09/06/10 FCST: KS, MO, IA

[Jeff Duda]

A synoptic scale disturbance will pass through the plains and Midwest on the 6th. Looks like one of the stronger late-summer shortwaves so far this year. It will be a strongly forced event (a cold front), and moisture will be somewhat limited, but shear will be pretty excellent along a strip ahead of the boundary from C/E IA SWWD through N MO and into E KS. The front might actually be "inactive" farther north in MO/IA due to veering winds ahead of the boundary, but if anything can go up ahead of the boundary and turn, it would have a good chance to rotate. Of course, one can't overlook the linear nature of the forcing and the subsequent chance that if anything even develops it could immediately be linear.

 

[Jayson Prentice]

While the forcing along the cold front does appear to be fairly strong, I would still have some major reservations on any severe potential based upon a few factors. The instability is likely to be meager at best, current model progs indicate a very narrow ribbon of ~1000-1500 J/kg of mixed layer instability. To make those matters worse, that is with forecast dew point values of 70-75 degrees! There is no way we see 70 degree dew points based upon what those values are at now across this area. The highest dew points anywhere on the plains are in the mid 50s, you don't see 70s until south Texas! The overall trajectory of flow isn't straight off the Gulf, which would likely be the only chance for good moisture return. Rather, any moisture from south Texas will have to ride northwest towards western Texas/Oklahoma then back northeast towards the area of interest. The shear is mainly behind the cold front, much of what you typically see, it is however strong enough and there does appear to be enough overlap (mainly over southeast Iowa and north-central Missouri) that may support some organized thunderstorms.

Overall I think this event has a long way to go to produce something of worthiness, simply based upon the operational models are likely quantifying to quick of a moisture return and thus are too high on any associated QPF values.

 

[Andrew Pritchard]

I definitely think this is at best a strongly forced thin band of thundershowers event at best across Iowa and northern Missouri. Synoptically it's certainly an impressive and very dynamic system but the juice just will not be there. The strong cold front lingering down near the Gulf coupled with high pressure over the eastern half of the country should keep any substantial moisture return at bay for the better part of Sunday, leaving Monday afternoon a moisture starved waste of a system.

Certainly very strong forcing should spark of a thin band of convection but seeing anything severe, let alone isolated and rotating will take more changes than I can see happening.

 

[Rocky Rascovich]

Lets not forget Minnesota too!

Potentially an interesting set up for the upper midwest, specifically southern MN. along and east of Hwy 169 and south of I-94. Deepening surface cyclone of about 996mb. by 0Z Tue. over west central MN. will provide alot of lift, more than ample shear; both speed and directional.. quite a cold punch in the mid levels around the 500mb. level. The Skew T's I saw based off of twisterdata.com look mighty interesting.. as far as dynamics are concerned.. except... moisture will be an issue. I think this will make or break this day because if we can get at least 1500 joules of cape, pushing the dews at least to near 60 or more and IF the cap which according to the NAM and GFS look formidable can break.. then any isolated convection which can initiate has an excellent chance of rapidly becoming supercelluar, capable of producing quick moving (40kts or better) tornadoes. Best chances as of the latest data lies between 20Z to 02Z Tue. in the area I mentioned above.

Moisture quality is key to whether this could verify or not. I included Wisconsin in my post as well because chances are what moisture that can be advected north maybe slightly deeper there so I would be watching in places like LSE , EAU to ISW after sundown. WRF model does break out precip. in all these aforementioned areas according to the radar model data so who knows. As of this forecast, I give it about a 40% chance of rotating updrafts triggering the TVS alarms at least a few times for the MSP&LSE offices, One thing I'm more confident of.. post frontal winds will crank as the system lifts toward my beloved Lake Superior.. then we repeat the process for later this week.. again with possible moisture issues. Anyway.. this day could be worth watching to see if Minnesota's record tornado count (which personally I question) can be raised further.

 

[Joel Wright]

After weeks and weeks of oppressive humidity over the Midwest, it's hard to believe that the lack of moisture becomes an issue with tomorrow's setup. However, that last cold front really scoured the deep moisture away from the area. Moisture return is currently taking place though, and with a pretty potent LLJ cranking up it's returning pretty quickly.

The latest NAM and SREF models lead me to believe there should be a decent chance for tornadic supercells over central Iowa up to southern Minnesota later tomorrow. The supercell mode may not last all that long given the incoming strong mid-level forcing, but there should be at least a brief window to where supercells flourish. Anywhere along the I-35 corridor from Des Moines to Albert Lea looks pretty good to me. Maybe just a tad east or west of there depending on the exact timing of the things.

 

[Kevin Walters]

WAA in progress overhead right now, elevated storms and a decent lightning show for the last 45 min or so. Tomorrow has potential with moisture and cap being the main issues. Looks like dp's will struggle into the low 60's in Eastern IA possibly into SE MN and SW WI. Strong cap might win out in the end, but strong forcing might get something going. Good shear in place with 50+knts of flow at 500mb. Certainly a more Fall like pattern with strong cyclones sweeping through pushing the good moisture far to the South.

