| Above: Visible satellite image of Subtropical Storm Debby, taken at 1:21 pm EDT Tuesday, August 7, 2018. Image credit: Levi Cowan, tropicaltidbits.com. |
Subtropical Storm Debby formed over the remote waters of the Central Atlantic at 11 am EDT Tuesday, and is headed northwards towards cold waters and a quick exit from the Atlantic named-storm stage. Satellite images on Tuesday afternoon showed that Debby has the classic appearance of a subtropical storm, with a large cloud-free center and the heaviest thunderstorms in bands well removed from the center.
Debby will be over marginal SSTs near 26°C (79°F) and under light to moderate wind shear of 5 – 15 knots through Tuesday night, but SSTs will fall to about 20°C (68°F) by Wednesday morning, as Debby heads north at about 16 mph, away from the warm waters of the Gulf Stream. Cold waters will likely put an end to Debby’s short life as a named storm by Wednesday night. With top winds of just 40 mph, and a lifetime likely to be measured in hours rather than days, Debby is the type of storm that would probably would not have been named in the pre-satellite era.
Debby’s formation date of August 7 comes a little over two weeks earlier than the usual August 23 formation date of the Atlantic’s fourth named storm. So far in 2018, we’ve had 4 named storms, 2 hurricanes, and no intense hurricanes; that is the level of activity typically seen by August 28. According to hurricane scientist Phil Klotzbach of CSU, Debby is the third named storm of the 2018 hurricane season to be classified as a subtropical storm at some point during its lifetime, along with Alberto and Beryl. The most Atlantic named storms to be classified as subtropical storms in a season is five in 1969.
While drifting high above a storm’s tangible fury, there is beauty to be found in the worst that nature has to offer. #HurricaneHector pic.twitter.com/fluMrJLUgD
— Ricky Arnold (@astro_ricky) August 7, 2018
Category 4 Hurricane Hector on track to graze Hawaii on Wednesday
Hurricane Hector is maintaining Category 4 strength as it steams west at 16 mph across the Central Pacific on a track that will take it about 150 miles south of Hawaii on Wednesday. A Tropical Storm Watch is in effect for the Big Island. Southern portions of that island, as well as higher elevations on its eastern side, could see tropical storm-force winds and a few heavy rain squalls on Wednesday. An Air Force hurricane hunter aircraft was in the hurricane Tuesday afternoon, and recorded surface winds near 130 mph. Hector peaked as a top-end Category 4 storm with 155 mph winds on Monday afternoon, and could have been at Category 5 strength between the two hurricane hunter missions that day. There have only been five Category 5 hurricanes in the Central Pacific ever recorded, so Hector is the sixth strongest hurricane ever observed in that region.
Intensity forecast for Hector
The SHIPS model predicts that Hector will be over marginally warm waters of 26.5°C (80°F) until it passes Hawaii on Wednesday. These temperatures are on the low end of what can sustain a major hurricane, and with Hector expected to move into a steadily drier environment, slow weakening can be expected through Wednesday, despite low wind shear of 5 – 10 knots.
Category 4 Hurricane #Hector overnight via #VIIRS on Suomi-NPP & #NOAA20. pic.twitter.com/rF4fwnm7Qo
— UW-Madison CIMSS (@UWCIMSS) August 7, 2018
Track forecast for Hector
The 12Z Tuesday runs of our top models for tracking hurricanes, the European, UKMET, GFS, and HWRF models, all showed Hector passing at least 150 miles south of the Big Island on Wednesday. The 5pm EDT Tuesday Wind Speed Probability forecast from the Central Pacific Hurricane Center gave the southern tip of the Big Island a 22% chance of seeing tropical storm-force winds 39 mph of greater; Hector’s tropical storm-force winds extended up to 90 miles from the center at 11 am EDT Tuesday. The 6Z Tuesday run of the HWRF model predicted that Hector’s heavy rains would remain south of the Big Island, with an inch or less of rain falling on the eastern sides of the Big Island and Maui. However, it would not be a surprise to see a band of heavy rain affect these islands on Wednesday, bringing isolated rainfall amounts of up to 4” that could cause localized flash flooding. For now, though, it appears that high surf will be the main threat Hector poses to Hawaii. Surf of 12 to 15 feet is predicted for the Big Island, mainly for the Puna and Kau districts, with surf of 6 to 10 feet for eastern Maui.
