Monday, October 8, 2012

Major Tornado Potential In Midwest Next Weekend??

There is potential for a tornado event over the Midwest next weekend, as a strong disturbance moves into the area.

500mb forecast of the GFS model has a negatively tilted disturbance centered over the Iowa/Minnesota border on Saturday, October 13. 500mb winds are being shown as strong as 80 knots, indicating that this trough is indeed pretty strong. The strongest winds are tilted to the southeast, indicating that this system will be more negatively tilted. A negatively tilted storm allows for cold air to breach the center of the storm and intensify instability. Some of the worst thunderstorms and severe weather events have occurred with negatively tilted storm systems.

The lower level jet stream is also at a heightened state, with winds surpassing 60 knots across much of the Midwest and western Great Lakes. The presence of such a strong lower level jet stream on Saturday evening is worrisome. It is known that the lower level jet stream (LLJ) is nocturnal, and provides fuel for summertime thunderstorms. If the LLJ strengthens overnight, the threat for strong storms and possibly tornadoes grows.

The jet stream itself in the 300mb level of the atmosphere is giving this system some love. The GFS is indicating that a jet streak will form over the Midwest as the system digs into the region Saturday evening. A jet streak is basically a part of the jet stream with stronger winds than the rest of the stream. This particular jet streak is progged at 90-100 knots- a jump of about 20 knots from yesterday's GFS forecast for the same time frame.

As of now, the atmosphere is looking very robust and supportive of some strong to severe thunderstorms. Let's look now at shearing- a crucial ingredient for tornadoes.

This is something called deep layer shear, and is commonly used to find the potential for supercells and tornadoes. A general rule of thumb is that supercells may be found in areas with deep layer shear (DLS) greater than 40 knots. Looking at the image of DLS above, valid for Saturday evening, we find a wide expanse of winds surpassing 60 knots, indicating that this environment is conducive for supercells.

We have now established that the jet stream, lower level jet stream, and deep layer shearing will be present. Already we have a modest set-up for severe weather and possible tornadoes. Let's now take a look at shearing in the lower part of the atmosphere.

This image, still valid for Saturday evening, depicts shearing as shown above, but at the lower levels. Where the above image shows surface-500mb shearing, this image shows surface-850mb wind shearing. This surface-850mb wind shearing can help us establish if there will be wind shear in the lower part of the atmosphere, which is also needed for tornadoes and helps severe weather.
This lower level shearing maxes out as high as 70 knots in the Midwest, now confirming my thoughts that this event has great potential.

This information has combined to tell me that this event does indeed have potential to be a tornadic event. However, there are some crucial pieces of the puzzle that lessen the threat.

•This is not spring/summer, therefore the atmosphere is not as dynamic as it could be.
•Precipitation out ahead of the actual event may dramatically lessen the threat.
•Models are projecting instability to be fairly low.

All in all, we are still a while out from this event. But, in my opinion, the Midwest needs to watch this very closely, as I see great potential for severe weather and possibly some tornadic action. Just how much potential actually comes to a point remains to be seen.



Anonymous said...

Chicago in trouble??

Anonymous said...

well, according to the spc, northeast iowa (where i live) seems to have a pretty good chance,and seeing that there will be a considerable amt. of negative tilt and wind shearing, could definitely be a good closer to the season, the only thing that might be holding this event back is how rapid the destabilization will occur, hoping that wont happen, anyways itd be awesome if u could post some video of chasing if u had the time :). btw i like the "expect the unexpected" banner too