If you played a drinking game while watching The Weather Channel yesterday and your keyword was "annular hurricane," I hope your insurance covers alcohol poisoning. Here's an explanation of this week's weather buzzword that few people have ever heard before.
Tropical cyclones come in many different shapes and sizes. Hurricane Sandy was the largest hurricane on record in the Atlantic Ocean, with tropical storm force winds extending nearly a thousand miles away from its center just before landfall back in October 2012. Tropical Storm Marco was the world's smallest tropical cyclone, with winds only extending 10 miles away from its center.
In addition to size, there's also the case of the storm's shape and structure. Just this week we've had two hurricane of very different shapes. In the Atlantic, we had that sad sack of clouds they call Bertha, which barely resembled anything but a messy blob producing strong winds.
Most people have an idealized vision of what a hurricane should look like. Think of Ivan or Katrina — the giant buzzsaw with a perfect eye.
On the other side of the continent from Bertha, we've got Iselle in the Pacific. Hurricane Iselle continues to churn towards the Hawaiian Islands, slated to slam the Big Island as a strong tropical storm later this week. If Iselle's current forecast holds, its track will bring the center of the storm over the Big Island as a 70 MPH tropical storm, battering most if not all of the major Hawaiian Islands with tropical storm conditions for a period on Thursday and Friday.
At its strongest, Iselle reached category 4 intensity with winds more than 140 MPH, and its structure was nearly perfect. It was an annular hurricane. To understand an annular hurricane, think back to Hurricane Katrina and similar buzzsaw"storms. Those storms feature an intense eyewall and many rainbands spiraling around like spokes, each containing strong winds and heavy rain, with conditions in the bands growing worse closer to the center.
Annular hurricanes don't feature much if any banding. The central dense overcast (the thick, deep thunderstorms around the eye) is the hurricane. The eye is large and almost perfectly symmetrical, and the entire eyewall is wrapped up in intense thunderstorm activity.
On satellite imagery it looks like a giant white wind bagel spinning over the ocean. Think of the thick eyewall as a fort, preventing many outside forces from weakening the storm. Annular hurricanes tend to weaken more slowly than "normal" hurricanes, but thankfully for land dwellers, storms don't stay annular for too long.
Above is a water vapor image of Hurricane Iselle last night. Warmer colors indicate drier air and dark green indicates very moist air. The dense ring of thunderstorms around Iselle's nearly-perfect eye shows that the storm was either annular or very, very close to it.
Here's another example of an annular hurricane:
This is 2003's Hurricane Isabel as it passed north of the Greater Antilles as a Category 5, and it was an extreme example of an annular hurricane. The sheer size of Isabel's eye made it so that the storm's intense eyewall was the hurricane.
Thanks to drier air and an encroaching ridge of high pressure, Iselle in the Pacific is starting to weaken and looks decidedly more ragged than this time yesterday, but it won't weaken quickly enough. The latest forecast from the Central Pacific Hurricane Center in Honolulu shows Iselle hitting Hawaii as a strong tropical storm.
The Big Island is now under a Tropical Storm Watch in anticipation of a direct hit from Iselle on Thursday night and into Friday. The rest of the islands should stay on high alert as the center may shift north or south of the current forecast track. The storm's effects will also extend beyond the center — especially to the north — so heavily populated areas may see a period of dangerous weather towards the end of the week.
[Images via GOES / NOAA / GOES / NOAA / CPHC]