Moderate severe storm risk - more tornadoes possible

Only in Minnesota can one be ankle-deep in mud with dust blowing in your face. The weather changes fast up here, if you blink, sneeze, turn away from just a second or two, the sky draped overhead is likely to change on you, often taking a sudden turn for the worse. Just last week the talk was drought, expanding across much of central and southern Minnesota. We were shivering (in June!) with a few days in a row of 50s, setting records left and right. There was even frost reported up north, with a rare June coating of snow over far northern counties! Many of us were experiencing a rare "where-am-I-living" moment!

Summer has arrived all right, in a hurry. Like turning on a light swtich. Seemingly overnight we went from drought-speak to tornado-talk, jackets gave way to shorts (and neighbors predictably complaining about the humidity). All those tired, tantalizing warm fronts that lurked to our south much of May and the first half of June finally surged north, sparking outbreaks of storms, and yesterday: the first official tornadoes of the year. During an average year Minnesota experiences about 25 tornadoes. Wednesday evening we saw nearly a third of that tally (at least 8 separate tornado reports).

Today will be muggy, murky and potentially violent, with a MODERATE risk of severe storms over roughly the southern third of Minnesota later today. As is always the case any severe weather will ultimately affect a tiny percentage of the state, but I'm pretty impressed with the dynamics, the ingredients lining up for later today. Strong instability, ample moisture near the ground, wind shear aloft (changing wind speed/direction with altitude). The result: a few isolated, spinning "supercell" thunderstorms capable of softball size hail and a tornado or two once again later today.



There's no way to predict exactly where a tornado will touch down. It's the meteorological equivalent of a SNEEZE. All we can do is tell when conditions are ripe for tornadic storms, in which case a tornado watch is issued for a big portion of the state, as many as 30-60 counties can be under a watch. If and when a rotating, tornadic storm is spotted on Doppler (or by professionally trained SKYWARN storm spotters) a warning is issued for counties, or specific parts of counties, so people aren't needlessly warned. A warning means that severe weather is imminent, time to seek shelter.

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.

Since the mid 70s the average lead-time for tornadoes, the time from when the tornado is detected/warned for, and when the violent winds actually strike, has expanded from 7 minutes to nearly 15 minutes, allowing more time for Americans to seek shelter and avoid injury.

In a tornado the safest place to ride out the storm:

* Basement, under the stairs. Statistically this is the safest place to be, under a desk or heavy piece of furniture if possible.

* Small, windowless room on the ground floor, like a closet or bathroom. People have survived F-5 tornadoes by hiding in their bathtubs, a mattress or blanket (or pillows) on top of them.

* Avoid outer walls/windows. Most tornado injuries are the result of flying debris and head trauma. That's why it's preferable to get below grade, underground if at all possible.

* In a school or office seek shelter near the interior of the floor, a concrete-reinforced stairwell or bathroom usually provides the best protection.

* Avoid mobile homes (which can become airborne at wind speeds as low as 80 mph). Avoid large rooms, gymnasiums, auditoriums - these are the first to collapse when a tornado hits.

* In a vehicle you can usually drive away from the storm. If that's impossible get out of your vehicle and seek shelter in a nearby building - OR - a ditch. Do NOT ride out the storm inside a car or truck or underneath a vehicle. Do NOT seek shelter under a bridge overpass (a wind-tunnel effect can increase the threat of flying debris hitting you there).

The good news? Watering the lawn/garden/field should be optional. Many yards will pick up a cool 1-2" of rain between now and Friday afternoon as a slow-moving storm tracks across the state. Skies should start to dry out (and sunny-up) Saturday - most of the day looks dry with only an isolated midday shower or two. Most of Sunday looks sunny, hazy and lake-worthy, with highs topping 80 (even up north at the cabin!) A few late-day T-storms may rumble in from the west late Sunday afternoon/evening, so keep that in the back of your mind.

No more cool fronts are brewing - in fact 90 degrees is not out of the question by Tuesday of next week. Computer models are hinting strongly at the rest of June being wetter than average, and warmer than normal, with highs mostly in the 80s by day.

Welcome to instant-summer, complete with all the unpleasant side-effects, including hail and high water. Thunder-wear gets a work-out through tomorrow, but our reward should be a warm, mostly-pleasant, mostly-dry weekend with temperatures warm enough to make just about everyone get a running start, and go jump in a lake!

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.




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).