10/18/07 FCST: WI/MI/IA/MO/IL/IN/OH/KY/TN

[Chad Cowan]

Deepening surface low (981mb!) associated with increasingly negatively tilted trough will move ne into MN and WI on Thursday as the diffluent flow aloft and cold front will provide the forcing for a potentially widespread severe wx event. Low-level gulf moisture will be advected north by strong LLJ resulting in low-60's dewpoints up towards the low. According to the 12z GFS, winds will be mainly unidirectional in the northern half of the forecast area, while the tornado threat should be highest in s IL, w KY, and w TN. Again, ongoing convection and cloud cover could hinder diurnal heating and instability.

 

[Justin E. Reed]

This is a fair system for this time of year. Winds are already sustained 15-20mph and gusty throughout mush of the South around and west of I-65. Dewpoints are still low, mid 50's in Mid-TN, but will increase. Atleast this gives a fair shot of rain.

 

[Travis Carlson]

WOW! Impressive deepening low pressure system on tap for Thursday and potentially a decent severe weather outbreak. Classic lee-side cyclogenesis takes place in eastern Colorado tues night. Looks to be a very dynamic and amplified system. GFS/ECM indicating this dynamic storm system very well with latest runs deepening the low from around 992mb to as low as 981mb. Either way it should be an "action packed" low pressure system. Strong isentrophic lift will develop ahead of this system wednesday and wednesday night as it ejects into the Midwest. Models are forecasting strong shear aob 50KT in the mid-levels. Moisture should build to aob 60 degrees back into Central Illinois in the warm sector. Concern this far north will probably be the warm front and any heating that we could get in the warm sector this far north due to the the strong dynamics and jet dynamics that will be present. Timing of this system will have to be watched very closely but nevertheless looks like a definate severe weather outbreak somewhere in the Midwest as timing and location gets narrowed down over the next couple of days.

 

[beaudodson]

Paducah Sounding for Thursday Evening - impressive

NAM brings one round on Wed Night (late) to the KPAH Region and then things calm down late on Thursday Morning. NAM would indicate a period of heating and then explosive thunderstorm development ahead of the frontal system. Storms would race through Southern Illinois, Southeast Missouri, Western Kentucky, and Indiana after 4 p.m. This is if the NAM holds true.

Sounding for KPAH

 

[V. Gensini]

I have 2 problems with the set up for Thursday. The first being timing. GFS/UKMET bring the system through much faster than the WRF. I suspect that the GFS may be too fast, but there are still considerable timing issues that need to be monitored. The other being morning convection. IF the WRF holds true, there will be plenty of morning convection ongoing during the beginning of the period. I suspect that the cloud cover may delay convective initiation and overall instability in the warm sector. Nevertheless, something to watch the next couple days!

 

[Justin E. Reed]

I'd go ahead and get my stuff ready for this day, just wish I was in Nashville to witness this, first April 7th 2006, now this, dang my military service!

 

[Justin E. Reed]

It appears more favorable for severe storms on Thursday as strong and persistent S/SW flow will bring ample moisture from the GOMEX to the TN Valley. It is possible for a few tornadoes in TN, MS, AL, and S. KY, if enough sun shows, if not, a healthy squall line or derecho is probable. This area had an unusually quiet Spring, so the 2nd season may be kicking off with a bang!

 

[Bryan Smith]

IMHO, I am optimistically reserved about this upcoming system. Based on my own experience forecasting these relatively "cool season" events, here are just a few of the many things that I believe are needed for a robust severe episode given this setup/time of year.

1) Increasing flow throughout the sounding profile. It seems that the juxtaposition of your upper jet and low level jet in some of the model output leads one to believe this may occur.
2) Adequate moisture. A basic rule of thumb I follow with these spring/fall/winter severe episodes is to follow the moisture. Based on surface obs along the LA coast and thereabouts 100 miles inland, there is a sufficient moisture reservoir in place (i.e. 62-68 deg. F at 16/02Z). Now, it just needs to be transported northward. I usually count on about a 36-48 hr window for a portion of this airmass to reach central Indiana (my chasing grounds) if a dynamic system can tap that air. Based on my prior forecasting experience, I would not be surprised to see dewpoint values reach 60, perhaps the lower to mid 60s in portions of central and southern IL and IN. These values in my opinion are pretty good given the strength of the wind field.
3) Instability. Given the above average temperatures this late in the year across much of the MS and OH Valleys, surface temperatures should make it easily in the 65-75 degree range given where temperatures will be on Wednesday. I can see temperatures reaching the 70-75 degree range with little help from low level WAA in some parts of the OH Valley/Mid MS Valley. The low level WAA regime kicks into high gear across the MS Valley region Wed. night and into the day Thurs. aiding in the deepening of the system. Height falls appear to be a distinct possibility based on the progged evolution of the system as a whole, most notably in the last few runs of the NAM. I would venture to say adequate conditional instability would be present in conjunction with each other based on those 2 factors.
4) CAP. Based on pattern recognition alone, it is usually a fortuitous/ominous sign depending on one's perspective, when the 700mb flow helps to transport a version of the Plains' EML eastward towards the MS river. Given the decent southwesterly 700mb flow (45-55 kts) progged to be in place by Thurs. across portions of IL/IN, this appears possible and may aid in helping to determine the overall convective mode by helping to limit widespread convection along the north-south boundary.

