Otto Expected to Strike Central America as a Hurricane

One of the strongest tropical cyclones on record so far south in the Caribbean, Tropical Storm Otto, was on the verge of becoming a hurricane Tuesday morning. In its 10 AM EST advisory, the National Hurricane Center pegged Otto’s top sustained winds at 70 mph, just short of the hurricane threshold. Otto was christened on November 21, an unusually late date for a Caribbean tropical storm; only eleven Caribbean storms since 1851 have had a later formation date.

Still holding stationary less than 100 miles north of the coast of Panama on Tuesday morning, Otto is expected to begin moving west on Wednesday. The storm will likely make landfall as a Category 1 or 2 hurricane on the coast of southern Nicaragua or northern Costa Rica on Thursday. A hurricane watch extends along the entire Caribbean coast of Costa Rica northward to south of Bluefields, Nicaragua. A tropical storm warning is in effect along the central coast of Panama from Nargana to Colon, with a tropical storm watch westward to the border of Costa Rica. These are startling locations for tropical cyclone advisories--understandably so. A storm of Otto’s expected strength has never made landfall so far south in the Caribbean, and there is no record of any hurricane striking Costa Rica.


Figure 1.Latest visible satellite image of Otto.


Figure 2. Enhanced infrared image of Otto as of 1545Z (10:45 AM EST) Tuesday, November 22, 2016. Image credit: NASA/MSFC Earth Science Office.

Over the past day or so, Otto has largely shaken off the elongating influence of the frontal zone on which it developed, which extends far to the northeast into the Atlantic. Showers and thunderstorms have become stronger and much more symmetric around Otto’s center since Monday night. The storm remains quite compact, with tropical storm force winds extending out to only 70 miles from the center. That small size is both good news and bad news, as it will allow Otto to either intensify or weaken more quickly than a larger storm would. Otto will be moving through a very concave section of the Central American coast, and the rugged topography may help impart some extra spin to Otto’s circulation as the storm gradually enlarges.

Intensity outlook for Otto
Otto is in a very favorable spot to intensify. Sea surface temperatures remain unusually warm for this time of year--around 29°C (84°F), or about 1°C above the seasonal average. The atmosphere will be moistening over time, with the 12Z Tuesday SHIPS model predicting that mid-level relative humidities around Otto will increase from about 60% to greater than 70%. The main negative factor for Otto is vertical wind shear of 15 - 20 knots resulting from upper-level southeasterly winds blowing over the storm as it sits and spins. However, that shear may abate slightly on Wednesday and Thursday, and it is not enough in itself to keep Otto from intensifying. Otto should reach Central America as at least a Category 1 hurricane, and Category 2 or even higher strength cannot be ruled out. The most reliable intensity model for existing tropical cyclones, the HWRF, has consistently projected that Otto will become a solid Category 2 storm. The SHIPS model gives Otto a 30% chance of reaching top sustained winds of 90 knots (105 mph) by Tuesday morning, and an 18% chance of reaching 115 knots (135 mph, minimal Category 4 strength) by Wednesday morning. The latest Category 3 storm ever observed in the Atlantic occurred at 00 UTC November 24, 1934, so Otto has a chance of beating the record for latest major hurricane ever observed in the Atlantic (thanks go to WU member Ryan1000 for this info.)


Figure 3. Track forecast for Otto as of 10 AM EST Tuesday, November 22, 2016.

Otto’s track: Central America, and perhaps the East Pacific
Stubbornly stationary Otto began to show signs at midday Tuesday of a long-awaited westward movement. Our most reliable track models--the European, GFS, and UKMET--are now consistent in bringing Otto onshore across southern Nicaragua or northern Costa Rica on Thursday. 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 has a shot at retaining its identity as a tropical cyclone 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.

