Jeff Masters

September 10 marks the traditional halfway point of the Atlantic hurricane season, and the first half of the hurricane season of 2013 is making its mark in the history books as one of the least active such periods on record. Going back to before when the Hurricane Hunters first began flying in 1944, there has been only one hurricane season that made it past the half-way point without a hurricane forming: the El Niņo year of 2002, when Hurricane Gustav formed at 8 am EDT on September 11. Tropical Storm Humberto is looking poised to become a hurricane later today, and 2013 will likely end up ranking in 2nd place for latest formation of the season's first hurricane, going back to 1941. Here are the Atlantic hurricane seasons since 1941 in which the first hurricane did not form until after September 7:

2002: September 11, Hurricane Gustav
2013: September 10+ (Nothing yet)
1984: September 10, Hurricane Diana
2001: September 8, Hurricane Erin

Prior to 1944, there were four hurricane seasons that had the first hurricane form after September 15:

1914: No hurricanes in the Atlantic
1907: No hurricanes in the Atlantic
1905: October 8
1941: September 16

According to NHC, August 10 is the average date the first Atlantic hurricane arrives, and the season's third hurricane usually develops by September 9. So, assuming Humberto makes it to hurricane status, we are two hurricanes behind the average season pace. An average season brings six hurricanes, two of them being intense hurricanes.

Remarkably low Accumulated Cyclone Energy (ACE) so far in 2013
The first half of 2013's hurricane season had one of the lowest Accumulated Cyclone Energy (ACE) totals on record for 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 through September 9 was 50. Through September 9 of 2013, we've managed just 9.6 ACE units, about 20% of average. Since the current active hurricane period we are in began in 1995, no other year had a lower ACE by this point in the year. Since reliable satellite-based ACE records began in 1966, there have been seven years with comparable levels of ACE by this point in the season. Four of these years were El Niņo years, when we expect hurricane activity to be low due to high wind shear. The other years had August ocean temperatures in the hurricane Main Development Region (MDR, from 10 - 20°N, 20 - 70°W) that were cooler than in 2013, though the ocean temperatures in 1988 were only 0.1°C cooler than in 2013. This year's combination of no El Niņo, warm MDR SSTs, and an exceptionally low first half of the season ACE is thus an event unparalleled in the historical record, going back to 1966.

Figure 1. Accumulated Cyclone Energy (ACE) totals for the first half of the Atlantic hurricane season, through September 9, 2013, were among the lowest on record, since reliable satellite data began in 1966. Other years with low first-half-of-the-season ACE occurred during El Niņo years, or when August sea surface temperatures (SSTs) were cooler.

Forecast for the next two weeks: below average activity
The main reason for the quiet first half of the season has been the large amount of dry, stable air over the Atlantic. The primary source of this dry air has been the Sahara desert of Africa. However, dry air from Northeast Brazil may also have contributed, argues wunderblogger Lee Grenci. That region of the country experienced a record $8.3 billion drought in 2013--the most expensive natural disaster in Brazil's history. Even with all the dry air we've seen over the Atlantic in 2013, it is really remarkable that activity has been so low when all of the other factors--lack of an El Niņo, wind shear near climatological averages, an active African Monsoon spitting out plenty of tropical waves, and above average ocean temperatures--have favored development. Instability increased over the tropical Atlantic over the last few days of August and the first week of September, thanks to the influence of the Madden Julian Oscillation (MJO), a pattern of increased thunderstorm activity near the Equator that moves around the globe in 30 - 60 days. Instability was also boosted by a Convectively Coupled Kelvin Wave (CCKW) that brought rising air to the Atlantic. This increase in instability helped the formation of Gabrielle, Humberto, and Tropical Depression Eight, and may help boost the odds of a potential tropical storm forming this weekend over the Southwestern Gulf of Mexico's Bay of Campeche. The influence of the MJO is fading over the Atlantic, though. Beginning next week, we will be entering a phase of the MJO where it will likely bring more stable, sinking air to the Tropical Atlantic. This suppressed phase of the MJO could last through the first week of October. The models are also pointing to another outbreak of dry air from the Saharan Air Layer (SAL) coming off the coast of Africa this weekend, which will keep the Tropical Atlantic dryer than usual next week. The steering pattern over the next two weeks features a strong trough of low pressure over the U.S. East Coast, giving high odds that any hurricane that manages to form and approach the U.S. will recurve out to sea, without affecting any land areas. So keep your fingers crossed--we're doing unusually well for this point in the hurricane season, with no landfalling hurricanes, and it looks like we have above average chances of keeping it that way deep into September.

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 was been much lower than average during most of August, primarily due to outbreaks of dry air from Saharan Air Layer (SAL). Low instability reduces the potential for tropical storm formation. During the last few days of August and the first week of September, an MJO event boosted instability. The MJO or Madden Julian Oscillation is 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 Western Hemisphere, it brings rising air over the Tropical Atlantic, increasing instability. Image credit: NOAA/NESDIS/CIRA.

Be prepared for a destructive hurricane in 2013
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 ruin an otherwise quiet hurricane season. Recall that last year's worst storm--Hurricane Sandy--didn't occur until the third week of October. Another thing to consider: the season with the greatest similarity to what we've seen during the first half of the 2013 season was 1988. That year, we also had unusual quietness before September 10, no El Niņo, and above average ocean temperatures in the MDR. But the most powerful Atlantic hurricane ever recorded up to that time ripped through the Caribbean, Hurricane Gilbert, as well as two other major hurricanes. I flew into Hurricane Gilbert at the height of its fury 25 years ago, and plan to recount the story of that amazing flight this Friday.