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Tropical Storm Hilda Degenerating as it Approaches Hawaii

By: Jeff Masters 3:26 PM GMT on August 12, 2015

A Tropical Storm Watch continues for the Big Island of Hawaii as Tropical Storm Hilda heads northwest at 5 mph towards the islands. Hilda degraded significantly over the past day due to high wind shear of 30 - 40 knots, but satellite loops on Wednesday morning showed that the storm continued to generate a respectable amount of heavy thunderstorm activity near its core. Shear will remain a high 25 - 40 knots through Friday, the surrounding atmosphere will grow increasingly dry, and sea surface temperatures will cool slightly, which should cause Hilda to weaken to a tropical depression by the time of its closet approach to Hawaii. The Wednesday morning runs of our two most reliable models for predicting hurricane tracks, the European and GFS models, both showed Hilda dissipating before making its closest approach to Hawaii on Thursday evening. However, even if Hilda dissipates before reaching Hawaii, it will still be capable of bringing heavy rains to the islands, particularly to the Big Island. In their 11 am EDT Wednesday Public Advisory, the Central Pacific Hurricane Center cautioned that Hilda could bring 6 - 12" of rain, with isolated amounts up to 18". These amounts will likely only fall over ocean areas, since the center of Hilda is expected to pass about 150 miles south of the Big Island. The 2 am EDT (06Z) Wednesday run of the HWRF model predicted that the Big Island would see 2 - 4" of rain from Hilda, with the extreme southeast corner getting 4 - 8". Maui would get less than 1". I expect that the Big Island will receive maximum rainfall amounts of 3 - 6" from Hilda, with Maui getting 1 - 2".

Figure 1. When a hurricane unravels: high wind shear due to strong upper-level westerly winds on Tuesday, August 11, 2015, exposed the low level center of Hurricane Hilda to view. Hilda's heavy thunderstorms were all on the east side of the center of circulation in this MODIS satellite image from 5 pm EDT, when Hilda had top winds of 75 mph. Image credit: NASA.

In the Western Pacific, the European and GFS models predict that twin tropical storms will form in the waters midway between Hawaii and the Philippines' Luzon Island this weekend. Both of these storms will have the potential to cause trouble for Asia late next week. The twin storms will be close enough together that they could influence each other, making prediction of their track and intensity more difficult than usual.

Figure 2. Tracks of the 22 tropical cyclones (tropical depressions, tropical storms, and hurricanes) to pass within 150 miles of the Hawaiian Islands, 1949 - 2015. Hawaii has seen an unusual number of these storms (three) affect them in the past three years: Flossie (2013), Iselle (2014), and Guillermo (2015). Hilda (2015) may become the fourth such storm if it survives into Thursday night. Hurricanes approaching from the east typically fall apart before they reach Hawaii due to the cool waters and dry air that lie to the east of the islands. Only two named storms approaching from the east has hit the islands since 1949, an unnamed 1958 tropical storm that hit the Big Island, and Tropical Storm Iselle of 2014. Hurricanes approaching from the south represent the biggest danger to the islands, due to the warmer waters and more unstable air present to the south. The only two major hurricanes to have affected the islands since 1949, Hurricane Iniki of 1992 and Hurricane Dot of 1959, both came from the south. Image credit: NOAA/CSC.

An unusual amount of hurricane activity for Hawaii in recent years
An unusually high number of tropical storms and hurricane have roamed the waters near Hawaii over the past three years. This year, there has been Guillermo and Hilda, last year had Iselle and Julio, and 2013 had Flossie. Since 1949, 22 tropical cyclones (tropical depressions, tropical storms, and hurricanes) have passed within 150 miles of the Hawaiian Islands, an average of one tropical cyclone every three years. So, to have three tropical cyclones (four if we count Hilda) pass so close in three years (see Figure 2) is an unusual amount of activity. Part of the blame for the activity can be placed on unusually warm sea surface temperatures: these temperatures were warmest on record for the waters south and east of Hawaii this summer, and were also well above average in 2014. It is also possible that we are seeing the beginning of a shift in the tracks of the Eastern Pacific hurricanes due to climate change, though it is too early to say. In my August 2014 blog post, Climate Change May Increase the Number of Hawaiian Hurricanes, I reported on a study done last year which projected a doubling or tripling of the number of hurricanes affecting Hawaii by the end of the century, due to climate change.

Figure 3. Double trouble for Hawaii: True-color VIIRS image of Hurricane Iselle (left) and Tropical Storm Julio (right) approaching Hawaii, taken between 3 - 6 pm EDT August 5, 2014. At the time, Iselle was a Category 2 hurricane with 110 mph winds, and Julio had 65 mph winds. Image credit: NOAA Visualization Lab.

Hawaii's hurricane history
On average, between four and five tropical cyclones are observed in the Central Pacific every year. This number has ranged from zero, most recently as 1979, to as many as eleven in 1992 and 1994. August is the peak month, followed by July, then September. Tropical storms and hurricanes are rare in the Hawaiian Islands. Since 1949, the Hawaiian Islands have received a direct hit from just two hurricanes--Dot in 1959, and Iniki in 1992. Both hit the island of Kauai. Only two tropical storms have hit the islands since 1949--an unnamed 1958 storm that hit the Big Island, and Tropical Storm Iselle of 2014, which hit the Big Island with sustained winds of 60 mph, causing $79 million in damage. A brief summary of the three most significant hurricanes to affect Hawaii in modern times:

September 1992: Hurricane Iniki was the strongest, deadliest, and most damaging hurricane to affect Hawaii since records began. It hit the island of Kauai as a Category 4 on September 11, killing six and causing $2 billion in damage. The filming of the original "Jurassic Park" was interrupted by Iniki.

November 1982: Hurricane Iwa was one of Hawaii's most damaging hurricanes. Although it was only a Category 1 storm, it passed just miles west of Kauai, moving at a speed of nearly 50 miles per hour (80 km/h). Iwa killed one person and did $250 million in damage, making it the second most damaging hurricane to ever hit Hawaii. All the islands reported some surf damage along their southwest facing shores, and wind damage was widespread on Kauai.

August 1959: Hurricane Dot entered the Central Pacific as a Category 4 hurricane just south of Hawaii, but weakened to a Category 1 storm before making landfall on Kauai. Dot brought sustained winds of 81 mph with gusts to 103 mph to Kilauea Light. Damage was in excess of $6 million. No Dot-related deaths were recorded.

Jeff Masters


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