Just a few thoughts and ramblings.
The continued deepening of a rather strong surface low in some of the model output is an encouraging sign if it occurs in reality. It would enable backing of the low level flow in the warm sector and inhibit the surface flow from veering to the southwest. This veering often seems to be a negative for the magnitude of OH Valley severe. I am particularly impressed by the magnitude of the low level profile in terms of low level shear and instability, especially in the 16/00Z NAM.

In reference to the KPAH sounding posted earlier, that looks like a classic severe weather sounding. If that were to occur in reality, look out. Notice how the CAPE and low level shear are juxtaposed, together one could make a strong supporting argument for the development of low level mesocyclones based on that sounding alone. But, that is model output 70+ hrs. in advance of the event. So we'll have to wait and see.
I am going to take the cautiously optimistic route and guess that there will probably be an upper end slight risk with the possibility* that conditions may warrant a moderate risk come Thursday. I'm going to wait for more fine-scale details to appear probable in the real-time fields and for more consistency to become evident in the model output before convincing myself to chase the cool season racers and their 40-50kt storm motions.

-Bryan

 

[Brandon Sullivan]

I as well am obviously very excited abou this event, I am fortunate enough not to have school on thurssday. My only concerns with the system is the widespread amount of precip, which may not allow for ample destabilization, and discrete activity as well.. However with the kinematic fields in place, looks to be a significant event on tap for the area. Local destabilization though will prove critical I think. I want to take a look at some models and get another look at 12z.

 

[V. Gensini]

Timing is now coming into better agreement and as I thought, GFS/UKMET are slowing the system down considerably! The WRF is also slowing the system down while deepening it, which is a great sign for svr potential. This will definitely be one to watch! I think its going to come down to overall destabilization in the warm sector after the morning convection...

 

[Robert Dewey]

I think we can add WI and MI to this thread per the latest SPC SWODY3 MDT.

06Z NAM showing LI of -9C across southern WI, northeastern IA, and northern IL between 18Z and 00Z... spreading eastward (although weakening) with time. NAM shows initiation around 21Z across IA/MO/IL, spreading rapidly northeastward, with further development to the south. If the 06Z NAM is correct, then I would expect the most intense action (including the possibility of strong tornadoes) to stretch from northeastern MO into southern WI. From that point on, things may progress into a more linear structure with the possibility for a very damaging squall line, but with strong low level shear (directional, speed) remaining, I still foresee a possible tornado threat.

Again, this is all based on the 06Z NAM, which is just one of many possible outcomes.

 

[David Draun]

Wow, this is starting to look a little bit like 9/22/06 in some aspects. On that day, two separate areas of supercells developed in IL and MO. One area of low-toppers fired along a secondary warm front in Northern IL and the Chicago Metro, and other supercells including an F4 tornado producer in the southern parts of IL and eastern MO. I think areas near Murphysboro, IL were hard-hit.

It appears instability will be even stronger than that day last year. I think if we can get enough surface heating, we could be looking at a nasty outbreak. A fly in the ointment would be the low level winds parallel to the front, favoring linear rather than discrete action. However even the SPC says that the general convective mode is uncertain. I think winds will be better backed near the warm front closer to the surface low, and if there is modest instability, putting the northern parts of the moderate risk area under the gun for tornadic supercells with more of a linear mode favored to the south. I will be watching the situation Thursday very carefully no doubt, and I may head out if things continue to look good. Geez, with the wind fields the way they are, these cells are going to be screaming to the NNE at over 40 mph. Gotta get in position early and let the storms come to you.

 

[Brandon Sullivan]

Upon looking at the new 12z Data I was pleased.. The WRF/NAM has slowed the system down bringing the surface front across the IL river around 15-17Z timeframe. The 12z NAM showed QPF values of 2+ inches along the front at 12z while it is in IA. Then totally diminishes on any precip by 18Z. Then it totally lights up things again along the fron after that.. 60+ DP's and LI's dipping all the way to locally -9 C !! 15z forecast image per NAM shows 2200 J/KG. Flow appears paralell to the front, and shear is mainly of the speed variety.

The GFS and UKMET have not caught onto the slowing trend yet, and we will have to wait to see what happens. But the forecast slowing would provide somewhat more favorable timing for me in West Central IL.

Right now Im looking at the Lincoln IL area, or maybe even more SE into Champaign. SPC SREF shows the Sig Param's most favorable in the southern half of IL. So looks like a pretty widespread threat, but it will be insolation and local modifications of atmosphere that will be key to this mid fall event...

 

[Adam Lucio]

Wow, this is starting to look a little bit like 6/22/06 in some aspects..

Did you mean 9/22?

Regardless, GFS looks a tad bit faster with this system than the NAM. NAM looks a little bit juicier with TDs. I havent had much time to pour over much else.

My original target was Champaign/Urbana as well, seems to me the best chance for discrete cells will be well ahead of the front before the squall line marches northeast. Then I thought about targeting northern MO despite all the hills due to slightly better shear. Embedded supercells should also be possible within any line segments.

Next few runs should be interesting.

 

[Craig Maire II]

I have to work tomorrow so I am grateful about the prospect of having a late season outbreak in my own back yard on Thursday! Since I do not have to work on Thursday I will be on HIGH alert and probably will be chasing. I'm hoping that the system slows down even more and places more of the moderate risk in my area (I know it doesn't mean much just words but it still makes me feel better). About half of Iowa is included in the slight risk so I'm pretty sure that at least some low-topped sups will probably fire in eastern Iowa before entering Wisconsin, Illinois etc. and the moderate risk is only around 100 miles east of me so with highway 20 giving me a straight shot east I should be set!