Classic Plains Tornado Outbreak Ingredients
Posted by JAC on 5/24/2011, 7:47 am
by Jonathan Erdman, Sr. Meteorologist

http://www.weather.com/outlook/weather-news/news/articles/tornado-outbreak-tue-classic-ingredients_2011-05-23

Updated: May 24, 2011 6:25 am ET
Sadly, given the events in Joplin, Mo. and Minneapolis this past weekend, we are not through with severe weather this week. Not by a longshot.

The ingredients are in place for a classic Plains tornado outbreak Tuesday. Let's first highlight the area of greatest risk, then step through why this is the case.

Tuesday's Outbreak Forecast

NOAA's Storm Prediction Center typically issues a high risk for severe t-storms only when a major severe weather outbreak is anticipated. This often includes a concentration of strong tornadoes.

Below is a clickable map highlighting the area of high risk Tuesday in the Plains states.

Cities in "high risk": Wichita | Oklahoma City | Tulsa

TWC Severe Weather Expert, Dr. Greg Forbes (Find him on Facebook) has updated the latest TOR:CON tornado-risk index values, as well.

http://www.weather.com/outlook/weather-news/news/articles/torcon-daily-forecast_2011-05-19

This is the fourth "high risk" day so far this spring. Here is a list of the other three days and how they turned out. While not all "high risk" days verify as major severe weather events, the record so far this spring is perfect.

SPC "high risk" days in 2011

Apr. 16: "Carolina Outbreak" (69 tornadoes...32 in NC...13 in VA...26 killed)
Apr. 26: "Mid-South Outbreak" (56 tornadoes....1 killed in AR)
Apr. 27: "Southeast Outbreak" (232 tornadoes...315 killed)

Why is Tuesday looking so dangerous? Put simply, it's a setup straight out of meteorology textbooks. Let's delve into the ingredients.

Moisture: Check!

First, you need rich low-level moisture to provide fuel for thunderstorms. Below is a current dew point map of the Plains showing surface moisture values. Values in the 50s are considered a minimum bound for significant severe t-storm development. Dew points in the 60s and 70s indicate more muggy air near the surface. Assuming the moisture is deep enough, the more humid the air, the lower the thunderstorm's cloud base, more favorable for not only supercells, but tornadic supercells (more on that later).






Instability: Check!

To support severe t-storms, you want cold, dry air aloft with warm, muggy air near and just above the earth's surface. Meteorologists use a parameter called convective available potential energy or "CAPE" to identify areas where that instability exists to support severe thunderstorms. The higher the CAPE, the stronger a thunderstorm's updraft is, assuming there's no warm air aloft to squelch the formation of thunderstorms.

As you can see highlighted below, the central Plains have plenty of instability, denoted by the purple and dark blue shadings.






For supercell thunderstorms, you also need strong wind shear.

Wind Shear: Check!

Put simply, changing wind speeds and direction with height, "wind shear" allows a thunderstorm's downdraft to deposit heavy precipitation away from its updraft. In addition to vertical pressure gradients generated, this allows a "supercell" thunderstorm to persist for hours at a time.

One way the so-called "deep-layer" shear is maximized is by a "streak" of jet-stream-level energy punching over the top of the warm, humid low-level air mass.

In the image loop below, you can see that jet streak coming out of the Desert Southwest, punching into the Plains states Tuesday afternoon and evening.





So now we've detailed some ingredients for supercell thunderstorms. But what is it that makes them tornadic?


Near-surface boundaries: Check!

Finally....you need a mechanism to lift the warm, humid air near the surface, a place where near-surface winds converge, or come together. In the Plains this time of year, this often happens along either the dryline (brown scalloped line in map below) or along cold or warm fronts.

In addition, research has suggested that what separates tornado outbreaks from severe thunderstorm outbreaks is the magnitude of the low-level wind shear, or how quickly the winds change from the surface to just a thousand feet or so aloft. Generally speaking, the greatest low-level wind shear can be found near surface low pressure systems or warm fronts. Indeed Tuesday, a dryline, surface low, and warm front will all be in play.






A strong cap: Check!

With all these ingredients in play, sometimes the atmosphere can't keep a lid on it long enough to realize its full potential.

The most classic Plains tornado outbreaks feature a strong "cap" or inhibiting lid, a few thousand feet in the atmosphere effectively squelching t-storm development until peak heating of the atmosphere occurs in late afternoon.

Think of this in terms of a pot of water on the stove. If you don't cover the pot of water, the steam escapes freely as the water boils. When the atmosphere remains uncapped in the morning or early afternoon, you may still get severe thunderstorms, but they may be more numerous and less vigorous, and may squash the potential for more dangerous storms later, or completely stabilize the atmosphere inhibiting any later-day storms.

However, add a cover to the pot, wait a period of time, then remove the cover, and the steam rushes up from the boiling water more vigorously than if it wasn't covered. This is the role of the "cap" in the atmosphere. The result, dangerous supercell thunderstorms erupt in the late afternoon or early evening.

You can see the cap holding over the Plains in the animated model forecast radar map below. Note the lack of rain/t-storms in southern Kansas, Oklahoma, and Texas during the day, followed by an eruption of storms by late evening.





Unfortunately, this isn't just limited to Tuesday. Dr. Forbes projects the tornado outbreak spreads east Wednesday. The peak tornado threat Wednesday appears to be from Iowa to northeast Texas.























80
In this thread:
OK & KS Under High Risk for Severe Weather Today - JAC, 5/24/2011, 5:44 am
< Return to the front page of the: message board | monthly archive this page is in
Post A Reply
This thread has been archived and can no longer receive replies.