Catastrophic Rains, Floods, Mudslides Possible as Lane Nears Hawaii

Still a powerful Category 4 storm, Hurricane Lane began its gradual turn northwest toward the Hawaiian Islands on Wednesday. On a broad looping path predicted from Thursday to Saturday, the hurricane could make landfall on any or none of the islands. Yet even a track that keeps Lane offshore will have the potential to bring damaging tropical-storm-force winds, torrential rains, and major flooding to large parts of Hawaii.

“Regardless of the exact track of the storm center, life-threatening impacts are likely over some areas as this strong hurricane makes its closest approach,” warned the National Weather Service office in Honolulu in a local statement issued Tuesday morning. Preparations are well under way across the islands, as reported by weather.com.

As of 11 am HST (5 pm EDT) Wednesday, Lane’s top sustained winds were rated by the NOAA/NWS Central Pacific Hurricane Center (CPHC) at 155 mph, at the very top of the Cat 4 range, although the showers and thunderstorms (convection) around Lane were showing some signs of weakening. The hurricane was located about 285 miles south of Kailua-Kona and about 420 miles south-southeast of Honolulu, moving west-northwest at 8 mph. Lane's central pressure was at 935 mb. Hurricane warnings were in effect for Hawaii and Maui counties, including the Big Island, Maui, Lanai, Molokaim and Kahoolawe. A hurricane watch was in effect for Oahu (including Honolulu) as well as for Kauai and Niihua.

A behemoth in the Central Pacific

As it pushed westward well southeast of Hawaii, Lane underwent a jaw-dropping round of sustained intensification on Monday that pushed the storm into Category 5 territory by late Monday night, with top sustained winds peaking at 160 mph. The storm benefited from warm sea surface temperatures of 27-28°C (81-82°F), light wind shear (around 10 knots), a moist mid-level atmosphere, and upper-level outflow that kept the storm’s heat-engine chimney working at fearsome strength. Lane’s central pressure dropped to as low as 922 mb. We’re fortunate that NOAA and Air Force Hurricane Hunters have been able to carry out frequent missions into Lane, made possible in part because the Atlantic tropics have been so quiet.

Amazing shots from today's mission into Hurricane #Lane. Pressure dropped like a rock throughout the 6-hr flight (940 to 929mb) with sfc winds up to 155mph (strong Cat 4). Saw 168mph at flight level! pic.twitter.com/FbSuCELlpm

— Jeremy DeHart (@the_realJeremyD) August 22, 2018

Intensity forecast for Lane

Fortunately, Lane has probably hit its peak intensity, but it will take time to weaken. The 18Z Wednesday run of the SHIPS model indicates that wind shear over Lane will remain in the moderate range (10-15 knots) through Thursday, and sea surface temperatures will remain in the 27-28°C range (81-82°F), so these should not induce any major weakening. The timing of eyewall replacement cycles is difficult to predict, but if one happens to take place on Wednesday or Wednesday night, which is quite possible, Lane’s strength could drop by roughly a Saffir-Simpson category (perhaps into the Category 3 range) for roughly a day. In this case, the hurricane would probably run out of time to regain its former strength before wind shear increases and Lane begins to interact with the Hawaiian Islands.

From Friday into the weekend, we expect Lane to undergo a more sustained weakening. The pace will be controlled mainly by wind shear, which will increase dramatically by Friday night, and by how closely Lane passes to any of the islands, a variable that is still impossible to nail down with certainty at this point. The CPHC forecast at 11 am EDT Wednesday predicted that Lane would weaken to Category 2 strength by early Friday and to strong tropical-storm strength by early Saturday.

Importantly, Lane’s swath of tropical-storm-force winds (sustained at 39 mph or more) is expected to extend 100 miles or more north of Lane’s center all the way from Wednesday through Saturday. This means that nearly all of the Hawaiian Islands have a better-than-even chance of experiencing winds this strong at some point. Such winds can be enough to bring down trees and power lines, especially after soils are saturated by heavy rain.

Figure 1. The probability of experiencing tropical-storm-force winds (sustained at 39 mph or more) at some point between Tuesday, August 21, 2018, and early Sunday, August 26, based on the current official forecast track. Image credit: NOAA/NWS/CPHC.

