Hurricane Hunters Find Tropical Storm Bonnie in the Caribbean, Will Impact Nicaragua and Costa Rica Starting Today
Elsewhere, the tropical Atlantic looks quiet into next week
Hurricane Hunters investigating Potential Tropical Cyclone Two this morning finally found a well-defined surface circulation during their first pass through the center, allowing the National Hurricane Center to upgrade the system to Tropical Storm Bonnie, the second named storm of the Atlantic hurricane season. Thankfully, Bonnie’s window over water to strengthen further is quickly closing and the primary threat to Nicaragua and Costa Rica will be heavy rainfall and potential flooding over the weekend. Bonnie is forecast to maintain its status as a tropical storm or tropical depression during its jaunt over Central America and reemerge in the Pacific, where it’s expected to restrengthen next week. If it does so, it would retain the name Bonnie rather than taking on a new name (Darby) on the Pacific side.
Over the past week in the daily newsletter, we’ve discussed how usual it is for tropical depressions, storms, and hurricanes to form in the deep tropical belt of the Atlantic – from the central Caribbean to Africa – during the month of June. Only a handful of tropical systems have ever been recorded this far south and east this early in the season. From hostile upper-level winds to a still dust-laden atmosphere, the obstacles are many early on.
Climatology – what we know to be seasonally typical – is the starting place for all weather forecasts, from which we layer in observations about the current state of the atmosphere and oceans to forecast what lies ahead. When our computer models suggest something that’s very usual given the climatology starting point, we take note. Such was the case with the disturbance we’ve been tracking since last week that became Bonnie this morning. According to our computer models, the formation potential was high from its early stages, but the environment was also unusually conducive for a tropical aberration – very light upper-level winds, very warm ocean waters, and a moisture-laden atmosphere resembling late July rather than late June.
On Monday the National Hurricane Center was forecasting Bonnie to form several hundred miles east of the Windward Islands in the less favored far eastern Caribbean and become the first Atlantic hurricane of 2022 by the end of the work week. Four days later and over 1,000 miles west of those same islands, Potential Tropical Cyclone Two only just strengthened to Tropical Storm Bonnie, despite the largely hospitable environment along its path. So how did climatology win out?
Early during the hurricane season, the easterly (winds coming from the east) trade winds in the deep tropics are especially strong. These brisk trade winds can whisk disturbances through the tropical belt too quickly to allow for concentrated thunderstorm growth, which in turn doesn’t allow pressures to fall and the low-pressure system to strengthen. This was the case with Potential Tropical Cyclone Two, which had been moving at speeds up to 30 mph, an incredibly fast pace for a would-be tropical cyclone.
These easterly trade winds accelerate through the south-central Caribbean and form a Caribbean Low-Level Jet – a local wind maximum – that is strongest in June and July. The acceleration of winds on the front side of this jet (to the east of the jet) results in a spreading out of air – something meteorologists call divergence – which subsequently produces sinking motions in the nearby area, capping thunderstorms that attempt to grow high and organize. It’s been hypothesized that this Caribbean Low-Level Jet in June and July is the primary reason we observe a dearth of tropical storm formations in the eastern Caribbean, which has garnered the moniker “Caribbean Graveyard” by hurricane forecasters over the years. By August and September, these easterly trade winds subside significantly, and tropical formations are generally a more common occurrence.
On the flip side, to the west of the Caribbean Low-Level Jet, easterly trade winds decelerate and air piles up, resulting in convergence that promotes rising air and thunderstorm growth. Potential Tropical Cyclone Two found itself on the conducive side of the jet this morning and took advantage of this factor, ultimately forming in a more climatologically favored part of the Caribbean.
Elsewhere in the tropics this weekend, things look quiet. A disturbance moving through the Lesser Antilles today isn’t expected to develop and the first full week of July appears to bring a rest to the unusually active Atlantic of the past week.
Hurricane Hunters Find Tropical Storm Bonnie in the Caribbean, Will Impact Nicaragua and Costa Rica Starting Today
Thank you for all the information you give not only on current storms but also how they form and why. As a fairly recent transplant to Florida (7 years) it is nice to learn what I need to about our possible weather problems.