4/30/10 FCST: TX, OK, AR, LA, MO, IA, MN, WI, IL

[John Farley]

Looking at this evening's NAM run, the area from central MO to eastern IA looks good. I'm liking the slightly backed surface winds in eastern MO, and the best instability appears to set up in a band through somewhere near COU. OTOH, the upper support looks a little better up in Iowa. If things stay looking tomorrow like they do now, I will probably try to make it to somewhere around Columbia by mid-afternoon. That's partly shaped by location and my ease of getting there on I-70, but it's also where the NAM is showing the best combination of instability and shear. The trick will be to be there when the main storms fire, as they could go linear quickly. But there's certainly enough directional shear to get discrete supercells for a while, and even after they go linear there could be embedded supercells. But with the crappy terrain of MO and the fast storm motions, it will be a tough chase once it goes linear, even if there are tornadoes to be had. Of course, we'll see what other models have to say - agreement has not exactly been good with this system.

 

[Jesse Risley]

I co-authored a forecast with Ryan Wichman, which we posted over on the site: http://convectiveaddiction.com/2010/04/29/fcst-043010-moiail/

The bottom line is that tomorrow could be very interesting, should the instability parameters materialize, though I don't like how the 00z NAM shows slightly more southwesterly 850mb winds than previous runs.

 

[Jason Berner]

I am feeling decent about this setup. I will give you that the instability is still showing a problem with the best CAPE values being in central missouri. I think I am shifting my setup west (Chillicothe) instead of north to hit the storms when they first form up there where the instability and wind shear come together for the best supercell composites. I am pretty new to forecasting and chasing so I guess its time to learn some lessons and make the most of it...

 

[Bill Schintler]

Chase Target for Friday, April 30

Chase target:
Birmingham, IA (southeastern corner of the state).

Timing and storm mode:
A squall line with embedded supercells will move into the area after 4 PM CDT. All modes of severe weather are likely, including tornadoes. Extremely poor chasing conditions will be the rule with very poor visibility due to low cloud heights, and very rapid storm motion in excess of 40 mph to the northeast.

Discussion:
The WRN CONUS trough will continue slow progression to the E while slowly amplifying. A 100kt H5 streak will eject from this feature over KS and then into IA by 00Z, 05/01/10. At the SFC, A CF will push though the area during the afternoon. The forecast is difficult due to uncertain air mass modification and subsequent recovery, and outflow boundaries from elevated morning convection. A negative factor is that convection will be widespread and synoptically forced. A large strongly forced line of storms will track through CNTRL IA during the mid-day hours. Meanwhile, MLCAPEs should increase to 1000J/kg ahead of this convection, despite both a CI shield and ST deck. Given mid-level lapse rates in the 7-7.5C/km range, SFC dewpoints of nearly 65F and temperatures approaching 80F will be necessary to realize these CAPE’s. Especially if partial clearing of the SC takes place ahead of the front in SCNTRL through ECNTRL IA as latest MDL indications suggest. Sufficient moisture return seems plausible given a robust SLY LLJ and upstream H85 dewpoints currently approaching 15C (00Z, 4/30 SHV, FWD, OUN, LMN, and TOP soundings).

Modest instability should be offset by extreme shear parameters and very low LCL levels AOB 400m AGL. SFC-6km deep-layer shear may exceed 100 kts as a highly diffluent ULVL jet overspreads the region! Hodograph curvatures and LLVL shear are also high with 35 kts of H925 flow overspreading backed SFC winds. Forecast composite SVR indices are impressive: the SPC SREF ensemble indicates a significant tornado index of 5 and a supercell composite parameter of nearly 15. Supercells should be embedded within the leading edge of a larger storm complex. Storms will evolve into a severe squall line by early evening, and track through IL.

- Bill
10:57 PM CDT, 04/29/10

 

[V. Gensini]

The trend via the SREF over the past ~3 runs has suggested that the best juxtaposition of CAPE/Deep Shear/SRH in the upper Mississippi Valley is trending a little further west with each run. I really like the lower LCL's, and steeper low-level lapse rates (contributing to LCLs and LFCs being nearly the same) as compared to Thursday's (29th) environment. Although the storm motions will be pretty fast up in this neck of the woods, its a little too close to home to plan to sit out. Initial target pending convective debris from the overnight activity: Ottumwa, IA

 

[John Farley]

Not much change in my target area from last night. I'm still thinking somewhere along I-70 between about Forestell and Columbia is the place for me. The RUC and NAM both show increasing instability and shear in east central and southern MO in the late afternoon into the evening. New storms should develop in this area and southwestward late in the afternoon, and the environment will be favorable for severe storms and tornadoes. This thinking seems to be reflected in the SPC upgrade of eastern MO and western IL to a moderate risk.

It looks to me like the best daytime storms might be a little to the southwest of my target area, but that is terrible terrain with lots of hills and trees. Even along 70 is bad enough in that regard, especially with the rapid storm motions we'll have today. The game may be to just find a good spot ahead of the hopefully-broken line of storms and let one storm after another pass, rather than trying to keep up, and hopefully catch something interesting. That strategy has worked for me before with fast-moving MO storms, but it won't be easy. And that is if the storms stay somewhat discrete for a while. Certainly the tendency will be for the storms to line out, albeit with embedded supercells, but I think given the model forecasts of new storms developing in the late afternoon in an environment with decent storm-relative helicity there might be a 2-3 hour window of somewhat discreet storms before either darkness arrives or the storms line out. Or is that wishful thinking?

 

[Adam Atkins]

It's all about boundaries today. Slightly veered low level jet and nearly parallel shear vector to the front will favor a quasi-linear storm mode. Stronger capping should help storms stay more discrete further south in the risk area. If I was chasing I'd probably head towards NC Arkansas. Nice boundary just north of LIT.