Wx4cast: May 2015

Hurricane season in the Atlantic begins June 1st and ends November 30th.

I posted my first thoughts on the 2015 Atlantic hurricane season. There haven't been any real changes in the pattern since then. We still have an El Nino Modoki developing into an east based El Nino; it's developing quicker than I thought it would. But I still think it will become moderate and most likely strong as we get into the Summer. We also still have the AMO in its negative state.

If you want to read or review my earlier post, you can find it here.

I've already heard some online weather outlets, starting to throw out the Super El Nino idea; because it's strengthening so quickly. Why do I think the El Nino will only become strong? The answer to that deals with yet another teleconnection; this one being the Southern Oscillation Index (SOI). The SOI gives an indication of the development and intensity of El Nino or La Nina events in the tropical Pacific Ocean. The SOI is defined as the normalized surface pressure difference between Tahiti and Darwin. When we have an El Nino we normally see a decrease in the strength of the Pacific trade winds.

The SOI has turned negative, A consistently negative SOI often indicates the presence of a stronger El Nino. The very warm SST's near Australia will have an impact on the current El Nino. Based on this I don't think we will see a Super El Nino. The warmer SST in the Southwest Pacific should keep the strength of the current El Nino to moderate (maybe stronger) levels. So most likely the SOI won't go as negative as it normally would. The SOI state, is also the reason I think we will see the El Nino weaken later in the Fall and heading into next winter (something I laid out in my Spring/Summer outlook).

Warming in the central equatorial Pacific( El Nino Modoki) can increase the number of Atlantic tropical cyclones. While a warming (El Nino) in the eastern equatorial Pacific tends to lower the number of Atlantic tropical cyclones.

When I did my winter 2014-2015 and Spring Summer 2015 outlooks three years showed up....1918,1919, and 1920. This hurricane season could be quite similar to 1918. Time will tell!

What will impact the 2015 hurricane season?

El Nino typically increases vertical wind shear in the tropical Atlantic Basin, which along with dry air, severely hampers the development of tropical cyclones. El Nino also normally increases the east to west trade winds. This year looks to see a fairly strong El Nino....how strong is the question. But a strong El Nino would make it difficult for east based Cape Verde systems. Dry air and higher wind shear would tend to rip them apart. Shear most likely will be very disruptive around the windward and leeward Islands.

The 2015 hurricane season will have some similarities to the 2014 season. One of these is cooler sea surface temperatures (SST).

Atlantic Multidecadal Oscillation (AMO), remember I said, in the first thoughts post, that the AMO affects the intensity of Atlantic tropical cyclones. One reason for this is when we have a negative AMO the waters off the West Coast of Africa is cooler. SSTs in the eastern Atlantic Ocean off the western African coast past the Cape Verde islands to about halfway to the islands of Dominica, St. Lucia, and Martinique. Another reason is when we have a negative AMO the number of Atlantic warm pools is a lot less than when the AMO is positive. So, less warm pools...less areas for tropical depressions and tropical cyclones to intensify.

The Inter Tropical Convergence Zone (ITCZ). the ITCZ is an area in the Atlantic where there is a confluence of the trade winds. The trade winds from the north and south hemispheres meet. The ITCZ has an impact on horizontal wind shear. The piling up of air near the surface due to the converging winds forces the warm, humid air over the tropical oceans to rise. As the air rises, it cools and water vapor condenses into clouds that become thunderstorms. If conditions are favorable, some of these thunderstorms can become tropical cyclones. The 2015 season looks to have a ITCZ that is below average.

The Saharan Air Layer (SAL). This is another similarity. Last year, saw dry dusty air get picked up by African Easterly Waves which pushed westward from Africa into the Atlantic Ocean. This year will see increased trade wind inversion. This will ensure that SAL dust will again be an issue for the 2015 tropical season. The dry dusty conditions help to weaken a tropical cyclone by promoting downdrafts around the storm, while its strong winds can significantly increase the vertical wind shear in and around the storm environment.

The set up in the western Atlantic, western Caribbean, and Gulf of Mexico (GOM) will be much more supportive for tropical cyclone development. This fact is in play right now. I've been talking about the warm SSTs in the GOM and off the Southeast Coast for a while now. I've also been taking about the likelihood of a subtropical/tropical storm forming off of East Coast of Florida or Georgia later this week. The National Hurricane Center (NHC) is now tracking a disturbance in the Northwest Caribbean. The NHC is currently giving it a 20% chance for development over the next 48 hours and 40% over the next five days.

Could this be a precursor of what the 2015 season could hold? The answer to that is yes. the western Atlantic, Caribbean, and GOM, will be at an increased risk for tropical cyclone development this season. With storms forming closer to the CONUS, we could see a greater risk for land falling systems.

I'm calling for a below average season, with 7 to 10 named Storms this season. There will be 2 to 4 Hurricanes of which 1 to 2 will become major hurricanes. I think we will see 2-3 CONUS land falling systems this season.

I've drawn a chart that shows the general setup for the 2015 hurricane season. the greatest potential for tropical cyclone development will be inside the red area.