Tuesday, July 13, 2010

Risk of a Stray Tornado

Tornado Watch until 11 am: Conditions are ripe for isolated tornadoes from near St. Cloud northward to Grand Rapids and Bemidji through the morning hours - the watch means "watch out", stay alert, and be ready to move to shelter if threatening weather approaches.

Tornado Day. Many ingredients are converging which may go on to spawn isolated tornadoes later today. Most of us will only see a generic thunderstorms, a tiny fraction of 1% of Minnesota may experience a tornado. Right now the best chance appears to be east of I-35, south/east of the Twin Cities during the mid/late afternoon hours. Expect watches and warnings - stay alert and check in for frequent updates.

Significant Severe Threat. SPC has a 45% hash mark over southeastern Minnesota and southwest Wisconsin, meaning a 45% risk of severe storms within 25 miles of any town in the purple-shaded region. The atmosphere floating over Minnesota will be volatile much of the day, but the greatest risk of hail, damaging straight-line winds and isolated tornadoes appears to be east of I-35, probably south/east of the Twin Cities. The latest from SPC is here.

Welcome to a potential wild Wednesday. Many of the ingredients are present for a severe weather outbreak, but (as always) there are a few meteorological wild cards. If clouds persist that will limit surface heating and (greatly) lower the risk of severe storms erupting this afternoon. The longer the sun is out during the midday/early afternoon hours the greater the potential for some rough storms popping after 3 pm or so. Any early-morning rain will cool & stabilize the atmosphere, also lowering the risk of severe storms later in the day. Morning storms will create "outflow boundaries" (sort of like miniature cool fronts). Any afternoon storms will probably be initiated along these boundaries, but trying to determine precisely where these boundaries will set up is next to impossible.

Tuesday Touchdown. Late Tuesday there was a reported tornado touchdown 7 miles east of Georgetown, in Clay county - a funnel cloud observed near Christine, in Wilkin county, very close to the Dakota border. At least 33-34 tornadoes have formed in the skies above Minnesota so far this year, well above the yearly average of 25-30 that normally touch down.

The level of granularity and precision necessary to pin down EXACTLY which counties will get roughed up the most is beyond the scope of the NOAA computer models we use to predict the weather. These "micro-meteorology" events literally fall between the cracks, a 5-10 mile diameter "supercell" thunderstorm much too small to show up on the simulations we look at (where the predictions are spaced 20-40 miles apart). I've said it before, I'll mumble it one more time, predicting when/where a tornado will touch down is like you telling me (right now) where/when you'll sneeze this afternoon. Good luck with that. All we can do is tell when conditions are ripe for tornadoes.

Today we have many of the necessary ingredients:

80-Degree Dew Point? The NAM/WRF model brings 80-degree dew point air into far southeastern MN Wednesday afternoon, off-the-scale levels of humidity that may provide the necessary fuel for a severe storm outbreak later in the day. Data courtesy of twisterdata.com.

1). Low-level moisture (translation: sticky, icky air direct from the Gulf of Mexico). The higher the dew point, the more moisture in the air. The NAM/WRF model is hinting at dew points in the mid 70s by midday. To put that into perspective, a dew point of 60 is considered humid, 70 is tropical/unbearable, once every couple of years we'll see a fleeting dew point of 80, which is unbearable (and potentially life-threatening).

Tornadic Thumbprint? This is a "hodograph" plot. Although the graphic looks vaguely incomprehensible, the red line shows the predicted change in wind direction with altitude for a hypothetical weather balloon rising above Rochester, MN around lunchtime today. Winds are forecast to "veer" with altitude, going from southerly near the ground to southwesterly a few thousand feet above the ground, to westerly at 10,000 feet. This "wind shear" may be sufficient to create a ripe environment for "supercell" T-storms, capable of large, damaging hail and isolated tornadoes, especially south/east of the Twin Cities metro area. For more on hodographs click here.

2). Wind Shear. When winds are not only strong, but shifting direction and speed (turning more clockwise with altitude) then conditions are ripe for horizontal wind shear to be "tilted and stretched" about a vertical axis. A violent thunderstorm updraft can transform this horizontally spinning air into a narrow column of rapidly spinning air shooting straight up at 100 mph. or more. Wind shear also allows T-storms to tilt over (slightly), protecting the warm updraft, preventing a thunderstorm from self-destructing as rain & hail-cooled air wraps into the updraft and kills the storm. These "unprotected updrafts" are prime candidates for storms that may eventually go on to spawn large, damaging hail and tornadoes. Today low-level winds will strong and shifting with altitude, creating a perfect environment for rotating "supercell" thunderstorms capable of tornadoes.

Sounding. Here is the NAM/WRF "sounding" valid for the Rochester, MN area around midday today, showing an "inversion" around 850 mb, roughly 3,000-4,000 feet above the ground (temperatures temporarily warming with altitude), what meteorologists call a "cap". Once warm thermals break through the cap the sounding hints at rapid, perhaps explosive growth of T-storms by mid afternoon). Data courtesy of twisterdata.com.

3). Instability. Temperatures almost always cool with altitude, but a "capped" environment can inhibit T-storm growth - an inversion (a warmer layer of air thousands of feet overhead) can act like a brake on rising thermals of warm air, preventing storms from firing. But if there's enough upward motion, an approaching storm aloft (or surface heating) this "cap" can be overcome. Once the updraft breaks through the inversion storms can mushroom in minutes. But to get tornadic storms you not only need buoyancy, but shear, and today we seem to have both ingredients.

I know - more than you wanted to know about "tornadogenesis." But I'm hoping that a sufficient number of readers checking in want to more know that just the basics, so I'll err on the side of providing TOO much background information. For more on tornadoes check out this excellent FAQ page from SPC, the Storm Prediction Center (the government division of NOAA that issues watches for the entire USA).

