Tuesday, March 25, 2014

2014 Severe Weather Season Outlook

Hello everyone, this is the 2014 Severe Weather Season Outlook, made by The Weather Centre. In this outlook, we'll go over current atmospheric factors playing into the upcoming severe weather season, go into a few patterns we saw during the winter that are likely to repeat themselves this spring, and look over analogs that we can use to help forecast this severe weather season.

Notice: If you don't want to read the technical discussion, you can scroll down to where it says 'Stop!' to get down to just the map and summary outlook.

We'll begin with an analysis of sea surface temperatures across the world.

If we take a look around the world on a sea surface temperature anomaly (SSTA) map, we find three areas of concern that we'll address today. We will discuss the temperature anomalies over the Northeast Pacific, anomalies over the eastern Pacific, and anomalies in the northern Atlantic.

We'll begin by looking at the temperature anomalies over the eastern Pacific, offshore of Ecuador. Looking over there now, we see widespread below normal water temperatures, with a few swaths of above normal water temperatures along the Ecuador. Everything seems normal, nothing really out of place, but there is quite a disturbance brewing just under the surface.

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The animation above shows a depth-by-latitude cross-section of the waters just below the eastern Pacific along the Equator. In simpler terms, this shows you temperature anomalies underwater, just below the Equator, if you were to look at the water from the surface down to about 450 meters. If we look at this animation, we find an enormous swath of anomalously warm waters about 100 to 200 meters under the surface, moving towards the surface. This swath of waters is called a Kelvin Wave (you can learn more about the Kelvin Wave by reading these two posts: Post 1 and Post 2), and is known as a kick-starter to an El Nino event.

An El Nino is one of two phases of the El Nino-Southern Oscillation (the other phase is a La Nina), also called the ENSO phenomenon. In an El Nino (La Nina), we see warmer (cooler) than normal temperatures placed in the waters along the Equator, west of Ecuador. These warmer than normal temperatures in an El Nino then create an overall pattern similar to the one pictured above. We see the main Polar jet stream displaced north, allowing warm air to flow into the North US, and also see a rejuvenation of the Pacific jet stream, which then flows into the Gulf Coast and along the South US, creating cooler and stormier conditions. The strong Pacific jet stream is what helps enhance severe weather during an El Nino.

Right now, I would expect the Kelvin Wave to hit the surface and initiate the El Nino in about 4-8 weeks, meaning we should start seeing the effects of an El Nino take shape around May and into June, thus possibly enhancing severe weather prospects for that time period, especially in the South.

The second area we want to look at is the Gulf of Mexico.

If we look over the Caribbean and Gulf of Mexico, we see slightly above normal SSTAs across the entire Caribbean and into most of the Gulf of Mexico, the exception being the coastline waters. For severe weather, we see the strongest systems pull up massive amounts of moisture and create immense instability thanks to above normal water temperatures in the Gulf of Mexico. The warmer than normal waters instigate tropical convection more often, leading to more instability and available moisture for passing storm systems, thus raising the severe weather threat. Right now, I'm only seeing meager above normal water temperatures, with a serious buzzkill in the cold waters along the Texas coast. If we saw warmer waters in the open Gulf, I would be more optimistic, but most of the above normal waters are only 0.0º to 1.0º C above normal- that's not as much as you'd think it is. Thus, I would temper my severe weather expectations accordingly.

Lastly, let's go over the Northeast Pacific.

In the Northeast Pacific, we currently see dominating above normal sea surface temperatures. This has been a major factor this winter, helping to keep high pressure in place in the NE Pacific, leading to the jet stream plummeting south and allowing Arctic air to flow freely south into the United States. We call this sort of set-up a northwest flow situation, because the air is coming from the northwest. This graphic from WHAS11 shows a typical northwest flow set-up well.

This winter, the ridge of high pressure was located a bit further west than what is depicted in the above example image, which allowed cold air to flow well into the Plains, Midwest, Great Lakes and all the way to the Mid-Atlantic.

If we see this body of above normal sea surface temperatures (SSTs) in the NE Pacific sustain itself into spring, it means we might be at risk for more than a few northwest flow severe weather events.

The image above shows five examples from the Storm Prediction Center of northwest flow set-ups, and their corresponding severe weather outbreak locations in the hatched areas. Let's analyze each set-up.

Top left: The top left set-up involves high pressure presiding over the Plains, with deep low pressure protruding into the Northeast. This results in severe weather for the Upper Midwest. This type of set-up is not expected to be dominant this spring, but will happen from time to time.

Top center: This set-up sees high pressure centered in the Southwest, with strong low pressure dipping south into the Great Lakes, producing severe weather in the Ohio Valley. Again, this set-up doesn't look to be dominant, but I could see it happening a few times.

Top right: This set-up is brought on by the "summer polar vortex" dropping anomalously far south, resulting in ridging along the West Coast and producing severe weather in the Central Plains. This set-up is likely to be seen multiple times this spring and summer.

