Hurricane Iselle a Threat to Hawaii; Latvia's 100°F an All-Time Record

Hurricane Iselle began a gradual weakening process overnight, falling from a Category 4 storm with 140 mph winds to Category 3 with 125 mph winds at 11 am EDT Tuesday. Ocean temperatures beneath the storm are about 26°C, which is marginal for maintaining a hurricane, and plots of Maximum Potential Intensity from the Center for Ocean-Land-Atmosphere Studies show that the Iselle should only be able to maintain Category 2 strength with these ocean temperatures and the current atmospheric background conditions. Iselle is headed westwards at 9 mph towards Hawaii, and will begin affecting the Hawaiian Islands Thursday night. Satellite images show an Iselle is an impressive storm with a large eye and intense eyewall clouds with very cold cloud tops, but the storm is no longer symmetric, due to wind shear and dry air eating away at its southwest side. The relative lack of spiral bands and large, thick eyewall qualify Iselle to be a rare breed of hurricanes known as "annular". Annular hurricanes are a subset of intense tropical cyclones that are significantly stronger, maintain their peak intensities longer, and weaken more slowly than average tropical cyclones. Only 4% of all hurricanes are annular hurricanes.


Figure 1. True-color MODIS image of Hurricane Iselle from 19:40 UTC (3:40 pm EDT) August 4, 2014. At the time, Iselle was a Category 4 hurricane with 140 mph winds. Iselle was showing an annual structure--a lack of spiral bands and large, thick eyewall. Image credit: NASA.

Forecast for Iselle
Wind shear is expected to stay moderate for the next four days, and ocean temperatures will remain near 26°C. However, the atmosphere surrounding Iselle will begin to dry considerably beginning on Tuesday afternoon, which should force steady weakening until the storm reaches the Hawaiian Islands on Thursday night. Due to is annular structure, Iselle will likely weaken more slowly than a typical hurricane, and it could still be a strong tropical storm capable of generating dangerous heavy rains when it reaches the islands. The Tuesday morning 06Z run of the HWRF model predicted that Iselle would dump widespread rains of 4 - 8" over the islands, with some isolated areas of 8 - 16". The 11 am EDT Wind Probability Forecast from NHC gave Hilo on the Big Island a 50% chance of experiencing tropical storm-force winds of 39+ mph, and a 4% chance of hurricane-force winds. These odds were 31% and 1%, respectively, for Honolulu. The Tuesday morning runs of our four top intensity models were in poor agreement, predicting Iselle would arrive in the islands with maximum sustained winds ranging from 45 mph to 90 mph. Historically, no hurricane approaching from the east has ever affected the islands, and I expect Iselle will weaken below hurricane strength before reaching the islands. The NOAA Hurricane Hunters' jet is scheduled to fly a dropsonde mission on Tuesday evening out of Honolulu, and an Air Force hurricane hunter aircraft is scheduled to fly a low-level mission into the hurricane early Wednesday morning. The data from these flights should help make better forecasts for Iselle beginning early Wednesday morning.

After Iselle comes Julio
After Iselle finishes its close encounter with the Hawaiian Islands late this week, the islands need be concerned with yet another tropical cyclone: Tropical Storm Julio, which had top winds of 60 mph at 11 am EDT on Tuesday. Satellite loops show that Julio is headed westwards towards Hawaii on a path very similar to Iselle's, and the storm should be able to take advantage of light to moderate wind shear and warm ocean temperatures to become a hurricane by Wednesday. The Tuesday morning 06Z run of HWRF model predicted that Julio, like Iselle, would be capable of dumping widespread rains of 4 - 8" in the vicinity of the islands, with some isolated areas of 8 - 16". The model had Julio's heavy rain swath barely missing the islands. However, given the large errors present in 5+ day hurricane forecasts, it is quite possible that Julio's heaviest rains will hit the islands, potentially falling on soils already saturated by Hurricane Iselle, resulting in extremely dangerous and destructive flooding. The latest 12Z (8 am EDT) Tuesday runs of our top hurricane track models (the GFS, European, HWRF, and GFDL) all show Julio missing the Hawaiian Islands to the north in their 5-day forecasts, so I am cautiously optimistic that Hawaii can avoid a devastating one-two punch from Iselle and then Julio.


