Apr 192014
 

Saturday, 19 April 2014

 

Here we are just past the middle of April and the storm chasing season has been pretty slow. The exceptions have been a day or two of supercells from north Texas into Oklahoma, with a few tornado reports. But for the most part severe storm activity has been infrequent at best across much of the central U.S., including the Great Plains. In fact, over the past couple weeks snow has fallen from central Oklahoma to Kansas! And not only that, but Detroit, MI officially broke their season snowfall record. They recorded 94.9 inches of snow for the season, beating the old record of 93.6 inches set way back in 1880-81! So, winter has sorta hung on into mid April and the severe storm/chase season has been sitting in the back seat, waiting for its turn at the drivers seat. Well, things are starting to come together and the chase season may get in the drivers seat by the end of the month.

A couple weeks ago I addressed the beginning stages of a MJO wave over the Indian Ocean and the Indonesia region. At the time it appeared like two areas of tropical convection would come together and propagate east. It hasn’t been a classic wave and plenty of other things are going on across the northern hemisphere that are complicating matters. But, tropical convection has reached the international dateline/western Pacific, and things are being set into motion (see image below).

 

Outgoing longwave radiation over a 3 day period (April 15-17). Notice the negative values over the western Pacific indicating where tropical convection has reached the international dateline.

 

Notice the MJO had a weak projection into phase 6 but has since weakened further.

 

For those of us hoping for the severe storm activity to pick up across the Great Plains (and that’s many of us), then it’s unfortunate the tropical convective signal (MJO) is weakening. But the good news is some strong jet energy coming out of eastern Asia and the northern Pacific will work in conjunction with this MJO signal. What this means is something that’s already being set into motion. We will see a couple Pacific jet stream extensions, which we will see in the form of troughs slamming into the western U.S. The first of which will occur next week (Week of April 21) and should bring a day or two of severe storms across the Great Plains/central U.S. But the larger, more significant jet extension will occur a few days later in late April (you can already see that beginning stages of that jet extension over the western Pacific east of Japan right now, see image below). This is the jet extension that should bring a series of troughs (some significant) into the western states during the last week of April into early May. And if everything worked out perfectly, this would bring several days of storm chasing/severe storm setups to the Great Plains during that time.

 

Water vapor image centered on the western Pacific valid 2032z April 19, 2014. The red arrow outlines the Pacific jet feeding into a low pressure system east of Japan (red circle). This combination should eject eastward across the Pacific in the form of a powerful Pacific jet extension.


 

GEFS Reforecast (initialized 00z 4/18/2014) 500mb height anomalies valid April 26, 2014. Notice the deep trough over the eastern Pacific extending into the western U.S. The red arrow is there to outline the Pacific jet extension, essentially the same ‘piece of energy’ that is currently over the western Pacific near and east of Japan. It should be noted the European Ensemble (not shown) is much more aggressive with the jet and troughing.


 

The biggest question in my mind now is how effective these troughs (storm systems) can eject east into the Great Plains. The concern is the projection of lingering tropical convection west of the dateline in late April/early May could promote ridging across the Rockies/Plains (some computer models suggest this trend). This would keep much of the troughing over the western states and could limit the severe potential across the Great Plains. But at the very least I think individual shortwaves will eject into the plains and bring a severe pattern equal to what climatology would suggest for this time of year. After all, there’s no sense in being picky here. The pattern has been very quiet and any uptick in activity is a welcome change for storm chasers!

But as can be seen in the images above, the Pacific jet stream is poised to extend into the eastern Pacific by the end of the month. As seen by computer models (one example shown above), this jet extension should bring a deep trough into the western states, which should then either eject east or send a series of shortwaves into the Great Plains. As I already said, the MJO is weakening, so just how deep of a trough we can get to eject into the plains is in question.

Though it seems clear the severe storm/chasing activity will come to life during the final week of April, perhaps extending into early May. If we can get another jet extension following this one, then maybe the increased severe activity will extend into the first ten days of May. We’ll just have to remain hopeful and keep our fingers crossed!!