Bottom left: In this scenario, we observed ridging over the Rockies producing a northwest flow over the Central Plains and Great Lakes, leading to severe weather in the Southern Plains. This set-up is not expected to be dominant.

Bottom right: In this scenario, we see strong ridging offshore and pressure from strong negative heights in Canada leading to a severe weather event in the Midwest. The expected ridging in the NE Pacific will likely see this set-up play out numerous times.

In summary, northwest flow events typically see severe weather pushed further north than the typical alignment of Tornado Alley, which is shown below.

Let's now move on to my analog set for this severe weather season. I created these analogs based on the observed and predicted conditions of the ENSO phenomenon, sunspot values and anomalies, the Quasi-Biennial Oscillation (QBO), the Pacific Decadal Oscillation (PDO), and the Atlantic Multi-Decadal Oscillation (AMO). Let's begin by looking over all years that placed into at least two of the categories that I outlined above.

The image above shows a composite of all the years that fit into at least two of the outlined categories. This composite shows you 500mb height anomalies, where the warm colors depict high pressure/ridging, and the cool colors indicate low pressure/troughing. In this image, we see the general idea is for this spring severe weather season to feature strong ridging in the northeast Pacific, possibly indicative of those warmer than normal sea surface temperatures we're seeing at the top of this post in the SST images. This strong ridging then leads to deep troughing over the Western US, as the widespread deep blues and purples show. We then see the emergence of ridging along the Southeast, as this pattern is exhibiting a typical negative Pacific North American (PNA) index pattern, as is shown below.

This sort of analog composite really fits the typical storm that favors tornadoes in Tornado Alley. If you take a look at that Tornado Alley graphic we looked at a little bit up from here, the jet stream is outlined as diving south through the Southwest before shooting north again as it enters the Plains. That tells me that the 'typical' Tornado Alley storm usually occurs in a negative PNA pattern, which my analog set says is likely to happen throughout the winter... or will it?

This composite, showing the same years as the one in the first composite year, but now projecting temperature anomalies at the surface, tells me we can also expect more than a few instances of the northwest flow regime. Why? Take a look at the deep negative temperature anomalies out West. We see the negative temperature anomalies along the West Coast (though I'm a bit skeptical of those, due to how high pressure has prevailed this winter in that area), but we also see negative temperature anomalies in the northern Plains, with warmer than normal temperatures in the East Coast. This temperature pattern is much more similar to this winter than the cold weather out West, and is indicative of a northwest flow pattern. This is why I still believe we will see a few northwest flow severe weather events, as well as the typical southwest flow events (the southwest flow pattern is depicted well in the 500mb composite image two images above this temperature composite).

Lastly, my analog set does keep the drought conditions around in the West, unfortunately.

I refined those analogs to a set that I felt fit the upcoming year better than all of those years jumbled together. The following images will show you my 'preferred' analogs.

This image shows 500mb height anomalies for my five preferred analog years. Once again, blues show negative height anomalies/stormy weather, while warm colors depict quiet, warm weather. We still see the general southwest flow regime outlined in my preferred analog set, with some troughing along the West Coast, and a stronger Southeast Ridge in place. We still see dominant high pressure out near the Bering Sea. This pattern actually tells me the severe weather season might be a little quieter than initially projected. The reason? This sort of pattern would likely produce a few instances of zonal flow, where we don't see any prominent high or low pressure systems- just plain west-to-east flow, nothing exciting. However, considering this map is made of five years of data averaged out, I'm not too concerned about that prospect for zonal flow ruining the season. 

The temperature composite for my five preferred analog years shows a cold spring ahead for those in the far north US, with warmer weather along the Gulf Coast. This sort of set-up tells me we'd be looking at a lot of cold air available for severe weather events, as well as abundant warm air from the south. All of this could add to the severe weather dynamics, and enhance the event's strength and expanse. Overall, these temperature boundaries could act to strengthen any severe weather events that hit the US, though we'll have to deal with each storm on a case-by-case basis, of course.

(If you scrolled down to see my severe weather season map and summary, STOP here!)

With all of this accounted for, let's look over my severe weather map for this season.

• I predict we'll see a higher than normal severe weather threat along the Central Plains and into the Midwest, thanks to instances of both southwest flow severe weather events and northwest flow events that would likely target this area. The temperature contrast along the jet stream (which is predicted to set up just south of this outline area, based on analog images I did not show here) looks to be pretty tight, enhancing the severe weather threat for that area.

• I also shaded in the Gulf Coast for an enhanced severe weather threat, as I expect we'll see an increase in activity as we move towards the spring and summer months in response to the expected emergence of an El Nino. If you recall, El Nino events help to strengthen the jet stream that runs through the South, thus strengthening severe weather prospects for that area.

• I colored in the New England area for a quieter than normal severe weather season. At this point in the game, I'm thinking much of the Northeast goes without more than a handful severe weather events this season, but it's the far Northeast I think will see the brunt of this quiet severe weather season. 

If you have any questions about this outlook, you may ask them in the comments below. 

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