Thursday, June 18, 2009

Moderate severe storm risk - more tornadoes possible

Weather Headlines

* At least 8 separate tornado touchdowns reported in Minnesota Wednesday, the first of the year. Hardest hit: the Austin, MN area, where a large, violent tornado was reported around the dinner hour, possibly a "multi-vortex" tornado comprised of multiple, smaller tornadoes all swirling around a common center.

* Slight to moderate risk of severe storms again today: potential for damaging hail and isolated tornadoes.

* Rainfall amounts of 1-2" quite possible by Friday as a slow-moving warm frontal boundary lifts northward across Minnesota.

* Very high dew points today: near 70 (tropical levels of moisture streaming northward).

* Drying out in time to salvage a sunnier, drier Saturday with lower humidity levels.

* More T-storms may rumble into Minnesota (from west to east) later in the day Sunday.

* Stormy patterns lingers into most of next week. Computers hinting at warmer-than-average temperatures and wetter-than-normal conditions through the end of June.

Doppler radar from 7:16 pm yesterday. Click on the image to bring it full screen, and you can see the "hook echo" bearing down on Austin. A tornado warning was in effect when the storm hit the Austin area, the same thunderstorm had a history of producing funnels and tornadoes near Waseca and Blooming Prairie. The storm was also a "right-turner", veering off to the right of the other storms in the area. These rogue, right-veering storms are often tip-offs of tornado-producing conditions, and the Austin storm was no exception.

Tornadoes are more likely to form along WARM frontal boundaries than cold fronts. Why? The wind profile is usually more favorable, "veering" winds changing in direction as you rise up through the atmosphere - blowing from the east/southeast at ground-level, then shifting around to the south and finally southwest as one reaches 5,000-10,000 feet. In this wind environment thunderstorms can begin to rotate, spin like a top, protecting the warm updraft, preventing it from being choked off by rain and hail-cooled air. The result: a long-lasting thunderstorm that can spin up enough energy to create a violent updraft funnel, tornadic circulation that manifests itself on the ground when debris gets caught up into the vortex. Boundaries left over from Wednesday's thunderstorms may result in uneven heating of the ground (less warming under cloudy areas, more warming where the sun is out). This, in turn, may spark a new crop of strong/severe thunderstorms later today, a tiny percentage of which may turn severe with large hail and potentially violent winds, even a few isolated tornadoes.

According to SPC, the Storm Prediction Center, the southern half of Minnesota is under a slight to moderate risk of severe storms, the threat of isolated tornadoes increasing the farther south you travel from the Twin Cities. There is roughly a 10% probability of a tornado touchdown within a 25 mile radius of any town south of Glencoe and Lakeville, which doesn't sound that threatening, until you really think about that statistic. A 1 in 10 chance of a tornado every 25 miles or so. Thursday may be a VERY active day for tornado spotting/chasing.
Less than 1 in 100 T-storms will ever go on to spawn a tornado, but roughly 10% of T-storms are capable of producing large, potentially damaging hail with 1"+ diameter hail. There is a 40% probability of large hail within 25 miles of towns across southern Minnesota on Thursday.

I counted 8 separate tornado touchdowns in Minnesota on Wednesday, the first twisters of the year observed in Wilkin county around 4:40 pm. Around the dinner hour a multi-vortex tornado developed from a wall cloud west of Austin, tracking across the northern and eastern suburbs of Austin, producing widespread damage, flipping cars, knocking out power to most of the town, leaving behind injuries.

Tornadoes were also observed near Blooming Prairie and Waseca, but the tornado that hit the Austin area must have been incredible. There were numerous reports of a "multi-vortex" tornado, multiple tornado funnels all rotating around a common center, each one of these "suction vortices" leaving behind extensive damage as it passes overhead.

The image below was created by Dr. Theodore (Ted) Fujita, a brilliant tornado researcher who studied at the University of Chicago. Fascinated by tornadoes and the damage they produce, he conducted extensive research, on the ground and in the air. His work is the basis for the F-scale that was used extensively from the late 50s until 2007, when it was replaced by the EF-scale, a more refined version of the damage scale. Even in 1951 Fujita theorized the existance of these rare "multi-vortex" tornadoes, literally tornadoes composed of smaller tornadoes!

(This is a file photo of a multi-vortex tornado. A similar tornado was observed in the Waseca-Austin area around the dinner hour Wednesday. Each one of the 3-5 individual funnels rotating around a common center would have produced extensive to near total damage, accounting for a wide discrepancy in damage where the tornado tracked across the Austin area).

Paul's Outlook

Today: Muggy and warm with intervals of sun. T-storms are likely, a few may turn severe with large hail, gusty winds, even an isolated tornado. Winds: SE 5-15 (gusts over 50 mph in strong to severe storms). High: 81

Tonight: Humid with lingering showers and storms, some locally heavy rains possible. Low: 62

Friday: Unsettled with more showers and T-storms, some heavy. Best chance of rain PM hours. High: 82

Saturday: More sun, drier. An isolated shower can't be ruled out, but most of the day looks dry. High: 83

Sunday: Hazy sun, quite warm and pleasant. A T-storm is possible by late afternoon. High: 85

Monday: Partly sunny, another round of late-day T-storms. High: 84

Tuesday: Hot sun, feels like summer! High: near 90

Wednesday: Sticky and warm, more T-storms likely. High: 85

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