Record-Setting Otto to Bring Thanksgiving Flooding to Central America

The latest hurricane ever recorded to form in the Caribbean Sea, Hurricane Otto, has thankfully weakened to a tropical storm, but is still poised to make a most unwelcome visit to Central America on Thursday, when it will move ashore over southern Nicaragua. Otto will become the first Atlantic tropical cyclone on record to make landfall on (U.S.) Thanksgiving Day. Otto’s intensification to a hurricane on November 22 came about a day later than the previous latest hurricane observed in the Caribbean, Hurricane Martha of 1969. The last time a hurricane was seen this late in the year anywhere in the Atlantic was in 2005, when Hurricane Epsilon meandered across the Central Atlantic as a Category 1 hurricane on December 2 - 7. Otto became a hurricane at a latitude of 10.5°N—unusually far to the south in the Caribbean. Only Hurricane Martha, which became a hurricane at latitude 10.3°N on November 22, 1969, and maintained Category 1 strength as it moved south to 10.0°N on November 23, was a hurricane at a lower latitude in the Caribbean.


Figure 1. MODIS satellite image of Hurricane Otto taken at 1:24 pm EST November 22, 2016. At the time, Otto was a Category 1 storm with 75 mph winds, and was the latest-forming hurricane ever observed in the Caribbean Sea. Image credit: NASA.

Hurricane Hunters find a weaker Otto
An Air Force hurricane hunter aircraft finished a mission into Otto around 9 am EST Wednesday, and found that the eyewall of Otto had collapsed. NHC rated Otto’s top surface winds at 70 mph at 10 am EST Wednesday, but the Hurricane Hunters did not find any surface winds in excess of 60 mph, and the NHC conceded in their discussion that their 70 mph estimate may have been too high. Moderately high wind shear of 15 - 20 knots was likely to credit for Otto’s weakening, as strong upper-level winds out of the south-southeast were able to drive dry air to the south of Otto into its core. Satellite loops on Wednesday morning showed a considerable reduction in Otto’s heavy thunderstorm activity near its core, but there was an increase in heavy thunderstorms along a band well to the north of the center, and these thunderstorms will bring very heavy rains to Nicaragua and Honduras on Wednesday and Thursday. Otto had decent moisture to work with—about 65% relative humidity at mid-levels of the atmosphere, and sea surface temperatures (SSTs) were very warm, near 29°C (84°F), which was about 1°C (1.8°F) above average. Panama radar showed that Otto’s heavy rain bands were mostly offshore from that nation on Wednesday morning.

Track forecast for Otto
The track forecast for Otto is straightforward. A ridge of high pressure is building in to the north of the storm, and this ridge will guide Otto west-northwest and then west at an increasing forward speed. Otto will make landfall in southern Nicaragua late Thursday morning. It should take about a day for Otto to cross southern Nicaragua, which is much less mountainous than other parts of Central America. In fact, much of Otto’s path could be over Lake Nicaragua, one of the top-ten biggest freshwater lakes in the Americas. With land influence at a relative minimum, Otto is expected to be a tropical storm when it enters the Pacific on Friday. In this case, Otto would keep its name in the Pacific. Should Otto dissipate, but its remnants manage to redevelop in the Pacific, the new storm will take the name Virgil from the Eastern Pacific list.

More than a dozen “crossover” tropical cyclones have been recorded, most of them moving from Atlantic to Pacific rather than vice versa. The most recent was Hermine (2010), which formed as an East Pacific tropical depression before entering the western Gulf and striking the northeast coast of Mexico as a tropical storm. Otto would be the first crossover storm in modern records to keep its name in going from one basin to another, since NHC’s previous practice was to rename such systems. All of Otto’s predecessors in this realm--including Hurricane Cesar-Douglas (1996)--underwent a name change when moving from Atlantic to Pacific or vice versa.

Intensity forecast for Otto
Conditions favor slow intensification of Otto until landfall. Sea surface temperatures will be unusually warm for this time of year--around 29°C (84°F). The atmosphere will be moistening through Thursday, and vertical wind shear is expected to abate slightly, to 10 - 15 knots, by Thursday morning. Our most reliable intensity model, the HWRF, predicted in its 06Z (1 am EST) Wednesday run that Otto would make landfall as a strong Category 1 hurricane with 90 mph winds. The 12Z Wednesday run of the SHIPS model gave Otto a 12% chance of reaching Category 2 strength with top sustained winds of 90 knots (105 mph) by Thursday morning. Regardless of how much intensification occurs, heavy rain will be the main threat from the storm, with rainfall amounts in excess of 10” in mountainous areas very likely to cause dangerous flash flooding and mudslides in parts of Central America.


Figure 2. Tracks of all hurricanes recorded in the Caribbean Sea in November and December between 1851 and 2016. Image credit: NOAA.

A rare Thanksgiving Day storm
Forecasters at the National Hurricane Center will be issuing storm advisories on Thanksgiving Day for the second year in a row on Thursday. Last year, record-warm ocean waters helped Hurricane Sandra off the Pacific coast of Mexico become only the second Thanksgiving Day hurricane in modern records for the Atlantic or Eastern Pacific, and the first major hurricane. Sandra set the record for the latest major hurricane ever observed in the Western Hemisphere, as the storm maintained at least Category 3 strength from 00 UTC November 26 through 00 UTC November 27 (previous record: an unnamed Atlantic hurricane in 1934 that held on to Category 3 status until 00 UTC November 24.) When Sandra peaked as a Category 4 storm with 150 mph winds at 1 am EST (06 UTC) Thanksgiving Day, November 26, it became the latest Category 4 storm ever observed in either the Eastern Pacific (previous record: Hurricane Kenneth on November 22, 2011) or the Atlantic (previous record: ”Wrong Way" Lenny on November 18, 1999.) The only other Thanksgiving Day hurricane ever recorded in the Western Hemisphere was Hurricane Karl of 1980, which spun harmlessly as a minimal Category 1 hurricane far out in the central North Atlantic on Thanksgiving Day that year.


Figure 3. VIIRS satellite image of the Eastern Pacific’s Hurricane Sandra taken on November 25, 2015—the day before Thanksgiving—when Sandra was an intensifying Category 3 storm. Image credit: NASA.

Several other weaker storms have had NHC forecasters issuing advisories on Thanksgiving Day. This includes 1987’s Tropical Storm Keith, which struck Florida as a tropical storm on Wednesday, November 23, and persisted as a strong tropical storm east of Florida until midday Thanksgiving Day (November 24). In 1998, minimal Tropical Storm Nicole weakened to a depression east of Bermuda early on Thanksgiving Day (November 26), with advisories discontinued at 10 am EST. Nicole did get a new lease on life several days later, becoming a hurricane on November 30 and persisting to become one of just five Atlantic hurricanes on record during the month of December. In the hyperactive Atlantic season of 2005, Tropical Storm Delta roamed the eastern Atlantic on Thanksgiving Day (November 24). And in 2011, a weakening Tropical Storm Keith well out to sea in the eastern Pacific prompted advisories on Thanksgiving Day (November 24). Prior to the establishment of NHC as we know it, an unnamed tropical storm dissipated on Thanksgiving Day 1953 (November 26) well east of Bermuda. Hawaii takes the cake for the worst U.S. hurricane-related impacts during Thanksgiving Week: Hurricane Iwa passed near Kauai on Tuesday, November 23, 1982, during the run-up to the “super” El Niño of 1982-83. Iwa caused one death and inflicted $250 million in damage in Kauai.

Have a great Thanksgiving holiday, everyone, and we’ll be back on Thursday to look at how Central America will do with Otto.

Jeff Masters and Bob Henson