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Slow-Strengthening Tropical Depression Forms in Southwest Caribbean

By: Bob Henson 5:07 PM GMT on November 21, 2016

A gradually organizing disturbance about 300 miles east of Nicaragua was designated Tropical Depression 16 early Monday morning. Little change in TD 16 was noted in the National Hurricane Center’s advisory at 10 AM EST Monday, with the depression nearly stationary and top sustained winds remaining at 35 mph. TD 16 is located in a “col” area between weather features, resulting in upper-level winds so weak that there is no definitive steering influence. As it sits and spins over very warm water (sea surface temperatures of 29°C, or 84°F, are about 1°C above average), TD 16 is very gradually becoming better organized. Showers and thunderstorms are consolidating around TD 16’s center, with upper-level outflow becoming more evident toward the west and north. Heavy rains on the periphery of TD 16 continue to affect parts of Costa Rica and Panama, as the depression pulls in moisture from the tropical Pacific.

Figure 1. Latest visible satellite image of TD 16.

Figure 2. Infrared satellite image of TD 16 at 1615Z (11:15 AM EST) Monday, November 21, 2016. Image credit: NOAA/NESDIS.

The outlook for TD 16
TD 16 is expected to become Tropical Storm Otto within the next day or so. Vertical wind shear will be in the light to moderate range (15 - 20 knots), and minor drying of the atmosphere will bring mid-level relative humidities down to about 60%, but neither factor should impede TD 16’s development. An upper-level high is projected to strengthen over the western Caribbean by midweek, which should hasten TD 16’s growth. Steering around the high will gradually impart a westward motion to the cyclone. NHC brings TD 16 to hurricane strength by Thursday as it approaches the Caribbean coast near the border of Nicaragua and Costa Rica, where landfall is currently expected on Thursday afternoon or evening. While all of our most reliable track models bring TD 16 toward Nicaragua, the 00Z Monday European run and its ensemble members are the further south (near or just south of the Costa Rica border), while the UKMET and GFS runs project a landfall closer to central Nicaragua. The ECMWF is also considerably weaker, with none of its ensemble members bringing TD 16 to hurricane strength; most of the GFS ensemble members call for a strong tropical storm or minimal hurricane before landfall. As TD 16 develops, we’ll want to start keeping a closer eye on the HWRF model, the most reliable intensity model once a system has developed.

Figure 3. Tracking map for TD 16 as of 10 AM EST Monday, November 21, 2016.

How rare is a hurricane landfall in Nicaragua?
Tropical storms and hurricanes move into Nicaragua from the east every few years, but one as far south as TD 16 is a more uncommon event. If a Hurricane Otto develops and strikes as currently predicted, it could be the southernmost hurricane landfall on record in Central America. There are no recorded Atlantic hurricane landfalls in Costa Rica or Panama. A weak tropical storm made landfall in Costa Rica in December 1887, and Panama: Hurricane Martha, which struck as a strong tropical storm in Veraguas Province, Panama, on November 24, 1969. “Undoubtedly, there have been other tropical cyclones that moved into Panama, but this was the first one that was definitely tracked,” said Robert Simpson and NHC colleagues in their roundup of the 1969 Atlantic hurricane season.

Noteworthy landfalls in Nicaragua, shown here in rough order from north to south, include:

Felix (September 4, 2007): Cat 5, landfall just south of the Nicaragua-Honduras border
Ida (November 5, 2009): Cat 1, landfall near La Barra del Rio Grande
Beta (October 30, 2005): Cat 2, landfall near La Barra del Rio Grande
1906 Florida Keys Hurricane (October 10, 1906): Cat 3, landfall on central Nicaragua coast
Cesar (July 28, 1996): Cat 1, landfall north of Bluefields
Gert (September 15, 1993): tropical storm, landfall near Bluefields
Joan (October 22, 1988): Cat 4, landfall south of Bluefields
Irene (September 19, 1971); Cat 1, landfall south of Bluefields
Bret (August 10, 1993): tropical storm, landfall near Punta Gorda Natural Reserve

Heavy rains the big threat from TD 16
TD 16 is likely to bring torrential rains, landslides, and flooding to parts of Central America. The region’s complex, rugged topography may lead to several widely dispersed areas of extremely heavy rain. The most confident outlook is for several inches of rain over nearly all of Nicaragua, with a core of 10” - 15” amounts very possible within TD 16’s circulation as it makes landfall and moves inland. The westerly flow south of TD 16 will impinge into parts of Costa Rica and Panama, leading to very heavy rain (10” or more) already under way on some of the region’s south- and west-facing slopes. Another core of torrential rain (again, 10” or more possible) is projected by models to develop later this week as TD 16’s outer circulation moves along the north coast of Honduras, perhaps extending into southern Belize and eastern Guatemala.

We’ll be back with our next update by midday Tuesday. Our post on GOES-R will appear after the threat from TD 16 has passed.

Bob Henson

Figure 4. Aerial picture taken on September 6, 2007, in the village of Sandy Bay, Nicaragua, after the passage of Hurricane Felix. The southernmost Category 5 Atlantic hurricane on record prior to Matthew in 2016, Felix caused 133 deaths, nearly all in Nicaragua, and destroyed thousands of homes in the city of Bilwi. Image credit: Oscar Navarrette/AFP/Getty Images.


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