It’s already been a record-setting season, with the Atlantic tipping into the Greek alphabet after exhausting its list of conventional storm names for only the second time on record. This year’s rambunctious hurricane season has seen the earliest-forming C, E, F, G, H, I, J, K, L, M, N, O, P, R, S, T and W storms on record — not to mention the fastest a season has ever made a run toward or into the Greek alphabet. Gamma is up next.
While the vast majority of systems this year have been quick-hitting and rather weak, some — like Laura, Sally and Isaias — have been more disruptive and damaging. And history teaches us to never let our guard down in October.
Beware of October
October is a month that has featured many of the United States’ most vicious and infamous systems, including Michael in 2018, the most recent Category 5 to make landfall in the Lower 48. Wilma in October 2005 remains the most intense hurricane ever recorded in the Atlantic from an air pressure standpoint. And other systems, such as Mitch in 1998, Opal in 1995 and Hazel in 1954 left a mark, too. Hazel even brought a 113-mph gust to the Battery in New York City after lashing Washington with gusts to 98 mph.
During October, wind shear — or a change of wind speed and/or direction with height — begins to ramp up over the central and eastern Atlantic. That can tear a system apart before it develops. And the chain of African Easterly Waves that seed storm development begins to peter out his time of year, shutting down the hurricane factory.
In the Gulf of Mexico and Caribbean, however, water temperatures are near their warmest, while wind shear remains near its minimum.
For states such as Florida, October is the most dangerous month for hurricanes. Of the 112 recorded hurricanes to have struck the Sunshine State since 1950, 38 have occurred in October.
October also features the southward advance of the jet stream as cold air begins to build over Canada. That can help fuel intense extratropical transitions as the remnants of hurricanes blast north. That was behind Sandy’s onslaught in the Mid-Atlantic in late October 2012.
By about Oct. 10, rising motion associated with the next convectively coupled Kelvin wave, a large overturning circulation that meanders about the tropics, will overspread the western Atlantic. That enhancement of upward motion will make it easier for thunderstorm clusters to become organized and a tropical system to mature, should one develop.
At the same time, the Madden-Julian Oscillation, or MJO — a similar circulation that traverses the tropics every month or two — is also posed to enter a state that would favor more storminess and rainfall in the Gulf of Mexico and the Caribbean.
It’s worth noting that, while some cooler waters linger in the northern Gulf after being churned up by Sally, the bulk of the Gulf of Mexico and the entirety of the Caribbean Sea remain much warmer than normal. There is plenty of heat energy to encourage a hurricane, should one form.
Putting it all together, the United States still can enjoy a respite from the tropics this week. There are no immediate concerns anywhere on U.S. soil of tropical development.
By this upcoming weekend, we should begin paying attention to whatever it is that attempts to organize in the northwest Caribbean. That will also usher us into our next active period, which looks to brew starting between Oct. 7 and 10.
There are signs that enhanced activity could last through the remainder of October.
One system to watch already
There is growing support from computer model guidance that a system may start to materialize late in the week over the western Caribbean. That’s where the American GFS model simulates an area of mid-level spin in the atmosphere developing coincident with a nascent zone of surface low pressure. And the ambient environment may be ripe for it to consolidate into a named system, though confidence in that is low.
In the shorter term, the Yucatán Peninsula may wind up with some heavy rainfall out of the system before it meanders northward or northwestward into the Bay of Campeche in the southwest Gulf of Mexico.
The National Hurricane Center estimates a 30 percent chance that the system will develop into a tropical depression — the precursor to a tropical storm — in the next five days, up from 20 percent odds touted Sunday night. While the probabilities may seem low, it’s important to note that they cover only the period through Saturday, at which point the tropical wave may still be getting its act together. The longer-term prospects of eventual development may be greater.
Down the road, weather models continue to work toward cranking out additional systems in this area. Historically, the western Caribbean and Gulf of Mexico have been areas to closely monitor this time of year, anyway. During autumn, activity typically wanes in the Atlantic’s Main Development Region, or the stretch of marine real estate between Africa and the Lesser Antilles, while the threat of “homegrown” storms closer to the U.S. shoreline increases.
And those are the same systems that often prove more tricky to predict, as they mature closer to land and often are harder to spot from a distance.