Little change to 90L; first half of October hurricane outlook

A low pressure system over the southeastern Gulf of Mexico (90L) has changed little today. The buoy 262 nm south of Panama City, FL had winds of 36 mph gusting to 45 mph this morning, but these winds have fallen some this afternoon. Satellite loops show some disorganized heavy thunderstorm activity that is not increasing in intensity. This system has the potential to become a subtropical depression by Wednesday, and a Hurricane Hunter aircraft is scheduled to investigate the system Wednesday afternoon. This afternoon's model runs continue to point to a landfall Thursday or Friday in Louisiana or Texas. I don't see this storm becoming a hurricane, due to the large amount of dry air to overcome, plus the extended amount of time it will take to transition from a subtropical to a tropical storm. The GFDL model probably has the right idea, bringing 90L ashore in Texas Friday with top winds near 40-45 mph.

First half of October hurricane outlook
In the first half of October, Atlantic tropical cyclone activity remains high, and is in the final two weeks of its peak phase. Since the current active hurricane period began in 1995, the first half of October has given birth to an average of 1.9 named storms, 0.75 hurricanes, and 0.3 intense hurricanes. For October through December, these figures are 3.8 named storms, 1.7 hurricanes, and 0.75 intense hurricanes. These numbers are nearly double the long-term climatological averages for the past 100 years. The final seasonal forecast from the Phil Klotzbach/Dr. Bill Gray team at Colorado State University, issued today, calls for four named storms, two hurricanes, and one intense hurricane for the remainder of this year.

October storms form 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 first half of October storms (Figure 1), 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.


Figure 1. Tracks of all tropical storms and hurricanes since 1851 that formed October 1-15. There are very few storms forming off the coast of Africa during this period.

Sea Surface Temperatures
Sea Surface Temperatures (SSTs) have been about 0.5 °C above average over the Caribbean and Gulf of Mexico the past few weeks (Figure 2). 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 Niña event.


Figure 2. Sea Surface Temperature (SST) departure from average for the end of September. Image credit: NOAA.

Tropical Cyclone Heat Potential
It's not just the SSTs that are important for hurricanes, it's also the total amount of heat in the ocean to a depth of about 150 meters. Hurricanes stir up water from down deep due to their high winds, so a shallow layer of warm water isn't as beneficial to a hurricane as a deep one. The Tropical Cyclone Heat Potential (TCHP, Figure 3) is a measure of this total heat content. A high TCHP over 80 is very beneficial to rapid intensification. There are similar levels of heat energy available in the Western Caribbean and Gulf of Mexico this year compared to the devastating Hurricane Season of 2005. Recall that Wilma, the most intense hurricane on record, formed on October 15 of that year.


Figure 3. Tropical Cyclone Heat Potential (TCHP) for September 30 2005 (top) and September 30 2007 (bottom). TCHP is a measure of the total heat energy available in the ocean. Image credit: NOAA/AOML.

Wind shear
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 Niña event the past month--which is supposed to bring lower than average wind shear to the tropical Atlantic--wind shear the past two weeks has been above average (Figure 4). The latest two-week wind shear forecast from the GFS model predicts near-average wind shear for the first half of October.


Figure 4. Wind shear departure from average for the 11 days ending September 29. Above average levels of wind shear were observed over the 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).

Steering currents
The steering current pattern for the last half of September has been dominated by a strong ridge of high pressure over the Eastern U.S. This ridge is expected to remain in place until at least October 12, and will reduce the tendency of storms to recurve out to sea. Long range forecasts from the GFS and ECMWF models foresee the ridge may break down beginning October 12, but it is too early to be confident of this.

Summary
I predict two named Atlantic storms will form in the first half of October. One of these storms will probably be 90L. Several models, including the GFS, have been hinting at formation of a tropical depression in the Bahamas, Western Caribbean, or Gulf of Mexico sometime in the next 4-7 days. The upper air environment is forecast to be favorable for intensification over the Gulf of Mexico next week, with low wind shear.

Jeff Masters