August Ends With a Whimper for the Atlantic Hurricane Season

The end of August is upon us, and with just one day remaining in the month, the remarkably quiet Atlantic hurricane season of 2013 is highly likely to be just the 6th season since the Hurricane Hunters began flying in 1944 without a hurricane forming by the end of August (the other years: 2002, 2001, 1988, 1984, and 1967.) Although there have been two tropical storms in August (Erin and Fernand), these storms were weak and short-lived, and August 2013 had one of the lowest Accumulated Cyclone Energy (ACE) totals on record for an August in the Atlantic. ACE is calculated as the square of the wind speed every 6 hours for every named storm with at least 40 mph sustained winds (scaled by a factor of 10,000 for usability.) Since the damage potential of a hurricane is proportional to the square or cube of the maximum wind speed, ACE is not only a measure of tropical cyclone activity, but also a measure of the damage potential. During the 20-year period 1981 - 2010, the Atlantic averaged 104 ACE units, and the 20-year average ACE by the end of August was 30. So far in 2013, we've managed just 9 ACE units, with only 1.9 of those occurring in August. Since the current active hurricane period we are in began in 1995, only 2002 had a lower ACE by this point in the year (4.9), and only 1997 had a lower August ACE (zero.) Both were El Niño years, when we expect hurricane activity to be low due to high wind shear. If we go back to the beginning of reliable Atlantic ACE statistics in 1966, when good satellite data first became available, only five other years had August ACE values comparable to 2013's. Three of those years were El Niño years, and the other two had ocean temperatures in the hurricane Main Development Region (MDR, from 10 - 20°N, 20 - 70°W) that were more than 0.5°C (0.9°F) cooler than in 2013. This year's combination of no El Niño, warm MDR SSTs, and an exceptionally low August ACE is an event unparalleled in the historical record, going back to 1966.


Figure 1. August Accumulated Cyclone Energy (ACE) totals in August 2013 were among the lowest on record for an August in the Atlantic. Other Augusts with low ACE in the Atlantic all occurred during El Niño years, or when sea surface temperatures (SSTs) were cool.

Why the quiet season?
The main reason for the quiet August has been the large amount of dry, stable air over the Atlantic. This dry air has two sources: the Sahara desert of Africa, and sinking air from aloft, which warms and dries as it sinks. Even so, I find it highly perplexing that activity has been so low when all of the other factors--lack of an El Niño, low wind shear, an active African Monsoon spitting out plenty of tropical waves, and above average ocean temperatures--have favored development. Instability has increased over the tropical Atlantic over the past few days, thanks to the influence of the MJO and a Convectively Coupled Kelvin Wave (CCKW) bringing rising air to the Atlantic. However, there do not appear to be any tropical waves positioned to take advantage of this increased instability and intensify to hurricane strength over the coming week (though 96L could become a tropical storm.) The next tropical wave with a shot at becoming a hurricane will not exit the coast of Africa until September 7. If we make it all the way to September 11 without a hurricane in the Atlantic, it will beat the record set by Hurricane Gustav of 2002 for latest date of formation of the season's first hurricane, going back to when the Hurricane Hunters first began flying in 1944. Even if we do get a first-half of September hurricane, the steering pattern features a strong trough of low pressure over the U.S. East Coast, which would have high odds of recurving any hurricane that manages to form out to sea, without affecting any land areas.


Figure 2. Vertical instability over the tropical Atlantic in 2013 (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 much lower than average during most of August, due to outbreaks of dry air from Saharan Air Layer (SAL) , and an unusual amount of dry, sinking air in the tropical Atlantic. Low instability reduces the potential for tropical storm formation. During the last few days of August, instability has increased, which should raise the odds of tropical storm formation. Image credit: NOAA/NESDIS/CIRA.

Quiet in the Atlantic
A tropical wave located about 900 miles east of the Lesser Antilles Islands is moving westward at 15 mph, and has changed little over the past two days. The wave has a modest amount of spin, but only a limited amount of heavy thunderstorm activity. In their 8 am EDT Friday Tropical Weather Outlook, NHC left the 5-day odds of formation of this disturbance at 20%, and the 2-day odds at 10%. Wind shear is a moderate 10 - 20 knots over the system, but there is an area of dry air and dust from the Saharan Air Layer (SAL) to the north and west that is interfering with development. Our three reliable models for predicting genesis, the UKMET, GFS, and European models, do not develop the disturbance during the next five days. The wave will likely spread heavy rains and gusty winds to the Lesser Antilles Islands by Monday, when the wave will encounter an area of high wind shear positioned over the Northern Lesser Antilles Islands. This high wind shear will likely decrease the odds of development beginning on Monday.


Figure 3. MODIS satellite image of the tropical disturbance 650 miles east of the Lesser Antilles, taken at 2:30 pm EDT on August 30, 2013. A small closed circulation had developed well to the west of an area of increasing heavy thunderstorms. Image credit: NASA.

A well-organized tropical wave emerging from the coast of Africa (Invest 96L) is headed west-northwest towards the Cape Verde Islands. This wave is expected to take a west-northwesterly track into an area with drier air and higher wind shear, and would likely not be able to make the long trek across the Atlantic to threaten North America or the Caribbean Islands. In their 8 am EDT Friday Tropical Weather Outlook, NHC put the 5-day odds of development at 60%, and the 2-day odds at 40%. The GFS model predicts that 96L will develop, but the UKMET and European models show little or no development.

Wunderblogger Lee Grenci has an excellent new post, "Atlantic Hurricane Season: The Saharan Air Layer and Vertical Wind Shear".

I plan on having a relaxing Labor Day weekend, but will do at least one update. Have a great weekend, everyone!

Jeff Masters