What's behind Hawaii's double hurricane hit? A sprinkling of heat, a dash of timing, and a big dose of bad luck

Friday, August 8, 2014, 2:26 PM - As Hurricane Iselle hits Hawaii, with Hurricane Julio following closely behind, this represents a rare event for the island chain - not only has it been over two decades since a hurricane made landfall there, but seeing two so close together is unprecedented. So, what is it that's bringing together this rare combo?

Hawaii is no stranger to tropical storms and hurricanes, or to the damages caused by them. Being a fairly small target in a very large ocean, though, the islands see far more storms sweeping just past them or the remnants of dissipated storms passing over, both delivering potentially-damaging rain and wind, than actual storms making landfall. Hurricane Iniki, from nearly 22 years ago - back in September 1992 - was the last hurricane-force storm to make landfall in Hawaii. To match the same scenario we're dealing with now, though, for a storm making landfall almost directly from the east (rather than Iniki's track of passing by to the south and then turning sharply to the north to pass across the islands), we need to look a lot further back, to an unnamed tropical storm in August of 1958.

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Now, having two storms arriving nearly on top of one another, and travelling along roughly the same track, is fairly unprecedented.

According to Kevin Roth, the lead meteorologist for The Weather Channel: "Dating to 1949, there is only one case where tropical cyclones with direct impact were even 10 days apart." This was the combination of Tropical Depression Daniel and Tropical Storm Gilma in 1982, on July 22 and August 1, respectively.

So, what's the deal with Iselle and Julio? What's behind these two storms hitting the islands in such close succession?

Partly, it's the temperature of the water in the Pacific Ocean right now. There's a weird 'delayed' El Niño in the works right now, with warmer sea surface temperatures, but a somewhat sluggish response from the atmosphere. These two factors typically progress along fairly well hand-in-hand to produce an El Nino, but according to the World Meteorological Organization, the delay seen in the atmosphere may be due to the fact that "sea surface temperatures are above average across virtually the entire tropical Pacific," rather than just in the traditional El Niño regions of the eastern and central tropical Pacific. While this means that we're not yet in an El Nino, and the ocean temperatures may actually be starting to back off now too, we're still seeing an active hurricane season in the Pacific from this warmer water. The map below shows the latest sea surface temperature anomalies in the oceans, as compared to the average set from 1995-2009. The region along the southwest coast of North America is quite warm compared to normal, and although there are a few cool spots in the region where Iselle and Julio formed and tracked through (in the red ellipse), that area is also showing slightly warmer temperatures than normal. This provided the fuel for both of these storms to spin up so close together, and to sustain them along their path towards Hawaii.

Credit: Environment Canada with edits by S.Sutherland

Two storms following such a close path is unusual, but not so much when you take timing into account. When a storm forms over the ocean, its path is largely determined by 'steering currents' in the atmosphere. These are the patterns of winds flowing at around 5.5 kilometres above the ground (what we meteorologists refer to as the '500 millibar level' or 'upper level). If you look at two storms that form over very different parts of the hurricane basin and/or with a week or more span of time between them, the steering pattern for each is typically sufficiently different that they'll follow different paths. However, with two storms forming in the same region within a day or two of one another, like Iselle and Julio, they both have roughly the same steering pattern. In this case, Iselle's track lined up so that the storm was aimed directly at The Big Island for landfall, and it should push off to the southwest of the island chain through the weekend. Meanwhile, Julio's track appears to be taking it on a path slightly more to the northeast of Iselle, so that it will likely swing past the island chain without making landfall. This will still mean more heavy rainfall, driving winds and storm surges due to this close-pass by Julio, which will add to the woes from Iselle, but it does appear as though the islands will at least be spared two direct hits in a row.

What about global warming/climate change? Could that be playing a role? Well, most of the current forecasts of how global warming will affect tropical cyclones state that we won't necessarily see the storms themselves show up with greater frequency, but we should see higher intensities happen more often. This will have the effect of having so-called '100-year-storms' show up every 50 years or so, while what we now think of as '50-year-storms' will hit every 20-years and so on. Thus, it's possible that Iselle and Julio are actually stronger than they would have been without the influences of global warming going on right now, but there isn't likely any contribution towards the two of them showing up in such quick succession.

However, mostly, the combination of these two storms back-to-back is just luck ... of the bad kind, unfortunately. The weather on our planet is a very chaotic system and eventually you're going to get a situation where the patterns line up just right to throw something unusual at you.

As Weather Underground's Dr. Jeff Masters told the Associated Press: "You roll the dice enough times you get something like this."