The last couple of Atlantic hurricane seasons have been especially active, with 2020 smashing all kinds of records, including 12 tropical systems making landfall in the U.S., five of which were in Louisiana.
This season has gotten off to a slower start compared to the last couple of years, but it is important to remember that June and July are only the first gear of hurricane season. Over the course of a season, those two months are expected to be slow. The period from mid-August to mid-October is when the season reaches full-throttle.
History has taught us this as well. Most of the tropical systems leaving long-lasting impacts in Virginia came during August, September, or October. There is the occasional exception, like Agnes in June 1972.
But even back to colonial days, that 90-day wiggle area between late summer and early fall is the critical time.
In 1749, historical accounts indicate that a sand area of 800 acres was washed up near modern day Norfolk during an October storm. An August hurricane in 1806 added to it, forming what we know today as Willoughby Spit.
Hurricane Hazel raced south to north across central Virginia in October 1954, and Camille devastated Nelson County in August 1969.
The same theme applies in the last quarter-century. In the late 1990s, flooding from Fran and Floyd came in early September. Isabel brought damage from the Blue Ridge to the Chesapeake Bay in September 2003.
Richmond’s 2004 flash flooding from Gaston came at the end of August. Heavy rain from Irene came the last week in August of 2011.
More recently, Matthew brought more than 9 inches of rain to Norfolk in October of 2016.
Most of this is due to the seasonality of the ocean water temperatures, reaching their peak through the entirety of the open Atlantic Ocean in September. The Gulf of Mexico, because it is shallower than the open Atlantic, warms up earlier in the season, and is often where some of the early-season storms generate.
Much is made of the influence of our warming planet on tropical systems, but in the Atlantic basin, there is scant evidence that hurricanes will become substantially more numerous on a regular basis. However, recent studies do indicate that storms are already tending to become stronger when they do form, producing more rain and stronger winds. There is also emerging evidence suggesting hurricanes are starting to intensify more rapidly, move more slowly, and remain stronger farther northward than they have in the past.
Considering how much variation there is within a season, it is far too early to say specifically where or when a tropical system will form in August. But in a couple of weeks, it will be a good idea to check on the tropical Atlantic on a much more regular basis.
On a week-to-week basis, there are several background influences involved in generating a tropical system. Dust from the Sahara Desert is regularly lifted and transported into the air over the tropical Atlantic, which inhibits development.
There is also a broad area of upward motion in the atmosphere that circles the tropics about every 30-60 days. Discovered about 50 years ago and known as the Madden-Julian Oscillation (MJO), it favors the development of tropical systems when it rotates over the tropical Atlantic Ocean.
Right now, Saharan dust amounts are relatively high, and the upward motion with the MJO is weak and far away, struggling to move eastward into the western Pacific.
These two elements will not last the entire season. Expect the activity across the Atlantic to pick up in 2-3 weeks, so enjoy the quiet tropical Atlantic while it lasts.
