Flood and Mudslide Threat Ramping Up as Lane Approaches Hawaii

Hurricane warnings are flying for the Hawaiian islands of Oahu, Maui, Lanai, Molokai, Kahoolawe and the Big Island, and a Hurricane Watch is up for Kauai, as Hurricane Lane moves slowly to the northwest at 7 mph. At 2 pm EDT Thursday (8 am HST), Lane’s top winds were holding at 130 mph, right at the bottom end of Category 4 strength, and the hurricane was located about 205 miles southwest of Kailua-Kona. A decrease in strength Wednesday night was due to increasingly strong wind shear of 25 knots from powerful upper level winds out of the southwest. Satellite loops show that this high wind shear has distorted the shape of Lane, so that the hurricane has less of a circular appearance. NOAA buoy 51002 southwest of the Hawaiian Islands is in the path of Lane, and it reported sustained northerly winds of 69 mph, gusting to 89 mph, at 1:30 pm EDT. Wave heights at the buoy had increased to 25.6 feet as of 12:40 pm EDT.

Radar imagery showed that the spiral bands of the hurricane were spreading dangerous torrential rains over the entire Hawaiian Island chain on Thursday, except for Kauai. The heaviest rains were falling on the Big Island and Maui. On the Big Island, Hawaii County Civil Defense reported ongoing flash flooding in Waipio Valley, with multiple road closures due to landslides and flash flooding. Numerous streams and drainages in the area around Hilo and northward up the Hamakua coast were overflowing, causing very dangerous conditions. Radar and rain gauges showed persistent rainfall rates of 1 to 2 inches per hour.

As of 5 am HST Thursday, the greatest 24-hour rainfall amount on the Big Island was 16.09” at Waiakea Experiment Station, according to the latest NWS Hawaii precipitation summary; Hilo airport received 12.75” of rain. The highest 24-hour rainfall amount on Maui was 4.10” at West Wailuaiki.

Radar images showed intense rainbands streaming onto the east coasts of Maui and the Big Island early Thursday, as the circulation around Lane pushes deep tropical moisture upslope and squeezes it out.

Figure 1. Lane’s outer rainbands were sweeping northward across the Big Island and Maui on Thursday morning, August 23, 2018. Shown here is a See this link for extended radar animations. Image credit: Courtesy Brian McNoldy, University of Miami/Rosenstiel School.

Forecast for Lane

There is high confidence that Lane will be affecting the Hawaiian Islands through at least Saturday with widespread torrential rains and potentially serious floods and mudslides. Lane will continue heading generally north to northwest, and it will likely weaken at a more sustained rate from Thursday into Friday as the increasing wind shear takes a toll. At some point before Saturday, Lane is expected to take a sharp left turn, as the circulation weakens enough to come under the influence of the prevailing trade winds. What’s not yet certain is whether Lane will actually make landfall on any of the islands before that happens.

The European forecast model (ECMWF) has been insisting that Lane will continue moving on a northwest or north-northwest track that would keep it well west of the Big Island and Maui, though perhaps drawing close to Oahu before the sharp turn west. In contrast, the GFS and HWRF models (including their most recent runs, from 12Z Thursday) have been predicting that Lane would maintain a stronger inner core and arc toward the north-northeast on Friday, coming much closer to Maui and perhaps making landfall before turning sharply west. The official forecast from CPHC falls in between these possibilities, close to the multi-model HCCA consensus. A more easterly track, as shown by the GFS and HWRF, would have a greater impact on the Big Island and Maui, but it could also result in Lane weakening more quickly at that point due to the effects of the higher terrain on Lane’s winds. A more westerly track, as portrayed by the ECMWF, would increase the odds that Lane could draw near Oahu as a strong tropical storm or even a Category 1 hurricane. Note: the 12Z run of the ECMWF featured a track further east than prior runs, now agreeing more closely with the GFS and HWRF on the potential for Lane to track closer to Maui and nearby islands late Friday or early Saturday.

Figure 2. Forecast track and “cone of uncertainty” for Lane as of 5 am HST (11 am EDT) Thursday, August 23, 2018. The cones are constructed based on typical forecast errors over the past few years such that a hurricane can be expected to fall within the cone about two-thirds of the time. The cone suggests that Lane’s center will most likely remain well south and west of each Hawaiian island. However, Lane’s effects will extend far from its center, touching virtually all of the islands in some form or fashion. Image credit: NOAA/NWS/CPHC.

Rain/flood/mudslide threat from Lane

Regardless of its exact track, Lane will continue to pull rain bands across the Hawaiian Islands through Friday and into the weekend. A long fetch of very rich atmospheric moisture (precipitable water of 2” to 2.5”) will be feeding into Lane from the east. The flooding threat will rise as soils become saturated, and mudslides and road washouts will be an increasing concern. Lane’s slow movement will only exacerbate the heavy-rain threat. Overall, we can expect widespread 10” to 20” amounts, as already observed on the eastern Big Island, and we would not be shocked to see a few localized storm totals in the 30” - 50” range. Massive flows from these rains will likely lead to floods, landslides, rock falls, and road washouts.

