A massive low-pressure system known as the Mesoamerican Gyre is developing over Central America and will bring heavy rain to much of Central America and southern Mexico next week. The Central American Gyre could bring more than a foot (305 mm) of rain over seven days to the Pacific-facing coasts of Mexico, Guatemala, El Salvador, Honduras, and Nicaragua, causing life-threatening flash floods and mudslides (Figure 1 ).
A gyre is a type of monsoon depression, a weak but broad area of surface low pressure that can persist for two weeks or more over Central America and adjacent areas of the Atlantic and Pacific Oceans, including the western Caribbean and southwestern Gulf of Mexico. They are most common in May, June, September, October and November. Circulations often produce smaller circulations that can become full-fledged tropical cyclones. One such circulation formed in the Gulf of Mexico in June and became the first named storm of the season, Alberto.
The Central American gyre typically takes many days to form, and once fully formed, it takes several more days to produce a tropical cyclone. The location of a tropical cyclone spawned by the Central American Gyre is difficult to predict more than two days in advance, but our top forecast models have been predicting it will spawn sometime during the Atlantic's next named storm, Helen. Last week.
It is currently unlikely that these models can converge over multiple days to determine the likely location and timing of potential Tropical Storm Helen's formation. The European model and its ensemble members favor weaker, slower-developing storms over the southwestern Gulf of Mexico; the GFS model and its ensemble members favor stronger storms farther east in the central Gulf. Since Thursday's run, trends in both forecast camps have slowed and moved farther west.
In its Tropical Weather Outlook, released at 8 a.m. ET Friday, the National Hurricane Center gave a two-day and seven-day probability of tropical cyclones forming in the western Caribbean or southern Gulf of Mexico at 0 percent and 40 percent, respectively. As of Friday, no Hurricane Hunter missions had been scheduled for the system. With record ocean temperatures and ocean heat content in the Gulf of Mexico, the ceiling for any storms that may occur in the Gulf of Mexico is high, and Gulf Coast residents should expect hurricanes to potentially hit the Gulf of Mexico.
Two systems that open the Atlantic are unlikely to develop
Two disturbances in the remote subtropical Atlantic pose little, if any, threat to land areas in the foreseeable future. The National Hurricane Center's Tropical Weather Outlook report, released at 8 a.m. ET on Friday, gave both systems a slim 20 percent chance of development on the 2-day and 7-day periods, respectively.
The remnants of former Tropical Storm Gordon continue to fester halfway between the Lesser Antilles and the Azores. Strong westerly shear around 25 knots moved showers and thunderstorms (convection) east of Gordon's weak surface low level (1008 mb). Shear is expected to weaken this weekend, with surface waters unusually warm (about 28 degrees Celsius or 82°F) but the atmosphere fairly dry (mid-level relative humidity about 45-50%). Still, former Gordon may briefly regain its tropical storm status as it drifts northward.
A few hundred miles west of Gordon is a disturbance called Invest 96L, which has a surface low of 1007 mb. It is located over the same warm subtropical waters and experiences slightly less shear than former Gordon. However, the atmosphere here is also quite dry (mid-level humidity is about 40-45%, and convection is limited, so any development will be gradual.
As of Friday morning, September 20, the only named tropical cyclone in the entire northern hemisphere was Plasang, which headed toward China as a tropical depression over the South China Sea after making landfall as a weak tropical storm near Shanghai on Thursday. Curved in the northeast direction. All four basins in the Northern Hemisphere – Atlantic, Northeast Pacific, Northwest Pacific and North India – are currently experiencing below-average accumulated cyclone energy (ACE) during this unusually quiet peak season. This year's hemispheric ACE is 212.7, according to Colorado State University calculations, while the average as of September 20 (1991-2020) is 364.4. To date, the total number of named storms worldwide is 31 (average 41.4) and 15 hurricane-force storms (average 22.1).
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