5/13/09 FCST: IA/IL/MO/OK

[Brandon Sullivan]

Early in the period a low around 990mb is forecast to be over the Canadian Provinces of Manitoba and Ontario.. Deep moisture will be advected northward in response to a very strong low level jet..

Depending on which model you would like to believe at this hour (84-96) there are a few different scenarios..

The Down Side (GFS) as per 12z and 18z GFS Data..
The GFS shows a 988mb low over the above mentioned areas. The model has 2000 MLCAPE developing by 12z, but the shear is not nearly as impressive as the NAM. Strong frontogenical forcing, and lack of good shear would suggest linear storm modes along the front in the afternoon and evening across the target area. SFC-H5 crossovers again are less than impressive.

The Wow Factor (NAM) as per 00z Data.
The NAM shows a 992mb surface low over the Canadian Provinces. By 12z the NAM also has 2k CAPE across MO. SFC-H5 Cross Overs are incredible, with a full 90 Degrees of Turning. 55 degree dews already evident over parts of the area. Helicity values of over 900 across some areas ahead of the boundary. Great loops to the hodographs across IL. H85 Jet is really cranking, which is great to see.. WNW flow at H5 provides good directional turning. It would be cool to see some more speed at H5 but those small details to be worked out later of course.. To top this all off, add in really slow forecast storm motions for a tantalizing picture!

As a caveat, as Rich T mentioned in another thread, the gulf will be pretty much cut off until tuesday morning, so it will be important to watch how the return flow commences.

Also, the CAP appears to be okay. I cant really look out past 12z on NAM, but it does indeed show a CAp which is good, as it will hopefully hold things off until later in the afternoon..

Haven't looked into the EC/UK/GEM much..

DVN highlighted this potential well in their afternoon AFD saying the Tornado Potential was "troubling"

Images from Sounding in Western Illinois

 

[Scott Weberpal]

This mornings NAM really spells trouble for E. MO into C. IL and IN. At this point, as with most very dynamic systems, the largest caveat appears to be whether or not cloud cover will clear in time for good insolation in the afternoon. H5 flow is oriented as such that there should be roughly 80-90 degrees of veering between sfc and H5 with unseasonably strong kinematics. If the juice is loose and the clouds go vamoose, there may be a significant tornado outbreak from E. MO through IL into IN.

 

[Derek Weston]

Going to be a much bigger severe day than 5/12

However, mode seems to be linear. Lots of linear forcing here with a strong cold front bearing down. However... extremely strongly sheared environment, even if largely unidirectional. Seems we'll have good enough CAPE with values > 2000 J/kg across a large swath of Missouri into Illinois. Most obvious worry is for a widespread damaging wind event...

However... cannot rule out embedded areas of rotation if we're able to get the right sort of shear profiles. That's the tricky question with this one, to me... whether or not we much of a tornado threat evolve. I'd have to do some more analysis in this area.

 

[Scott Weberpal]

extremely strongly sheared environment, even if largely unidirectional.

According to the 00z NAM, the shear is far from largely unidirectional. Southerly SFC winds (180 degrees), and WSW H5 winds (260-270 degress) means 80-90 degrees of veering with height...not exactly unidirectional.

Regardless, it should be a fairly significant event for a good part of Central Illinois. The NAM 3 days out is likely not going to be a great indicator of storm mode.

 

[Adam Lucio]

THe 850-500 crossovers on the 0Z run showed almost 90 degrees of turning. I think there may be some warm front play where winds may locally back. T/td spreads look good for surface based storms on this one. Im hoping for something isolated infront of said MCS.

If not this looks like one of those days where we could see some mesovorticies within the line to enhance areas of intense damaging winds. The 500 jet max is over 100Kts but centered mostly in northern IL and southern WI.

Certainly setting up to be a widespread event of some sort. Exact storm mode will become clearer but I wont rule out embedded sups or even an isolated one just yet. Some very nice hodos around the champaign area too!

 

[Bob Hartig]

THe 850-500 crossovers on the 0Z run showed almost 90 degrees of turning. I think there may be some warm front play where winds may locally back. T/td spreads look good for surface based storms on this one. Im hoping for something isolated infront of said MCS.

