## Friday, April 13, 2012

### High Risk Severe Storms/Tornadoes Saturday

A HIGH RISK for severe storms and tornadoes Saturday and Saturday night for the middle of the country was issued by the Storm Prediction Center.  High risks are very rare, normally reserved for drastic severe weather situations. No doubt many long path tornadoes will develop tomorrow.

Historically, high risk areas are relegated to the traditional "tornado alley" areas. Below is the HIGH RISK climatology put together by University of Oklahoma graduate student Patrick Marsh who runs a blog about meteorology data, severe storms and a slew of other meteorology related topics.

Upon first inspection, the map seems straight forward.  Yet the work involved in creating this easy-to-read map is not so simple. Normally I steer clear of detailing complex descriptions due to their esoteric nature. However this time, I made an exception. Here is Patrick Marsh's explanation on how he took almost 20 years of HIGH RISK data and morphed it into the above graph.

I took the polygon outlining the first Moderate [High] risk during a given year, regardless of time of issuance. This means I treated a 12 UTC issuance the same as a 1630 UTC issuance and 2000 UTC issuance. I took the risk polygon, placed it on a 4km grid (specifically grid number 240), and activate all grid points that fell inside the risk polygon. This left me with a grid of 1′s (inside outlook) and 0′s (outside outlook). I created a grid for each year and then summed all the grids together. This gave me a grid containing the number of times each grid point was within the first Moderate [High] risk of the year. I then divided each grid point by the number of years I was examining. This left me with the probability that a grid point would be contained in the first Moderate [High] risk of the year.

Got all that?

What about the areas where the FIRST HIGH RISK is issued each year? The same technique was applied.

The same system will move through OHIO and CLEVELAND on Monday.  Even if the parent cold front is a shadow of its Saturday self, we can anticipate some severe storms. More updates this weekend