Tropical Cyclones Iselle And Julio: A Double Whammy For Hawaii


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Iselle and Julio head for Hawaii. Satellite image courtesy of NOAA

Iselle and Julio head for Hawaii. Satellite image courtesy of NOAA

Hawaii isn’t often troubled by tropical cyclones, despite the fact that the island chain sits in the middle of the subtropical Pacific Ocean.

But this year, storms seem to have Hawaii in their bull’s-eyes; two storms are now heading directly towards the islands.

Breeding Ground For Eastern And Central Pacific Storms

The water south and southwest of Mexico is normally very warm in the summer. This year it is even warmer than normal. In addition, atmospheric conditions have been favorable for tropical cyclone development. Basically this means low vertical wind shear (little change of wind with height).

As a result, there has been a train of storms, several of which have developed into hurricanes.

Typical Movement Of Eastern Pacific Storms

Most hurricanes in the eastern Pacific move northwest and quickly encounter cold water, which leads to their demise.  Even storms that head in the direction of Hawaii normally succumb to colder water.

This year, the water is slightly warmer than normal between the breeding ground and Hawaii. This slight uptick has been enough to allow storms to survive and threaten the islands. The boundary of the warm water is sharp, so storms must walk a tightrope to get to Hawaii without being disrupted.

Hurricane Iselle: The First Threat

Iselle reached category four strength yesterday, with winds of 140 miles per hour. The storm now feels some effect of the cooler water, dry air infiltrating from the north, and a little wind shear; top winds are now 125 miles per hour and slowly diminishing. The latest forecast takes Iselle directly over the Big Island of Hawaii as a strong tropical storm.

Tropical Storm (Soon To Be Hurricane) Julio: Double Whammy

Julio is practically duplicating the path of Iselle, and this will result in mixed effects. Iselle will have pumped moisture into the atmosphere, so that Julio will not be affected by dry air. On the other hand, Iselle will have stirred up the water and brought colder water to the surface from below.

Julio is now forecast to track slightly north of where Iselle is going. If this forecast holds — and it is still quite far away — it could be a more troublesome storm than Iselle, even if it is slightly weaker.

Iselle And Julio: The Wind

A hurricane-force wind in Hawaii is an extremely rare occurrence. It has happened only three times since 1950. All of these storms took a path well south of the island chain and turned sharply north, remaining over warm water for the maximum amount of time possible. The primary effects were on the northwesternmost island of Kauai.

Iselle and Julio are close enough to the cold water to have their strength diminished. Wind is unlikely to be a serious problem. However the path creates an additional danger.

Iselle And Julio: The Rain

The tandem deluge from Iselle and Julio will create flooding problems in parts of Hawaii that are not accustomed to much rainfall. Honolulu normally receives less than an inch in August, though it gets over four inches in some winter months.

Possibly more  significant will be the effect if one or both of the storms pass north of the islands, bringing west winds on their south sides. Since the winds are almost always from the east over Hawaii, cities on the west sides of the islands are very dry: the rain is squeezed out as the winds traverse the mountains.

Kailua-Kona, on the west shore of Hawaii, receives less than two-thirds of an inch of rain in a typical August. Even in winter, monthly rainfall barely reaches an inch and a half. A heavy tropical deluge in these communities could result in serious flooding.

The 2014 Eastern And Central Pacific Hurricane Season

The conditions that produced Iselle and Julio show no signs of changing, so we can expect more storms to affect Hawaii. The last season to produce a lot of storms in Hawaii was 1992, a year which brought very little activity to the Atlantic — but included Hurricane Andrew. Just thought I’d mention it.

*Editor’s Note: Updated 8/6/2014 to reflect a geographical error

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