2012-10-13 FORECAST: IA/NE/MO/MN/IL

[Bob Hartig]

Everybody's been watching it; it's time we got the discussion started. Friday's setup in the Plains looks poised to intensify into a dynamic system that will provide some action Saturday in the Corn Belt. Consecutive runs of the GFS have corroborated a surface low setting up in central NE/SD and strengthening to around 996 mbs as it moves rapidly northeast to IA/MN. It is collocated with a 500 mb trough/closed low with a jet max of 75 kts overriding southwest 850 mb winds upwards of 55 kts and southerly surface winds of 10-15 kts. Forecast dewpoints in the mid-60s and adequate SBCAPE over 1,000 J/kg should do the job for moisture and instability. The ingredients are all coming together; it's just a matter of waiting to see where.

Two possible chase scenarios will be (1) near the low center, which could offer a cold-core play with more modest storm speeds; and (2) the warm sector where the low-level jet is able to hook up with good instability. It's too early to get more specific than that, other than to state the obvious, namely, that it'll be an early show and storms will be moving fast, upwards of 55 kts.

 

[Skip Talbot]

The GFS has been showing a beautiful cold core for days. I've been waiting until the NAM was in range to really give it a serious look though. These setups have tendency of dissolving from the models like a mirage even 48 hours out. Sure enough, the 12z NAM isn't really showing it, keeping that cut off 500 mb low a bit further to the west with the surface moisture confined to a more conventional warm sector instead of wrapping it around the low.

A few chasers have pointed out the similarities of this setup to November 12, 2005. Here's the SPC storm event data if you want to take a look yourself:
http://www.spc.noaa.gov/exper/archive/event.php?date=20051112
Pretty spooky similarities between that event and what the GFS has been showing the past few runs including the placement, shape, and magnitude of the trough, jet streaks, and surface low with an 80 knot midlevel jet ejecting across Iowa in the evening with a deepening 996 surface low up by the MN/SD/ND corner, and very strong low level jet.

The NAM agrees on the jet, but it's not bombing out the surface low like the GFS is. Still, very impressive shear profiles. Also like 11/12/05, the NAM is firing an arc of convection by late morning that fans out across Iowa. The atmosphere destabilizes behind the line with the cold air advection aloft from that cut off low nudging into the warm sector and new storms fire by early evening. The timing is going to be critical on that one. Just in time CAA and insolation, maybe like March 8, 2010 in Hammon, OK? On the other hand, if the timing is a little off, a lot of chasers are going to be sitting under misting skies in Iowa and then driving home after the "imminent outbreak" had been hyped for a week: October 22, 2004.

 

[Jesse Risley]

I've been watching the ECMWF more than the GFS due to historical consistency being better in the long-range. The 0z/10 ECMWF is more synchronous with the 12z/10 NAM run and the 00z/10 GFS ensemble runs in depicting a weaker surface low (1000 - 1004 mb) than previous runs of the regular GFS. However, the Euro has been consistently advertising surface dewpoints >/= 55-60+ F advecting into the upper Missouri River valley courtesy of the strong LLJ and surface flow in advance of the low, despite a more modest deepening trend than the bullish runs from previous GFS progs.

While not as magnanimous with CAPE forecasts as the NAM, the ECMWF is still advertising 1000 - 1500 J/KG SBCAPE parameters across even far northern parts of the warm sector atop that cold upper low with H5 temps AOB -15C to -20C. Needless to say, this is definitely something to be watched. The models generally do not have a good handle on ongoing convection and true instability trends this far in advance, so that seems to be the one major caveat here, as Skip noted. That convection and cloud cover will be critical to the evolution of Saturday's threat. There's also the classic warm sector further south, though shear profiles further south into parts of S NE/KS/OK and points proximal to there have more unidirectional shear profiles owed to surface winds relative to the forcing mechanisms being more veered with a rather stalwart forcing mechanism. The ample shear profiles and potential instability, even if short-lived, make this a synoptic system that is not to be ignored.

 

[Logan Karsten]

Looking at the SPC analysis from 11/12/2005 and looking at the models for Saturday is giving me serious hesitation for considering going out chasing. While there are similarities between the two days, the 500 hPa trough on 11/12/2005 seems to have had more of a negative tilt moving into IA, leading to much more veered flow with height compared to this upcoming Saturday. The other thing to remember about 11/12/2005 is the instability came together at the last minute to make things happen. Destabilization is key, which has already been noted. In addition, I am not seeing pronounced backed flow in the models. However, that could easily swing either direction between now and Saturday.

