Friday, June 14, 2013

Forecasting Tornadoes In Oregon

It was just last month that I wrote about why the Pacific Northwest is not a tornado "hotspot". Of course, as a meteorologist, I should know better than to challenge Mother Nature like that. On Thursday, McMinnville was hit with an EF-1 tornado. No injuries were sustained but a good amount of damage was done to several buildings and highway 99W was re-routed around Lafayette. The twister was the first in Oregon for 2013 (Washington had an EF-0 storm in Battle Ground in March) and the first tornado since 2011. The Willamette Valley last saw a tornado back in 2010 when an EF-2 tornado struck in Aumsville. So they DO happen! But why can't we forecast these "cold-core" twisters that touch down in the Pacific Northwest?

Here is a snip-it of an essay by John Sauder, a Canadian meteorologist, who describes a "cold core funnel":
"A cold core funnel is a vertically tilted rotating column of air under a rapidly growing convective cloud, but the atmospheric conditions are different than those conditions that produce typical funnel clouds or tornadoes"

Imagine a typical Oregon spring rainy day. You know, those days where the weather doesn't seem to be able to make up its mind? Rain, sun, clouds, hail, sun, rain. The environment described by Suder above is very similar to those kind of Pacific Northwest days. Tornadoes in the Midewest and Southeast do not form this way. Tornadoes in "Tornado Alley" are much easier to see coming, not to mention much larger, and thus it is easier to forecast and issue warnings. No tornado or severe thunderstorm watches or warnings were issued with yesterday's McMinnville cell. 

Radar is an amazing weather product that helps meteorologist identify characteristics of a tornado. This radar image has almost NO characteristics of a typical tornado. Take a look at the radar comparison between McMinnville's tornado and the tornado that ripped through Moore, Oklahoma last month:

Glaring differences! On the right, the Moore tornado signature is a classic! This is what is known as a "Hook Echo", the rain shown wrapping around some very intense rotation. No such signature in the McMinnville radar image. The only similarities between the two appear to be rain intensity. The pink on both images show heavy rainfall and most likely hail present as well. The Moore radar had a Tornado Warning associated with
it, while the McMinnville did not. Why was no warning issued for yesterday's storm? Sauder offers up an explanation:

"...these cold core events happen on a small scale (mesoscale) and are typically rather short lived."

This would answer why it is so difficult to issue warnings on Oregon twisters. They just pop-up! Just like those showers and sun-breaks that are so common during our spring weather. Another tool radar offers up is called "velocity". That helps identify wind directions in a storm. Here is a typical tornado signature using the velocity tool: 
Tornadoes are often found right were green meets red. The different colors mean that winds are moving in opposite directions, and where they touch is where winds are spinning in a tight rotation. This velocity signature is one most tornadoes in the Midwest and Southeast contain. But here in the Northwest, we don't often see this strong of a signature. Sauder says,

"The velocity signatures are very small and usually blend in with background noise...makes cold core funnel or cold core tornado detection using radar almost impossible. This difficulty in forecasting such events results in short , if any, warning times for the public"

On top of the weak signatures, the elevation at which these signatures occur are rather close to the surface. The radar beam that sweeps over McMinnville hits at somewhere near 4,000 feet. That is too high to detect significant velocity circulations at the surface. 

All these factors challenge any meteorologist in the Northwest when it comes to tornadoes. Seeing the radar triggers described above in real-time make issuing watches and warnings much easier in other parts of the country than in our neck of the woods. So while these events are very rare and not significantly strong, the tornadoes we see still present a very real danger when the spawn. 

Wednesday, May 22, 2013

Understanding Tornadoes

It's something that native Oregonians don't fully understand--the anatomy and history of tornadoes.On average, Oregon sees one to two tornadoes every year. However, there is no comparing our twisters with those in the Midwest and South. A chief example, Moore, Oklahoma. The monster twister that rolled through the Oklahoma City suburb is just the latest in a storied history of the most active-weather state in the U.S.

