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October hurricane season outlook

By: Dr. Jeff Masters, 2:24 PM GMT on October 03, 2006

Hurricane Isaac is no more. The storm skirted the southeast coast of Newfoundland, bringing wind gusts up to 60 mph to the island. No damage was reported.

It's a different story in the aftermath of Typhoon Xangsane for the Philippines and Vietnam. These countries suffered heavily from Xangsane. The death toll has risen to 207 in the Philippines, with hundreds injured, 146,000 homes damaged or destroyed, and 171,000 people left homeless. In Vietnam, the death toll has risen to 42, 1200 are injured, and heavy flooding is still a serious problem. Xangsane destroyed or unroofed 200,000 houses in Vietnam, causing at least $300 million in damage.

The tropical Atlantic is quiet
A non-tropical low pressure system is about 500 miles south of the Azores Islands. This low may gradually acquire tropical characteristics over the next few days as it drifts over the open Atlantic.

The computer models are indicating some development is possible by Thursday in the region between the Bahamas and Bermuda, along an old cold front. Any development here would probably move northeast out to sea, and could be a threat to Bermuda.

October Atlantic hurricane season outlook
We're in the home stretch now! For the first time since 1997, we've made it through September without a landfalling hurricane in the Atlantic (Ernesto and Florence came close, but didn't hit land as hurricanes). We now have only two more weeks of peak hurricane season left. While the season technically ends November 30, we can see that hurricane season slows down drastically around October 18 (Figure 1), thanks to increasing wind shear, cooling Sea Surface Temperatures (SSTs), and an end to the tropical waves coming off the coast of Africa. With no major changes expected to the steering patterns or general atmospheric conditions over the Atlantic the next few weeks, I believe that we will go the remainder of this season without a landfalling hurricane in the Atlantic. It still pays to be vigilant, though--recall that at this time last year, Hurricane Stan was killing 1500 people in Central America, and we still had ten more named storms to go, including the strongest hurricane of all time, Wilma.

Figure 1. Climatological hurricane and tropical storm activity for the Atlantic.

Steering pattern
The steering pattern for October over the Atlantic will remain similar to what we've seen all of hurricane season. The jet stream is expected to stay active, bringing frequent troughs of low pressure over the Atlantic that will act to recurve any storm that approaches the Caribbean or U.S. East Coast. This pattern has been in place since early June. As we progress deeper into October, the troughs grow stronger and extend further south, making it very unlikely anything developing between Africa and the Lesser Antilles Islands will make landfall. Any landfalling storms will have to form from the the remains of old fronts that push off the U.S. coast.

Dry air
Dry air should not be as detrimental for tropical cyclone formation in October compared to previous months. The Saharan Air Layer (SAL) is most prevalent over the Atlantic in June and July, and is usually not in evidence much in October. The current SAL image shows very little dry air over the tropical Atlantic. The SAL is pretty much confined to the waters near the African coast. The 2-week outlook from the GFS model shows near-normal SAL activity, with no major SAL episodes over the tropical Atlantic.

Wind shear and El Ni�o
Wind shear over most of the tropical Atlantic has been near or below normal since early July. Climatologically, wind shear reaches its minimum in September, which is the same time that SSTs reach their peak. In October, wind shear normally begins to rise, which one can see in the plot for the eastern Caribbean (Figure 2, black line). The blue line in Figure 2 shows that in the past few days, wind shear has spiked to above normal values, and this is also occurring throughout the rest of the tropical Atlantic. This appears to be a temporary increase for the Caribbean; the latest 2-week wind shear outlook from the GFS model shows below-normal shear over most of the Caribbean next week. However, wind shear over the Gulf of Mexico and remainder of the tropical Atlantic is expected to be higher than normal the next two weeks.

Part of this increase in wind shear is probably due to El Ni�o. An El Ni�o event was officially declared by NOAA's Climate Prediction Center on September 13, when SSTs in the Equatorial Eastern Pacific reached +0.5�C above normal, the threshold for a weak El Ni�o. El Ni�o continues to strengthen, and SSTs are almost a full degree Centigrade above normal now, the threshold for a moderate El Ni�o. SSTs are forecast to continue much above normal (Figure 3) through the next six months (if they increase to 2�C above normal, then this will qualify as a strong El Ni�o event). It is well known that El Ni�o events tend to suppress Atlantic hurricane activity by increasing upper level westerly winds at about 40,000 feet (200 mb). These strong westerly winds create a high wind shear that prevents tropical storms from forming.

Sea Surface Temperatures
Sea Surface Temperatures in the tropical Atlantic are slowly cooling now that it is October, but are still 0.5 - 1 �C above normal. These above normal temperatures are expected to persist through the remainder of hurricane season (Figure 3).

Figure 2. Observed wind shear in 2006 (blue line) and climatological wind shear (black line) for the eastern Caribbean. Image credit: NOAA/CIRES.

Figure 3. NOAA's Sea Surface Temperature forecast for the next four months. Note the much warmer than normal SSTs over the Equatorial Eastern Pacific, indicating a moderate El Ni�o event occurring. Note also that SSTs are expected to remain 0.5 - 1 �C above normal over the tropical Atlantic for the remainder of hurricane season.

Because of high wind shear over the Atlantic due to the expected jet stream pattern and a strengthening El Ni�o event, I am forecasting at most two more named storms this year and no hurricanes. I think it most probable we will get just one more named storm. The steering pattern we've seen all hurricane season is not expected to change much, so any storm forming during the remainder of the season is likely to stay over water. If we do get a landfalling storm, the most likely targets are the Carolinas, Bermuda, or the Gulf Coast of Florida. Perhaps the best analogue year to compare with is 1997, a strong El Ni�o year, when we had two 45-mph tropical storms form in October. Neither of these storms hit land.

Jeff Masters

The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.