Tropical storm Aletta (Cooperative Institute for Meteorological Satellite Studies)

The Western Hemisphere’s first tropical storm of 2012, named Aletta, formed just yesterday from one of several well-organized thunderstorm clusters in the eastern Pacific. Aletta, a modest 40 mph storm, is not expected to intensify much more before it dissipates over the open ocean in the next couple of days.

Aletta’s development coincided perfectly with the official May 15 start of the hurricane season in the eastern Pacific. Is Aletta a harbinger of tropical development in the Atlantic?

The hurricane season in the Atlantic, Caribbean, and Gulf of Mexico officially begins June 1. Given that we are now only half way through May, you might think it’s a bit early for weather forecasters to start anticipating tropical-cyclone formation in that part of the world. Well, think again.

During the last week or so, there has been more than a little expectation –presumably inspired by predictions from a few of the global weather models- that development will indeed occur in the Caribbean and/or Atlantic during the last third of the month. And though I find it tempting to completely ignore dire forecasts like these, some of the reasoning behind them does have merit.

For starters, the fact that the hurricane season hasn’t even begun doesn’t mean our shores are immune from tropical threats. There is, after all, nothing “magical” about the season’s start date. Though not common, it is not unprecedented to have a tropical storm or hurricane in spring. According to the Hurricane Research Division (HRD), 3% of the annual tropical activity occurs out of season.

The birth of Aletta in the eastern Pacific signals activity is picking up in tropical regions around the world. That’s after a record 41-day period (in at least 70 years) with no tropical storm formation anywhere globally according to the UK Met Office.

The agitated appearance of the cloud field in the eastern Pacific suggests there may be more development there in during the next few days.

Upper-level velocity potential (contoured) and high-level clouds, valid today. Yellow arrow shows expected movement of disturbed weather. (NOAA)

As long as this feature remains nearly stationary, widespread thunderstorm activity -and the chance for a tropical cyclone- will remain over the east Pacific. Yet during the next week or so, there are signs that this area of disturbed weather will propagate eastward over the Caribbean Sea (as shown by the yellow arrow above), where water temperatures (in the low 80s) remain warm enough to support tropical development.

With that said, it is not particularly surprising that some of the weather models have recently spun up tropical systems on the Atlantic side in their long range outlooks. And though it’s not hard to believe the atmosphere over the Caribbean will be stormier next week than it is now, it is nearly impossible to place confidence in the existence of any one of the tropical systems the global models have recently spit out. Even in peak season, under seemingly ideal circumstances, the development of a tropical cyclone is an uncommon event. It requires a special set of conditions unclearly seen by the global models as little as a day in advance, let alone a week.

A forecast of upper-level winds (blue arrow) and surface pressures (black contours) early next week from the GFS ensemble mean. White “L” marks an area of relatively low pressure. (NOAA)

A more likely scenario, as many of the models still seem to indicate, is that a broad, elongated area of low pressure may form in the Caribbean (marked by the white “L” in the image above). Its shape and proximity to relatively-high wind shear suggest that the development may not be strictly tropical. Rather, it may be largely driven by non-tropical processes tied to the upper low. This would mean that what some of the models are hinting at may be subtropical (or hybrid) development instead.

Whatever the case may be, whatever the models have in mind, it’s all expected to happen before the official start of the season. That means we have roughly 97% of the annual tropical activity still to look forward to.