4/25/08 FCST: IL/IA/WI/MO/AR/OK/TX

[Chad Cowan]

Potent shortwave trough with attendant surface low and cold front will slowly eject into the upper MS valley on Friday. Moderate instability will be present as far north as central WI owing largely to a very moist air mass. The triple point should be in eastern IA around 18z and lift ne into WI as the day progresses and the cold front should drift into the Chicago area by 00z/26. Amount of shear in the northern target should be sufficient for supercell structures if (as usual) we can get some cloud breaks and insolation after the morning convection moves out. My second concern is the 85ht LLJ veering progressively as the day goes on, so I'll be hoping for early clearing and initiation, which isn't unlikely given the meager forecast CINH.

 

[Adam Lucio]

The potential for ongoing crapvection and cloud cover in the morning [as has been the case each time this year UGH!] will play a huge role in the outcome.

Large differences in CAPE values between WRF and GFS with GFS bringing in as much as 2500 by 18z friday. GFS also brings in 60s Td across much much of northern IL during the same time frame. WRF wants to keep the best moisture and the best CAPE further west it seems.

Based on that, and the presumption that early morning activity will exit areas to the south earlier in the day allowing for a better chance of clearing, I will make my preliminary target around the Dekalb oasis near I-88 and I-39. Road options here are excellent and I can adjust quickly in any direction based on live conditions.

with 3K SRH values around 300 storms should have no problem acquiring rotation, and I do see the potential for some tornadic supercells across northern IL, we just need that clearing, surface temps should warm into the low 80s across much of central IL.

If the warm front fails to light up, the strong cold front sweeping in behind it will offer a second chance for at leaste a good shelf/hail/wind event.

Overall a chase-worthy setup IMO.

 

[Andrew Pritchard]

I don't think you've got the option of playing the warm front down here in Illinois, since it should be in northern Wisconsin and Michigan by evening. It's likely going to be picking your favorite location along the cold front.

I was partial to northern Illinois earlier, but now I don't see any real difference in directional shear along the front, so I may play the slightly better moisture in west central/ southwest Illinois. Both models are pooling 65-70F dew points around Jacksonville.

Looking closer at the new wrf it looks like it may actually be trying to fire convection in the early afternoon ahead of the front. This would be the ideal solution for tornadic supercells as we'll likely get a squall line along the cold front with the insane forcing.

At this point I'll leave myself with two prelim targets. The northern target being somewhere along Interstate 80 near the Illinois and Iowa border by early afternoon, or south near Jacksonville - Quincy. I've got an exam at 8, but should be done with that by 9 and will likely leave shortly after.

I think overnight convection for the most part should get lifted north into Wisconsin along with the low so areas in Illinois should see fairly good destabilization. The convection could actually lay out an ofb where we could see that pre-frontal early afternoon show develop along the ofb, or regenerate along the southern flank of the precip shield with southward development through the day. I think that's our best bet right now for anything discrete. SW flow at most levels will still be hard to overcome.

 

[Scott Weberpal]

I think the mention of an OFB would be a moot point. The majority of overnight convection will be limited to Iowa, SE MN, and N. WI, with perhaps some smaller coverage convection in IL and WI. On the off chance there is a nice OFB, that will be the game. Biggest problem I see with tomorrow is strongly veering sfc and LL winds throughout the day, and the ability for the airmass to destabilize sufficiently before LL flow veers. I am VERY strongly considering sitting about 20 mi inland from Lake Michigan and intercepting storms as they cross the Lake Breeze boundary. This reminds me a bit of 3-8-2000 when a tornado developed in the MKE metro in a situation much like this with slightly veered sfc winds and strong kinematics aloft.

 

[John Farley]

I agree on the veering surface winds as a concern - may not be much directional shear by the time the afternoon storms get going. As to overnight convection, I would not write off possible impacts, either bad (stabilization of the airmass, as has happened so many times before this year in IL) or good (laying down outflow boundaries). There is model disagreement on this point - WRF has a lot, GFS much less. In both cases, this is more of an issue northward, but WRF shows overnight and/or morning precip over most of the northern 2/3 of IL. Best case scenario would be good destabilization south of the precip area, and then some OFBs along its southern edge. That might favor a more southern target like Jacksonville, which would be good for me. Right now I'm thinking somewhere around the Jacksonville-Peoria-Lincoln triangle, but lots can (and probably will) still change. I do worry about the limited directional shear, though.

 

[Andrew Pritchard]

I think the mention of an OFB would be a moot point. The majority of overnight convection will be limited to Iowa, SE MN, and N. WI, with perhaps some smaller coverage convection in IL and WI.

