Ask Eric: Has It Been A Quiet Hurricane Season?

Eric,

 Watch your forecasts every night.  Very informative and educational. Is this a slow hurricane season?  And if so, why?

 Thanks,

Alan

Thanks for the note, Alan! It's definitely that time of year when our attention turns to the tropics, and with good reason. I'm always amazed how the 'switch is flipped' when we hit mid-August. Almost always on cue, clusters of storms start spinning in the MDR (main development region) as big clusters blow off of Africa and over the warm Atlantic waters. We haven't been talking about the tropics much this year, but that's not too unusual. 90% of all tropical activity occurs after the start of August. So while some early season storms are always possible (indeed we had 3 of them - Ana, Bill and Claudette) they're more the exception than the rule.

Animated loop of Danny as it churns in the Atlantic

When you look at 30-year averages, the 4th named storm of the season occurs on August 23rd. So having our 4th (Danny) form on the 18th is just about on schedule! In terms of named storms, it's a typical season. However, that's not the whole story. One way of looking at tropical activity is ACE - accumulated cyclone energy. ACE is a way of measuring the overall intensity and duration of tropical activity in a particular basin. It gives a more clear picture as to what type of storms have been forming and how much energy is being released/transported. After all, tropical activity is all about moving heat poleward and away from the tropics - a constant battle for planetary balance. And when we look at this metric, we find that the Atlantic has indeed been lagging far behind.

Tracks of the first 3 named storms of the 2015 season. Source: National Hurricane Center

The Atlantic basin is sitting at approximately 1/3 of its average ACE to date. The three named storms that formed earlier in the season never reached hurricane status, and none of them endured for very long. Danny has the potential to stack up some ACE, as it will survive for at least 5 days, if not longer, and should strengthen into the season's first hurricane.

Source: Weatherbell.com

Meanwhile the Pacific has been off the charts. Powerful typhoons have been constantly rolling through the West-Pac, and there has been considerable action in the central and eastern Pacific. None of these figures is all that surprising with a strong El Nino developing. When El Nino is in charge, we tend to see hyper-active Pacific tropical seasons and below average Atlantic seasons. The reason for this is a large pool of above-average sea surface temperatures in the Pacific, and increased wind shear across the Atlantic.

Typhoons Atsani and Goni as seen by Japan's Himawari satellite on Tuesday. Each of these were the equivalent of 'Major' Category 3+ hurricanes at the time.

So what does the future hold? Danny will be battling off some Saharan dust and, down the road a few days, increasing wind shear as it approaches the Windward Islands. But models suggest it will continue to chug westward and perhaps be able to enter the Caribbean early next week. If it can survive, it's not out of the question it could pose a threat to the mainland U.S. The timing on that would be into the middle/end of next week, so there is a lot of time to watch this one. In general, models are spinning up several tropical systems in the Atlantic over the next few weeks. We're approaching the peak of the season and there's some climatology built into the models, so they may not come to fruition. But we're approaching the peak where the vast majority of development takes place, and it's a good time to think about hurricane preparedness and to keep an eye on whatever is spinning out in the ocean!

Tuesday 18z GFS forecast shows several tropical systems. Take exact locations and intensity with a grain of salt, but general idea is that tropical development looks more likely in the weeks to come. Source: WSI Energycast