I haven’t been active on this blog the past couple of weeks as I’ve been in Costa Rica, however, the unfolding situation with Hurricane Iselle warrants a posting. I’ve observed Iselle as it formed from a tropical wave and then a tropical depression along the Inter-Tropical Convergence Zone off of the coast of Central America where I’m currently staying. Now, Iselle is closing in on the Hawaiian Islands and is set to make landfall on the Big Island this evening.

Despite the fact that the Hawaiian Islands lie near an active area for tropical cyclones, they are rarely impacted, in part due to the sheer vastness of open ocean versus the relatively small land area of the islands. During most years, a semi-permanent high pressure area that resides near the Islands also precludes the formation, strengthening, and penetration of cyclones into the region. This changes dramatically when El Nino conditions prevail, as they seem to be currently. Iselle will be the first landfalling hurricane to affect the Hawaiian Islands in 22 years since the devastating Hurricane Iniki of 1992. Fortunately, Iselle will be making landfall as a much weaker storm than Iniki, but regardless, it will still pack a serious punch.

Track Forecast

With less than 24 hours before landfall, the forecast track for Iselle is essentially zeroed in. The official track forecast cone has Iselle hitting the Big Island straight on. The center of the current forecast track envelope, should it hold up, has Iselle potentially making a direct hit on the eastern portion of the Big Island, with the eye of the storm skirting just south or perhaps directly over the Hilo area. I would hope for a northward deviation of this track, as this would spare the region from the strongest quadrant of the storm’s winds and waves (in the northern hemisphere, the strongest quadrant of a cyclonic storm is generally the northeastern quadrant). In the case of Iselle, due to its forecast west-northwest motion, the strongest quadrant should actually be the northwest, since the forward speed of the storm is added to the winds in that area.

After making landfall on the Big Island, Iselle is forecast to move to the south of the rest of the islands, passing closest to Maui. Because of uncertainty in the track forecast, it is entirely possible that Iselle could end up making multiple landfalls on different islands. However, I would anticipate these subsequent landfalls to be less serious in nature as Iselle should weaken substantially due to the influence of interaction with the mountainous terrain of the Big Island as well as increasing vertical wind shear from the north and west. There could still be some damaging storm surge and high winds in the other Hawaiian Islands, especially if the center of Iselle passes to the south of them.

Intensity Forecast

Iselle has been stubbornly persistent in maintaining minimal Category 1 status. Forecast models have been giving conflicting signals as to whether Iselle will be impacted by increased wind shear. Forecasters had not anticipated that Iselle would enter into a col between two high pressure centers, essentially an area of very low wind shear, think of it as a calm spot between two swirling eddies. Different forecast models are providing two divergent scenarios: (1) that Iselle continues to move in tandem with the col, allowing it to maintain current strength, or (2) Iselle leaves the col and experiences much higher wind shear.

No matter what, Iselle will be making landfall on the Big Island as either a minimal Category 1 hurricane or a very strong tropical storm. The real impacts of either scenario will be hardly discernible. Wind over the Big Island should pick up substantially this afternoon and evening first from the northeast, then north, and then switching to the south and southeast overnight. Sustained tropical storm force winds in the range of 50-65mph are expected depending on the specific locale. Wind gusts of up to 90mph are also possible in some locations. Offshore waves will build from 6 feet to as high as 31 feet closer to the center of the eye. I hope all people on the islands are making adequate preparations. Wind speeds as high as the ones forecast could easily shred even a decently well constructed roof. I’m not sure if the local government has issued evacuation orders, but if so, I hope all citizens of Hawaii will heed the call and take every necessary precaution to secure life and property. Let’s hope this Iselle weakens some more before it makes landfall.

Notes

In contrast to the Atlantic Hurricane Season thus far, this year’s Pacific Hurricane Season has been quite active, in part due to the influence of anomalously warm sea surface temperatures over the Eastern Pacific. These temperatures are indicative of what could be the start of an El Nino event. Hurricane Iniki also formed during a very strong El Nino event.