August hurricane outlook, Part I: SSTs

Before we get into the outlook for this August, let's discuss the current area of concern. A tropical wave near 16N 57W, about 300 miles east of the Lesser Antilles Islands, continues to be a threat as it moves west-northwest at 15-20 mph. Visible satellite imagery shows a very robust surface circulation, but little thunderstorm activity associated with the wave. Strong upper level winds from the northwest are creating about 20 knots of wind shear over the low, and this is keeping what little thunderstorm activity it has confined to the southeast quadrant. A 5:30 am EDT pass from the QuikSCAT satellite revealed surface winds of about 30 mph in this region. The low is embedded in a large area of dry, dust-laden Saharan air that moving west along with the low, limiting any chance the system has for intensification. In addition, wind shear is increasing in the region just ahead of the low's track, and should be high enough to prevent it from becoming a tropical depression today. The shear may relax down to the 15-20 knot range on Tuesday as the storm passes through the Lesser Antilles Islands near Guadaloupe, but dry air should still be a problem for it then. We still have one model that develops the system into a tropical storm--the GFDL model predicts that the system will hit Puerto Rico as a tropical depression on Wednesday, then intensify into a tropical storm that hits the Dominican Republic on Thursday. None of the other models buy this solution, and neither do I. Wind shear and dry air will probably combine to keep this wave from developing. Even if the wave does develop, a strong upper-level cold low north of the Bahamas will bring very hostile wind shear to any storm that tries to approach the U.S. East Coast through the Bahamas. The only chance the storm appears to have is if it can stay south of the Dominican Republic in the Caribbean Sea.

The Hurricane Hunters are on call this afternoon and tomorrow in case a reconnaissance flight is needed into the system. Let's hope they get to enjoy the beach instead!


Figure 1. Preliminary model tracks for the tropical wave approaching the Lesser Antilles Islands.

New development off the Carolina coast
A non-tropical low pressure center has developed about 150 miles east-southeast of Cape Hatteras North Carolina. Wind shear is a very high 30-40 knots today over the disturbance, but may slowly decrease over the next few days, possibly allowing some slow development. The low is currently moving slowly east-southeastward, but is in an area of weak steering currents.


Figure 1b. Preliminary model tracks for the low off the Carolina coast.

August Sea Surface Temperature Outlook
Sea Surface Temperatures (SSTs) throughout the tropical Atlantic are very near normal for this time of year (Figure 2). Contrast this with last year, when we had the highest SSTs ever recorded in the tropical Atlantic--about 2 degrees C higher than normal. This difference in SSTs is primarily due to the stronger than normal trade winds we've had during June and July this year. A much stronger Bermuda High than last year's has been driving increased trade winds and higher evaporative cooling of the ocean.


Figure 2. Departure of SST from normal for July 28, 2006. Image credit: NOAA.

What are SSTs likely to do in August?
As we can see from the long range forecast from NOAA, SSTs are expected to remain near normal for the duration of hurricane season. The Bermuda High and the trade winds it drives are expected to be near normal, resulting in normal SSTs. Now, these long range forecasts are not that reliable, but do have some skill compared to flipping a coin. I believe that for at least the month of August, the forecast is correct, since the 2-week GFS forecast is indicating near-normal winds and pressures across the Atlantic.


Figure 3. Forecast departure of SST from normal for August through October , 2006. Image credit: NOAA's Climate Prediction Center.

Total heat content of the ocean
As most of you are aware, hurricanes generally require sea surface temperatures (SSTs) of at least 80 F (26.5 C) to exist, and the hotter the water, the better.Hurricanes also like to have these warm ocean waters extend to a depth of several hundred feet, since the winds of a hurricane generate ocean turbulence that stirs up colder water from the depths to the surface. Hurricane that pass over a region of ocean with very deep warm waters can intensify explosively; this happened with Katrina, Rita, and Wilma in 2005. A good way to monitor this total oceanic heat is with the Tropical Cyclone Heat Potential (TCHP) imagery prepared daily by NOAA using satellite measurements of the height of the ocean surface. Hotter water expands, creating a higher water surface that the satellite can measure.