Track forecast for Lane

The leading computer forecast models stuck largely to their guns on Wednesday, with enough difference between them to throw a frustratingly large amount of uncertainty into Lane’s track. For days, the European model (ECMWF) has insisted that Lane would stay well west of the Big Island and Maui, bending northwest on a path that could bring it over or near Oahu or Kauai by Saturday, most likely as a strong tropical storm or perhaps a Category 1 hurricane. In contrast, the GFS and HRWF models have kept Lane heading much closer to the west coast of the Big Island, with the potential for an unprecedented hurricane-strength landfall on Maui before a sharp turn west. These differences remained stark in the Wednesday morning round of model runs. The GFS and HWRF trended somewhat further away from the Big Island in their 12Z Wednesday runs (though not as far away as the ECMWF solution), but both the GFS and HWRF continued to call for a Maui landfall on Friday.

All three of these models are excellent performers, and we have no experience with any of them for a storm of this magnitude in the midst of the Hawaiian Island chain. The difference may come down to how the models are handling the flow around Lane and how that flow will interact with the Big Island. The easterly winds flowing toward Lane will be shunted around the two great volcanic peaks of the Big Island, Mauna Loa and Mauna Kea, which will help create a zone of low pressure just west of the island. It’s possible Lane will feel a “tug” from this wind-induced pressure perturbation and get pulled toward Maui. At this point, it’s hard to say which models might be handling this interaction the best, but it is safe to say that a Maui landfall cannot be ruled out at this point. Maui remained on the right-hand edge of the “cone of uncertainty” on Wednesday morning, and any given hurricane will stray outside the edges of the cone about one out of three times on average.

If the European solution proves to be more correct, then the odds of a landfall or close brush by Oahu and Kauai late Friday or Saturday will increase. As Lane weakens, it will experience a greater steering effect from the prevailing trade winds, which will tend to cause an increasing westward turn. The stronger Lane remains, the greater the chance it will continue moving northwest and draw closer to Oahu and Kauai.

Rains, floods, and mudslides: The most concerning threat

Assuming that Lane follows a rough model consensus, as suggested in the CPHC forecast, it may not strike any of Hawaii’s islands directly—but it could still be a catastrophic hurricane. Lane’s powerful and well-established circulation will send huge amounts of tropical moisture flowing against the islands’ rugged terrain. This will lead to periods of torrential rain that could extend over several days in the most favored spots, especially across north- and east-facing slopes. Totals of 10” – 20” can be expected across large parts of Hawaii, and I would not be at all surprised to see localized amounts well above 30”, especially along east-facing slopes of the Big Island and Maui. Massive flows from these rains could lead to floods, landslides, and road washouts.

Models are already suggesting that a broad fetch of tropical moisture feeding into Lane from the east will translate toward the islands as Lane moves north. The pressure gradient north and east of Lane may lead to the development of a convergence zone that would focus the rainfall along an east-west band north of Lane. Wherever this band happens to intersect with the islands, rainfall could be especially heavy. Among the potential impacts noted by NWS/Honolulu in its Hurricane Local Statement on Lane:

• Extreme rainfall flooding may prompt numerous evacuations and rescues.
• Rivers and tributaries may overwhelmingly overflow their banks in many places with deep moving water. Small streams, creeks, canals, arroyos, and ditches may become raging rivers. In mountain areas, deadly runoff may rage down valleys while increasing susceptibility to rockslides and mudslides. Flood control systems and barriers may become stressed.
• Flood waters can enter numerous structures within multiple communities, some structures becoming uninhabitable or washed away. Numerous places where flood waters may cover escape routes. Streets and parking lots become rivers of raging water with underpasses submerged. Driving conditions become very dangerous. Numerous road and bridge closures with some weakened or washed out.

Figure 2. Very rich tropical moisture will extend along a zone of converging air extending east from Lane, as shown in this GFS-model depiction of flow at 850 millibars (about a mile above the surface) and precipitable water (the amount of moisture in the air, in centimeters, above a given spot). This forecast for 2 am EDT Friday, August 24, was generated on Wednesday morning. Image credit: tropicaltidbits.com.

