Saturday, August 8, 2009

Will we break the cap? Severe risk drops off by the minute

(Update: 7:15 pm). Where are the storms? Great question. The atmosphere over southern Minnesota is primed for a potentially wild display of severe storms. But the atmosphere is also "capped". A layer of warmer air has "put a lid" on the airmass, at least temporarily preventing warm, rising thermals from mutating into thunderstorms. The thought was that an upper level storm, a wrinkle of cold air aloft approaching from Montana, would be strong enough to break the cap, but now SPC meteorologists in Oklahoma aren't so sure and neither am I. Conditions for severe weather are off the scale, but if we can't get the process started, then it will be a much quieter evening than just about anyone anticipated. Stay tuned...

Tornado Watch for east central and southeastern MN and much of western WI until 10 pm Saturday night
. Many ingredients are in place for a significant outbreak of tornadoes, even a couple of longer-lasting, long-track, violent tornadoes capable of significant damage. Go about your normal activities, but keep an eye on the sky, and stay connected with local media for possible warnings, issued on a county by county basis. Most towns and suburbs will merely see a generic thunderstorm or two, any severe weather will impact less than 1/2 of 1% of the watch area. But much like a sneeze or hiccup, it's impossible to tell precisely where Mother Nature may launch deadly winds, much more than 20-25 minutes in advance.

That said, I think the MN River Valley, from Redwood Falls into the southern suburbs of the Twin Cities, is most ripe for a family of tornado touch downs.

SPC Update. Risk for much of southern/central MN has been upgraded from slight to moderate

Risk of Tornadoes (composite index from SPC in Norman, OK below). This is an experimental forecast tool, but one of the more reliable indices from SPC is highlighting a corridor from Windom and Redwood Falls into the southwestern suburbs of the Twin Cities as "most ripe" for tornadoes into the late afternoon/evening hours. A tornado watch remains posted for much of central and southwestern MN through 7 pm and the risk of large, damaging hail, straight-line wind damage, even 2-4 tornadoes, is potentially higher than it's been all summer. Stay alert, and stay tuned to local media, including Internet for updates - be ready to move to a shelter, even if a warning hasn't been posted or the sirens aren't sounding. No substitute for common sense. I have a hunch someone south/west of the Twin Cities is going to get blasted by a pretty strong (EF2-3) tornado by 6 pm. Just a strong gut feel.

VGP, or Vorticity Generation Parameter. Usually anything greater than .2 m/s2 gets our attention. Note the .3 field expanding across southern Minnesota. Another ominous sign.

Doppler Radar at 3:45 pm, showing a "supercell" thunderstorm gaining strength near Marshall and Tracy, dropping estimated 2" diameter hail, tracking toward Redwood Falls. This cell, or a nearby cell, is capable of spinning up a tornado in the coming 1-3 hours. Stay alert, especially if you live near the Minnesota River. There is enough low-level wind shear (helicity), instability and moisture (dew points in the mid 70s) for something more than just a tiny, brief, garden-variety twister.

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