Halfway Point of Hurricane Season Arrives; TD 6 Forms in Central Atlantic

September 11 marks the halfway point of the Atlantic hurricane season (based on the past 100 years of data, 1914-2013)--and we're doing much better than usual so far. Only four named storms have formed, with three becoming hurricanes (and no major hurricanes.) An average Atlantic hurricane season has 6 named storms, 3 hurricanes, and 1 intense hurricane by the mid-point of the season. The four storms so far in 2014 have inflicted much less punishment than usual for half of a hurricane season. Hurricane Arthur made landfall in North Carolina as a Category 2 hurricane then blasted the Maritime Provinces of Canada as a powerful hurricane-force extratropical storm, but damage was low by Category 2 hurricane standards--just $14 million, with most of the damage occurring in Canada. Hurricane Bertha caused two deaths along the U.S. East Coast due to rough surf and strong rip currents, but did insignificant damage as it recurved out to sea, just off the coast. Hurricane Cristobal also did minimal damage, but killed a total of seven people--three swimmers in the U.S., and flood victims in Haiti (2), the Dominican Republic, and the Turks and Caicos Islands. Tropical Storm Dolly, which made landfall in northeastern Mexico on September 2 with 50 mph winds and torrential rains, killed one person and did millions in damage. Residents of Hurricane Alley shouldn't assume the rest of the season will end with a whimper, though. All it takes is one bad hurricane to make a ruinous hurricane season. Recall that 2012's worst storm--Hurricane Sandy--didn't occur until the third week of October!



Bahamas disturbance 92L struggling to develop
A small area of low pressure over the Northwest Bahamas (Invest 92L) has become more organized since Wednesday, but has limited heavy thunderstorm activity. Long range radar out of Melbourne, Florida shows that 92L has low-level spiral bands with a good degree of rotation, and satellite loops show that a small surface circulation has developed. However, strong upper level winds out of the north-northeast are creating a high 20 - 25 knots of wind shear, and the atmosphere is quite dry to the north, making development of 92L unlikely today and Friday. The current westwards 5 - 10 mph motion of 92L will carry the disturbance over Florida on Friday, and the storm should emerge over the Gulf of Mexico on Saturday. Ocean temperatures are a very warm 30°C (86°F) over the Eastern Gulf, but the presence of dry air and high wind shear of 15 - 25 knots should keep any development of 92L over the Gulf of Mexico slow. The disturbance will likely bring rains of 1 - 3" to much of Florida through Sunday. One of the three reliable computer models for predicting tropical cyclone genesis, the UKMET model, does develop 92L over the Gulf of Mexico. In their 2 pm EDT Thursday Tropical Weather Outlook, NHC gave the system 2-day and 5-day odd of development odds of 30% and 30%, respectively.


Figure 1. MODIS satellite image of Invest 92L off the coast of Florida, taken at approximately 11:30 am EDT Thursday September 11, 2014. Image credit: NASA/GSFC.

Central Atlantic Tropical Depression Six not a threat to land
Tropical Depression Six has formed in the Central Atlantic about 750 miles west of the Cape Verde Islands, and is headed northwest at about 15 mph. Satellite images show TD 6 has a moderate amount of spin and heavy thunderstorm activity, and these thunderstorms are becoming more organized. The disturbance is embedded in a moderately moist air mass, has marginally warm (SSTs) of 27°C (81°F) beneath it, and is experiencing light wind shear. These conditions favor continued development. The 8 am EDT Thursday run of the SHIPS model predicted that wind shear would remain light to moderate (5 - 20 knots) and the atmosphere at mid-levels of the atmosphere (between 500 - 700 mb) would remain moderately moist this week, favoring development. TD 6 does not appear to be a long-range threat to the Lesser Antilles Islands, U.S. East Coast, or Bermuda. It remains to be seen if TD 6 will be a threat to the Canadian Maritime Provinces late next week.


Figure 2. Latest satellite image of Tropical Depression Six.

