3/22/11 FCST: IA/NE/KS/MO

[Skip Talbot]

12z NAM and GFS are in fairly good agreement with position of upper level and surface features as well as their magnitudes. The NAM is quite a bit more favorable with the instability, but both models are trying to push 60 degree dewpoints into Iowa by early evening Tuesday. The NAM has a nice dryline/warmfront triple point setting up near Omaha while the GFS places it a bit further north in northwest Iowa.

NAM paints a pretty picture with modest surface based cape and good low level instability pooling near the triple point and strong directional shear along the warm front. The 80 knot midlevel jet overtakes the warm sector by afternoon as that trough is finally ejecting after its delayed arrival looks to shortchange the previous days' setups. The cap looks to erode by mid afternoon with this setup too, which is nice change from the previous day's stout cap forecasts. Marginal instability may be an issue however with forcing from the trough kicking up a lot of cloud cover and crapvection on and north of the warm front. The GFS shows a lot of ongoing convection in the warm sector and that may be the reason for the lower instability its plotting. The NAM instead seems to have the ongoing convection to the north reinforcing the warm front to the south, which storms going in the warm sector could interact favorably with. Dynamic lift from that midlevel jet may be enough to overcome the marginal instability.

I like Tuesday the best out of any day associated with this system, mainly because it will have had more opportunity for moisture advection, shows the least capping issues, and is finally working with the energy of that ejecting trough. I'd play closer to the triple point (wherever it winds up) where surface winds will be most backed and directional shear strongest for the greatest tornado potential. The dryline extending southwest into KS could see some discrete cells as well although the mode may be a little more linear with surface winds and 850's more veered.

 

[Jeremy Den Hartog]

The 19.00z run of the ECMWF has a 995mb low pressure in eastern Nebraska - similar to the 19.18z NAM but slightly further north. This right now is probably a pretty good compromise between the GFS and NAM. I also noticed that the 18z NAM appears to be slightly more north then the 12z was. In any case though, looks like a good system to keep an eye on.

 

[Jeff Duda]

...Yeah, but Iowa is involved...need we say more?

In all seriousness, I just spoke with one of my chasing partners about this and we agreed that this setup resembles that of April 6th last year in almost the same place. The problem is with the orientation of the storm motion vector, the warm front, and the total lack of instability north of the warm front. The storms that do form are only going to pose the risk of going tornadic during the few minute period where they are interacting with the warm front. After that, they will quickly stop being surface based if they even live for much longer. If the storm motion vector veered more or the front became more E-W or SW-NE oriented, then we'd be looking at a different story. It certainly should be watched, though.

 

[Jeremy Den Hartog]

Yeah, it does look similar to April 6. Here are some surface obs for 00z on April 6:

We will see how things play out though. The low pressure location and surface winds seem more favorable IMO then April 6th of last year. Hopefully the storms will ride along the warm front and not cross north of it into more stable air...

 

[Jeremy Den Hartog]

Based on the latest model runs, I think the NAM is to far south and west. The GFS has pushed this system even further north as did the ECMWF but I believe the ECMWF is more realistic then what the GFS shows (severe weather would make it all the way up into southern MN if the GFS verifies). It seems to be a good bet that the triple point will end up somewhere in northwest Iowa. Another issue being depicted on some of the models is a capping issue. Of course we are a couple days away yet so hard to tell if capping will really be an issue or not...but it wouldn't surprise me.

My initial (very early) target is Storm Lake, IA based off the ECMWF

 

[Jeff Duda]

Having had a look at the 12Z GFS, 12Z NAM and 09Z SREF, none of the three models (when considering the ensemble mean for the SREF) agrees on the placement of the low. While the GFS has it straddling the IA-MN border at 00Z, the NAM has it straddling the Missouri River just north of Omaha at the same time, and the SREF has it far to the east over Waterloo at the same time. The other thing I noticed is that the GFS and SREF have the winds above the surface veering out quite a bit so that there is next to no directional shear above 850 mb. Although there is plenty of speed shear resulting in 0-6 km bulk shear values up to 80 kts, it looks to be mostly speed shear above 850 mb, but it isn't that great in magnitude. That kind of shear profile tends to support bow echoes and line segments more than it does supercells. The NAM does not show this veering and has by far the best looking shear profiles. Also, all the models agree that instability will be hard to come by. Most don't have instability getting above 1000 J/kg (the NAM is the one that has it higher, up to 2000 J/kg, in some areas). I'm a little worried about the upper level shortwave causing a threat in the northern part of SPC's SLGT SWODY3 becoming too negatively tilted and closing off before the peak instability hits so that QG forcing is less, and I can see ongoing convection killing off instability in the area as well (although the models do not explicitly predict this as of now). It still looks like anywhere from SE NE through S IA and N MO are in the greatest threat, but how much threat is still in question.

