November Atlantic hurricane outlook

Hurricane Rina is gone, and the tropical Atlantic is quiet, with no threat areas to discuss, and no models predicting development of a tropical depression during the coming seven days. So, are we all done for 2011? Or will this seventh-busiest hurricane season of all-time spawn a Tropical Storm Sean? Let's try and come up some answers. Since the active hurricane period we are in began in 1995, ten of the sixteen years (62%) have seen one or more Atlantic named storms form after November 1, for a total of fifteen late-season storms:

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 (20%) caused loss of life: Hurricane Ida of 2009, which killed one boater on the Mississippi River; Tropical Storm Odette of 2007, 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 both the deadliest and the 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. However, as I noted in the wake of last year's Hurricane Tomas last November in my 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.


Figure 1. The strongest hurricane on record in the Atlantic in November, Hurricane Lenny, takes aim at the Lesser Antilles on November 17, 1999. Image credit: NOAA.

Forecast for November 2011
The oceans are certainly warm enough to support continued development of tropical cyclones. Sea Surface Temperatures (SSTs) over a wide area of the tropical Atlantic are 0.5 - 1.0°C above average, and are well above the 26°C (79°F) threshold typically needed to support tropical storm formation (Figure 2.) However, wind shear is starting to rise over much of the tropical Atlantic as the jet stream moves farther south in its usual seasonal cycle. Wind shear over most of the Atlantic will be too high to support tropical storm formation over the coming two weeks, according to the latest run of the GFS model (Figure 3.) Only the southern Caribbean and a few transient pockets in the middle Atlantic east and southeast of Bermuda will have low enough wind shear to support tropical storm formation over the next two weeks. The African Monsoon is quiet this time of year, and we no longer have African waves coming off the coast of Africa that can act as the seeds for formation of a tropical storm in the Caribbean. If we do get a tropical storm, it will probably be to the northeast of the Lesser Antilles, far from land, in a region where an extratropical low pressure system gets cut off from the jet stream and lingers long enough over warm waters to acquire tropical characteristics and get a name. Both the GFS and ECMWF models are suggesting a system like this may take form 7 - 10 days from now. Taking all these factors into account, I predict we are all done this hurricane season with storms that will cause loss of life, but there is still a 70% chance that we will get one or more named storms in the middle Atlantic that will stay out to sea and not affect land.


Figure 2. Sea surface temperatures in the Atlantic on November 1, 2011. The black dotted line is the 26°C (79°F) isotherm, which marks the boundary where tropical storm formation can typically occur. A large portion of the Atlantic is still capable of supporting tropical storm formation.


Figure 3. Wind shear forecast for November 11, 2011, as predicted by the 2am EDT November 1, 2011 run of the GFS model. The model is predicting low wind shear of less than 4 m/s (about 8 knots, light red colors) in the southern Caribbean and southern Lesser Antilles Islands. Very high wind shear in excess of 44 m/s (85 knots, orange colors), associated with the jet stream, will protect regions north of the Caribbean.

I'll have a new post Wednesday or Thursday.

Jeff Masters