Hector likely to be extremely long-lived
The latest long-range forecasts from the 12Z Tuesday runs of the GFS and European models predict that Hector will stay far enough south during the coming week to avoid recurvature to the north and a death over the cold waters of the North Pacific. This track will likely allow Hector to pass near the U.S. military facilities on Johnston Island on Friday, August 10. The 5pm EDT Tuesday Wind Speed Probability forecast from the Central Pacific Hurricane Center gave that island a 59% chance of seeing tropical storm-force winds, and an 14% chance of seeing hurricane force winds. Hector will be over steadily warming waters and under light wind shear after it passes Hawaii, so is still likely to be a hurricane--and possibly a major hurricane--when it makes its closest approach to Johnston Island.
Hector is expected to cross the Date Line around August 13, when it would become the Northwest Pacific’s Typhoon Hector. The 12Z Tuesday run of the GFS model had Hector finally recurving to the north and dying on August 22, in the waters over 500 miles east of Japan. Hector became a tropical depression on July 31 and became a tropical storm on August 1, so it is possible the storm could be around more than 20 days, making it one of the longest-lived tropical cyclones and named storms on record.
The longest-lived tropical cyclone in world history was another Eastern Pacific hurricane that crossed the Date Line and became a typhoon: Hurricane John of 1994. John followed a 7,165-mile (13,280-km) path from the eastern Pacific to the western Pacific and back to the central Pacific, lasting 30.0 days as a tropical cyclone. John brought a few heavy rains showers and high surf to Hawaii, and caused $15 million in damage to U.S. military facilities on Johnston Island. Including John, there have been at least nineteen tropical cyclones that have lasted at least 20 days as a tropical depression or stronger.
Two more named storms in the Eastern Pacific
In addition to Hector, there are two other named storms in the Eastern Pacific: Tropical Storm Kristy and Hurricane John. A third tropical storm, Ileana, was destroyed on Tuesday morning by upper level outflow from John that caused high wind shear, ripping the storm apart.
Hurricane John is the larger of the two storms, and is expected to intensify into a Category 3 hurricane by Wednesday. John is headed to the northwest, parallel to the coast, and is far enough offshore so that it will likely not bring strong winds or heavy rains to the coast of Mexico. John will move over cooler waters on Thursday, and will likely die this weekend without affecting any land areas.
Tropical Storm Kristy is far out at sea, and not a concern for any land areas this week. Depending upon its interaction with big brother Hurricane John, Kristy may be a concern for Hawaii next week, if the storm can stay disentangled from John and maintain a westerly track, as the 12Z Tuesday run of the European model predicted.
The high activity in the Eastern Pacific this week is due to several factors. High sea surface temperatures (SSTs) are one reason; SSTs beneath John are near 86 - 88°F (30 - 31°C), which is about 1 - 2°F above average. The other main factor is the Madden-Julian Oscillation (MJO), a pattern of increased thunderstorm activity near the Equator that moves around the globe in 30 - 60 days. When the area of increased thunderstorms associated with the MJO is located in the Eastern Pacific, the associated rising air and low pressure can help spawn hurricanes, as is occurring now. According to weather.com, an enhanced phase of the MJO over the Pacific in late July 2017 helped trigger and outbreak of eight tropical cyclones in the Pacific, from southeast Asia to the Pacific Coast of Mexico. That was the most tropical cyclones to coexist in the northern Pacific Ocean since 1974.