Limited storm surge risk for Hawaii

Flash flooding from torrential rains is the main threat from Hurricane Lane in the Hawaiian Islands, followed by wind damage. Storm surge is less of a threat compared to Atlantic hurricanes hitting the U.S. coast, since the Hawaiian Islands are surrounded by deep water that prevents a storm surge from building to large heights. The worst-case surge from a mid-strength Category 2 hurricane hitting Oahu, for example, is only four feet in Pearl Harbor, which is the most vulnerable portion of Oahu to storm surge (see wunderground’s Maximum of the "Maximum Envelope of Waters" (MOM) storm tide image).

The latest Hurricane Local Statement from CPHC said to expect locally hazardous storm surge impacts across south and west facing coasts of the islands:

• Localized inundation with storm surge flooding mainly along immediate shorelines and in low-lying spots, or in areas farther inland near where higher surge waters move ashore.
• Sections of near-shore roads and parking lots become overspread with surge water. Driving conditions dangerous in places where surge water covers the road.
• Moderate beach erosion. Heavy surf also breaching dunes, mainly in usually vulnerable locations. Strong rip currents.
• Minor to locally moderate damage to marinas, docks, boardwalks, and piers. A few small craft broken away from moorings.

Elsewhere across the Hawaiian Islands, little to no impact from storm surge is anticipated.

A bigger threat than the storm surge is the waves Hurricane Lane will generate. The latest marine forecasts for the leeward waters of the Big Island and Maui call for waves of up to 25 feet at the peak of the storm.

Figure 3. In records going back to 1851, only five hurricanes before Lane were known to have reached Category 5 strength while prowling the central Pacific: Patsy in 1959; Emilia, John, and Gilma, all in 1994;  and Ioke in 2006. (Many hurricanes went undersampled or underreported before the era of satellite monitoring.) Image credit: NOAA Historical Hurricane Tracks, courtesy Jon Erdman, weather.com.

Recent Hawaiian hurricane history

After seeing three consecutive years of unprecedented hurricane threats in 2014, 2015 and 2016, the Hawaiian Islands got a break in 2017, when no named storms passed nearby. The most recent named storm to make landfall on a Hawaiian island was Tropical Storm Darby, which hit the Big Island of Hawaii with sustained winds of 40 mph on July 23, 2016, becoming just the fifth named storm since 1949 to make landfall on a Hawaiian Island. The next day, Darby made the closest approach on record by a tropical storm to the island of Oahu, which resulted in torrential rains in excess of 10 inches. Darby passed just 40 miles to the south and west of Honolulu with sustained winds of 40 mph. Darby’s formation was aided by ocean temperatures that were more than 1°C (1.8°F) warmer than average. Darby caused no deaths and only minimal damage.

Prior to Darby, the most recent landfall by a named storm in Hawaii was Tropical Storm Iselle, which hit the Big Island on August 8, 2014, with sustained winds of 60 mph. With estimated damages of $79 million (2014 USD), Iselle surpassed Hurricane Dot of 1959 as the third-costliest tropical cyclone to ever hit Hawaii. The only more damaging Hawaiian storms were Iniki in 1992 and Iwa in 1982.

Prior to Hurricane Lane this week, the most recent hurricane warning in Hawaii was for the Big Island on August 31, 2016, for Hurricane Madeline, which peaked as a Category 4 storm with 130 mph winds on August 30, 2016. The Big Island was fortunate to receive only a glancing blow from a weakening Tropical Storm Madeline, with a few wind gusts in excess of 40 mph and some minor flooding from heavy rains in excess of 5”. Just three days later, on September 3, 2016, the outer rainbands from Hurricane Lester, which had also peaked as a Category 4 storm east of the Hawaiian Islands, produced heavy showers and minor flooding over the leeward slopes of the Big Island and portions of east Maui. Winds were light, however.

As we explained in a August 2014 post, Climate Change May Increase the Number of Hawaiian Hurricanes, warming sea surface temperatures may help shift the tracks of Eastern Pacific hurricanes closer to Hawaii.

What about Kilauea?

Experts say there's little risk that the effects of the hurricane will make things worse at the Kilauea volcano on the Big Island, where hundreds of homes have been lost from this summer's lava flows. "Effects of hurricane-related torrential rainfall on the lower East Rift Zone lava flow will be minimal," Janet Babb, a geologist at the Hawaiian Volcano Observatory, told weather.com. "Where the interior of the flow is still hot, heavy rain will likely result in the formation of steam."

For more details on potential impacts from Lane, see the frequently updated weather.com feature story.

Dr. Jeff Masters co-wrote this post.