by Jeff Masters

3:04 PM GMT on September 11, 2014

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!



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 0.6C (1F) 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 0.6C (1F) 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.