Three Factors Explaining Oklahoma's Quiet Storm Season
Posted on Tuesday, May 27, 2014 at 3:00 PM MST
It's no secret by now that the severe weather season in Oklahoma this year has been nearly non-existent. The tornado count for the entire state so far this year can pretty much be counted on one hand. So what has caused the storm season to be so quiet? There are plenty of theories, but I will look at three of the more obvious ones.
Reason 1: Extreme Drought in Oklahoma
As of May 20, 2014, 95% of Oklahoma is experiencing some sort of drought, and more than 60% of the state is in either extreme or exceptional drought, the two most severe categories. This area consists mainly of the western 2/3 of Oklahoma, plus the panhandle.
When areas such as western Oklahoma have been under a severe drought for as long as they have, there is hardly any soil moisture. When the spring winds blow over such a large area of extreme drought, it acts like a hair dryer to dry out the lower levels of the atmosphere. No low-level moisture means there are no storms, and the ones that do form are often high-based and poorly organized.
Reason 2: Out of Phase Large Scale Systems
With severe weather, the term "out of phase" refers to the timing of the severe weather ingredients occurring at different points during the day. It seems like most of the powerful storm systems this year have been out of phase as they crossed Oklahoma. Remember there are four main ingredients required to generate severe weather: instability, moisture, lift, and shear.
Let's take a look at the April 27, 2014 severe weather event that began in Oklahoma and continued into Arkansas and Mississippi. April 26th saw ample moisture return to Oklahoma and Arkansas for severe weather, and with the jet stream directly overhead, there was plenty of shear over both states. Daytime heating is the primary driver of instability, peaking around 4-5 PM, and reaching its minimum around dawn. Forecast high temperatures across Oklahoma and Arkansas were forecast to be well into the 80's on April 27th, providing plenty of instability for severe weather.
The only ingredient that's missing is the lift. Lift is provided by the main upper-level storm system as it passes overhead. On April 27th, the main upper-level low passed over the Oklahoma City area at 6 AM, when instability minimized. This is as close to completely out of phase as you can get. There was not enough instability at that hour of the morning to support supercells, so we ended up with a squall line that had some gusty winds and large hail.
As that system progressed east throughout the day, instability in the moist sector skyrocketed with the daytime heating. By the time peak heating rolled around and everything was then in phase, the main upper-level low was over western Arkansas, where there was more than enough instability to support supercells. Supercells exploded on the dryline and a tornado outbreak began shortly thereafter, with a few violent tornadoes occurring that evening in the Little Rock area. Shift the timing of the upper-level low 12 hours earlier and that tornado outbreak would have been occurring along the I-35 corridor in Oklahoma.
Reason 3: The Unseasonably Cold Winter
It's also no secret that this past winter was very unseasonably cold for nearly the entire USA. This included the Gulf of Mexico, the southern plains' main source of rich tropical moisture in the spring. As a result, water temperatures in the Gulf of Mexico were much colder than normal. Warmer air can hold more moisture, so colder Gulf temperatures meant less moisture return to the southern plains. Combine this with the extreme drought in Oklahoma, and that's all she wrote.
Has This Happened Before?
Believe it or not, this has happened before. I'm not going to go into the details, but there were zero tornadoes in Oklahoma in May, 2005, 4 tornadoes in Oklahoma in May, 2006, and 3 tornadoes in Oklahoma in May, 2012. Years such as 2010, where there were 91 tornadoes in May, seem to more than make up for it, though. You can see the tornado statistics at the National Weather Service's Monthly Tornado Statistics for Oklahoma.
As of this posting, there have only been 5 tornadoes in Oklahoma in 2014 (4 in April and 1 in May). Since 1950, the record low for Oklahoma tornadoes is 17, which occurred in 1988. If the drought really takes hold this summer (it sure seems headed that way), it would not surprise me at all to see that record fall.
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