Heavy rains a major threat from Otto
Otto is likely to bring torrential rains, landslides, and flooding to parts of Central America. The region’s complex topography may lead to several widely dispersed areas of extremely heavy rain. The most straightforward part of the outlook is for several inches of rain over nearly all of Nicaragua, with a core of 10” - 15” amounts very possible just north of Otto’s circulation as it makes landfall and moves inland. The broad circulation enveloping Otto has been impinging on parts of Costa Rica and Panama, a process leading to very heavy rain that could go well beyond 15” in some areas, especially western Panama. Another narrow zone of torrential rain (again, 15” or more possible) is projected by models to develop later this week as Otto’s outer circulation moves along the north coast of Honduras, perhaps extending into far southern Belize and extreme eastern Guatemala.


Figure 4. Total rainfall for the 150-hour period ending at 7:00 AM EST Sunday, November 27, 2016, projected by the 06Z Monday run of the GFS model. Image credit: tropicaltidbits.com.

Climatology of November and December Atlantic tropical cyclones
Since the active hurricane period we are in began in 1995 (including 2016), 13 of the 21 years (62%) have seen one or more Atlantic named storms form after November 1, for a total of eighteen November/December storms:

2016: Tropical Storm Otto on November 21
2015: Hurricane Kate on November 8
2011: Tropical Storm Sean on November 8
2009: Hurricane Ida on November 4
2008: Hurricane Paloma on November 6
2007: Tropical Storm Olga on December 11
2005: the "Greek" storms Gamma, Delta, Epsilon, and Zeta
2004: Tropical Storm Otto on November 29
2003: Odette and Peter in December
2001: Hurricane Noel on November 5 and Hurricane Olga on November 24
1999: Hurricane Lenny on November 14
1998: Hurricane Nicole on November 24
1996: Hurricane Marco on November 19

Only three of these storms caused loss of life: Hurricane Ida of 2009, which killed one boater on the Mississippi River; Tropical Storm Odette of 2003, whose floods killed eight people in the Dominican Republic; and Hurricane Lenny of 1999, which killed fifteen people in the Lesser Antilles. "Wrong-way Lenny" was the second deadliest and the second strongest November hurricane on record (Category 4, 155 mph winds).

There have been only seven major Category 3 or stronger hurricanes in the Atlantic after November 1. Part of the reason for the relatively low loss of life for November storms is that they tend to form from extratropical low pressure systems that get cut off from the jet stream and linger over the warm waters of the subtropical Atlantic. These type of systems typically get their start in the middle Atlantic, far from land, and end up recurving northeastwards out to sea. Tropical Storm Sean of 2011 was an example of this type of storm. However, as noted in the wake of Hurricane Tomas of November 2010 in our blog post, Deadly late-season Atlantic hurricanes growing more frequent, "It used to be that late-season hurricanes were a relative rarity--in the 140-year period from 1851 - 1990, only 30 hurricanes existed in the Atlantic on or after November 1, an average of one late-season hurricane every five years. Only four major Category 3 or stronger late-season hurricanes occurred in those 140 years, and only three Caribbean hurricanes. But in the past twenty years, late-season hurricanes have become 3.5 times more frequent--there have been fifteen late-season hurricanes, and five of those occurred in the Caribbean. Three of these were major hurricanes, and were the three strongest late-season hurricanes on record". Dr. Jim Kossin of the University of Wisconsin published a 2008 paper in Geophysical Research Letters titled, "Is the North Atlantic hurricane season getting longer?" He concluded that yes, there is an "apparent tendency toward more common early- and late-season storms that correlates with warming sea surface temperature but the uncertainty in these relationships is high". The recent increase in powerful and deadly November hurricanes would seem to support this conclusion.

We’ll be back with an update on Wednesday and will be tracking Otto throughout the Thanksgiving holiday period.

Bob Henson and Jeff Masters
 

Figure 5. The second strongest hurricane on record in the Atlantic in November, Hurricane Lenny, takes aim at the Lesser Antilles on November 17, 1999. Image credit: NOAA. (Thanks go to WU member elioe for pointing out that the November 1932 Cuba hurricane, which peaked at 170 mph winds, was the strongest and deadliest hurricane to occur in November.)