Hurricane Iselle Bearing Down on Hawaii

Hurricane warnings are flying for the Big Island as Hurricane Iselle bears down on Hawaii as a Category 1 hurricane with 80 mph winds. This is the first hurricane warning issued for the main Hawaiian Islands in 21 years--since Hurricane Fernanda in 1993 (which ended up missing.) Thursday morning satellite images showed very little weakening of Iselle. The eyewall's heavy thunderstorms continued to maintain their intensity and areal coverage, and the eye was still prominent. The outer spiral bands of Iselle had already arrived on the Big Island as of early Thursday morning, as seen on Hawaii radar. Wind shear was moderate, about 15 knots, due to strong upper-level winds out of the north, but this shear is having only a minor impact on Iselle.


Figure 1. Hurricane Iselle, with 90 mph winds, and Hurricane Julio, with 75 mph winds, steam west-northwest towards the Hawaiian Islands in this GOES-West image taken at 8 pm EDT Wednesday, August 6, 2014. Image credit: NASA/Goddard Space Flight Center.

Forecast for Iselle
Wind shear is expected to stay moderate until Iselle reaches the Big Island Thursday evening. Ocean temperatures will remain a marginal 26 - 26.5°C, and the atmosphere surrounding the storm will be fairly dry, resulting in continued slow weakening of Iselle. Our top intensity models are in decent agreement on Iselle's strength, predicting that the storm will weaken by 5 - 10 mph, to top winds of 70 - 75 mph, before landfall. The official CPHC forecast for a 75 mph minimal Category 1 hurricane at 8 pm EDT Thursday is reasonable. The main threat from Iselle will be heavy rains leading to flash flooding and mudslides. The Thursday morning 06Z 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 11 am EDT Thursday Wind Probability Forecast from Central Pacific Hurricane Center (CPHC) gave Hilo on the Big Island a 88% chance of experiencing tropical storm-force winds of 39+ mph, and a 16% chance of hurricane-force winds. These odds were 45% and 2%, 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. Predicted rainfall along the track of Hurricane Iselle from the 06Z (2 am EDT) Thursday August 7, 2014 run of the GFDL model. The model forecast that Iselle would be a strong tropical storm with 65 mph winds at landfall Thursday evening. Widespread rains of 4 - 8" were predicted over the islands, with some regions seeing 8 - 16". Image credit: NOAA/GFDL.

Hurricane Julio intensifies, expected to skirt Hawaii
Hawaii's other hurricane threat is Hurricane Julio, which intensified to a Category 2 hurricane with 105 mph winds at 11 am EDT on Thursday. Satellite loops show that Julio now has 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 Category 2 status until Friday, when a slow weakening trend should begin. With the notable exception of the GFDL model, our top track models continue to predict that the center of the storm will pass about 100 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.


Figure 3. True-color MODIS image of Hurricane Genevieve from 22:45 UTC (6:45 pm EDT) August 6, 2014. At the time, Genevieve was intensifying into a Category 4 hurricane with 135 mph winds. Image credit: NASA.

Hurricane Genevieve becomes Super Typhoon Genevieve
Farther west in the Pacific, what was formerly Hurricane Genevieve has now become Super Typhoon Genevieve, after the storm crossed the International Date Line from east to west early Thursday. Genevieve put on an amazing display of rapid intensification, going from a tropical storm with 60 mph winds to a Category 5 super typhoon with 160 mph winds in just 27 hours, from 09 UTC August 6 to 12 UTC August 7. Satellite images show an very impressive storm with a large eye surrounded by a giant area of intense eyewall thunderstorms with very cold cloud tops. Fortunately, Genevieve is not expected to threaten any land areas. Genevieve reached major hurricane status shortly before crossing the Date Line, so the Eastern Pacific has now had 10 named storms, 6 hurricanes, and 4 intense hurricanes so far in 2014. On average, we expect to see just 7 named storms, 3 hurricanes, and 1 intense hurricane by August 7. It is rare for a tropical cyclone to cross the Date Line; the last storm I'm aware of to do so was Category 5 Hurricane Ioke of 2006, which was renamed Super Typhoon Ioke when it crossed the Date Line on August 27. There is no difference between a North Pacific hurricane and a typhoon other than its location--if the storm is west of the Date Line, it is called a typhoon, and if it is east of the Date Line, it is called a hurricane. This only applies to storms in the Pacific in the Northern Hemisphere; in the Southern Hemisphere's Pacific Ocean, everything is called a Tropical Cyclone regardless of which side of the Date Line it falls on.


Figure 4. Typhoon Halong as photographed and tweeted by astronaut Reid Wiseman at 09 UTC August 7, 2014. At the time, Halong was a Category 1 storm with 85 mph winds.

Typhoon Halong drenching Japan
In the Western Pacific, Typhoon Halong was a Category 1 storm with 85 mph winds at 8 am EDT Thursday, and was spreading heavy rains into Southern Japan. Halong's center passed 25 miles east of Minamidaitojima (South Daito Island) between 3 and 5 pm JST (2 and 4 am US EDT) today. According to Japan's AMeDAS observation network, the peak 10-minute sustained wind at Kitadaito (North Daito) Island was 72 mph from the NNE, with a peak gust of 106 mph also from the NNE, both close to 3 pm JST. Minamidaito reported a peak gust of 100 mph and a minimum pressure of 953.7 millibars (28.17 inches of mercury) along with about 3 inches of rain so far. Satellite loops show that Halong has lost most of the heavy thunderstorms along the northern side of its circulation, but the typhoon still has a large eye. Halong is moving north at just 5 mph, and will bring Southern Japan an extended period of heavy rain today through Saturday as a Category 1 typhoon. Thanks go to TWC's Nick Wiltgen for Halong wind stats.

The Atlantic is quiet
In the Atlantic, there are no threat areas to discuss, and none of the reliable models for predicting tropical cyclone genesis predicts development over the next five days.

Interesting links
Storm surge history of Hawaii from Dr. Hal Needham
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

I'll have a new post later today.

Jeff Masters