Hurricane Iselle Lashing Hawaii's Big Island With Heavy Rain

Hurricane warnings continue for the Big Island as Hurricane Iselle closes in as a Category 1 hurricane with 80 mph winds. The latest center fix from the Hurricane Hunters at 22:55 UTC Thursday (6:55 pm EDT or 12:55 HST) found the pressure had risen to 992 mb, up 7 mb since this morning. They also noted that the eyewall had collapsed, so Iselle is weakening. However, this weakening had yet to translate to a reduction in the maximum sustained surface winds, which were still 75 - 80 mph during their last two passes through the center. Thursday afternoon satellite images showed that Iselle's eyewall thunderstorms continued to be very vigorous with cold cloud tops. The outer spiral bands of Iselle were lashing the Big Island, as seen on Hawaii radar.


Figure 1. Radar image from the South Hawaii radar showing the outer spiral bands of Hurricane Iselle affecting the Big Island. The North Hawaii radar will be intermittent today, due to high winds blowing the radome hatch open, according to the NWS.

Forecast for Iselle
There isn't much time for Iselle to change much before the center makes landfall on the Big Island, which should occur near 10 pm HST (4 am EDT.) Iselle's top winds at landfall will likely be between 65 - 75 mph. The main threat from Iselle will be heavy rains leading to flash flooding and mudslides. The Thursday morning 12Z run of the GFDL model predicted that Iselle would dump widespread rains of 4 - 8" over the islands, with some regions seeing 8 - 16". Wind damage is also a concern from Iselle; the 5 pm EDT Thursday Wind Probability Forecast from Central Pacific Hurricane Center (CPHC) gave Hilo on the Big Island a 94% chance of experiencing tropical storm-force winds of 39+ mph, and a 12% chance of hurricane-force winds. These odds were 35% and 0%, respectively, for Honolulu. On the higher terrain of the islands, winds will be up to 30% stronger than what is observed at sea level. High surf of 10 - 20' and higher will also pound the islands, causing erosion problems and coastal flooding. Since accurate landfall records began in 1949, only one tropical storm (an unnamed storm in 1958) and no hurricanes have ever hit the Big Island.


Figure 2. True-color MODIS image of Hurricane Julio from 19:30 UTC (3:30 pm EDT) August 7, 2014. At the time, Julio was a Category 2 hurricane with 105 mph winds. Image credit: NASA.

Category 2 Hurricane Julio expected to skirt Hawaii
Hawaii's other hurricane threat is Hurricane Julio, which remained a Category 2 hurricane with 105 mph winds at 5 pm EDT on Thursday, but appears to be intensifying. Satellite loops show that Julio has maintained a respectable area of heavy thunderstorms and well-formed eye. The storm should be able to take advantage of light to moderate wind shear and marginally warm sea surface temperatures near 26°C and maintain at least Category 2 status until Friday, when a slow weakening trend should begin. With the notable exception of the GFDL model, which forecast a direct hit on Kauai, the 12Z (8 am EDT) Thursday runs of our top track models predicted that the center of Julio would pass 50 - 200 miles northeast of the Hawaiian Islands on Sunday. On this path, Julio's core of heavy rains and wind would miss the islands, and high surf would be the main impact of the storm. The edge of Julio's cone of uncertainly for Sunday still lies over the islands, so we cannot yet be confident of this track, but at this time it appears that Hawaii will avoid torrential rains from Julio falling upon soils already saturated by Iselle. The 5 pm EDT Thursday wind probability forecast from NHC gave Honolulu a 25% chance of receiving tropical storm-force winds of 39+ mph from Julio, and a 1% chance of hurricane-force winds.


Figure 3. Webcam view from the top of Mauna Kea at 1:40 pm HST August 7, 2014, as the rains from Hurricane Iselle were falling. Image credit: Mauna Kea Weather Center.

The winds at 13,000 feet on Mauna Kea
The weather on top of the highest point in Hawaii, the Big Island's Mauna Kea, elevation 13,796' (4,205 m), will be interesting to follow as Iselle makes landfall. Winds have risen steadily today, and were 25 mph, gusting to 40 mph, at 2:10 pm HST (8:10 pm EDT) at the University of Hawaii 88" telescope (UH88.) There are six anemometers reporting winds on top of Mauna Kea today, but beware of the data from the Canada - France - Hawaii Telescope (CFHT). The Mauna Kea webcam page says that those winds are highly exaggerated due to location of the anemometer tower between two large telescope domes. You can see this tower on Google Maps.

If the center of Iselle passes directly over Mauna Kea, the mountain may set a new record for highest location ever to encounter a direct hit by a tropical storm or hurricane. There are no peaks in the Caribbean as high as Mauna Kea--the highest being Pico Duarte in the Dominican Republic, at 10,161' (3,098 meters). Taiwan's highest point is Yu Shan (Jade Mountain) at 12,963' (3,952 meters). In the Indian Ocean, no tropical cyclone has ever come close to reaching the Himalayas.  The closest approach was either Aila (2009) or an unnamed storm in 1925, both of which only reached maximum elevations of about 400 meters before dissipating--near Mal Bazar, India and Sanischare, Nepal, respectively. ‪Pico de Orizaba (18,486', 5,636 meters) in Mexico, about 70 miles west of Veracruz, had Hurricane Karl (2010) pass within 25 miles of the peak (to the south) as a Category 2 storm, according to the ‪NOAA/CSC hurricane database. However,‬ I doubt the peak experienced the calm of the eye. Thanks go to Phil Klotzbach, Will Komaromi, and Brad Barrett for helping with these stats.

Links
Weather on Mauna Kea
Live stream from KHON2 TV in Honolulu
Central Pacific Hurricane Center
2-km resolution WRF model output from the University of Hawaii for Hawaii
Storm surge maps for Oahu
Storm info from Tropical Tidbits
NWS Honolulu

Jeff Masters