I think the potential for a couple of weak tornadoes is there on Saturday, but for me, I just don't see the potential for discrete supercells capable of producing more than a quick spinup. I see things going linear pretty quickly given the more unidirectional shear that will be in place compared to 11/12/2005.

 

[Derek Weston]

Looking at the SPC analysis from 11/12/2005 and looking at the models for Saturday is giving me serious hesitation for considering going out chasing. While there are similarities between the two days, the 500 hPa trough on 11/12/2005 seems to have had more of a negative tilt moving into IA, leading to much more veered flow with height compared to this upcoming Saturday. The other thing to remember about 11/12/2005 is the instability came together at the last minute to make things happen. Destabilization is key, which has already been noted. In addition, I am not seeing pronounced backed flow in the models. However, that could easily swing either direction between now and Saturday.

I think the potential for a couple of weak tornadoes is there on Saturday, but for me, I just don't see the potential for discrete supercells capable of producing more than a quick spinup. I see things going linear pretty quickly given the more unidirectional shear that will be in place compared to 11/12/2005.

Right. Currently too unidirectional for me to get too excited. I've seen quite a few upper-midwest shear-bomb setups fizzle out where tornado potential is concerned... That said... could be an appreciable severe threat, regardless mode, if we do get the destabilization needed. (course just a bit more backed and it gets really interesting ... so waiting and watching)

 

[Robert S. Thompson]

I agree with Derek and Logan. Moisture is another concern of mine.

In looking at the SPC event database, some reasonably close analog events are 10/2/2007 and 10/29/2004. One produced tornadoes, the other mostly wind damage. Of course, busted events are not in the database at all.

 

[Bob Hartig]

Give it time, gents. This system is just on the fringe of the short-range models, and we've got a good number of runs to go. It's a shear-driven system, so it doesn't need a ton of instability. There are indications that serviceable instability will show up, at least if you go by the NAM, and that instability may coincide with decent SRH. I wondered about that at first, and I still wonder, but it's what I'm seeing now, anyway, after sampling a couple of RAOB model soundings for 21Z around central Iowa. From Bufkit NAM data at P#A north of Des Moines, here are a few figures: MLCAPE, 929 J/kg. 3km SRH, 265. 1km SRH, 194. STP, 2.5. That's with 75 kt H5 winds and 45 kt H85s. There's a good 45 degrees of veering from the surface to around 800 mbs--enough to slightly curve a hodograph whose overall linearity suggests the potential for right-movers. And one or two of those is all it would take given so much shear.

Of course, that's just one forecast sounding, and it will change. Moreover, I note that the GFS isn't as ambitious as at first with the surface low. But it's still a strengthening low, and the low-level jet is strong enough to deliver moisture where it's needed.

My point: Again, give this thing time. Maybe the tornado potential will prove to be a no-show. But I surely wouldn't start wringing my hands about that just yet.

 

[Jeff Duda]

Very interesting characteristic change in the forecasts has occurred over the last few model cycles. I first noticed this in the 00Z models last night, but now it's pretty robust in the 12Z models this morning. A secondary vort max develops within the larger upper-level low and swings around to the south of the disturbance, eventually growing into a linear vort max with the main trough staying open instead of staying closed off like had been previously progged. My experience with open waves is that they result in more impressive storms (both warm and cold season) than closed lows. I'm also seeing a tad bit of slowing, especially on Saturday. A few days ago it looked like one would have to be at least in IA/MO/AR to be ahead of the front and within the good shear. Now it seems that an area from SW IA through W MO, E KS, and C OK is now in the greatest threat area Saturday. Forecast hodographs around Norman show some insane shear (granted, there's still less turning than farther to the west on Friday, so storm motions will be quick), but there could be a dangerous situation setting up in OK/KS on Saturday.

 

[Tim Supinie]

My main concern for tornado potential on Saturday is the orientation of the 300 mb wind and 0-6 km shear vector to the dryline. Looks like about 30 degrees difference, which makes it easier for discrete storms to grow upscale. Also, last night when I was looking at the wind profiles, some of them looked kind of wonky, almost favoring the left-mover more than the right-mover. This might be partially because the dryline is so close to the trough axis; the profiles near, say, Fort Smith, AR, looked great, but the dryline was about 200 miles farther west. I'd love a raging squall line Saturday afternoon, though.

That being said, the 00Z ECMWF seems to have had the same change in thinking that the 00Z GFS had (more open wave, jet concentrated farther south than previous runs). The NAM will lag behind the GFS in thinking because it's using the previous GFS for its initialization. Interestingly, the low is not that much better sampled now than it was 24 hours ago. We're still not sampling half the low, including most of the entrance region of that jet. We'll see how the forecast changes up until Saturday.