...Certainly setting up to be a widespread event of some sort. Exact storm mode will become clearer but I wont rule out embedded sups or even an isolated one just yet. Some very nice hodos around the champaign area too!

Mesos buried in the line will likely be the order of the day. With good moisture in place, the veering surface winds aren't a deal-breaker for me. They've produced in Illinois often enough, and in this case winds continue to veer on up to H5. I agree, some nice hodographs in central Illinois.

Embedded supes are what we get so often in the Great Lakes. I'm not crazy about them, but I think this setup could have something to offer. And if we could just get a smidgen of clearing before the CF moves in, we might see an isolated cell or two.

 

[Jesse Risley]

I agree that WF play will probably be an option, although surface-based instability is also something that needs to we watched, along with timing of the cold front. It doesn't appear that shear will be the biggest concern this time around, and 0-1 km SRH per the latest NAM is progged to be in excess of 200 m**2/s**2 (300+ progged for 0-3 km SRH).

If the WF clears Central IL and a respectable dry slot is able to develop, it should greatly enhance the severe potential. However, limited instability certainly appears possible if morning clouds do not clear well enough in advance of the approach cold front. The GFS and the NAM are differing in regards to the timing and placement of the best SBCAPE between 18z and 23z on Wednesday.

 

[Jeremy Den Hartog]

The SPC has painted an area of moderate risk of severe weather for eastern Missouri (around St louis) into central Illinois. As im sure many of you know, this doesn't happen to often 3 days out...while this obviously doesn't necessarily mean tornados, it does mean it will probably be an interesting day - how interesting will depend on a few things as stated above...

 

[Scott Weberpal]

On thing to note...with veering flow aloft (260-270 degrees at H5), storm motions should roughly parallel an E-W oriented warm front or outflow boundary which is a big plus for tornado production. With somewhat weaker flow at H5 vs H7 and H85; HP mode is a likely scenario. Get on the storms early and hope the produce early.

 

[John Farley]

Naturally this setup happens on a day when my wife and I have tickets for the Beach Boys concert in St. Louis. And it was my idea, too - not sure what I was thinking making such a commitment during the heart of the chase season, especially when we have commitments with out-of-town relatives the next couple days.

Of course, if something pops close to STL earlier in the day, I can still chase. I would think that for those with greater flexibility than I have that day, the warm front could be a better play. Ahead of the cold front, tornadoes associated with mesovortices in bow echoes are a very good possibility with this kind of setup, and they in fact account for a large minority of tornadoes that occur in the IL/MO region. But they are very difficult to chase - you have to be in the right place at the right time, usually at or to the left (relative to storm motion) of the apex of the bow. And you will, sooner or later, get overrun by the storm. Supercells, probably of the HP type, embedded in the line are also a possibility, but these aren't easy to chase either, especially if they are fast-moving as they may be in a strongly-forced environment. I agree with Scott that the better chance for more chaseable storms could be on the warm front - but that may not be an option for me. And while those may initiate as supercells, they may quickly become HP or morph into a more of a bow-echo type of MCS. So, as has been mentioned, get on those storms quickly once they initiate.

 

[Ryan Wichman]

I have been watching Wednesday for a few days now and I was hoping the picture would be a little bit clearer by now. Any who, here is a bit of thinking:

12zGFS: Shows that an MCS will move through southern IL on Wed morning and show clearing occurring behind this. CAPE on the order of near 3000 at KUIN and +1500 throughout central IL at 18Z indicates the region has cleared out nicely. Boundaries from said MCS will need to be identified for possible points of initiation AOA 19Z. Unidirectional wind flow indicates that a linear line of storms will move through along the cold front. But, local backing of winds out in front of the line will allow any storm which can fire to be in a very favorable environment.

12zNAM: shows a much more chaotic outlook than the GFS, limited heating behind an MCS which takes a more northern track through central Illinois. This means instability will be harder to come by. Wind shear will likely still make up for this lack of instability as far as producing severe storms, but will really limit super cell potential out in front of a linear line.