It takes a perfect atmosphere for a tornado to form. Plenty of moisture in the air, instability, strong winds and ultimately, the trigger needs to be pulled to initiate storm formation. There is no better set up in the world than right here in the U.S. More specifically, east of the Rocky Mountains. The moisture roll player is the warm Gulf of Mexico. With mild water temperatures year around, the Gulf of Mexico provides plenty of "fuel" for storm development. All that warm air can travel freely over the land and deep into the Plains due to the lack of "elevated terrain". There is no physical barrier that would wring out moisture or block it from proceeding North or East or any direction until it reached the Rockies.

All that warm air then needs to be lifted. A typical lifting mechanism is simple day-time heating. As the sun breaks through the clouds, it warms the air and causes bubbles of lifting air. Those lifting bubbles then grab the moisture from the Gulf and sends it upwards. A cold front moving through the area will also do the trick. The boundary will help push the warmer, less dense air up and over the colder, more dense air.

So we now have moisture and lift. Next we need to create instability. Those bubbles of air won't continue to rise unless the air surrounding the bubbles is cooler than the bubble itself. That cold air comes compliments of Canada. The still-cool spring air sinks down from the north behind our cold front and interacts with those rising warm bubbles. This creates instability and allows storms to being to form. Those big, towering, white puffy clouds are a result.

Tornado Ingredients
Almost there. We have the storms brewing, probably just dropping heavy rain and hail along with some lightning and thunder. In order for a tornado to spin up, we require spinning air above the ground. That is provided by a westerly jet stream, or fast moving pocket of air way up in the atmosphere. That gets the air surrounding these big thunderstorms rotating at different speeds and at different directions. This is called "wind and speed shear". Once all of these ingredients are present, tornado watches and warnings must be considered.

Oregon lacks several of the ingredients previously listed. We lack a significant warm body of water to provide moisture. The Pacific Ocean along the U.S. west coast is a cold current body of water. Cool water helps stabilize the air passing over it. Thus, the air coming onshore in Oregon is more stable than the air coming in from the Gulf of Mexico in the South. Coastal and Cascade mountain ranges also pose an issue to the formation of tornadoes. The rugged terrain often disrupts organization of all the ingredients required. Lastly, we just don't have a good source of warm, moist air that can filter in to our area. The most common form of active weather we Pacific Northwesterns' get are summer time thunderstorms. A good southerly push of warm, more moist air from California will typically help spark those storms. I think most of us will take the grey, drippy days instead of a regular threat of severe weather.

The Moore, Oklahoma tornado was just a perfect recipe of weather. The tornado wound up being ranked at the top of the Enhanced Fujita (EF) tornado scale, which is a 5. The EF scale, a re-tooled scale of the original Fujita scale, remains based upon damage caused from the storm. National Weather Service workers will asses storm damage and then use that damage to estimate wind speeds from the twister. A common misconception is that tornadoes are ranked on wind speeds. Hurricanes are classified using their wind speeds. Back in 2007, the NWS implemented the EF rankings, slightly adjusting damage-to-estimated wind speed scale. You'll notice that the new scale does not include winds of 300+ estimated winds but the old Fujita did. It is widely suspected that tornado winds are the fastest winds on earth. The confusion between Fujita and Enhanced Fujita is a bit perplexing but it really shouldn't be. In my mind, they are almost interchangeable but I refer to them using "EF" just to follow along with the NWS.

I had mentioned above that the Moore tornado was officially classified as an EF-5. Since 1950, when the NWS implemented the Fujita ranking scale, 58 tornadoes received the top classification. Moore, Oklahoma made it 59. In fact, of those 59 F/EF-5's, 7 of them have ravaged the state of Oklahoma. That is more than any other state and compromises just over 8% of all F/EF-5 tornadoes in U.S. history. Needless to say, Oklahoma is "Tornado Alley".