I guess that depends on which model you are buying into. The wrf has quite a bit of convection over the entire state of Iowa, moving into most of Wisconsin and NW Illinois.
http://wxcaster2.com/CENTRAL_ETA212_1000-500_SLPPRPTHK_27HR.gif

http://wxcaster2.com/CENTRAL_ETA212_1000-500_SLPPRPTHK_30HR.gif

I wonder if this won't hurt chances further north into Wisconsin, as has been the case with most northern targets in the midwest so far this year. It's unfortunate, because that's where the only hint of directional shear will be located during peak heating, and even then it isn't much.

If you're buying the GFS more, then yeah, it lifts more of the convection north with the low into northern Wisconsin.

I guess I'm torn between the two solutions, but given the general trend with these systems I'm not going to get too confident in the drier solution and expect a rainy start to the NW.

 

[Brandon Sullivan]

To Be Brief..

Well, tommorow continues to grow in confidence for me...

The models are picking up on the drier air being wrapped in by 18z in ESE IA... CAPE values jump drastically to someplaces over 3000 in SE IA.. This is a possible target... Or possibly playing along the I-80 Corridor near Lasalle,IL where CAPE values are also high, and SigTor And Sup composites are elevated....

The SPC likes the Central IL route....

 

[Joel Wright]

Hmm. I'm really hoping the storm slows up a bit. Winds really begin to stack up later in the afternoon. NAM shows dewpoints mixing out slightly by late afternoon too, which obviously decreases instability somewhat.

I have a job interview at 3:30, so if we go out we'll be getting a late start anyway. That's another reason I hope this thing slows up some lol. I'm sure we'll make a run at it if things look halfway decent.

The early guess would be just to head on east down I-80 towards the Lasalle area. This could all change though depending on what's going on. Chances are, the game should have already begun by the time we're ready to chase.

 

[Chad Cowan]

I'm losing hope in the potential tomorrow. Recent model runs have the sfc low screaming into northern WI in the mid-afternoon, taking the best shear profiles with it- far away from the best thermodynamics. My current thinking is that there will be a brief window of opportunity where tornadoes will be possible in the early afternoon near the MS river (Davenport, Dubuque area), and I can't rule out a rouge tornado in central WI before 00z, but that isn't exactly chaseable terrain. If we get enough sfc heating, the cap could be overcome out away from the cold front and produce a discrete storm before the cold front mashes it into a messy line, but that's very conditional.

 

[Scott Weberpal]

and I can't rule out a rouge tornado in central WI before 00z, but that isn't exactly chaseable terrain.

Actually, much of C. WI is very chaseable. From Coloma north to Wausau and west to Eau Claire is very flat and treeless in many spots. Lots of sandy, dusty fields in that area where it's tough to grow trees. But there again there are patches in the region with trees where you don't want to be.

 

[Bill Schintler]

Chase Target for Friday, April 25

Chase Target for Friday, April 25

Chase target (north):
Galesburg, IL.

Timing and storm mode:
Storms will develop in the area at 3 PM CDT. Storms will form into a north/south-oriented line with imbedded supercells possible.

Chase target (south):
Ennis, TX (20 miles south of Dallas).

Timing and storm mode:
A line of storms will back build during the afternoon, developing in an unstable airmass in the target area through 3 PM CDT. Embedded supercells and/or bowing structures are likely. Hail to golf-ball size and winds to 70 mph will be the primary severe hazards with these storms.

Synopsis:
The WRN CONUS trough will slowly shift EWRD over the next 24 hours as the first of two S/WVs lifts E towards IA. ULVL energy diving S through WRN CAN will phase with this through late in the period. A cold core is associated with this system, however during the FCST period the coldest H5 temperatures of -26C will lag well behind the moisture and SFC features. The latest MDLs are earlier with the arrival of the strongest H5 flow in the Upper Midwest. An axis of deep moisture now extends from TX and OK N into IA. During the FCST period, SFC low pressure will deepen and track NE through IA and then towards the Great Lakes through 00Z Saturday. Further S, a trailing CF will push SE through NERN TX.

Discussion (IA, IL):
The FCST is a challenge because of the uncertain details associated with a morning MCS. LLVL cloudiness will persist courtesy of the deep moisture in place. Current reasoning is that widespread convection will exit ERN IA to the NE during the morning, while broken SC will persist over the entire area with breaks in the cloud cover over SERN IA and WRN IL between 18Z and 22Z. This should allow for at least some partial insolation. Capping will persist with a mid-level thermal ridge advecting an axis of 6-7C H7 temperatures into ERN IA, however forcing along the CF should be sufficient to overcome inhibition. For the same reasons, discrete prefrontal convection should remain capped. Deep moisture beneath steep mid-level lapse rates in excess of 8C/km will contribute towards MLCAPE’s (SFC-100mb) in excess of 1500J/kG, especially where any SFC heating occurs. Shear parameters should support rotating updrafts with deep-layer (SFC-6km) shear increasing to 50-60kts as an 80kt H5 streak rounds the base of the S/WV over IA. LLVL hodograph curvatures will increase as a 40kt LLJ flow becomes increasingly WRLY.

