November Atlantic hurricane season outlook

A small area of disturbed weather in the extreme southern Caribbean has brought heavy rains of up 4-6 inches to Costa Rica and southern Nicaragua over the past two days. This disturbance should persist for the next 3-5 days, and the UKMET and NOGAPS models continue to forecast that a tropical depression could form in this region 4-7 days from now. Steering currents are weak in the area, and any storm forming there would move slowly. Nicaragua would be at greatest risk from such a storm.

Late season Atlantic tropical storms
What are the odds of getting a late-season November or December tropical storm? In the active hurricane period that began in 1995, we've had nine tropical storms in November, and four in December, for an average of one late season storm per year. Six of these late season storms have become hurricanes. The record for late season named storms is four, which occurred in 2005, when three November and one December storm formed. The typical formation location for these late-season storms is the Western Caribbean or the middle Atlantic (Figure 1). The Western Caribbean storms are the most dangerous. There have been two Category 4 hurricanes that have formed in November in that region, Hurricane Lenny of 1999 (the strongest late-season hurricane on record, with 155 mph winds), and Hurricane Michelle of 2001. November storms are primarily a theat to Central America, Cuba, the Cayman Islands, the Bahamas, and the Gulf Coast of Florida.


Figure 1. Historical tropical cyclone tracks in the Atlantic for storms that formed in the first half of November. The Western Caribbean is the preferred formation region.

Sea Surface Temperatures
Sea Surface Temperatures (SSTs) are cooling, but are still warm enough to support a tropical storm over the Caribbean (Figure 1). The Gulf of Mexico will cool significantly in the coming week, due to the presence of cold air and northwesterly winds. It is now too late for the Gulf to spawn a tropical storm, and any hurricane or tropical storm that passes into the Gulf will likely weaken due to the cool SSTs there. The total heat content of the ocean is still high enough to support a major hurricane in the Caribbean.


Figure 2. Sea Surface Temperatures (SSTs) on October 29, 2008. The 26 °C isotherm (red line that separates blue colors from yellow colors) marks the boundary where SSTs are warm enough to support a tropical storm. Image credit: NOAA/AOML.

Wind shear
High wind shear is main reason we don't get many November tropical storms. Wind shear is currently very high north of the Caribbean Sea, and is forecast to remain high for the next two weeks (Figure 3). However, shear is low over the Caribbean, and is forecast to remain low for at least the next two weeks.


Figure 3. Forecast wind shear (in meters per second) for Friday, November 7, at 06 GMT. This is an 8-day forecast generated by the 12Z GMT run of the GFS model on Thursday, October 30. The Caribbean is forecast to remain under low shear for the first half of November, while the U.S. will be protected by very high shear, thanks to the presence of the jet stream.

MJO
We are currently in the phase of the Madden-Julian Oscillation (MJO) that suppresses Atlantic tropical storm formation. The MJO is a pattern of enhanced rainfall that travels along the Equator, and can act to boost hurricane activity when it propagates into the Atlantic. The MJO has a period of about 30-60 days. According to the latest 15-day GFS model forecast and the MJO discussion from the National Weather Service's Climate Prediction Center, we are expected to remain in the inactive phase for the MJO for the next two weeks. By mid-November, we may transition to a positive MJO again. This year, the active phase of the MJO has been strongly correlated with formation of named storms in the Atlantic. Thus, the chances for a late-season named storm may increase by mid-November.

Summary
Given past climatology, warm SSTs in the Caribbean, and forecast low wind shear over the Caribbean for the first half of November, I put the odds at 50% we will see a named storm in the western or south-central Caribbean in the first half of November. Given the recent history of such storms, there is a 50% chance that such a storm would become a hurricane.

I'll update this blog over the weekend if there's any developments in the tropics worth reporting.

For those interested, the portlight.org charity now features a blog that details the recent Hurricane Ike relief effort they undertook.

Jeff Masters