Tornado Tip-offs:

If you see any of the following, it's time to head for the basement (or any other sturdy shelter), even if there are no warnings, and the sirens aren't going off.

1). Large Hail. Any thunderstorm updraft violent enough to keep golfball to baseball size hail several miles aloft is potentially strong enough to spin up a tornado. I usually don't get nervous until/unless large hail begins to fall.

2). Greenish Sky. You've heard this one before - if the sky turns an eerie shade of green or yellow, time to head for the basement. Meteorologists don't completely understand why the sky turns a sickly shade of green before a tornado - it may have something to do with white sunlight being refracted as it passes through a cloud of hail towering overhead.

3). Rotating Wall Cloud. A tornado usually forms on the southwestern flank of a severe thunderstorm. Rain gives way to large hail, skies may brighten before the tornado moves through. Watch for any lowering, rotating cloud base - here is where a tornado is most likely to form, on the edge of the updraft.

* Remember, the outdoor emergency sirens were NEVER meant to be heard indoors. They are for outdoor use only. The secret to weathering Minnesota's manic, occasionally violent weather, is to have multiple safety nets: TV, radio, web, cell phone alerts, and the best (cheapest) life insurance you'll ever buy: NOAA Weather Radio, which can be set up to sound an alarm for only your county when a warning is issued.

Where Should You Go?

* At Home: A basement is still your safest bet, under the stairs, under a heavy piece of furniture, like a table or desk (if available). You don't want to hide in the southwest corner, statistics show that the safest place is under the stairs.

* No Basement? No worries. You're still relatively safe if you can find a small, interior room, like a closet or bathroom. Avoid outer walls and windows. People have survived F-5 tornadoes by going into their bathtubs, reinforced by pipes - it's usually the last thing to become airborne.

* Office. Avoid elevators if a tornado warning is issued, find a (concrete-reinforced) interior stairwell - OR - an interior bathroom. Again, avoid outer walls and windows. There's little risk of the office tower collapsing, the real risk (as is always the case) is being hit by flying debris. A tornado is a giant swirling landfill. The danger is not getting sucked up into the heavens like Dorothy in the "Wizard of Oz." The real risk is standing up and being hit on the head by a little rock, nail or other piece of debris, turned into a missile, a projectile by 100-200 mph. winds.

* In a Vehicle. If you can't outrun the tornado or find a building/store/building nearby, get out of your car or truck and seek shelter in the nearest ditch. Curl up in a ball and try to stay as low as possible. Do NOT hide under a bridge overpass (this actually increases the threat of being hit by flying debris). Don't hide under or in your vehicle, which can become airborne once winds top 90-100 mph.

Enough tornado-babble for one day - just know that it may be an "active" day. Expect watches and warnings later today, go about your normal activities, but keep an eye on the sky and stay tuned for updates. As I tell my kids (without freaking them out) it's all about situational awareness. Keep asking yourself: what would you do, where would you go if a tornado was sighted right NOW? Have a continuous action plan, just in case. Chances are (very) good you'll never need to use that plan, but my Boy Scout days come to mind, summed up in two words. "Be prepared."

Good news: after today's fireworks things settle down Thursday, as winds turn around to the west, pumping drier air into Minnesota. Under sunny skies temperatures reach the 80s Thursday and Friday, in fact blue sky should linger into most of Saturday, as winds begin to blow from the south, pumping sticky air back into town just in time for the weekend, setting the stage for more scattered T-storms late Saturday and Saturday night (especially up north). Models are still hinting at a string of 90s the latter half of next week. Enjoy our relatively "comfortable" weather this week - next week may bring the Dog Days of Summer - finally.

Heat Wave Next Week? The CPC, Climate Prediction Center, is predicting temperatures well above average from July 21-27. We may experience a string of 90-degree days, especially the latter half of next week.

8 Injured as German Tornado Hits North Sea Island. After nearly 100-degree heat severe storms ripped across much of Europe late Monday, producing rare tornadoes. The story is here.

Long-Range Outlook: More Heat. The frequency and intensity of heatwaves is forecast to increase in the coming decades - more on the reasons behind the forecast of escalating heat here.

12 Month Running Mean Global Temperature Has Reached a New Record in 2010. The latest data from NASA here.

* Climate Change Needs a Plain English Guide. Amen to that. The story is here.

* Scientists Quantify Global Warming's Threat to Public Health. Here is the story in Scientific American

Study: Global Warming Could Negatively Impact Yellowstone Ecosystem. Climate change isn't a theory - it's already taking place, evidence on display at one of America's most cherished national parks. More from the yellowstoneinsider.com here.

Paul's SC Times Outlook for St. Cloud and all of central Minnesota

Today: Very humid, windy and warm with T-storms likely (best chance early, again this afternoon). Some midday sun is possible, and storms redeveloping this afternoon will probably be severe. A few isolated storms may produce large, damaging hail and isolated tornadoes - watches and warnings are likely later today. Stay alert for rapidly changing conditions). Wind: S 15-25. High: 85

Wednesday night: Evening T-storms (best chance southeastern MN and western WI). Clearing late. Low: 59

Thursday: Mostly sunny, warm and dry. High: 82

Friday: Blue sky, seasonably warm. High: 85

Saturday: Partly sunny, hazy and humid with a stiff south wind (15-25 mph). T-storms are possible late in the day and at night, especially over northern & central MN). High: 88

Sunday: Plenty of sun - an isolated storm can't be ruled out, but most of the day should be dry. High: 84

Monday: Sunny start, then increasing clouds, T-storms arrive late. High: 82

Tuesday: More clouds than sun, still humid. High: 84

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