 

Until next time…

 

Jim

Apr 042014
 

Friday, 4 April 2014

 

It’s been a real battle up until the past couple days to bring about spring storm chasing setups to the southern plains. But at least the brutal cold winter pattern finally gave way to spring in a meaningful way. There were severe storms including supercells, which did produce a few tornadoes from Texas to Missouri in recent days. But I do believe this is only a small taste of what mother nature has in store for parts of the Great Plains as we head into mid and late April.

For months, tropical convection near and west of the international dateline has been a dominant feature. This has been primarily responsible for a persistent ridge over western North America and Alaska, promoting cold, arctic air intrusions into the central and eastern U.S. And through most of March, this cut off the rich, low level moisture supply from the Gulf of Mexico and can be at least partially blamed for a very slow start to the spring severe weather season across the central U.S. (namely the Great Plains).

Over the past few weeks new tropical convection has emerged over the Indian Ocean, whereas the dateline convection stubbornly remains in place. Over the past several days it’s becoming more and more clear that the Indian Ocean convection is growing and may be emerging into a new MJO wave. You can see the two areas of convection in the images below. The first is a satellite image centered on Indonesia, while the other shows outgoing longwave radiation anomalies over a 3 day period (March 31-April 2).

 

Infrared satellite image valid 02Z April 4, 2014. Notice the two areas of tropical convection. These may come together over the next few weeks as the main area over the eastern Indian Ocean moves east.


 

Outgoing longwave radiation (OLR) anomalies valid March 31-April 2. Areas of blue and purple indicate negative anomalies, or areas where tropical convection has occurred.


 

It’s probably going to be a slow and kinda messy process. But over the next few weeks the ‘blob’ of convection in the Indian Ocean should propagate eastward and slowly consolidate with the dateline convection in the form of a MJO wave moving into the western Pacific. And I use the word ‘messy’ because all throughout winter and early spring the dateline convection continued to fire and dominate the tropical Pacific, impacting the jet stream pattern. So it’s hard for me to imagine that dateline convection going away without a fight.

In a perfect world the MJO would make it out to the western Pacific in mid to late April. This in turn would drive deep troughs into the western states and yield severe storms/chase setups across the Great Plains. Although, given this is April and not May or even June, there would be intermittent western ridges developing between major western troughs (see computer model forecasts for next week, western ridging building ahead of Pacific troughs). That’s just a factor of the jet stream wavelengths still being short, a leftover from winter. Anyways, as the MJO moves further east and makes it out to the western Pacific, more significant troughs should crash into the western U.S. and we should also see more southeastern U.S./Gulf of Mexico ridging as well. Essentially, a pattern that would allow rich low level moisture to lift north into strong westerly winds aloft, promoting severe storms across the southern plains.

 

MJO plot as of April 3, 2014. Notice the latest plot is over the eastern Indian Ocean, indicating that area of convection may become the most dominant. I drew in the black arrow myself, showing how this new MJO wave should move east, eventually towards the western Pacific later in April.


 

BUT, this is a not a perfect world and the dateline convection IS a messy issue. What will probably happen is the dateline convection will bring a convective response in the form of intermittent ridging across the northwestern U.S. into western Canada, perhaps even into Alaska. But the Pacific jet stream will increasingly cut underneath this ‘block’ and crash into the southwestern U.S. We will also continue to see a subtropical jet stream as warm sea surface temperatures in the western tropical Pacific will support that.

I think this is all very positive news for storm chasers..at least it is for me. The storm chasing season has been slow as we were frozen by the never ending winter. Predictions of El Nino have brought a lot of doubt to the future of the chase season going into May and June. And while this post has nothing to do with that, I’m simply pointing out a sub-seasonal weather event that may bring some decent severe storm events to the Great Plains in mid and late April (perhaps into early May?). Whether or not El Nino develops is a topic for another blog post (if I ever get around to that!). I will say this: If El Nino does indeed begin developing in earnest by May, that is not a good sign for the remainder of the chase season.

Until next time…

 

Jim