 

[Jayson Prentice]

Have to admit I'm a little more intrigued with the setup currently than I was previously, mainly for areas of southern Minnesota and northern Iowa closer to the surface low. Wind fields will obviously be very strong and thus shear will be more than sufficient for thunderstorm organization along most areas of the front. I do still believe that the models are over-estimating surface moisture with dew points only now reaching near 60. Still, mid 60s dew points will likely be present over the area and be sufficient for thunderstorms... However, along with those strong winds bringing in the moisture they also look to bring in some hefty warm air aloft for any thunderstorms to battle against. Thus, I believe the best hopes for any severe weather will be further north where at least some cool air aloft can aid in creating steeper lapse rates and a little more instability. Still, the big question mark here is how much low level instability can we really get with this warm air aloft...

Personally if I was in the region and did not have to work I don't think it would be a terrible idea to be somewhere along the Iowa/Minnesota border. Perhaps based on storm speeds of 35-45 mph I would play along I90 so you have a quicker east/west option.

 

[Jeff Duda]

Tonight's 00Z NAM seems to have increased mid-level flow to 60-70 kts and backing surface winds just ahead of the front somewhat, resulting in pretty spectacular shear and helicity values ahead of the front. As others have mentioned, it seems like the parameter that will make or break this day is low-level moisture. If enough moisture gets far enough north, there will be a good opportunity for some supercellular activity early on.

The 00Z EMC 4km WRF seems to indicate isolated rotating storms for a few hours tomorrow near 00Z.

 

[Andrew Pritchard]

Again - with less than 24 hours of even halfway decent WAA, there was just NO way the NAM or GFS forecasted moisture or instability was going to make it in there. Even the RUC is forecasting 3,000 j/kg of surface based CAPE to go along with 70F dew points. 70F? In an area that has mid-upper 50s shallow dew points at best 6 hours before show time. Even down in ARKANSAS you're going to struggle to find 60F. To simply buy into the computer forecast instability numbers is just a lack of situational awareness.

Perhaps I'll be proven wrong, but right now I really doubt it. At this point I stand by my earlier forecast from a few days ago; strong forcing + minimal low level moisture/instability = strongly forced thin linear band of shallow convection. Until after dark this is about all I see happening. Once the sun sets and the LLJ really cranks perhaps we'll start seeing some elevated convection get going along the cold front. Before you start looking at what kind of storms a particular model is spitting out you need to look at what kind of data the model is predicting that could hint at such a possibility and ask yourself if it's a reasonable scenario. Perhaps I'll eat crow later, but looking at current data I'd say isolated rotating storms or anything worth a drive is a long shot.

 

[Jayson Prentice]

Somewhat agree... I do think there will be a chance over the far northern reaches of the threat (southern Minnesota and maybe far northern Iowa). This will not necessarily be because there is any significant moisture, but the cooling in the upper levels will likely yield a little bit higher instability values. With such dramatic wind fields over these areas, the combination of strong shear and marginal instability may be able to yield a few rotating updrafts. Still, I have reservations about the actual tornado threat given low level instability will likely still be lacking over these areas as well.

 

[Kevin Walters]

I would think any updraft will rotate without a problem given the impressive shear. The problem will be how long an updraft can survive before being torn apart. There may be a brief tornado or two as a storm gets stretched out. As another poster stated, all summer we have endured mid to upper 70 dp's. It would be a potentially dangerous situation if we had that kind of juice today. We are entering our Autumn mini spike so maybe there will be chase opportunities yet this year.

 

[Jeff Duda]

I think the difference between this event and one that could seriously go bonkers and would be deserving of a MOD or HIGH risk is the boundary right along the gulf coast. It has remained stationary and, as Jayson said, kept the Gulf from opening up and throwing much higher moisture into the threat area. I agree with Andrew that this will probably be a weenie of an event. Strangely, the 12Z EMC 4km WRF still develops discrete intense storms along the front.

 

[John Lavin]

I have the same concerns as well. Initially I felt KS would see some elevated convection by around 6-7 pm but even now the lack of low-level moisture is discouraging anything around here. However, I do like the setup for central and eastern Iowa. Still need to get more moisture up there though.

 

[Joel Wright]

With the intense LLJ over the area, surface moisture is returning. A few locations in the Missouri Valley have dews in the 62-66 degree range already. This corridor will shunt to the east later this afternoon, and with the LLJ remaining quite strong I think the dews will reach the 64-68 degree range in central Iowa later. While the moisture is clearly the lacking feature here, it may be just enough to kick things off. RUC forecast CAPE values of over 3,000j/kg look way too high, but I think a narrow corridor of 2,000+j/kg is very doable. Looking at the RUC, it appears to be about 3-6 degrees too high on the dewpoint readings.

I still expect a broken line of supercells to fire near the I-35 corridor very late this afternoon. The show may not last long, but I think there will be a brief window of opportunity for tornadoes.