3/11/10 FCST: IA/IL

[Skip Talbot]

I'm eyeing central IA into west central IL for possible cold-core like activity in the early afternoon including an isolated risk for large hail and tornadoes.

The 12z NAM is in fairly good agreement with previous runs that an elongated area of low pressure at the surface is going to setup over central IA extending SE into IL. A closed low at 500 mb with temps below -25 C looks like its going to slowly track north near Kansas City. Dewpoint temperatures approaching 55 F are forecasted to advect during the morning and early afternoon from west central IL into central IA. Coupled with very cold (-20 to -25C) temps advecting in from the west at the midlevels, this area should destabilize during peak heating hours in the early afternoon. Surface based CAPE values may only hit 750-1000 J/Kg, however, with the low topped convection and any strong solar heating, the resulting low level lapse rates and instability values should be more than enough to put down a couple tornadoes and some large hail. Closer to the center of the surface low in central IA, dewpoints are lower, but this area may be more of the classic cold core type, as its closer to the upper low where temps aloft are even colder. Speed shear looks fairly light and storm movement looks like its going to be mostly north (maybe even northwest in IA), but this is pretty typical for a cold core setup. Given the lack of cap I expect low topped storms to fire in Missouri in the early afternoon in the vicinity of the surface low and in the wake of some morning precip. Hopefully these storms will continue to organize as they track north into IA and IL with moisture advecting in from the south and cold air aloft from the west. Further north it looks like something like a warm front is setting up with surface winds much more strongly backed and helicity is markedly better. The tornado threat should increase if storms hold together as they approach this boundary.

The setup reminds me of what we saw yesterday in Oklahoma. I'm not saying there is going to be a long-track, significant tornado, but with the closed 500 mb low well to the west, advecting cold air over relatively meager dewpoints in the warm sector, we're looking at very similar setup. If the models maintain these parameters and the morning of it looks like we can a dry slot into the area from MO, I'll probably be out on my first chase on Thursday (or at least an equipment test).

55 Td's advecting in:
http://www.twisterdata.com/data/mod..._221_2010030912_F57_DPTF_2_M_ABOVE_GROUND.png

Cold air aloft advecting in from the west:
http://www.twisterdata.com/data/mod.../09/12/NAM_221_2010030912_F57_TMPC_500_MB.png

Modest instability but ample for low topped convection:
http://www.twisterdata.com/data/mod...09/12/NAM_221_2010030912_F57_CAPE_SURFACE.png

 

[Jesse Risley]

The upward motion in vicinity of this shortwave trough could indeed net favorable parameters for severe weather in Central and Southwest Illinois on Thursday afternoon and evening. However, the GFS and NAM offer different solutions.

The NAM is forecasting CAPE values in excess of 1000 J/KG, with low level winds on the order of 20-25 kts, whereas the GFS is much less prodigious, with maximum surface-based CAPE values being much lower, along with less than ideal forecast soundings, which indicate less favorable turning along with slightly weaker wind fields (10-20 kts). Since this is still early March, one has to question whether or not the NAM is being overly magnanimous about moisture availability and the resulting instability.

Nonetheless, this certainly bears watching. If thunderstorms can organize and move into this region, assuming it remains more favorable (as noted with more of a NAM bias, showing potential clearing entering the region between 15Z and 18Z) in the late afternoon, the potential will exist for some storms to produce lower-end damaging winds, hail and possibly brief tornadoes.

 

[Adam Lucio]

Todays 12z NAM IMO is a bit less favorable than the pervious 3 runs, however it still shows some potential. The GFS is not coming in line at all on some things. It is beginning to line up the lows and all the other parameters but doesn't want to bring in nearly as much warm and moist air. Convective feedback perhaps? Or is the NAM being too generous.

The latest NAM has mis-aligned things like the best bulk shear, pushing that out into Indiana while maintaining the cape in W IL. Speed shear values appear to be at their minimum over the areas of best instability. However directional shear looks good, and there are some nice curved hodographs. with bulk shear just under 30kts this may be enough to get the job done if we can maximize instability.

