Wednesday, June 3, 2015

Stormy Spring Pattern to Persist?

What in the world is going on with this weather?

I've gotten some variant of that question quite a bit in recent weeks. The questions started with the fierce storms on April 27 that made 9 a.m. look more like 9 p.m., continued through several bouts of heavy rains and severe weather in May, and once again popped up this past weekend as torrents of rain wreaked havoc on the baseball regional at Alex Box Stadium.

Lightning strike during April 27 severe storms.
(Credit: Chad Forbes)
So what is to blame? 

Any extended run of active weather likely has several causes behind it, but it's becoming increasingly clear that a strengthening El Niño is likely a big factor in the stormy weather in recent weeks, not only locally, but across much of the Southern Plains and northern Gulf Coast.

What is El Niño?

Without getting too bogged down in the details, El Niño is an oceanic phenomenon reflected by warmer-than-normal sea surface temperatures in the central and eastern equatorial Pacific Ocean. Most people think of the ocean and atmosphere separately, but in the worlds of weather and climate, the two are very much intertwined. During El Niño events, the 'typical' states of the ocean and atmosphere are changed such that weather patterns are altered over much of the globe.

NOAA image from the week of May 18-24, 2015 showing sea surface temperature departures from normal. The area of red labeled 'Warmer than average' is indicative of a strengthening El Niño.

Evidence of El Niño's Impact?

One well-established pattern during El Niño events is a general trend toward wetter conditions across much of the southern U.S.
It's a pattern that is typically more evident in the fall and winter months, but it is sometimes found in the spring and summer as well.

And unless you've been asleep for the last 2 months, you know it's been awfully wet around much of south Louisiana. You've probably also seen the remarkable flooding in parts of Texas and Oklahoma in recent weeks, areas that had previously been experiencing significant drought conditions.

The map below shows departure from normal precipitation since the beginning of April. Areas in purple have seen at least an 8-inch surplus in rainfall during that stretch, with some spots receiving more than 15 inches above-normal! You can see that shades of purple extend across much of south Louisiana, a large portion of Texas, and most of Oklahoma.

It's obvious to anyone in Oklahoma, Texas or our part of the world that the last couple of months have been very wet. But how do we know that El Niño is (at least in part) to blame for the constant soakings in recent weeks? The answer lies in something known as the subtropical jet stream.

Chances are you've heard of the jet stream and may even know that it's essentially a 'river' of fast-moving winds at high altitudes. But there's another 'branch' of the jet stream known as the subtropical jet that is typically present at lower latitudes than the main polar jet. The subtropical jet is usually most active in fall and winter, but meteorologists have noted that it has been a big player in the big rains parts of the southern Plains and Gulf Coast have seen in recent weeks.

The map below shows departure from normal wind speeds at the approximate altitude of the subtropical jet. The area of yellow, orange and red shadings that extends from near Hawaii across Mexico into Texas represents wind speeds that were well above-normal in April and May. It also provides more evidence to me (and other meteorologists) that the subtropical jet (and therefore El Niño) was a large factor in the big rains during that stretch.

Wet Pattern to Continue?

We're enjoying a much-needed break in the wet weather pattern during the first week of June, but chances are this drier pattern won't last in the long-term. Most model forecasts are calling for the current El Niño to strengthen further through the summer and into at least the fall. If that occurs, we're likely to see more periods where the subtropical jet is active, delivering rounds of rain (sometimes heavy) to the southern Plains and Gulf Coast. And, remember, El Niño's impacts are typically more pronounced in the fall and winter, so that would also lend credence to a continued wet pattern later in the year.

The upside to El Niño? It typically results in fewer tropical storms and hurricanes in the Atlantic basin. However, we have still seen significant hurricane impacts during El Niño events, including Betsy (1965) and Andrew (1992).

Credit: LSU Earth Scan Lab

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