Above: Rainfall outlook (in inches) for the week-long period from 8 am EDT Friday, May 29, 2020, through 8 am EDT Friday, June 5. This outlook is derived from all members of the GFS model (GEFS) from 12Z Friday. Ensemble output tends to smooth out errors that can crop up in a single run, so it tends to be more reliable than an individual model run for large-scale patterns going out more than several days. Exact locations of the heaviest rain are likely to shift over time. (tropicaltidbits.com)
A large, complex area of low pressure—a recurring feature called the Central American Gyre—(CAG) will slog from the Northeast Pacific into the Caribbean and Gulf of Mexico over the next few days, potentially causing a variety of trouble. One or more tropical cyclones may spin off from the gyre, and there is a good chance of torrential rains, flooding, and mudslides in the coming week regardless of any tropical development. Update (5 pm EDT Sunday): Tropical Storm Amanda developed early Sunday in the Northeast Pacific and quickly dissipated a few hours after making landfall on the south coast of Guatemala.
CAGs tend to develop on either end of the Atlantic hurricane season, as large-scale weather features rearrange themselves during the waxing and waning of northern summer. CAGs tend to have relatively weak surface winds, but they can sprawl over hundreds of miles, and they are notoriously long-lasting and slow-moving, both of which make them dangerous rain producers.
As smaller-scale vortexes spin around the gyre, there’s always the chance that one or more will consolidate into a tropical cyclone and eventually break away from the gyre. This happened with a CAG in October 2018; after a week of gestation, a disturbance on the north end of that gyre became a tropical depression. Three days later, that depression had detached from the gyre and barreled into the Florida Panhandle as fearsome Category 5 Hurricane Michael.
Days of life-threatening flooding are possible next week over Central America and far southern Mexico
The most confident outcome from the upcoming CAG is a prolonged period of heavy—possibly extreme—rainfall across much of Central America and far southern Mexico as the gyre shifts slowly northward over the next week. Some of the heaviest showers and thunderstorms (convection) may shift location from day to day, while other areas of convection may lock into one location for days on end, depending on how small-scale features interact with the gyre and the rugged topography.
Based on output Friday morning from the GFS ensemble model, rainfall could easily total 1 to 2 feet over the next week across much of El Salvador, central Costa Rica, southern Guatemala, and/or far southern Mexico. While we can’t say exactly which areas are at greatest risk just yet, it’s clear that destructive, dangerous floods and mudslides will be a real hazard, especially across south-facing terrain.
Tropical cyclone likely to develop over the Northeast Pacific this weekend
Update (5 pm EDT Sunday): On the southern flank of the gyre, a large, loosely organized area of convection named Invest 91E consolidates into Tropical Storm Amanda this weekend (see above). Amanda's remnants will move slowly northwest, likely making their way into the southern Bay of Campeche early in the week. Relentless bouts of rainfall can be expected in the coastal terrain on its north side, and perhaps some distance inland.
Longer-range possibilities over the Caribbean and Gulf of Mexico
We’ll have to wait to get a more solid sense of whether a tropical cyclone might form in the Atlantic basin as the CAG drifts north. Our most reliable long-range models for tropical cyclogenesis, the GFS and European, have a recurring signal for development next week into the week after, but with large variations across models, runs, and ensemble members as to where and when a system might develop.
About half of the members of the 12Z Friday European ensemble indicate a potential tropical cyclone moving north from the Bay of Campeche into the western Gulf of Mexico next week, strengthening only gradually. The 12Z Friday GFS ensemble has a more diverse range of locations across the Gulf, and it delays development till at least a week from now. Update (5 pm EDT Sunday): In a tropical weather outlook issued at 1:55 pm EDT Sunday, NHC pegged the odds at 40% through Tuesday and 50% through Friday that Amanda's remnants would reorganize into at least a tropical depression over the southern Bay of Campeche.
Brief potential for subtropical development in the Central Atlantic with 92L
A broad arc of convection several hundred miles east-southeast of Bermuda, Invest 92L, has a fifty-fifty chance of becoming at least a subtropical depression on Friday night or Saturday, according to a special tropical weather outlook issued at 9 am Friday by NHC. Satellite images show this complex already had a comma-shaped subtropical look to it on Friday afternoon. Relatively cool sea surface temperatures of around 25°C (77°F) and strong wind shear also suggest that any development would be subtropical rather than tropical. Update (5 pm EDT Sunday): This system is no longer expected to become a tropical or subtropical cyclone.
The next name on the Atlantic list is Cristobal. If 92L manages to become a depression, it will be only the second time in Atlantic tropical records that any year has spawned three tropical or subtropical cyclones prior to the official start of hurricane season (June 1). The year 1951 saw Tropical Storm One in January (though this system had subtropical characteristics) and a tropical depression as well as Hurricane Able in mid-May. Note that the January 1951 tropical storm could easily be considered a laggard system from the 1950 season, just as Hurricane Alex in January 2016 was arguably the last tropical cyclone of the 2015 season.
An agukabam in Oman?
Multiple models are suggesting that Invest 92A, a disturbance near the southern coast of Oman Friday, will intensify slightly over the next few days as it works its way inland—an odd prognosis given the scorching, sandy desert terrain. Tropical cyclones normally draw energy from the warmth and moisture of the ocean. There are a few examples of tropical cyclones that have weakened upon landfall and then restrengthened when passing across unusually wet terrain, drawing on the “brown ocean” effect.
This example might be called the “sandy ocean” effect. It’s been documented for years with tropical cyclones that move across the scorching soils of northern Australia. In a 2008 Monthly Weather Review paper, eminent tropical researcher Kerry Emanuel (Massachusetts Institute of Technology) and coauthors Jeff Callaghan and Peter Otto (Bureau of Meteorology, Australia) proposed a name for them: “Because such redeveloped warm-core cyclones are apparently nearly unique to remote desert areas of northern Australia, we here call them ‘agukabams,’ from the aboriginal word roots ‘agu,’ meaning land, and ‘kabam, meaning storm.”
Using simulations from a simplified soil-atmosphere model, Emanuel, Callaghan, and Otto theorize that the initial rains from an approaching tropical cyclone can add just enough moisture to hot, sandy soil to generate the warm, moist air needed to sustain or intensify a tropical cyclone.
The European model is among those that’s been predicting that the disturbance off Oman would intensify after landfall, in agukabam fashion.
It’s also possible that greener, wetter landscapes in the higher terrain just north of the coast will play a role as well.