Flood threat to Mexico from Tropical Storm Odile diminishes
In the Eastern Pacific, Tropical Storm Odile formed on Wednesday morning a few hundred miles southwest of Acapulco. Satellite loops show that Odile has a large area of heavy thunderstorms that are slowly organizing, but the heavy rains of the storm are remaining just offshore of the Pacific coast of Mexico. If Odile follows the current projections from our two top track models, the GFS and European, these rains will remain offshore as the storm moves northwest, parallel to the coast. If the storm deviates to the right of its expected path, it will be capable of dumping 5 - 10" of rain along the coast from Acapulco to Puerto Vallarta.

Why has the Eastern Pacific been so active?
It's been a remarkably active hurricane season in the Eastern Pacific; Odile's formation gives the basin 15 named storms, 10 hurricanes, and 7 intense hurricanes so far this year. An average Eastern Pacific hurricane season sees 15 named storms, 8 hurricanes, and 3 intense hurricanes during the entire year, with about 2/3 of that activity occurring by September 9. Since July, the Eastern Pacific has had ocean temperatures about 0.6°C (1°F) above average and wind shear about 20% below average. The region has been dominated by moist, rising air and low pressure, leading to above average vertical instability. All of these factors are favorable for an active hurricane season. The Atlantic and Eastern Pacific are usually out of phase with their hurricane seasons--when one is active, the other is inactive. This occurs because when the large-scale atmospheric circulation favors rising air and low pressure over one ocean basin, there must be high pressure and dry, sinking air elsewhere to compensate--which typically occurs over the neighboring ocean basin, suppressing hurricane activity there.


Figure 3. Sea Surface Temperatures (SSTs) over the Eastern Pacific in 2014 (blue line) compared to average (black line.) SSTs have been up to 0.6°C (1°F) above average during the summer, increasing the potential for tropical storm formation. Image credit: NOAA/NESDIS/CIRA.


Figure 4. Vertical wind shear (in knots) over the Eastern Pacific in 2014 (blue line) compared to average (black line.) Wind shear has been about 20% below average during the summer, increasing the potential for tropical storm formation. Image credit: NOAA/NESDIS/CIRA.


Figure 5. Vertical instability over the Eastern Pacific in 2014 (blue line) compared to average (black line.) The instability is plotted in °C, as a difference in temperature from near the surface to the upper atmosphere. Thunderstorms grow much more readily when vertical instability is high. Instability has been higher than average during most of the summer, increasing the potential for tropical storm formation. Image credit: NOAA/NESDIS/CIRA.

Tropical Depression 15 forms east of the Philippines
In the Western Pacific, Tropical Depression Fifteen is organizing in the waters east of the Philippines, and is on course to intensify into a typhoon and potentially affect the northern portion of the Philippines' Luzon Island on Sunday. TD 15 will then potentially impact China early next week.

Twenty-five years ago on this date
On September 11, 1989, Tropical Depression Twelve continued to grow more organized, building a large region of heavy thunderstorms near its center. Two hooking spiral bands formed, prompting the National Hurricane Center to upgrade the depression to a tropical storm in their 11 am advisory. The new storm's name: Hugo. Tropical Storm Hugo headed westward across the open Atlantic at 20 mph, still four days from the Lesser Antilles Islands.

That day at NOAA's Miami-based Office of Aircraft Operations--the hurricane hunting division of NOAA--I joked with my colleagues about the fearsome new storm with the same name as the director of NOAA's Atlantic Oceanographic and Marine Laboratory (AOML), Hugo Bezdek. AOML housed the offices of NOAA's Hurricane Research Division, whose scientists would decide whether or not our hurricane hunting group would intercept the new storm once it got close enough to the Lesser Antilles Islands. Even if Hugo was a dud, we figured we'd be flying the storm for sure, since it shared the same first name as the big boss of the hurricane research scientists. We did not suspect at all that this storm named Hugo would go on to be the most destructive hurricane ever seen in the Atlantic.


Figure 6. AVHRR visible satellite image of Tropical Storm Hugo taken on September 11, 1989. Image credit: Google Earth rendition of the NOAA HURSAT data base.

Hurricane expert Steve Gregory has a more detailed look at 92L and the rest of the tropics in his latest post.

Jeff Masters