 

[Scott Weberpal]

...Yeah, but Iowa is involved...need we say more?

In all seriousness, I just spoke with one of my chasing partners about this and we agreed that this setup resembles that of April 6th last year in almost the same place. The problem is with the orientation of the storm motion vector, the warm front, and the total lack of instability north of the warm front. The storms that do form are only going to pose the risk of going tornadic during the few minute period where they are interacting with the warm front. After that, they will quickly stop being surface based if they even live for much longer. If the storm motion vector veered more or the front became more E-W or SW-NE oriented, then we'd be looking at a different story. It certainly should be watched, though.

I'm not seeing the northerly winds north of the wf, and south-southwesterlies in the warm sector with this system as they were on April 6 last year. This appears to be a more classic setup with southeasterlies in the warm sector, and easterlies along and north of the wf. Given the southeasterlies in the warm sector, these storms should be able to tornado well south of the warm front - that wasn't the case last April. The NAM is showing a gradual drop off of moisture north of the warm front, whereas April 6 the gradient was a significant 15-17 degrees in a matter of miles. I guess I'm just not overly concerned with that aspect on Tuesday. I'm more concerned about the Missouri River.

 

[Mark Ellinwood]

Omaha looks like a pretty good staging area for Tuesday, with some E/SW/W options should the energy eject into the Plains a bit earlier/later than progged. Wouldn't stray much further east than Omaha as extensive LL clouds and fading daylight would make for some pretty poor visuals.

 

[Logan Karsten]

There are two things I have a problem with for this day, even though it's a nice looking warm front:

1.) As Jeff stated earlier, the air just across the WF will be much cooler and stable. With storm motion going NE across the boundary, any storms that do form in the warm sector will quickly move into that air mass.

2.) Storm motion will be 30-50 kts. With that kind of motion, you really have to be at the right place at the right time.

I definitely see the potential for some isolated tornadoes, but I think I will pass due to the previous two reasons listed.

 

[Jeff House]

Regarding how far north or south the warm front WF ends up, it will highly depend on overnight or pre-dawn rain Tuesday. The NAM has more precip; therefore, paints the WF farther south. GFS has some precip. My gut would be to target from I-80 south as opposed to north. Even if I'm wrong about the synoptic WF, outflow boundaries OFB should be in place south of the WF due to morning precip. I'm not chasing this event, and have no dog in the hunt; therefore, hope I can provide some good input without location bias.

Regarding the dry line, agree winds at sfc and 850 are too veered. Would target the OFB intercept with the surface low or just east of the low. I share concerns about the stable layer north of the WF. Normally with fast-moving cells, one might position farther northeast and let them approach. Trouble will be ongoing precip or at least poor visibility in that stable air. Just have to be there as the cell traveling at 40 mph crosses the front at just the right moment.

Good luck to y'all going out and chase safe!

 

[Mike Smith]

I'm going to do my usual and go out on a limb here and forecast good hunting conditions from south central Kansas into the northern Flint Hills.

1. The models tend to be too far east (as does SPC until Day 1) in this type of situation. Scoot everything 40 mi. farther west which is my standard correction factor.

2. There will be plenty of instability in the area.

3. The NAM forecast sounding from today's 12Z run is dry adiabatic with PLENTY of moisture at 33 hr. just southwest of ICT (21Z Tuesday).

4. The high res WRF convective from both 06Z and 12Z break out supercells in the region and move them rapidly northeast.
06Z version valid 4pm CDT Tuesday

12Z version valid 4pm CDT Tuesday

Upper level winds are plenty strong enough and the high res. models are showing local 850mb backing along the KS-OK border southwest of ICT. So, I think a tornado is possible.

