Iselle made landfall in Hawaii Friday morning as only the second tropical storm the Big Island has witnessed in over half a century of landfall records. The storm, with maximum sustained winds of 60 mph, is lashing the island with a band of torrential rainfall. Radar estimates suggest that over five inches of rain has already fallen on the southeast side of the Big Island.
A flash flood warning is in effect for the Big Island until 5:30 a.m. Hawaii time (11:30 a.m. Eastern). Rain rates greater than four inches per hour have been measured between Volcano and Wood Valley, according to the National Weather Service in Honolulu. In addition to the flood threat, the Weather Service also warns that heavy rainfall could generate rock and mudslides in the steep terrain.
Strong winds have also been battering the Big Island. Around the Mauna Kea Observatory (13,796 feet) unofficial reports indicate winds gusting over 50 mph. Hilo, which is at a low elevation on the east side of the Big Island, has reported wind gusts up to 47 mph. The Pōhakuloa Training Area, which is further north on the island at 6,800 feet, has so far experienced gusts up to 43 mph.
The Weather Channel reports that despite weaker than expected winds, the Big Island is taking a beating:
There have already been reports of structural damage, trees downed, power outages, and some flooding. Roofs were reportedly removed from homes just southeast of Hilo late Thursday night. As of early Friday morning, parts of the Big Island had already picked up over eight inches of rain.
Though weakening, Iselle has also slowed down in forward speed. “I’d argue that the stalling will make a worse impact than if it had maintained intensity and speed due to increased exposure to winds and more rainfall,” CWG tropical weather expert Brian McNoldy writes on Facebook.
Water vapor loop of Iselle’s approach and landfall in Hawaii, via Andy Revkin (of NY Times’ DotEarth)
Strong winds and heavy rain will continue for the Big Island on Friday, as the storm spreads to the other islands in Hawaii. Ocean swells of around 10 feet are possible north and east of the Big Island, with wind waves of up to eight feet.
As Iselle tracks west, the island of Oahu should expect heavy rain and winds around 40 mph, with stronger gusts at higher elevations.
— Jeff Piotrowski (@Jeff_Piotrowski) August 8, 2014
Beyond Iselle, Hurricane Julio has taken advantage of favorable winds and warm ocean water, and strengthened to a category 3 with winds of 120 mph. Julio is expected to weaken to a category 1 hurricane over the next couple of days, and close in on Hawaii on Sunday and Monday, but is not expected to make landfall on the islands at this time.
While current forecasts suggest that Julio’s strong winds and heavy rain will remain mostly off-shore, it’s still possible that this forecast could change over the next couple of days. The islands are very close to the southern edge of the forecast cone, and residents of Hawaii should pay close attention to future updates.
Should Julio pass close enough, it could bring heavy rain to islands already saturated from Iselle, posing a risk for more flash flooding and mudslides.