Gamma satellite image
Infrared satellite image of Tropical Storm Gamma over the Caribbean at 11:30 a.m. EDT October 3, 2020. (Image credit: RAMMB/CIRA/Colorado State University)

An intensifying Tropical Storm Gamma was barreling ashore in Mexico’s Yucatan Peninsula early Saturday afternoon with top winds of 70 mph, just below hurricane strength. Gamma was bringing heavy rains in excess of an inch per hour to portions of the Yucatan Peninsula on Saturday, and flash flooding from these torrential rains is the greatest threat from the storm. Some 24-hour rainfall amounts from Gamma at Weather Underground personal weather stations as of noon EDT Saturday included:

Puerto Morelos, Mexico: 15 inches
Cozumel, Mexico: 11.15 inches
Cancun, Mexico: 5.64 inches
Roatan, Honduras: 4.79 inches

The 2020 parade of record-early named storms continues

Gamma developed into a tropical storm on Friday evening, and its October 2 arrival marked the earliest date that any Atlantic season has produced its 24th tropical storm, topping the record held by Beta from October 27, 2005.

With the Atlantic hurricane season three-quarters done, we’ve already had 24 named storms, eight hurricanes, two intense hurricanes, and an accumulated cyclone energy (ACE) index of 106 (24% above average for the date). Only one Atlantic hurricane season since 1851 has had more named storms during an entire season: 2005, with 28 named storms. According to Colorado State University hurricane scientist Phil Klotzbach, the averages for this point in the season are nine named storms, five hurricanes, two intense hurricanes, and an ACE index of 85.

Gamma’s landfall in Mexico gives the “Greek” storms of 2020 three consecutive landfalls, along with Alpha in Portugal and Beta in Texas. The only other year with Greek-named storms, 2005, also had three of these late-season storms make landfall: Alpha, Beta, and Gamma, all in the Caribbean.

Figure 1. Track forecasts out to eight days for Tropical Storm Gamma from the 6Z (2 a.m. EDT) Saturday, October 3, run of the new version of the GFS ensemble model (GEFS). The black line is the mean of the 31 ensemble members; individual ensemble member forecasts are the thin lines, color-coded by the central pressure they predict for Gamma. The long-range forecasts predominately call for a weak storm with a pressure greater than 1000 mb (green and blue colors). (Image credit: Tropical Tidbits)

Forecast for Gamma

Conditions for development were less favorable on Saturday than on Friday, with wind shear a moderately high 15-20 knots and a drier atmosphere (a mid-level relative humidity near 60%). Despite these factors, satellite images showed that Gamma continued to grow more organized right up until landfall, with an eye beginning to appear.

The trough of low pressure pulling Gamma to the northwest is expected to move eastwards on Sunday and leave the storm behind, putting Gamma in an area of weak steering currents. As a result, Gamma’s forward motion is expected to slow to less than 5 mph Saturday night through Monday, resulting in very heavy rains across much of Mexico’s Yucatan Peninsula, portions of Central America, and western Cuba. The current model consensus is that a weak ridge of high pressure will build to the north of Gamma on Sunday and Monday, forcing the storm to move west or west-southwest Monday through Wednesday.

Conditions for development of Gamma will become more hostile on Sunday and Monday, when high wind shear of 20-30 knots and dry air over the Gulf of Mexico will interfere with development, as will land interaction with the Yucatan Peninsula. Because of these obstacles, the top intensity models generally show weakening of Gamma over the next five days.

Figure 2. Rainfall outlook (in inches) for the two-day period from 2 a.m. EDT (6Z) Saturday, October 3, 2020, through 2 a.m. EDT Monday, October 5, from the 6Z Saturday, October 3, run of the GFS model. The model predicted rainfall amounts in excess of 12 inches (yellow-orange colors) for portions of Mexico from Gamma. (Image credit: Tropical Tidbits)

Central Caribbean tropical wave 92L may develop

Further complications on predicting Gamma’s track may arise by Monday or Tuesday as another tropical wave, now moving through the central Caribbean, arrives in the western Caribbean and potentially develops into a tropical depression or tropical storm. This wave, which was designated 92L by NHC on Saturday morning, was located south of Haiti on Saturday afternoon and was moving west-northwest at about 10 mph. Satellite images showed the wave had a modest amount of heavy thunderstorms that were poorly organized and the wave was under a moderate 10-15 knots of wind shear, which was slowing development.

Also see: Bidding farewell to Dr. James McFadden, the longest-serving NOAA Hurricane Hunter

When 92L reaches the western Caribbean on Monday, the shear is expected to drop to a low 5-10 knots, ocean temperatures beneath its path will warm to 30 degrees Celsius (86°F), and 92L will be in a moist environment — conditions that favor development. The top three models for predicting tropical cyclone genesis were giving modest support for development to occur on Monday or Tuesday in the western Caribbean, and in an 8 a.m. EDT Saturday Tropical Weather Outlook, the National Hurricane Center gave 92L two-day and five-day odds of development of 20% and 50%, respectively.

There were two other areas of interest in the central Atlantic that were being given low odds of development by NHC on Saturday morning, but neither of these two systems are a threat to any land areas.

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Posted on October 3, 2020 (12:44pm EDT).

Jeff Masters

Jeff Masters, Ph.D., worked as a hurricane scientist with the NOAA Hurricane Hunters from 1986-1990. After a near-fatal flight into category 5 Hurricane Hugo, he left the Hurricane Hunters to pursue a...

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