Thursday, March 19, 2009

Criteria For Severe Thunderstorms; It's Changing For Some

Right now, most of you probably know the three criteria for a thunderstorm to be classified as severe.

  • Hail 3/4" or greater in diameter
  • Winds to 58 mph or greater
  • Producing a tornado

    Well this year, the central region of the National Weather Service, which covers states from Missouri to Minnesota and Colorado to Illinois...will be changing the minimum hail criteria from 3/4" to 1" or greater in diameter.

    Each weather office in the central region will change this criteria at their own pace, sometime between April 1 and July 1.

    By July 1, the whole region will be on the same page.

    One main reasoning behind this is extensive study has shown that damage really doesn't begin until hail reaches 1" in diameter, and that the warning frequency will decrease by raising the minimum -- since the majority of warnings that get issued, especially in this part of the country, are due to hail.

    Here is a great link with a short video that talks more about this change and the history behind it.

    Speaking of warnings, the recent poll has ended...and the majority of our 75 voters get their information from either NOAA weather radio or the internet. Here are the results of the poll...

    Where do you get your weather warnings?

  • NOAA Weather Radio -- 37%
  • Text Message -- 2%
  • Email -- 2%
  • Internet -- 41%
  • Television -- 14%
  • Radio -- 1 %

    I've posted a new poll for you, asking about the greatest 24-hour precipitation you have recorded since joining CoCoRaHS.

    And looking ahead to next week, storm chasers all over are licking their chops...and making plans to come to the high plains.

    We could be looking at the first widespread severe weather outbreak of the season on Monday. Areas that may see active weather stretch from south-central South Dakota to north Texas.

    We'll have to wait and see how it all comes together.

    1. By 24-hour precipitation, are you talking 7 AM to 7 AM, or any 24 hour period? My biggest number is 4.39", but the 24-hour period was actually something closer to 7"...

    2. The largest amount of rainfall in 24 hours occurred where I lived in Montgomery in the tear 1997. 11" of rain fell from 7:30 in the evening thru the night to 7:00 the next morning. It was a 24 hour thunderstorm. Flooding was immense.