I also haven't considered the potential cold core setup yet with the center of the low being over the IA/NE border, h5 temps to -20 still wrap around over IL above surface temps up to and over 60 and this is probably where the NAM is getting its high cape values from.

If the GFS could come into line, and the NAM can shift the maximum speed shear back to the areas of greatest instability I think C IL is in line for severe storms. Eagerly awaiting the next series of runs.

 

[Derek Weston]

noticed this a few days back... was surprised to see that much cape working its way north into Iowa. NAM has been quite a bit more optimistic than the GFS. Shear isn't too strong, but like Adam mentioned, nice looking hodographs, at least in eastern Iowa when I last checked. We'll see... could materialize, basically defaulted into chasing given the location.

 

[Bob Hartig]

The 18Z NAM looked less optimistic, and the 00Z kind of scuttles it--shifts everything east, and weakens CAPE and drops it south. The MIC from KIND mentioned that NAM has been too aggressive, and the SPC has been conservative about its SWODY3, so maybe this latest incarnation of NAM is a more realistic one. I still have hopes, but not high expectations. Cold core by southeast Iowa may be the most viable option. Based on the latest, I may just hang out by the Michigan border where the winds are backed and see what happens.

 

[Skip Talbot]

It is a little disheartening to see the dewpoints and instability drop off on this run. It does, however, look much more like a classic cold core setup now. The surface low and 500 mb closed low are nearly stacked. With low 50's Td's advecting in around the surface low and -25 C aloft at 500 mb, conditions are still favorable for low topped convection with a tornado threat given the steep lapse rates, and provided we get some strong solar heating. Despite what seams like a negative trend, the movement of the 500 mb low further east is a big positive, and I remain optimistic for chase ops on Thursday.

50's Td's advecting into the surface low in central IA:
http://www.twisterdata.com/data/mod..._221_2010031000_F42_DPTF_2_M_ABOVE_GROUND.png

-25 C temps aloft at 500 mb almost immediately over the surface low:
http://www.twisterdata.com/data/mod.../10/00/NAM_221_2010031000_F42_TMPC_500_MB.png

 

[Bob Hartig]

After looking at this morning's 6Z, even the cold core doesn't look all that compelling. The 500 mb low/-25 C temps are now perched near the KC area, west of the surface low and moisture lobe. Maybe that will change, but at this point I think that my plans for Illinois are off. I may head down toward New Buffalo just to see some lightning, with at least a very slim chance of a brief spin-up.

 

[Andrew Pritchard]

Per the 12z NAM run, I'm intrigued by northwest Illinois. Anywhere from Galesburg to Peoria to Springfield looks interesting, but I may try and hug the northern portion of that arc. It's not a textbook cold core setup, especially further south into Illinois, but we may be able to benefit from some of the same ingredients. Nearby cold H5 upper low is really helping with lapse rates and LI values reaching even as low as -6c by mid afternoon. A mid level dry punch is evident kicking into the area around 18z. The vort max should be situated over the Missouri and Iowa border by this time as a wave kicks out over the surface boundary in SE Iowa arcing into western Illinois. A mid level dry punch is evident kicking in during this time out of the south as well. I may try to be on the nose of this punch as it kicks in to the Quad Cities to Peoria region early in the day. Convection should begin early in the day in SE Iowa, and spread SE along the arcing boundary to western Illinois by early in the afternoon. Too early to pin down targets as things seem to alter slightly in each model run, but I'm intrigued by a northern target near Galesburg or Peoria early tomorrow afternoon. The other potential target is near the more obvious instability bulls eye along Interstate 72 in central Illinois. Really liking the Galesburg target though.

Check out the hodo for GBG at 18Z

http://beta.wxcaster.com/cgi-bin/parse_skewt_trace.cgi?model=NAM&fcsthr=030&STATIONID=kgbg

Notice the cape maximum near 750 mb suggesting rapid stretching of updrafts, low level buoyancy that would otherwise go largely undetected using traditional parameters. Very nice low level hodographs, and upper level winds intersecting perpendicular with the surface boundary. This is very near the cold H5 core and vort max just across the border into Iowa and needs to be watched with methods other than traditional severe parameters or a tornado or two could sneak by in this otherwise overlooked area near the quad cities.

