TropicalAnalystwx13's WunderBlog

2013 Atlantic hurricane season preliminary forecast

By: TropicalAnalystwx13, 12:29 AM GMT on March 12, 2013's March. Daylight Saving Time was yesterday, meaning we have lost an hour of sleep. But it also means we're getting closer to hurricane season, which now begins in less than three months. Speaking of the season...this one looks to be active. The variables I used for my forecast, an overview of them, and what they are forecast to be for this upcoming hurricane season are listed below.

El Niño-Southern Oscillation (ENSO)

One of the most significant factors one must take into consideration when forecasting a hurricane season is the state of the ENSO, or El Niño-Southern Oscillation. Simply put, the ENSO is a quasi-periodic climate pattern observed across the equatorial Pacific. Within this pattern lie three phases: a warm stage, a cool stage, and a neutral phase. As the name would suggest, a warm phase, or El Niño, is characterized by anomalous sea surface temperatures at or above 0.5°C across the equatorial Pacific for at least three months. During the cool phase, or La Niña, anomalous sea surface temperatures are -0.5C or below for three consecutive months; the neutral phase, known as Neutral, represents the range between the two. It should be noted, however, that Neutral phases are typically short-lived and do not have as much of an influence on global patterns.

As aforementioned, all three phases of the ENSO have a significant influence on a particular hurricane season; we'll focus on the Atlantic for this post. During an El Niño, above-average sea surface temperatures across the eastern Pacific promotes the growth of convection--shower and thunderstorm activity--across the basin. Outflow as a result of this disturbed weather enhances vertical wind shear across the western Atlantic, therefore making it more difficult for tropical cyclones to develop. Oppositely, a La Niña promotes cooler-than-average sea surface temperatures in the eastern Pacific, focusing convergence in the Atlantic and allowing the development of anticyclones thanks to rising motion across that basin.

Another variable, the Southern Oscillation Index (SOI), comes into play when dealing with ENSO. Simply put, the SOI index is a value representing the air pressure difference between Tahiti and Darwin. During the positive phase of the SOI, higher pressures across the aforementioned locations promotes increased trade winds along the equatorial Pacific. Increased winds upwell water through evaporational cooling, leading to below-average values; that is, a La Niña. As one would guess, the exact opposite occurs during the negative phase of the SOI: lower pressures over Darwin, and to a lesser extent Tahiti, favor weaker trade winds along the equatorial Pacific. This allows no change or gradual warming of the sea surface temperatures along that portion of the basin, which may lead to an El Niño; sometimes conditions fall between 0.5C and -0.5C and this is, as stated, known as the Neutral phase.

The consensus forecast for this upcoming hurricane season seems to be for neutral--near normal--sea surface temperatures across the equatorial Pacific. However, it should be noted that during the January-April timeframe, computer models historically preform less-than-adequate.

Figure 1. Mid-February 2013 model predictions

North Atlantic Oscillation (NAO)

Another significant indicator for the upcoming hurricane season is the NAO, or North Atlantic Oscillation. This variable describes the strength of the surface pressure difference between two weather features: the Icelandic Low and the Bermuda High. Similar to the SOI, there are two phases: a negative and a positive. A stronger-than-average high across the Atlantic increases trade winds to the south of it--across the eastern portions of the basin and the Caribbean--allowing for more evaporational cooling, and thus cooler-than-average sea surface temperatures. During a negative NAO, the Bermuda High and Icelandic Low are weaker-than-average. A weak high favors calmer trade winds, allowing for substantial increase in sea surface temperatures. The importance of this phase peaks during the pre-season, though it is important year-round.

The NAO is also not only a big player in sea surface temperatures, but in tropical cyclone development itself. When trade winds are quicker than average, they allow for rapid movement of any tropical waves coming off the western coast of Africa. It is hard for a disturbance to consolidate when it is moving quickly--typically 20 mph or higher--and can actually lead to what is known as speed shear.

As if that were not enough, the NAO is also very crucial in the track of a tropical cyclone. During a negative NAO, the Bermuda High is weaker, but oriented west-to-east and closer to the Southeast USA coastline. In addition, all troughing on the East Coast is typically weaker. This makes it harder for hurricanes to recurve; in fact, many of our greatest hurricane strikes--Ivan for example--have occurred during this phase. The opposite is true for the positive phase.

That being said, the NAO has impressively been negative for much of the winter and looks to remain that way throughout the spring.

Figure 2. Observed NAO and forecast trends.

Other factors

Mean Sea Level Pressures also play an important role in tropical cyclone development, but during March, are typically not too terribly useful. When MSLP is above average across the Atlantic, subsidence is likely. Subsidence favors dry air, which is not favorable for formation. When Mean Sea Level Pressure is below average, upward motion is favored across the Atlantic, which leads to moist conditions; this is favorable for development.

Rainfall patterns over western Africa give us a clue to the strength of tropical waves emerging off the coast, but this is typically not important until the beginning of the season. Above average rainfall enhances the temperature gradient and therefore allows for stronger waves.

Landfall risk

Everything about the NAO being said, it is too early to determine where tropical cyclones will track this season. We will have to wait and see how the overall pattern evolves into the spring and beginning of the season. The last three years--barring Irene, Isaac, and Sandy--have been filled with many, many recurvatures. This is due to the presence of strong troughing along the East Coast for much of the summer. Unless a shallow hybrid (ex. Sandy) is phasing with the trough, it is nearly impossible to get a USA strike. If we take a look at the 700mb Geopotential Height anomalies from July to October during 2003, 2004, 2005, 2007, and 2008--all of which featured many major landfalls--then the mean pattern was ridging between 50-60N, with little troughing otherwise.

We shall see.

Figure 3. 700mb Geopotential Height anomalies from July-October 2010-2012. The presence of East Coast troughing directed the majority of tropical systems away from the USA coastline and out to sea.

Figure 4. 700mb Geopotential Height anomalies from July-October 2003-2005, 2007-2008. The absence of troughing and presence of strong ridging (north of Newfoundland) directed many powerful hurricanes into the coastline.

Final forecast

* 15-19 named storms
* 7-11 hurricanes
* 3-5 major hurricanes


Tropical Weather Tropical Cyclones of 2013

Updated: 10:44 PM GMT on March 22, 2013


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Teenager. Weather aficionado. Soccer fan. Realist. Posts subject to sarcasm. Goal: National Hurricane Center.

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