trying to understand everything, especially early on. I appreciate those who have taken time to help explain things.

Thanks again.

And true, I guess IA and NE have been quite a bit more active this year than TX/OK/KS... And MUCH closer to home!

 

[Andy Wehrle]

Well, funny you mentioned that since it looks like the most favorable target zone has once again shifted westward well into Iowa/perhaps eastern Nebraska. With work at 3AM tomorrow, I will be sitting this one out.

*Edit - At least based on 06Z NAM. GFS still holds out some hope for NW/NC IL. Guess will have to watch and see how things evolve over the next few hours.

 

[kevin-palmer]

Last night I did something I told myself I would never do. I attempted to chase a tornado at night. After shooting this storm at sunset near Rock Island, IL, it started weakening and turned into a mess of rain. So I started driving home. But as I drove, the storm rapidly intensified. It was shooting out very close, frequent, and powerful cloud to ground lightning bolts. At one point it seemed like lightning struck the same place 5 times in the span of a minute. I checked the radar and saw that this storm was showing strong rotation, and a tornado warning was issued soon after. I exited the freeway, and with about 20 minutes before the tornadic circulation arrived, I placed myself 5 miles south of it. At this point the lightning had calmed down somewhat and the storm started to weaken. I didn't notice this until I got home, but I may have captured the wall cloud on the right side of the image. What do you guys think, does this look like a wall cloud?


Wall Cloud? by kevin-palmer, on Flickr

This is what the velocity radar looked like at the time:


Tornado Warning by kevin-palmer, on Flickr


I don't plan to try to chase a tornado at night again. But I felt that for this storm, I was able to attempt to spot the tornado without putting myself in too much danger. I was ready to call it in to the NWS if I spotted it, since it was passing right over the town of Woodhull. Even though the SPC stated a 5% tornado risk, and issued 2 tornado watches covering large areas, this was the only tornado warning of the day. That doesn't seem to happen very often.

 

[Skip Talbot]

but I may have captured the wall cloud on the right side of the image. What do you guys think, does this look like a wall cloud?

Difficult to say with certainty what kind of lowering that is, but it looks like its either a wall cloud or part of the RFD gust front. I usually just call them lowerings when I'm not certain. It may be what I like to call the rear flanking downdraft gust front, which is more of a shelf like feature, but often starts out as or incorporates a wall cloud on the northern end. You can see that bowing shape on your velocity image, which is probably the RFD fanning out from the storm, and it makes this big arcing gust front lowering.

 

[John Bowles]

Out of desperation to see something, I headed out to Iowa on Saturday. Got to Des Moines area just at storms started popping up, but none of them really did much before merging into a weak MCS. At least I got to test out some new equipment.