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 , 2:46 PM GMT on October 16, 2007
A low pressure system (99L) over the Gulf of Mexico, about 500 miles south of the Texas/Louisiana border, is headed to the north at about 10-15 mph. QuikSCAT showed top winds of about 30 mph near this low. Satellite loops show that the thunderstorm activity has increased slightly this morning near the low, but remains disorganized. Wind shear is about 15 knots, and is expected to increase to 30 knots by the time 99L crosses the Louisiana coast Thursday morning. The shear should be high enough to prevent 99L from forming into a tropical depression. However, the Hurricane Hunters are on call to fly the storm Wednesday afternoon, if necessary.
The storm and an associated trough of low pressure extending northward to the Gulf Coast will bring heavy rains to the northern Gulf Coast today through Thursday, and should bring welcome rains to the drought-parched Southeast U.S. on Thursday and Friday.
Outlook for the remainder of hurricane season
Atlantic tropical cyclone activity finishes its peak phase in mid-October, and takes a major downturn during the final half of October (Figure 1). Since the current active hurricane period began in 1995, the last half of October through the end of hurricane season has given birth to an average of 1.9 named storms, 0.8 hurricanes, and 0.3 intense hurricanes. These numbers are about double the long-term climatological averages for the past 100 years. So, we're probably not done with the season yet, since this is not an El Nino year--El Nino years typically bring higher wind shear to the Atlantic, and an early end to hurricane season.
Climatology of major hurricanes
Let's examine the possibilities of getting a major hurricane this year, since those are the storms we care most about. Since 1960, there have been ten hurricanes that have existed as major Category 3 or higher storms after October 15. Six of these have occurred since 1995: Wilma of 2005 (Cat 4, Mexico; Cat 3, SW Florida), Beta of 2005 (Cat 3, Nicaragua), Michelle of 2001 (Cat 4, Cuba), Lenny of 1999 (Cat 4, northern Lesser Antilles), Mitch of 1998 (Cat 5, Honduras), and Lili of 1996 (Bahamas, Category 3). The other four were Joan of 1988 (Cat 4, Nicaragua), Kate of 1985 (Cat 3, Gulf of Mexico), Ella of 1962 (Cat 3, west of Bermuda), and Hattie of 1961 (Cat 4, Belize). Wilma of 2005 was the only major hurricane since 1960 to hit the U.S. after October 15. The highest risk region for late season major hurricanes is the Western Caribbean, along the coasts of Nicaragua, Honduras, Belize, Mexico, and Cuba. So, we can say with high confidence that most of the U.S. coast can relax. Only the west coast of Florida, Florida Keys, and South Florida need to still be concerned about the possibility of a major hurricane. The Lesser Antilles Islands, Puerto Rico, and Hispaniola are also at low risk for a major hurricane the remainder of the season.
Figure 1. Atlantic hurricane season activity over the past 100 years.
October storms tend to form both from tropical waves that come off the coast of Africa, and from the remains of old fronts that push off the coast of the U.S. As we can see from the track plot of all last half of October storms (Figure 2), there is a lot of activity during the period, but relatively few storms form out near the African coast. The water temperatures off the coast of Africa are starting to cool and be marginal for hurricane formation, and wind shear is starting to pick up in its normal fall cycle.
The jet stream is now more active and extends further south, which brings higher levels of wind shear to the Atlantic. The more active jet stream also acts to recurve storms more quickly. Any system penetrating north of about 20 degrees north latitude we can expect to recurve quickly to the north and northeast this late in the season.
Figure 2. Tracks of all tropical storms and hurricanes since 1851 that formed October 16-31.
Sea Surface Temperatures
Sea Surface Temperatures (SSTs) have been about 0.5 ° C above average over the Caribbean the past few weeks (Figure 3). This is the primary formation area for October storms. Note also the tongue of colder than average SSTs extending out into the Pacific Ocean from the coast of South America. This is the signature of a moderate strength La Nina event.
Figure 3. Sea Surface Temperature (SST) departure from average for the first portion of October. Image credit: NOAA.
Wind shear is usually defined as the difference in wind between 200 mb (roughly 40,000 foot altitude) and 850 mb (roughly 5,000 foot altitude). In most circumstances, wind shear above 20 knots will act to inhibit tropical storm formation by tearing a storm apart. Wind shear 10 knots and lower is very conducive for tropical storm formation.
Despite the presence of a La Nina event the past month--which is supposed to bring lower than average wind shear to the tropical Atlantic--wind shear the past month has been near average (Figure 4). The latest two-week wind shear forecast from the GFS model predicts near-average wind shear for the last half of October.
Figure 4. Wind shear departure from average for the 31 days ending October 13. Near average levels of wind shear were observed over the primary hurricane formation regions of the tropical Atlantic, Caribbean, and Gulf of Mexico. Wind shear is the difference in wind between 200 mb (roughly 40,000 foot altitude) and 850 mb (roughly 5,000 foot altitude) in meters per second (multiply by two to get the approximate wind shear in knots).
We've gotten very lucky this hurricane season since the departure of Hurricane Felix in early September. We had a record eight named storms form in September, yet we had only four hurricane days that month. Wind shear has been strategically high at the right time and right place, and storms have tended to form too close to land to develop. A good case in point is the current storm 99L. Had that system formed just 200 miles further east, it would have spent five days meandering over the Western Caribbean instead of over the Yucatan Peninsula, and could have easily grown into a Category 4 or 5 hurricane. There are still several low-shear periods ahead for the Western Caribbean this hurricane season, and I expect our luck may not hold for one of these periods. There is still one hurricane likely to form this season, possibly a major hurricane. The GFS model is predicting a large area of low shear will develop over the Caribbean around October 30, and I expect further low shear periods may occur through the first half of November.
I thank Margie Kieper for helping out the major hurricane seasonal stats.
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