Saturday, 2 April 2011
We are starting to get into the time of year when severe storm/chase setups become more common across the plains. From what I’m seeing there should be a pretty good jet stream extension across the northeastern Pacific over the next few days, which will result in a significant trough to dig into the western states and eject into the plains mid to late this coming week. With adequate moisture already available in the Gulf, there should be at least two to three chase setups leading up to the main trough, and of course as the trough ejects through the plains.
There a few different reasons for the pattern becoming more conducive for severe setups. The first is the MJO, which has been moving swiftly eastward across the Indonesia region over the past week. It is now making it’s way towards the western Pacific, and the tropical convection associated with this wave is feeding into the Pacific jet. You can see this in the waver vapor image (below). Over the next few days, the MJO will help to extend the Pacific jet (briefly) into the Gulf of Alaska and extreme western Canada. This will force a deep trough into the southwestern U.S. and help set the stage for severe storm setups later this week over the southern plains.
Below is the PNA from the CPC website. Usually, when there is a ridge over the western states the PNA is positive, and when there is a trough over the western states the PNA is negative. As you can see, all the GFS Ensemble members are forecasting the PNA to return to a negative phase after being positive throughout the entire month of March. This is a good sign that Pacific energy will dig into the western states and then eject eastward through the plains over the coming week or so. There are a couple other things going on, but I don’t really want to spend anymore time analyzing the reasons why the pattern is going to behave. Let’s just say a lot of things are going on right now.
Having said all of the above, the computer models are showing a few different potential severe storm/chase setups mid to late this upcoming week through the weekend, from Texas and Oklahoma into Kansas, Nebraska and Missouri. I could go through each setup individually, but that would not only take a long time, but also be rather fruitless at this juncture. We are still several days away from these potential setups and the models need time to get a better handle of the immense amount of energy coming out of the Pacific. But I will say this — the CAP is likely to be a big problem on the days leading up to main shortwave before any significant upper air support is available.
Below is the U.S. drought monitor valid March 29, 2011. Notice all of the southern plains and parts of the southwestern U.S. are in a severe drought. Some areas — including parts of Texas and Oklahoma — are in an extreme drought. Notice the extreme drought covers southwest Texas and southern New Mexico. All this does is create a breading ground for an enormous capping inversion (‘CAP’), or for the meteorologists reading this, a substantial elevated mixed layer. In order for a strong CAP to be broken, you need good daytime heating with either substantial low level moisture or adequate upper air forcing.
I will say that the GFS is forecasting a strong 850mb jet on both Friday and Saturday (below). But on Friday there won’t be much, if any, upper air support because the main trough will still be over the interior West. But by Saturday, the main trough will be ejecting through the southern plains, so that could be the main event. But again it’s too early to talk details since the timing of this main trough is still a bit uncertain. I think every setup should be analyzed closely over the next week for any potential supercell and tornado risks across the southern and even central plains.