Either way you slice it, as Scott has already mentioned these storms appear to quickly become engulfed with rain. PW values approaching 1.75-2.00" means these will be super soakers.

Lots of time to sit around and see what the models spit out.

 

[Andrew Pritchard]

I've been eyeballing southwest or south central Illinois for a couple days now, and stand by my target as of this morning's run. I have a final exam Wednesday, but it will be done no later than 1 PM so with prime location here in Champaign I should be fine with time and travel distance needed.

The way things look now, we'll likely see a cluster of thunderstorms across western Illinois early in the day, then tracking across central Illinois around noon (enough for constant thunder to disrupt my concentration). I actually don't mind this scenario. The way it appears in this mornings NAM the MCS would lay out a nice OFB along a line between Interstate 70 and 72. We then see new explosive development along that boundary by 3 PM in southwest Illinois. However, when we're dealing with potential morning convection, 60 hours is a pretty big range to be trying to nail down how much of an effect it will have on our chase prospects. I'm pretty convinced this will not be a major linear event. Models have, and are still showing a good area of convection and UVV's spiking well ahead of the cold front, which is located back towards Iowa and Missouri. The cold front will likely have a solid band of thunderstorms along it later in the evening, but I'm seeing plenty of signs of pre-frontal initiation.

No sense in nailing down a final target yet, but I'm sticking to my favored area as of now. I'd look for healthy supercells near Interstate 55 between STL and SPI tracking across central/south central IL during the evening. Something always goes on Interstate 72 during these bigger days. With an EHI and SigTor bullseye over Champaign at 7 PM, maybe I'll just sit out on my front porch.

 

[Derek Weston]

According to the 00z NAM, the shear is far from largely unidirectional. Southerly SFC winds (180 degrees), and WSW H5 winds (260-270 degress) means 80-90 degrees of veering with height...not exactly unidirectional.

Regardless, it should be a fairly significant event for a good part of Central Illinois. The NAM 3 days out is likely not going to be a great indicator of storm mode.

didn't look at both runs really closely... GFS seemed more unidirectional... agree that there is a bit more turning with NAM... Had just been eyeballing the 500 jet streak and 850s for a while on this, and never thought I saw significant turning.

Looking at the crossovers 850/500 crossovers... I've seen supercells/tornadoes in such a setup... but it's not ideal. (though I've also noted that we've seen some huge outbreaks with shear profiles that were *near* unidirectional -- I just have a hard time picking out the situation in which they come to fruition)

Perhaps the warm front can get something going early enough before what looks to be a major line sweeps through the region.

Looking at the 12z model runs now...

Big difference seems to be timing of MCS and other junk convection. GFS seems to want to have Illinois relatively dry from the 12z-18z time frame... apparently allowing for decent instability to develop. (of course, seems often GFS is more optimistic on CAPE) NAM very much more pessimistic on CAPE, keeping most of Illinois without good instability. Shows quite a bit of precip in Illinois/E Missouri from 18z...

Long story short -- will we be able to clear out and develop respectable instability over the most strongly sheared region in Illinois? I don't know. Could be a big day if things come together just right.

Going to be a pain to chase, likely, with (possible) storm mode and speed... but we'll take what we can get closer to home, eh? (surprise surprise -- upper mississippi river valley crowd is all over this one)

 

[Jared Farrer]

Looking at the 12z models, I think it looks good. Good turning in the lower levels in IL, central to northern IL. Its slight between the 850s and 500s but theirs some. The NAM really breaks out good shear, but with less instability, which its probably underdoing alot anyway. But even with 1500 joules I am pretty confident in some decent storms for once in our area.

 

[Skip Talbot]

Illinois looks like its definitely in play, sandwiched between the LLJ over IN and the upper level jet/trough over IA. I think the specifics of picking a target are going to depend on the location of the morning convection, which will have to wait until the morning of the event. I concur with Andrew's assessment of the situation. Instability might be greater between Jacksonville and St. Louis where the morning junk has had more time to clear out. Also if there is no prefrontal initation in the warm sector, the cold front might be playable where it intersects an outflow boundary or the quasi warm front of the recovering warm sector. This triple point play might work during a brief window before a solid line congeals down the cold front into MO.