Figure 2. Tracks of all tropical cyclones (tropical depressions, tropical storms, and hurricanes) to pass within 100 miles of the Hawaiian Islands, 1949 - 2013. 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 one named storm approaching from the east has hit the islands since 1949, an unnamed 1958 tropical storm that hit the Big Island. 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.

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 one tropical storm has hit the islands since 1949--an unnamed 1958 storm that hit the Big Island. 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.

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.


Figure 3. True-color MODIS image of Hurricane Bertha at 1:50 pm EDT August 4, 2014. At the time, Bertha was a Category 1 storm with top winds of 80 mph. Image credit: NASA EARTHDATA.

Bertha headed out to sea
In the Atlantic, Tropical Storm Bertha is on its way to the North Atlantic graveyard for the tropical cyclones--the cold waters south of Canada. Bertha was able to hold onto hurricane status for 18 hours on Monday and Tuesday morning, but higher wind shear has taken its toll on the storm, reducing it to a 60 mph tropical storm as of 11 am EDT Tuesday. Visible satellite loops on Tuesday morning showed the typical signature of a weak tropical storm struggling with wind shear--a low level center that was exposed to view with only a small area of heavy thunderstorms that was limited to one side of the circulation. High wind shear and very cool waters of 20°C will convert Bertha into a powerful extratropical storm on Wednesday, and its remnants could bring some heavy rain showers and tropical storm-force winds gusts to Southeast Newfoundland on Thursday. Along with Hurricane Arthur, Hurricane Bertha gives us two Atlantic hurricanes so far this year, matching the total number of hurricanes during the entire 2013 Atlantic hurricane season. The second (and final) hurricane of the 2013 season (Ingrid) did not arrive until September 14. On average, the second hurricane of the Atlantic season arrives on August 28. The last time the first two named storms in the Atlantic became hurricanes was in 1983, when Alicia, Barry and Chantal all became hurricanes (kudos to TWC's Stu Ostro for this stat.)


Figure 4. Typhoon Halong as photographed and tweeted by astronaut Reid Wiseman at 09:35 UTC August 5, 2014. At the time, Halong was a Category 2 storm with 105 mph winds.

Weakening Typhoon Halong headed towards Japan
In the Western Pacific, Typhoon Halong, formerly a mighty Category 5 super typhoon with 160 mph winds on Sunday, had weakened significantly to a Category 2 storm with 100 mph winds at 11 am EDT Tuesday morning. Satellite loops show that Halong no longer has an eye, though a new eyewall is trying to build. Halong is expected to affect Southern Japan as a Category 1 typhoon late this week.

Latvia sets a new national heat record
On Monday, August 4, 2014, for the second consecutive day, the nation of Latvia recorded its hottest temperature in its recorded history. The mercury hit 100.0°F (37.8°C) at Ventspili (also spelled Ventspils), the first 100°F reading ever recorded in the Baltic countries (Latvia, Lithuania, and Estonia.) Latvia's previous all-time heat record of 98°F (36.7°C) was set just the day before, on August 3, 2014. Prior to that, the hottest temperature ever recorded in Latvia was 97.5°F (36.4°C) in August 1943 at Daugavpils. Wunderground's weather historian Christopher C. Burt has more details in his latest blog post.

According to weather records researcher Maximiliano Herrera, one other nation has seen an all-time national heat record in recent weeks--Iran, where the mercury rose to 127.4°F (53.0°C) at Gotvand on July 17, 2014, tying the record set at Delhoran, Iran in July 2011. I hear you saying, yes, but it was a dry heat. True enough. In fact, the air was so dry over Iran during the July 17, 2014 heat wave that in Delhoran, where the temperature topped out at 125°F (51.5°C), an astonishingly low relative humidity of 0.8% was recorded. There was a difference of 131°F (73°C) between the temperature and dew point.

Jeff Masters