Jeff Masters, 1:45 PM GMT on November 08, 2010
Hurricane Tomas is gone, and good riddance to this deadly late-season storm that hit the southern Lesser Antilles and Haiti hard. While Tomas thankfully spared Haiti a flooding catastrophe, it may yet cause heavy loss in that beleaguered nation by worsening their cholera epidemic, which has already claimed over 500 lives. So, with Tomas gone, are we all done for 2010? Or will this third-busiest hurricane season of all-time spawn a twentieth named storm, Tropical Storm Virginie?
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.
Since the active hurricane period we are in began in 1995, seven of the fifteen years have seen an Atlantic named storm form after November 8: 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 Olga on November 24), 1999 (Hurricane Lenny on November 14), 1998 (Hurricane Nicole on November 24), and 1996 (Hurricane Marco on November 19). Only two of these storms caused loss of life: 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 the only major hurricane of these very late season storms.
Figure 2. Wind shear forecast for November 24, 2010, as predicted by the 2am EDT November 8, 2010 run of the GFS model. The model is predicting low wind shear of less than 4 m/s (about 8 knots, light red colors) over a small region of the Central Caribbean. Very high wind shear in excess of 44 m/s (85 knots, orange colors), associated with the subtropical jet stream, will protect regions north of the Caribbean.
So, judging by the recent history of late season tropical storms, there is about a 50% chance that we are all done this season. The odds of a significant storm that causes loss of life are much lower, less than 15%. The oceans are certainly warm enough to support continued development of tropical cyclones. Sea Surface Temperatures (SSTs) in the Atlantic's Main Development Region for hurricanes had their warmest October on record, according to an analysis I did of historical SST data from the UK Hadley Center. SST data goes back to 1850, though there is much missing data before 1910 and during WWI and WWII. SSTs in the Main Development Region (10°N to 20°N and 20°W to 80°W) were 0.95°C above average during October, beating the previous record of 0.93°C set in October 2003. Wind shear will also be low enough in the Caribbean to support tropical storm formation over the coming two weeks, according to the latest run of the GFS model (Figure 2.) However, the subtropical jet stream is forecast to slowly edge southwards over the next few weeks, in keeping with its usual seasonal cycle. The Caribbean will gradually see wind shear increase, until only the extreme southern Caribbean near the coast of Panama can support tropical storm formation. Taking all these factors into account, I believe we are all done this hurricane season with dangerous storms capable of causing loss of life. The latest long-range runs of the GFS and ECMWF models don't hint at anything developing over the next seven days, and I give a 30% chance we will see one more inconsequential named storm, which will not cause loss of life if it forms.