Quiet in the Atlantic; Hilda Poised to Bother Hawaii

With mid-August at hand, the stress level of residents along the Atlantic's Hurricane Alley rises as African tropical wave season enters its climatological peak period. A steady supply of spinning disturbances emerge from the coast of Africa from mid-August through early October, providing weeks of suspense as we watch them develop and decay as they march across the hurricane Main Development Region (MDR) from the coast of Africa and into the Caribbean. However, this year is atypical--there has only been decay. This, despite the fact that wind shear has been a moderate 10 - 20 knots over the eastern portion of the MDR, and sea surface temperatures have been near average. The credit for the quiet start to hurricane season goes to an atmospheric circulation that has brought high pressure and dry, sinking air to the tropical Atlantic--due in part to one of the strongest El Niño events in recorded history that is underway in the Eastern Pacific. In addition, frequent outbreaks of dry, dusty air from the Saharan Air Layer (SAL) have brought even more dry air, making the atmosphere so stable that tropical waves coming off the coast of Africa have quickly decayed. El Niño is creating strong upper-level winds over the Caribbean that were generating a very high 30 - 50 knots of wind shear over the Caribbean on Monday, making tropical storm formation virtually impossible there. The high wind shear and low instability is forecast to persist in the Caribbean and tropical Atlantic for at least the next week. Wind shear will be lower, at times, in the waters of the Gulf of Mexico and off the U.S. East Coast, so if we get any tropical storms forming in mid-August, those would be the most likely locations. Keep in mind, though, that the last time we had an El Niño event this strong--back in 1997--no named storms formed in the Atlantic during August, and only one named storm (Hurricane Erika) formed in September. It would not be a surprise to see similar behavior in 2015. In their 8 am EDT Monday Tropical Weather Outlook, NHC continued to predict quiet conditions in the Atlantic for at least five days.


Figure 1. Vertical instability as of August 9, 2015 over the tropical Atlantic, from the coast of Africa to the Lesser Antilles Islands. 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. Normal instability is the black line, and this year's instability levels are in blue. The atmosphere has been dominated by high pressure and dry, sinking air all summer, which has made it difficult for thunderstorms to develop. Instability has also been unusually low in the Caribbean, but has been near average over the Gulf of Mexico and waters off the U.S. East Coast. Image credit: NOAA/CIRA.


Figure 2. Saharan Air Layer (SAL) analysis for 8 am EDT Monday, August 10, 2015, from the University of Wisconsin CIMSS, shows plenty of dry air dominating the tropical Atlantic.

Hilda poised to bother Hawaii
In the Eastern Pacific, Hurricane Hilda continues to weaken as it heads northwest at 9 mph towards Hawaii. Hilda is under high wind shear of 15 - 25 knots, and the shear will increase to 30 - 40 knots by Tuesday. This increasing shear should cause the hurricane to weaken to a tropical depression by Wednesday. The speed with which Hilda weakens will be crucial for determining whether or not the storm will track over the Hawaiian Islands late this week; a weaker Hilda will tend to track more due west, caught in the low-level trade wind flow near the surface, while a stronger Hilda will tend to track more to the northwest, potentially leading to a landfall on Thursday. The Monday morning run of the European model favored this latter scenario, while the GFS model run showed Hilda turning due west and missing the Hawaiian Islands to the south. Either scenario is possible, and we will have to wait and see how the situation plays out.


Figure 3. Latest satellite image of Hilda.

In the Western Pacific, all looks to be quiet until late this week, when the European and GFS models predicts a new tropical depression will form in the waters midway between Hawaii and the Philippines' Luzon Island.

Jeff Masters