Ask Weather Dan: Hurricane season

One reader asks: “Why is hurricane season when it is, and what makes it hurricane season? Is it hurricane season at the same time for everyone everywhere?” Read on to find out the answer and for what we can expect in the Commonwealth this year.

June 1, 2010; 6:00 AM • by Weather Dan

One of the advantages of having a non-science-y girlfriend is that she gives me plenty of material for these columns. This week, Nicole asks:

Why is hurricane season when it is, and what makes it hurricane season? Is it hurricane season at the same time for everyone everywhere?

Let’s start by defining exactly what a hurricane is. The term ‘hurricane’ is one of several specific names given to the general class of storms known as tropical cyclones. Like other storm systems, a tropical cyclone is a strong area of low pressure. Unlike other low pressure systems, tropical cyclones are described as warm-core systems. Tropical cyclones draw in warm, humid air over ocean waters typically warmer than 80 degrees Fahrenheit. These warm winds rise and cool near the center of the low pressure, convecting in the form of thunderstorms. These winds kick up spray near the surface of the ocean; the warm air evaporates the spray, feeding the convection process and causing the storm to grow. The warmest air is found near the center of the storm, which is why it is designated as “warm core.”

Tropical cyclones are known by different names in different ocean basins. With the exception of the South Atlantic, all oceans see tropical cyclone activity at different points in the year. In the North Atlantic and eastern Pacific, cyclones are designated as tropical depressions, tropical storms, or hurricanes, depending on intensity. In the northwest Pacific ocean, they’re known as typhoons or supertyphoons (again, depending on intensity), and in the remainder of the world they’re just called cyclones.

Hurricane season also varies between ocean basins, based on when the historical maxima and minima have occurred. The National Hurricane Center, part of the National Oceanic and Atmospheric Administration (NOAA), has jurisdiction over the Atlantic and northeastern Pacific ocean basins. The eastern Pacific season began on May 15, and the Atlantic season begins today, June 1. Both run through November 30. Seasons in other basins are much less defined, and in some cases, do not exist at all; rather, the threat for cyclones exists nearly year-round. This is especially true in the western Pacific.

In fact, the eastern Pacific saw their first named system develop over the weekend. Tropical Storm Agatha developed southwest of the coast of Guatemala and made landfall Saturday afternoon, before dissipating over Central America.

Tropical cyclones form in most global tropical ocean basins – including the western Pacific and most parts of the Indian Ocean. The southern Atlantic is the only basin where cyclones don’t form with any regularity, though they’re not unheard of. Tropical Storm Anita formed off the southern coast of Brazil back in March of this year, the first to form since 2004.

Meteorologist Brian Peters, a contributor to the Weatherbrains podcast, took the time to put together an assortment of records for the Atlantic basin. Here they are, for your enjoyment:

• Earliest hurricane to form in a calendar year: Storm 1, March 6 1908
• Earliest Category 5 to form in a calendar year: Hurricane Emily, July 17, 2005
• 2005 set several records: Most hurricanes (15), Most retired storm names (5), Most category 5 storms (4)
• 2005 was the most active season, with 28 named storms. It was the first to use both the V and W names, and the first to use the Greek alphabet.
• 2005 also saw the formation of the costliest storm in history. Hurricane Katrina caused $81.2 billion in damage along its track.
• 2005’s Hurricane Wilma holds the record for lowest central pressure at 882mb.
• Hurricane Ivan (2004) holds the record for most tornadoes spawned with 117. Hurricane Beulah (1967) comes in a close second with 115 tornadoes.
• The 1983 season was one of the quietest on record; Hurricane Alicia was the only major hurricane that year.

As an aside, for anyone who considers themselves “into” weather, I recommend taking some time and listening to episodes of Weatherbrains. It’s a weekly podcast put together by several TV and former National Weather Service meteorologists, and best of all, it’s both entertaining and extremely informative.

Looking ahead: 2010

There are a number of topics that will be of issue in 2010 that I’d like to discuss.

NOAA has published its outlook for the 2010 season. Their outlook is is for a highly active season, with a 70% chance for the following :

• 14-23 Named Storms
• 8-14 Hurricanes
• 3-7 Major Hurricanes (rated at Category 3 or higher)

This would parallel well with the 2004 season, which saw 15 named storms, 9 hurricanes and 6 major hurricanes. No less than five of these storms impacted the Commonwealth that year, including the unforgotten Hurricane Gaston.

Since 1973, tropical cyclones in the Atlantic and eastern Pacific basins have been rated on the Saffir-Simpson Hurricane Scale. Named for structural engineer Herbert Saffir and meteorologist Bob Simpson, this rated hurricanes on a scale of 1-5 based on 60-second sustained wind speed and expected storm surge. Unfortunately, history has proven that storm surge and wind speed in hurricanes are not dependent; a storm with category 3 winds (111-130 mph) can have a storm surge that would rate as category 1 (4-5 feet), or a category 5 storm surge (more than 19 feet) with only category 2 winds (96-110 mph), or really any combination one can think of. The problem here is that this rating system did’t accurately portray the threat posted by any single storm to the public. Beginning with the 2010 season, the Saffir-Simpson Hurricane Scale has been modified as the Saffir-Simpson Hurricane Wind Scale, and the storm surge component has been removed from the criteria.

Finally, as we approach June 1, there’s been increased discussion of the Deepwater Horizon oil leak in the Gulf of Mexico and its potential impact on or by any tropical systems. The National Hurricane Center has put together a fact sheet about the impacts of the hurricane upon the oil spill and vice-versa. Dr. Jeff Masters at Weather Underground has taken a look at both of these topics in depth, and has spent time exploring both what the oil slick might do to a hurricane, and how a passing hurricane would impact the oil slick.

Regardless of the intensity of the season, its important to note that it only takes one storm to cause serious damage to both life and property. Isabel was the only hurricane to cross Virginia in 2003, and I’m sure many of us still have vivid memories about how that turned out. If you haven’t already, I highly urge you to review the National Hurricane Center’s Hurricane Preparedness Week presentation. Locally, make sure you monitor local media outlets, and you can always get the latest information from the National Weather Service forecast office in Wakefield.

What are your hurricane memories, and what advice would you pass along to others?

For more information :

• National Hurricane Center
• National Weather Service – Wakefield, VA
• NOAA Oil Spill & Hurricanes Factsheet
• NHC Hurricane Preparedness Presentation
• Dr. Jeff Masters’ Blog

(The featured image shows the eye of Hurricane Isabel as seen from the International Space Station.)

June 1, 2010 6:00 AM 1