Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.
By: Dr. Jeff Masters , 8:15 PM GMT on September 15, 2006
Could Ernesto end up being the worst the hurricane season of 2006 has to dish out? With the season more than half over, and no landfalling storms in sight, this year in no way resembles the Hurricane Season of 2005, when we had 15 named storms, 8 hurricanes, and 4 major hurricanes by this date. Our tally so far this year is a relatively meager 8 named storms, 2 hurricanes, and 1 major hurricane, right near average. Looking at the plot of typical hurricane activity for the Atlantic (Figure 1), we see that we are almost a week past the peak of hurricane season, which was September 10. We still have a long road ahead--hurricane activity stays high for another four to five weeks, on average. Let's analyze analyze what the remainder of hurricane season may have in store for us.
Figure 1. Ernesto--the main event of the hurricane season of 2006? Image credit: NOAA Visualization Lab.
Figure 2. Climatological Atlantic hurricane and tropical storm activity.
Sea Surface Temperatures (SSTs)
It's mid-September, and SSTs have peaked in the Atlantic and are on a slow decline. Still, water temperatures will remain warm enough to support hurricanes throughout usual Atlantic development areas through the end of October. SSTs are 0.5-1.5°C warmer than normal right now (Figure 3), which is a huge amount of extra heat energy. The Bermuda High has stayed relatively weak all of August and September, leading to lighter trade winds that have kept the ocean warm due to reduced evaporative cooling.
Figure 3. Difference of SSTs from normal for Sep 11, 2006.
Wind shear and El Niņo
An El Niņo is on the way, according to the latest El Niņo discussion posted September 7 by NOAA's Climate Prediction Center. A steady warming of the waters in the Equatorial Pacific near the coast of South America, combined with stronger than usual westerly winds over the Equatorial Pacific, point toward the emergence of a weak El Niņo episode beginning in October or November. As most of you know, El Niņo conditions put a major damper on both the number and intensity of Atlantic tropical cyclones. This is primarily due to increased wind shear. The upper air winds that develop when one heats the Equatorial Eastern Pacific waters tend to blow from west to east over the Atlantic at high speed. Since the tropical Atlantic trade winds near the surface typically blow the opposite direction, this creates a lot of shear that makes it difficult for a tropical cyclone to survive. Since the peak portion of hurricane season began in late August, wind shear over the tropical Atlantic, western Caribbean, eastern Caribbean, and Gulf of Mexico has been near or below normal, so there is no sign that a developing El Niņo is suppressing hurricane activity yet. When I eyeball the shear forecast for the coming two weeks from the GFS model, I don't see any sign that an El Niņo-induced increase in shear is in the offing this September.
While the lack of wind shear would seem to favor an active hurricane season, what wind shear we have had has been of exceptionally high quality. The jet stream has spun off a continual series of upper level "cold lows" over the Bahamas and central Caribbean that have brought hostile wind shear to any tropical waves or tropical storms that have tried to approach. This jet stream pattern has been in place since early June, and shows no signs of changing through the end of September.
Dry air and vertical instability
Hurricanes like plenty of moist, unstable air. This has been lacking this year, thanks to an above-average amount of dry Saharan air coming off the coast of Africa. For example, take a look at the instability plot for the eastern Caribbean, where many of last year's fiercest storms formed. Instability has been well below normal this year. Model projections through the rest of September show no change to the basic atmospheric pattern over the Atlantic, and I expect lack of instability will continue to inhibit hurricane development.
The large scale jet stream pattern and associated positioning of the Bermuda High has remained unchanged since early June, and is forecast to remain the same through the end of September. This pattern puts a trough of low pressure over the U.S. East Coast, which will act to recurve storms approaching the U.S. or Caribbean. Now that we are entering late September, the troughs are getting stronger and extending farther south, making recurvature even more likely. The current pattern has been for tropical waves that form into tropical storms to emerge from Africa at about the latitude of the Cape Verde Islands (12-15 north latitude). This is far enough north to make recurvature very likely. This pattern is forecast to continue through the end of September, and I don't expect any of these African systems will be able to avoid recurvature. We will probably get two or three more of these recurving storms before the usual end to the African tropical storm season in early October.
Given that the current jet stream pattern that favors recurving storms and shear-producing upper level lows over the Bahamas and central Caribbean is forecast to continue until the end of September, plus Bill Gray's September 1 forecast of only two named storms and one non-major hurricane in October, I don't believe any major hurricanes will affect the U.S. or Caribbean the remainder of hurricane season. I expect one or two tropical storms or Category 1 hurricanes will form in October from the remains of old cold fronts that push off the coast of the U.S. A hurricane of this nature is most likely to affect the west coast of Florida or the Outer Banks of North Carolina, and rarely has enough time over water to make it to Category 3 status. In total, I expect 5 more named storms, 3 hurricanes, and 1 major hurricane this season.
I answered a series of questions on what the rest of this year's hurricane season might be like for Texas, the Katrina disaster, and the global warming/hurricane connection for houstonist.com that appeared in today's edition.
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