Let's take a look at the TCHP data from July 28 this year, and compare it to last year (Figures 4 and 5). The units of measurement are in kilojoules per square centimeter, and any value greater than 20 kJ/cm**2 (a medium blue color) is high enough to support a Category 1 hurricane. A TCHP greater than 90 kJ/cm**2 (orange color) can lead to rapid intensification of a hurricane. The TCHP image from last year shows a large area of oranges and reds in excess of the 90 kJ/cm**2 threshold for rapid hurricane intensification covering most of the Gulf of Mexico, Caribbean, and tropical Atlantic. A red bullseye in the Gulf of Mexico marks the Loop Current Eddy that broke off in the Gulf last July, and helped fuel Katrina and Rita to record intensities.

In contrast, the TCHP image for this year shows the oranges and reds covering just a portion of the western Caribbean and southern Gulf. There is much less heat energy available to fuel intense hurricanes, and thus we should see fewer of them this year than in 2005 (or 2004, which also had very high TCHP values). However, we again see an ominous looking red bullseye in the central Gulf of Mexico this year, similar to what was there last year.


Figure 4. Tropical Cyclone Heat Potential (TCHP) for July 28, 2005.


Figure 5. Tropical Cyclone Heat Potential (TCHP) for July 28, 2006.

Another dangerous Loop Current Eddy in 2006
As I described in my May 2006 Gulf of Mexico Loop Current outlook, the Loop Current is an ocean current that transports warm Caribbean water through the Yucatan Channel between Cuba and Mexico. The current flows northward into the Gulf of Mexico, then loops southeastward through the Florida Keys (where it is called the Florida Current), and into the Bahamas. Here, the waters of the Loop Current flow northward along the U.S. coast and become the Gulf Stream. During summer and fall, the Loop Current provides a deep (80 - 150 meter) layer of vary warm water that can provide a huge energy source for any lucky hurricanes that might cross over. Every 6 to 11 months, the Loop Current sheds an eddy that moves westward at 3-5 km per day across the Gulf of Mexico. These eddies can double the area of the Gulf where explosive hurricane intensification can occur. When the loop current sheds an eddy at the height of hurricane season, it's bad news for the residents along the Gulf Coast. This occurred in 2005, when a Loop Current Eddy separated in July, just before Hurricane Katrina passed over and "bombed" into a Category 5 hurricane.


Figure 6. Currents in the Gulf of Mexico on July 28, 2006. Red colors indicate fastest current speeds. Note the two eddies in the Gulf of Mexico that have separated from the Loop Current. Image credit: Navy Research Lab.

Unfortunately, there's bad news again this year. Another Loop Current Eddy has just separated, and is now spinning in the central Gulf of Mexico, ready to fuel explosive intensification of any system that might cross the Gulf. The position of this year's eddy makes it primarily a threat for hurricanes that would hit Mississippi, Louisiana, or the upper Texas coast. The behavior of the Loop Current over the past year can be viewed at Navy Research Lab's web site (51 Mb). This movie has arrows showing the direction of the current, plus a color coding that represents the height of the sea surface above mean level. The higher the height, the warmer the water (since warm water expands and thus raises the sea level where it is at). One can see near the beginning of the animation that the Loop Current Eddy that fueled Katrina and Rita that broke off in July 2005. Another eddy breaks off in March 2006, and the final eddy during the past two weeks. The old March eddy has only a little extra heat content, but the new eddy is similar in heat content and only about 25% smaller in size than the eddy that fueled the intensification of Katrina and Rita in 2005. Let's hope we don't get a hurricane in the Gulf this year that crosses over this eddy! I'll talk about the possibilities of that happening in Part II of this blog on Tuesday.

Jeff Masters