2015-04-09 EVENT: IA/IL/MO/AR/MI/WI

[Andrew Herron]

So you're saying the 34F along the coast isn't going to dramatically affect the storm? Why is that the case?

 

[rdale]

I wouldn't stand on the beach in South Haven expecting a supercell tornado (although even with a wind from the west, temps along the LkMi shore are in the mid-upper 50s today so it isn't hurting too much) but I don't see how those conditions will impact anything happening across the rest of S MI (again assuming model trends with the WF continue - I'm not blaring the "tornadoes coming" horn quite yet )

Here's a paper looking at the impact of LkMi on MCSs (in the spring the storms likely become elevated when crossing the lake so not affected by low-level cold.) https://ams.confex.com/ams/pdfpapers/81343.pdf

 

[Jesse Risley]

Some facets of Thursday have the look of a classic severe weather outbreak for this region, especially if one goes with the NAM, although details are going to change over the next 84 hrs, so I'm not sure how this is going to evolve.

We've got the classic spring ingredients here with a rather stalwart perturbation at 500 mb, and I'm liking what the last few runs of the GFS and NAM are showing, i.e., a strengthening surface cyclone and position of attendant, sharply defined frontal boundaries. Both models are indicating that, for once, there will be ample destabilization of the warm sector south of the surface WF, with seasonably favorable instability parameters. However, the models do not yet know how Wednesday evening's convection will evolve, nor can they ascertain surface feature placement yet, and these are going to be big factors with how Thursday evolves. Timing will be crucial, though taken at present face value the GFS and NAM are in relatively decent agreement with evolution of the surface low, with the usual small differences there that are to be expected 3+ days out. Last nights ECMWF and GEM were somewhat delayed on the timing of upper-level and surface features, though I hadn't yet gotten to see 12z data from today to compare to the American models.

Nevertheless, what really stood out to me on the 12z runs today was the surface and low-level wind field differences. The NAM has much more favorable surface-850 mb wind profiles associated with the strengthening cyclone, where the GFS has an orientation more parallel to the main forcing, and these are painting two very different possible scenarios for the evolution and maintenance of discrete storm modes on Thursday. So the evolution of overnight convection, or lack thereof, and the stature of the low-level wind fields will be something to watch as we get nearer to the event, but now that the NAM is within range it certainly has my attention.

 

[rdale]

It's remarkable how much the Euro has changed for Thursday... Hard to swallow saying "The GFS has been the most reliable" with a straight face

 

[David Draun]

Was looking at CIPS analogs for Thursday based on the 12Z GFS run. I noticed 4/20/04 was the third closest analog. The warm front lit up that day without thunderstorms being in the forecast. CAPE could be as high as 2000J/kg which was much higher than that day of the Utica, IL tornado. The surface low tracking through central/eastern Iowa as it intensifies is almost ideal placement for northern IL tornadoes. Though the surface winds don't seem to back that much. Still with a pre frontal trough, I am quite concerned for the possibility of rush hour supercells in the metro area on Thursday. If I was chasing, I would target Galesburg, IL and adjust east from there.

 

[chrisbray]

Hate to say it, but GFS has been trending worse and worse every run. Hope it's flat out wrong and NAM is more in the ballpark otherwise it may just turn into a typical cold front squall line, all too common around these parts.

 

[Joel Wright]

Hate to say it, but GFS has been trending worse and worse every run. Hope it's flat out wrong and NAM is more in the ballpark otherwise it may just turn into a typical cold front squall line, all too common around these parts.

You can blame me, I put in for Thursday off this morning lol.

NAM is definitely looking the best at this point for Thu, which is a bit scary considering it's the NAM beyond 48hrs. If it is correct though, we should have a pretty decent event over the northern half of IL. Loving the strengthening surface low, and the shear vectors pointing away from the cold front. Deep moisture availability won't be an issue by the time we get to Thu, so if the instability axis doesn't get too muddled up from the Plain's leftovers we should be in decent shape. As is typical for IL, this is looking like a conditional setup dependent on the prior day's evolution of convection. Let's hope for the best.

 

[Bob Hartig]

I'm glad you've got Michigan in the discussion. I agree that there's some potential, and it may be my best hope for a slice of the pie after I meet with my tax man in the afternoon. NAMM soundings for KAZO and KBTL looked surprisingly good, with MLCAPE approaching 3,000 J/kg and sweetly looped hodographs at 23Z. Naturally the 00Z NAM quelled that extremely buoyant (ahem) assessment of instability by the off-hour run, and it favors strongly veered surface winds till after dark. But it doesn't kill things entirely, just tones them down, and we've still got a few days for the models to settle in. Grand Rapids soundings point to elevated instability with the warm front draped not far to the south. With a potent low moving through the area, I've got to think veering won't be as much of a problem as the GFS (or even this latest run of the NAM) suggests. The I-94 corridor could prove to be a sleeper for this system.

Some facets of Thursday have the look of a classic severe weather outbreak for this region, especially if one goes with the NAM, although details are going to change over the next 84 hrs, so I'm not sure how this is going to evolve.

I agree. This has the earmarks of an eastern corn belt/Great Lakes outbreak, at least if you go by the NAM.

 

[chrisbray]

NAM and GFS continue to paint different pictures as of this morning. GFS taking the system farther North and East by late afternoon, with a broader area of low pressure leading to much more veered lower level winds, as well as a longer-capped and less bouyant warm sector. NAM shows much more instability, better wind field, and just seems to be more favorable overall. I am really hoping GFS is wrong at this point, I would like a shot at some discrete cells before that cold front blasts through.