Rainfall records for August may fall in a number of locations, including Honolulu and Lihue. Honolulu’s current August rain record is 7.63”, set in 2015, and Lihue’s is 9.86”, also from 2015. Hilo is one of the wettest towns in the United States; breaking its August rain record of 26.92” from 1991 would be more of a stretch, though it can’t be ruled out.

I despise slow-moving tropical cyclones, and #Lane will be another one for the next 2-3 days. All Hawaiian Islands are on the stronger, wetter side of the hurricane, and the terrain will enhance rainfall and bring water and debris downslope. Potential flooding disaster.

— Dr. Rick Knabb (@DrRickKnabb) August 23, 2018

Wind and tornado threat from Lane

The chance of sustained hurricane-force winds from Lane is quite low for most of Hawaii, but the risk of tropical-storm-force sustained winds (39 - 73 mph) is quite high. Nearly all locations apart from the easternmost Big Island and Kauai County have a greater-than-even chance of experiencing tropical-storm-force winds. As Lane weakens, its wind field will spread out, so that it could continue to pack sustained winds of 40 - 55 mph as far as 100 miles north of its center into Friday night and early Saturday, when Lane is predicted to be a strong tropical storm within about 100 miles of Honolulu. Such winds can be enough to bring down trees and power lines, especially after soils are saturated by heavy rain. Widespread power outages may develop.

The strongest wind gust on record for Honolulu is 82 mph, recorded as Category 1 Hurricane Nina (1957) approached from the south and then veered west just in the nick of time. The population of Oahu and Honolulu has nearly doubled since Nina delivered its record 82-mph gust, and there are many more people living and working in high-rise buildings, where winds can be considerably stronger than at ground level.

Tornadoes are somewhat more likely with Lane than with Hawaii’s other recent brushes with tropical cyclones, in part because of the storm’s strength and path. As it approaches, Lane will put each island in its right front quadrant, the most favored location for the wind shear that can lead to tornadoes. On average, Hawaii sees less than one tornado a year, with 42 recorded from 1955 to 2015, according to the Tornado History Project. Most of these occurred in association with winter storms rather than tropical cyclones. Four of Hawaii’s tornadoes have been rated F2, the most recent being on May 18, 1982.

Figure 3. The probability of experiencing tropical-storm-force winds (sustained at 39 mph or more) at some point between Thursday morning, August 23, 2018, and Tuesday morning, August 28, based on the forecast track as of Thursday morning. Image credit: NOAA/NWS/CPHC.

Storm surge: a lesser concern

Storm surge is generally less of a threat in Hawaii 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).

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 22 feet at the peak of the storm.

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.

Typhoon #Cimaron is making landfall on #Japan with Category 1 intensity. (while #Soulik moves further inland on #SouthKorea). https://t.co/qGdHTcDE3G pic.twitter.com/tSrFqiQgoU

— UW-Madison CIMSS (@UWCIMSS) August 23, 2018

Twin cyclones Soulik and Cimaron slam into Korea and Japan

In a rare juxtaposition of landfalls, Typhoon Cimaron struck the coast of Japan’s Honshu Island as a Category 1-strength typhoon late Thursday local time at the same time that Tropical Storm Soulik (which peaked as a Category 3-strength typhoon with an enormous 75-mile-wide eye) moved onto the southwest coast of the Korean Peninsula. The Associated Press reported one death and one injury from Soulik in South Korea, while hundreds of flights were cancelled across western Japan.

Both Cimaron and Soulik will be weakening quickly as they move over rugged terrain and accelerate toward the northeast, eventually swinging around a large upper-level low in the far North Pacific. By later next week, whatever remains of Lane will likely be swept northward into the flow around that same upper low.

Slight chance of development in the tropical Atlantic next week

A disturbance moving off the west Africa on Friday will be moving through the deep Atlantic tropics over the weekend and into next week. It’s been a very quiet period in the Atlantic tropics: the last named storm to develop south of latitude 30°N was Hurricane Beryl in early July. Conditions have been unusually hostile for development, with stronger wind shear and cooler sea surface temperatures than average. Even so, the waters are now sufficiently warm to support a hurricane in the western Atlantic tropics, although strong wind shear remains widespread.

In its Tropical Weather Outlook on Thursday morning, the National Hurricane Center gave the new disturbance a 10% chance of becoming a tropical depression in the central Atlantic at some point between Saturday and Tuesday. In the longer-range, the GFS and ECMWF ensembles suggest that the deep Atlantic tropics may become somewhat more favorable for development around the Labor Day weekend.

Dr. Jeff Masters co-wrote this post.