Of course sunshine in central Illinois while still benefiting from the CAA regime could net some impressive soundings by mid afternoon where low level hodographs are not awful either. Just not sure I like the upper winds paralleling the boundary down there. The sounding is really impressive near Champaign at 21z itself. Decisions will have to be made the day of.

 

[Andrew Pritchard]

Not impressed with tonight's model display. Soundings are showing pockets of warm air in the mid levels and are now lacking the large low level hodographs. Not a surprise that the models are holding off robust convection along the boundary in central Illinois now. Last nights model suite was not impressive at all, but the morning run recovered, and now we've reverted back to a stay at home day so I suppose there is time for change.

You can look at the forecast models until your eyes bleed, but you won't have a grasp on tomorrow's outcome until you wake up in the morning and start taking a peek at some soundings. At this point I'm very unimpressed, but I've got the luxury of waiting til the last second for departure should things change.

 

[Danny Neal]

I, too, am not liking what the 0z was showing. There is no sense in going over what the 0z data shows, because AP did so above me and the 6z will be out soon anyway. I like how the convection now is moving into SW IL and should be out of the area of interest by 6-9 AM. :crosses fingers: I guess the question is will things clear out to give a shot in the arm to the meager instability. With warm/dry mid levels any SBCAPE that does exist at the surface will have one hell of a time being realized. Maybe this mornings runs will be more optimistic. Meso-analysis/surface obs will be the name of the game tomorrow. 4 KM WRF (God model) shows a thin line of showers forming from SE IA down into C IL by 21z. If low topped sups/convection were to form it would form then and in that region. As Andrew noted the intensity is being held down due to the uncertainties involved with the mid level temps. I think Skip and I are going to go for Effingham, IL and hope that thin band of showers is a little more

 

[Andrew Pritchard]

Still not overly impressed, but there is time for improvement and being a local day I have all the time in the world (or until sunet, whichever ends first) to sit and wait for that improvement to occur. If I had to pick a target today, it would be somewhere near Mattoon, IL. That's about 40 miles south of me here in Champaign so I'm content sitting on my butt until things become clear.

Warm air is still killing off instability, with a nice thick warm layer at H7. In addition to that, hodographs are as good as they're going to get right at this very moment. I'm only concerned about the lower levels, but even that bottom portion is not all that impressive. Areas near I-57 still have decent turning, but really lack on the velocity end towards afternoon. Both of these factors don't leave much room for hope. The RUC is especially negative in keeping warmer H5 temperatures than the NAM, but seems to be falling more in line with the NAM on subsequent runs allowing for slightly better low level cape values. It's probably about time to be done looking at the models though, and just start hoping and praying and wishing or what have you that the inversion at 700 mb slacks and the low level wind fields do not. If we can get a little help, I could still see a minisupercell or two forming near Interstate 57 from Champaign to Effingham. My hopes are not all that high right now, and I'm only giving it this much attention for the lack of effort it will take on my end to intercept if things do turn for the better. This isn't the best stomping grounds as things can get hilly once you get closer to the Wabash River, but it is what it is.

http://beta.wxcaster.com/cgi-bin/parse_skewt_trace.cgi?model=NAM&fcsthr=009&STATIONID=kcmi

That would get it done if we're talking low topped supercells, but this is assuming less screwy H7 temperatures as well which remains to be seen.

EDIT: For what it's worth, the RUC is continuing to improve the situation across IL, increasing the instability a bit more further north. I'm still not convinced that the warm inversion will be overcome very far north, but the further north it can go the better. Your odds at getting something to rotate really increase when you start getting north of Interstate 72. I'm still looking at a Champaign to Mattoon target, but am keeping an eye on my initial northern target still in case things continue to recover up there. Pontiac towards to Quad Cities is my ideal target, but Mattoon to Champaign is what I'm assuming I will be dealt.