6/9/06 FCST: NE / IA / IL / MO

[Mike Hollingshead]

The sfc boundary shows up well on Sioux Falls radar right now, just south of town. There's some cu developing north of it. I'm pleased to see dews holding their own in w IA in that tongue poking nnw. I'd certainly say the boundary between Sioux City and Sioux Falls will be the obvious spot and maybe even north of it. Looping both the 700 and 850 temps on the spc mesopage you can see some cooling there. There's also some weakening cin in sw IA in that tongue poking nnw. I'd hate to be late for something so I plan to leave soon and head to at least Sioux City. I'm sure it's way too early yet, but would rather bake in the sun for a bit than risk being late for an early show.

 

[Scott Olson]

Well the moisture situation looks fairly grim, leading to a smaller and weaker axis of instability than originally forecasted. NAM is obviously overdoing surface evaparation to a large extent and even though the RUC has in the past overmixed moisture, I think based on this mornings soundings and current obs that it seems much closer to reality. Even so, effective shear and storm releative helicity still look fairly potent. LCL's on the other hand are quite high with RUC >2400m. I will most likely find myself chasing anyway, since it's only a 2 hour drive to Sioux City. RUC (and some support from SREF/ 0z NMM-WRF) suggest possible intiation near Onawa/Sioux City just prior to 0z moving NE and hopefully being able to to root on the WF/right turn and take advantage of the 500-1000j/kg MLCAPE that will be avaliable and the particularly strong helicity. More likely I expect that intiation will not occur in time or that the continued mixing and lack of significant moisture return will not allow enough instability to develop. Nonethless I feel like I should be chasing this one, so good luck to all who are out!


-Scott

 

[Edward Ballou]

Going from the latest models Mike, I agree with you. This event isn't written off yet. I have a good feeling about Sioux Falls to Sioux City. I'm afraid my wife and I aren't going to make the timing of the event. Hopefully it will hold off long enough. My target has shifted east of Yankton - somewhere on I-29 at least 15-30 miles north of Sioux City.

Anyone want to pick up some Chick fil-A while you're coming through Sioux City?

 

[nickgrillo]

Yeah, I honestly don't think the day is "written off" yet, although it does have some serious issues (LOL like most days this year I suppose). I agree with Mike H and everybody that virtually everything is pointing towards the Sioux City area for today.

At any rate, current surface analysis shows a well defined warm front stretching from the surface low (northeast of KANW) and then curving southeastward into western IA -- where it intersects the low-level moist axis. Latest RUC mesoanalysis shows some boundary layer destabilization has occured on the western frindge of the low-level kinematic transition zone (with the strongest and most favorable deep-layer kinematic profiles resting on the cool side of the boundary) with ~1500j/kg of SBCAPE developing near the warm front in western IA (along the low-level moist axis). However, a major decrease in CINH will be needed to initiate surface-based convection -- with current RUC mesoanalysis showing widespread 200-350j/kg of SBCINH across pretty much the whole area. Nonetheless, the pervasive low-level moist advection/WAA (given the pronounced veering profiles across the whole area) and insolation resulting in diabatic surface heating should weaken CINH (and increase CAPE) through much of the afternoon -- with RUC initiating precip east of the surface low by 00z -- near the surface moist axis. Overall, the tornado threat should be maxamized once

a) SBCINH effectively erodes so moist surface parcels can attain their LFCs (which can become increasingly likely through further heating and moistening of the boundary layer) -- with the best area for tornadic supercells existing on the cool side of the boundary -- where perhaps less convective mixing will result in lower LCLs (compared to the open warm sector where temps will be in the 90-100F range) and less evaporative cooling potential.
b ) near the triple point (particularly along/just north of the warm front) where low-level convergence will be maxamized -- and where storms will be able to meander through a more favorable thermdynamic and kinematic environment (and may ingest the richer surface vorticity along the warm front).

Given the incredibley tight window for anything worthwhile -- I decided to NOT make the ~800 mile trip to the TA area for today.

 

[afischer]

Decided to sit this one out.

The model biases of the RUC and NAM are definitely showing themselves in their solutions for this event. For those chasing and using the SPC mesoanalysis page... two aircraft soundings from SUX over the past hour indicate the mesoanalysis has a good grasp on the 850 T/Td and 700 T field and thus the near-term cap evolution.

 

[nickgrillo]

Latest RUC mesoanalysis shows CINH weakening slightly the past couple of hours near the warm front (particularly in western IA) given sufficient low-level heating and moistening. The 20z RUC doesn't show precip breaking out until after 00z -- with the brunt of the precip well southeast of I-90 (where significant SBCINH and nearly non-existent SBCAPE exists) by 03z. The earlier WRF run showed convective cells blowing out near I-90 around 01z (and the NAM initiates precip all along/north of the warm front by 00z). The RUC forecasts a gradual decrease in CAPE (and increase in CINH) in much of western IA (near the warm front) through 00z given the continued insolation/convective mixing -- eroding better low-level moisture. At any rate, this is becoming far from a nice setup (LOL like the majority of the setups I've chased in 2006) -- and the small time/spatial scale for a supercell with surface-based inflow (and a low-base) will make today a very difficult setup to deal with.

One situation I could think of is for an area along the warm front (east of the surface low) near the I-29/I-90 intersection (along the front in slightly lower LCLs, per RUC mesoanalysis -- from Sioux Falls to just east of Storm Lake) to locally breach the capping inversion -- and for a storm to "ride" the boundary (and stretch and tilt the low-level vorticity -- and ingest the richer thermodynamics along the boundary as well). Otherwise, any convection further south of the triple point will be confined to crap multicellular (and perhaps brief supercell structures) high-based garbage with extreme evaporative cooling potential given the very dry boundary layer (with the weak tropospheric flow further resulting in outflow dominant storms -- with a quick upscale-growth to linear structures).

 

[J.B. Dixon]

Sitting in Sioux City now, and I was initially cussing out the Missouri River, thinking she was going to be messing with me to decide to be on one side or the other. It looks like the N side is the place to be, though, if anywhere. LCL's are still in the stratosphere, and 98 feels all of 98 right now. Cu field has finally appeared, but it's got some more work to do to bust the cap. If I go north from here on the Iowa side, I'll be flying blind. Likely heading up to Hwy 50/Vermillion area over the next hour to find anything that wants to anchor itself on the front.

MD out for the area....just about time for a NOW thread.

 

[Jayson Prentice]

Currently north of Le Mars also watching the CU field that has finally shown itself this afternoon, also watching the cells that are trying to get going in northeast NE right now. I was further south, but along with JB feeling the heat and no moisture whatsoever was enough for me to move north and get on the north side of things...

Not sure on the next move quite yet, still waiting to see if something in the CU field can take shape.

 

[Benjamin Sipprell]

Dewpoints in and around the region of Sioux City on the order of 61-63F ... MSAS surface analyses showing 343K thetaE corresponding with Moisture Flux Divergence values of -40 g/kg/12hr ... SPC Mesoscale Analyses showing strong 0-1 and 0-3 SRH along the warm front extending eastward (and of course northward)

Here's to hoping something localized will occur.