 

[Jesse Risley]

I agree that there are still rather significant surface flow differences between the 00z/07 GFS and the 00z/07 and 12z/07 NAM. All of the major models are showing a more neutral orientation to the main upper-level energy, though they seem to be locking on to a solution whereby strong vort max approaches eastern IA/MO by peak heating, with sufficient destabilization and a deepening low.

I really believe, as mentioned yesterday, that the evolution of any convection Wednesday evening (or lack thereof) will be key to determining the extent of destabilization on Thursday across the warm sector, as well as the placement of residual boundaries and, ultimately, perhaps the northward extend of warm frontal placement.

There are some subtle differences on timing and placement of the surface low and cold front too. Surface flow on the 12z/07 NAM is somewhat anemic proximal to the warm front itself, and, although stronger, oriented more parallel to the forcing further down into SE MO, SW IL and points southward. Surface flow patterns on the NAM had been suggesting the possibility of a pre-frontal wind shift, which could ignite discrete convection in the warm sector before the main line of forcing ignites a linear mode prior to the main frontal passage, and this 'may' be what's being reflected in simulated reflectivity precip signatures on the recent runs. As of right now, if the NAM remains the more correct solution, discrete supercell and tornadic potential looks to be maximized in a narrow corridor from EC IA/NW or WC IL into S WI - I expect to see further changes over the next 48 hrs.

 

[chrisbray]

I agree that there are still rather significant surface flow differences between the 00z/07 GFS and the 00z/07 and 12z/07 NAM. All of the major models are showing a more neutral orientation to the main upper-level energy, though they seem to be locking on to a solution whereby strong vort max approaches eastern IA/MO by peak heating, with sufficient destabilization and a deepening low.

I really believe, as mentioned yesterday, that the evolution of any convection Wednesday evening (or lack thereof) will be key to determining the extent of destabilization on Thursday across the warm sector, as well as the placement of residual boundaries and, ultimately, perhaps the northward extend of warm frontal placement.

There are some subtle differences on timing and placement of the surface low and cold front too. Surface flow on the 12z/07 NAM is somewhat anemic proximal to the warm front itself, and, although stronger, oriented more parallel to the forcing further down into SE MO, SW IL and points southward. Surface flow patterns on the NAM had been suggesting the possibility of a pre-frontal wind shift, which could ignite discrete convection in the warm sector before the main line of forcing ignites a linear mode prior to the main frontal passage, and this 'may' be what's being reflected in simulated reflectivity precip signatures on the recent runs. As of right now, if the NAM remains the more correct solution, discrete supercell and tornadic potential looks to be maximized in a narrow corridor from EC IA/NW or WC IL into S WI - I expect to see further changes over the next 48 hrs.

Thanks for your input, I know last night in chat we were discussing the probability that convection will be restricted to the cold front and pretty quickly forced into a line before making it's way to Illinois. I am not trying to "wishcast" only but I am looking for any signals of prefrontal convection, or even cells along the front keeping tornadic potential as they move across the Mississpi because I don't have a realistic shot of making it into Eastern Iowa by 1pm.

 

[John Wills]

Definitely favoring the timing and placement of the 12ZNAM over the GFS. Chasing the hills of WI is not how I would like to start my chase season, however, I'm not completely impressed with the situation as a whole. There are definitely a few things that are going to have to go right for things to work in our favor for a good chase day Thu. First, we need the overnight convection to clear and lift north earlier rather than later to induce sfc heating and allow for a sufficient CU field to setup in the warm sector. I would personally like the WF to stay further S into Nctrl IL rather than lifting into N IL/S WI.

I do like the amplified LLJ concurrent to the progression of nose of the 500mb jet across the region as well as a large swath of nicely oriented vorticity maxima in association with the UL wave. The way I currently see things I would play the triple point in NW IL/E IA, however, it is too early and much too inconclusive to make a good forecast att imo.

 

[Adam Lucio]

12z runs seem to have sped things up just a tad especially on the GFS. This makes things a little less favorable from a quality chasing standpoint. The surface low would end up in WI by 21z as things are lining out and about to plow through IL. I think any chance for a tornado will be early on (perhaps as early as 18z) near the surface low. I would almost like to see less of negative/neutral tilt in the 500mb flow, that way we could keep the winds more westerly and up the directional shear. Things are pretty unidirectional from 850 up so its going to come down to critical surface backing which we will only have very early on in the setup.

Tonights convection is going to play a huge role in how things play out tomorrow.

A) We don't need that junk sticking around too long killing off the instability until its too late. If this were to occur we would just end with a sloppy linear complex spawned by last minute recovery and cold frontal forcing.

B) WAA storms seem a good bet. I would love to see the advancing boundary get re-enforced but that seems like a pipe dream with an ejecting system such as this. In fact, now that I've typed that out it doesnt even make sense. Still, anyway it can slow down will certainly up the chances for a good tornado. I don't like seeing that WF lifting into C WI by 0z. Right now I am banking near the surface low or east along the warm front early on. Make sure you are in your target areas by 18z ready to chase!

 

[Dan Rupnow]

After looking at the 12z NAM, I'm concerned about the backing of winds in the 250-300mb levels. This, of course, would lead to veer-back profiles that aren't exactly conducive for discrete supercells. Like many others, I have a hard time trusting the NAM much at anything over 48 hrs. 12z GFS isn't advertising this, so hopefully its just a one run fluke. This trough isn't completely onshore yet, so upcoming model runs will undoubtedly change some as more of the system gets sampled.

 

[chrisbray]

Just perused the 18z Nam and Nam4km. Regular NAM seems to be trending faster and a little more GFS like sadly, but the 4km looks pretty good in my opinion. Here is simulated reflexivity

And here is the 1km EHI at that time

Could get some pretty good storms, albeit moving very quickly, through some pretty unstable air for this time of year up here.
Plus, both nams have considerable better supercell profiles than the GFS currently does.