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First tornado of 2009?

(Update: 5:05 pm. A tornado warning is posted for Grant, Otter Tail and Wilkin counties until 5:45 pm. People living in the Elbow Lake area are potentially in the path of this tornadic storm, which is producing 3/4 to 1″+ hail. As a rule the larger the hail the greater the potential for the same violent updraft spawning a tornado. The threat of severe weather will continue, spreading from west central into central Minnesota later this evening. Computer models suggest that storms will become even more widespread during the evening and early nighttime hours, spreading from west/north toward the east/southeast, reaching St. Cloud after 8 or 9 pm, into the Twin Cities by 11 pm or midnight. Most of us will NOT see anything severe, but isolated reports of damaging hail, straight-line winds over 70 mph, and an isolated tornado [first of the year in Minnesota] can’t be ruled out).

Latest Supercell Composite Index from SPC, the Storm Prediction Center. The greatest risk of severe “supercell” thunderstorms is over far southwestern MN and much of central and western Iowa this afternoon. With murky sun, a very unstable airmass, and sufficient wind shear aloft conditions seem ripe for strong/severe T-storms. A tiny percentage may produce large, damaging hail, damaging straight-line winds, even an isolated tornado. The greatest threat of violent weather appears to be south/west of the Minnesota River, including Albert Lea, Mankato, Worthington, Jackson and Windom later today. Although the greatest risk appears to be across southern Minnesota, a few strong to severe storms may form over central Minnesota, from St. Cloud to the Twin Cities area – the best chance of severe weather coming from 4 to 7 pm. Stay alert, and check in often for possible warnings later today. It’s entirely possible that Minnesota will see it’s FIRST tornado touchdown of the year later today).

(Midday visible satellite image showing some hazy, murky sun breaking through over much of southwestern Minnesota. This is important, because any heating of the ground and the air immediately above the ground will increase the instability of the airmass over the southern third of Minnesota, increasing the potential for severe storms later today).

(Probability of a tornado touchdown within 25 miles of any point – a 5% risk for much of southwestern and central Minnesota. Many of the ingredients for isolated tornado-producing, supercellular thunderstorms are present. I do expect watches and warnings across parts of central and southern Minnesota later today. Stay tuned).

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