Update at 5:20 p.m. ET, Wednesday: A hurricane warning has been issued for the Big Island of Hawaii, and a tropical storm warning is also in effect for the islands of Molokai, Lanai, Kahoolawe, and Maui.  Iselle has strengthened slightly on Wednesday afternoon despite forecasts, though gradual weakening is still expected as the hurricane approaches Hawaii. As of 5 p.m. ET, Iselle was a category 1 hurricane with maximum sustained winds of 90 mph.

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A pair of hurricanes are lined up in the Pacific on Wednesday, and one is poised to slam Hawaii with torrential rain and massive waves. Hurricane Iselle’s heavy rainfall will begin to impact Hawaii’s Big Island on Thursday or Friday, putting it at risk for life-threatening flooding and mudslides. Hurricane Julio, which is behind Iselle as they move west, remains weak and is expected to track north of the islands early next week.

Hurricane Iselle

Hurricane Iselle remains a category 1 hurricane with sustained winds of 85 mph on Wednesday morning. Iselle peaked in intensity on Monday as a strong category 4. Despite its ragged appearance on satellite, hurricane hunters passed through Iselle last night and encountered a storm that had not yet weakened below hurricane status.

Iselle has moved into an environment that is far from favorable for tropical cyclones. Extremely dry air is wrapping around the storm, cutting off the moist air needed to maintain a higher intensity. Iselle has also encountered higher wind shear which has played a role in the deterioration of the hurricane. The eye wall has begun to collapse, and the strong thunderstorm activity has decreased. It’s likely that Iselle will be downgraded to a tropical storm today.

Dry air (yellow) is wrapping into Iselle on Wednesday, as shown on water vapor satellite imagery. (NOAA)

Track forecast for Iselle. (National Hurricane Center)

Hawaii will begin to feel impacts from Iselle on Wednesday. Ocean swells of 10 feet will increase to more than 20 feet over the next 24 hours as the hurricane approaches. While it’s too soon to tell whether a direct landfall will occur, if there is a landfall, it will be on Hawai’i Island (the Big Island) near Hilo. Either way, heavy rainfall with potentially life-threatening flooding and mudslides will be the main impacts from the storm.

Though the final outcome depends strongly on Iselle’s actual path, recent model runs have been suggesting a massive amount of rain could fall on the Big Island. Some models have suggested that over 12 inches of rain is possible. The largest amounts of rain will fall in the high elevations, where terrain will enhance the thunderstorms — this could create life-threatening flash flood situations.

In addition to tropical storm watches for the islands east of and including Oahu, a flash flood watch has been issued for all the islands of Hawaii, effective until 6 a.m. Hawaii time.

Possible rainfall accumulation for Iselle from the HWRF model. (NOAA)

Hurricane Julio

Julio remains a weak category 1 hurricane on Wednesday morning, with sustained winds of 75 mph. The hurricane looks disorganized on satellite and appears to be encountering it’s own share of dry air, though not as much as Iselle. Environmental conditions are expected to improve as Julio tracks west toward Hawaii. The National Hurricane Center forecasts that Julio will intensify to an 85 mph category 1 hurricane over the next day or two.

Julio is still expected to track north of Hawaii as a tropical depression on Sunday and Monday, though this forecast could change over the next few days. Hawaii remains in the forecast cone for Julio, and Hawaiians should remain attentive to future forecasts. Whether or not the storm approaches the islands directly, impacts such as heavy rainfall and large waves are possible.

Julio on enhanced infrared satellite on Wednesday morning. (NOAA)

Track forecast for Julio. (National Hurricane Center)

As we have discussed, hurricanes in Hawaii are relatively rare and back-to-back tropical cyclone landfalls would be unprecedented.