Mike

 

[Jeff Duda]

I am surprised to see the quantity of moisture that has made it to the warm front. As of right now, Tds around 60-61 have made it all the way to the warm front and are widespread across KS/MO/OK. This makes it look like the NAM's forecast of those higher dewpoints making it to the boundary will verify. Instability may end up being higher than forecast here. However, it is quite warm at 700 mb for this time of year and there could be quite a cap over the threat area until cooler mid-level temperatures move over.

 

[Greg Blumberg]

Ouch. Can you say CAP?

http://www.spc.noaa.gov/exper/soundings/11032200_OBS/OUN.gif

Thank goodness the DDC sounding looks a bit more decent for tomorrow, so I'm agreeing with Mike Smith and the WRF. Just from a real quick analysis of current water vapor and some of the model's story of how this situation will play out. To put it short, the models are doing a horrible job. I'm very suspicious of this setup, as it has some key characteristics of good severe weather days. One, it's got a subtropical jet and a very nice polar jet nearby. There's nice, deep moisture about the Southern Plains (as Jeff pointed out.)

Just a quick look at SPC's 700 mb analysis.

http://www.spc.nssl.noaa.gov/obswx/maps/700_110322_00.gif

If this is the initialization tonight, be prepared for more errors and junk forecasts being thrown out by the models. The height contours are pretty far off from where they ought to be (i.e. parallel to the winds.) Garbage in, garbage out.

 

[Brandon Sullivan]

Ouch. Can you say CAP?

http://www.spc.noaa.gov/exper/soundings/11032200_OBS/OUN.gif

Thank goodness the DDC sounding looks a bit more decent for tomorrow, so I'm agreeing with Mike Smith and the WRF. Just from a real quick analysis of current water vapor and some of the model's story of how this situation will play out. To put it short, the models are doing a horrible job. I'm very suspicious of this setup, as it has some key characteristics of good severe weather days. One, it's got a subtropical jet and a very nice polar jet nearby. There's nice, deep moisture about the Southern Plains (as Jeff pointed out.)

Just a quick look at SPC's 700 mb analysis.

http://www.spc.nssl.noaa.gov/obswx/maps/700_110322_00.gif

If this is the initialization tonight, be prepared for more errors and junk forecasts being thrown out by the models. The height contours are pretty far off from where they ought to be (i.e. parallel to the winds.) Garbage in, garbage out.

The NAM breaks the cap after 21z with a strong vort max coming through... NW IA is a play off the 00z nam, but I cant data I'm a huge fan of the tiny warm sector plays...

Of a bigger note, the RUC picture for tomorrow at 2pm is not even close to the NAM picture! This is a wake up in the morning and decide day!

 

[Bill Schintler]

Chase Target for Tuesday, March 22:
Lake City, IA (70 miles northwest of Des Moines).

Timing and storm mode:
Surface-based storms should develop between 4 PM and 5 PM CDT. An isolated tornado or two cannot be ruled out, especially with storms that interact with backed surface flow along the warm front.

Discussion:
A warm front will provide the focus for severe storms during the late afternoon hours Tuesday. Low pressure should be centered 50 miles west of Fort Dodge at 21Z, with a sharp warm front extending to the east-southeast. A dryline in western IA will mix eastward during the afternoon, and the warm front/dryline triple point should be the focus for surface-based initiation between 21 and 22Z as lift increases.

Low clouds will persist north of the warm front during much of the day, enhancing the temperature gradient; while broken SC should linger further south. CI will also blanket the region until mid afternoon, at which time clearing will take place from southwest to northeast. Dewpoints should increase into the mid-50’s; however, moisture depth is a concern. The 00Z upper-air analysis indicates 12-13C H85 dewpoints over extreme southwest IA, and strong moisture transport will occur over night as the LLJ increases to 60 kts. SFC-100mb MLCAPE’s should increase to 1000-1500 J/kg with steep lapse rates in the 800-600 mb layer. Shear parameters are impressive, with large hodograph curvatures along and north of the warm front. Extremely low LCL levels along and north of the front will also enhance tornado probability.

- Bill Schintler
9:59 PM CDT, 03/21/10