Discussion (TX):
The airmass will become increasingly unstable ahead of an advancing CF, while LLVL flow locally backs along a developing SFC wave about 50 miles SE of DAL. Overall deep-layer shear and forcing will be considerably weaker then to the N, however SFC-6km shear of 30-40 kts should be sufficient for storm organization. Forcing should be also enhanced by jet coupling and attendant ULVL divergence aided by an 80kt H25 streak in the SRN branch.

- bill
9:15 CDT, 04/24/08

 

[Scott Weberpal]

With the NAM slowing the system vs. previous runs, the liklihood of a C. IL chase is going further down the tubes. LL flow in IL begins to veer by 18z, and only in far N. IL into WI do the sfc winds remain southerly by 00z. Unfortunately, it appears ongoing convection will limit insolation through at least 18z limiting additional convection further south. I'll likely play NE IA into S. WI unless fantastic clearing occurs in IL pre-20z. I have no reason to jet south when it appears the better kinematics and instability will be in C. WI by late afternoon.

 

[John Farley]

The instability doesn't look all that good anywhere in this evening's NAM run. While I agree that the better directional shear in Wisconsin could result in a tornado if strong storms can get going, that is really too far for me to go in such a marginal situation. I'm actually debating between the area just west of STL and farther north between Jacksonville and Galesburg, but actually leaning a bit toward the MO target as the NAM is showing slightly better SRH there. Not sure why, as there is rather limited directional shear anywhere until you get up into WI, although there is a LITTLE directional shear and some speed shear with SSW winds at the surface and strong SW winds at 500 MB and up in both east-central MO and west-central IL. Looks like a situation where a relatively local chase would be OK, but a longer drive would not be warranted - especially given that gas just jumped to 3.79 here in Edwardsville - Yikes! I do think there will be severe storms with the forcing along the CF, and maybe just enough wind shear for a tornado or two in bow/LEWP structures along the squall line somewhere in eastern MO or western IL. Won't be easy to find the needle in the haystack, though.

 

[Scott Weberpal]

I like the wording of the AFD issued by MKX...

...CAPE JUMPS TO NEAR 1500 J/KG BY 21Z. 1KM AND 3 KM EHI
PARAMETERS ALSO CONTINUE TO POINT TOWARD SOME TORNADO
POTENTIAL..BOTH AROUND 2 M2/S2...

...THE SLIGHT RISK OF SEVERE...ACTUALLY BORDERING ON MODERATE STILL LOOKS
REASONABLE. SEVERITY SHOULD STILL REST ON IF CONVECTIVE CLOUD
DEBRIS FROM TONIGHTS...AND FRIDAY MORNINGS CONVECTION DISSIPATE IN
THE WARM SECTOR. LOCAL 4KM WRF MODEL BASED OFF THE GFS DEPICTS
CELLULAR PRECIP PATTERNS...INDICATING INDIVIDUAL STORMS RATHER THAN
WIDESPREAD UNIFORM PRECIPITATION...FAVORING SEVERE POTENTIAL.

I tried to link to the AFD, but couldn't get it to work (I trimmed down the actual product as much as possible). Hope that isn't a problem!

 

[Andrew Pritchard]

Can probably count me out on chasing tomorrow. I just have a feeling storms that flare up tonite to our west are going to ruin the day. Not that there is much to ruin with the terrible directional shear. If I could go anywhere, it would likely be towards southern and southwestern Illinois along Interstate 70 near St. Louis.

More than likely though I think we'll see another precip mass ruling the day as we have with most setups in the midwest this year. I hope I'm wrong, but my guess is when the sun comes up there will be a large area of rain and thunderstorm from eastern Iowa down through Missouri. The NE advancement of this could very well kill off any severe chances for Wisconsin and northern Illinois where the better shear would exist. We'd likely end up seeing a flare up of convection on the leading edge of the precipitation with a mostly wind threat across central and southern Illinois. The storms may eventually outrun the better dynamics and forcing along the cold front and be gone before sunset, leaving a worked over atmosphere along the front which sees no more development. The only place I think that will see any shot at momentarily discrete storms is going to be close to the St. Louis area and points immediately east of there.

Areas near the low in Wisconsin and far northern Illinois may eventually clear out late in the afternoon, but the large area of convection to the south over Illinois and Missouri should effectively disrupt low level flow. That in combination with plenty of left over subsidence should kill off any real chance of deep convection in the better sheared environment.

Another low instability day, only this time we don't even have good shear. Count me out for now until I wake up and see that my guess on the precip shield location is totally off.