Thursday, October 11, 2012

Damaging Winds, Tornadoes Likely Saturday

Damaging winds and tornadoes are among the likely events that will strike Saturday into Sunday in the Midwest and Plains.

A strong disturbance will move into the Plains and fire off strong to severe thunderstorms in the afternoon and evening. The storms should start as supercells, with a tornado threat present. Iowa and Missouri could catch some of the tornadic action. However, as night falls, a more linear storm threat will take shape, and possibly extreme damaging winds will evolve as the primary threat.

Lower level winds will be strong over the expected squall line, which will start in western portions of the orange area in the evening and strike the Midwest overnight. As these lower and mid level winds strengthen with nightfall, any bow echoes that emerge from this situation have great potential to do harm across the region, with damage to structures and a risk to life present.

Tornadoes will be most probable in Iowa and Missouri, where a jet streak and favorable shearing will combine to create a feeding ground for tornadoes. However, given that instability will top out below 2000 j/kg, tornadoes that do form should be weak to moderate. Stronger cells that develop may be able to get a stronger tornado out.

I am declaring Saturday a Storm Action Day across the following states:


This means that people within the mentioned states should make preparations for severe weather. If the models continue to trend strong with this system, I may have to issue a Critical Storm Action Day bulletin, but that will be decided tomorrow around the 12:00-1:00PM CT timeframe. The announcement will be posted on the Facebook page, which you can find on the right sidebar.


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LRC May Be Active This Winter

Above are ensemble forecasts for various time periods throughout the next 5 to 14 days. As you can see, there are many instances of ensemble members showing deep depressions in the US and Canada. I'm going to tell you what that means for us, and what it may do to the Lezak Recurring Cycle.

The Lezak Recurring Cycle, commonly abbreviated as the LRC, is an annual pattern that was first recognized by Gary Lezak. Lezak found that a pattern sets up in late fall (October into November) that repeats itself throughout winter. Each time this repetition happens, it is a cycle. Here's the low down.

•Each cycle lasts 40-60 days.
•Storm systems, high pressures, warm ups and cold snaps are included in these cycles.
•The pattern, while annual, is never the same as the last.

We are in the observation period of the LRC, looking for signs of the first cycle and trying to get the pieces of the puzzle to fall into place so forecasters can begin using the LRC. Above are individual ensemble member forecasts for different time frames through the next 5-14 days. As you can see, many members have deep depressions in the lines, which are forecasting the 500mb heights. These depressions are storm systems- the deeper the depression, the stronger the storm.

The fact that we are seeing such strong storm forecasts in the next couple of weeks makes me think that the Lezak Recurring Cycle may be playing a part here. Because these systems should strike when we are still in the observation stage, I find it very plausible that these systems may be found in January, when the next cycle most likely repeats.


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