Monday, May 28, 2012

What's In A Name, Extratropical, Tropical, and Subtropical Storms.

Hi, Rebecca here again,  The 2012 hurricane season is starting off with a bang. We've had two storms in the Atlantic basin, and two in the eastern Pacific. The official hurricane season is typically from June 1 to November 30, but as most of us are aware, Mother Nature does what she wants, when she wants to do it. With the season officially almost ready to start I thought I would post this entry about tropical weather, I will talk about Tropical and subtropical storms as well as some of the terminology. Often Meteorologist throw terms around you might not understand,  It's my hope this blog post will help clear up some of that confusion. This will be a quick a dirty explanation, so it won't go into a lot of detail.

The season so far:

Hurricane season in the Atlantic Basin begins June 1st and ends November 30th. The Atlantic Basin consist of the Atlantic Ocean, the Caribbean Sea, and the Gulf of Mexico. The Eastern Pacific Basin hurricane season begins May 15th and also ends November 30th.

What is the statistical peak of the Atlantic Hurricane Season September 10th

The Eastern Pacific Basin:

Tropical Storm Aletta formed on May 14, one day before the official May 15 start of the Eastern Pacific hurricane season. This was only the third system since 1971 to develop before the official May 15 start date.

Tropical Storm Bud formed on May 22. Bud also had some distinction bestowed . The 22nd of May is the earliest date on record for the second named storm in the eastern Pacific. Bud also became the earliest major hurricane on record in the eastern Pacific.

The Atlantic Basin:

Alberto's was the first tropical storm of the 2012 Atlantic hurricane season. The May 19 date is historic, it marks the first time in recorded history we had named storms in both the eastern Pacific and Atlantic basins before the official start of their respective seasons.

Subtropical Storm Beryl formed late on May 25. The formation of Beryl  is also noteworthy, There have only been two other years in recorded history where the Atlantic Basin has seen two named storms before June 1st. At the time of this writing her max winds were 65 mph.

All of this early tropical activity begs the question, does the early start of the 2012 hurricane season mean we will see a busy season ahead? The short answer is no. There is absolutely no correlation between early-season tropical activity and the rest of the hurricane season.

The Storms:

I've been fielding lots of questions about tropical and subtropical storms the last few days. Mainly everyone wants to know what's the difference between the two?

Before I answer, I will talk about cyclones,  cyclones are large revolving areas of low pressure  that form near the boundary between warm and cool air masses. The vast majority of cyclones are classified as Extratropical. However they can also be Tropical or Subtropical in nature.

Extratropical Cyclones:

An extratropical cyclone has  cold air at their core, they get their energy from when cold and warm air masses interact and release energy . Extratropical storms  can occur over land or ocean. An extratropical cyclone can have winds as weak as a tropical depression, or as strong as a hurricane. Extratropical systems form from the top down.  Examples of extratropical cyclones include blizzards, Nor'easters, and the ordinary low pressure systems that we hear about in the daily weather forecast. It can also be a former tropical storm or hurricane that has lost its tropical characteristics.

Tropical Cyclones:

A tropical cyclone forms from the bottom up usually over water that's at least 76 F.  In addition they usually form at a distance of at least 300 miles away from the Equator, in areas of weak wind shear and where low level wind currents are converging beneath higher level winds that are diverging. Also wind shear must be very weak.

Subtropical Cyclones:

Subtropical storms have characteristics of both a tropical cyclone and an extratopical cyclone. The thunderstorm activity along with the stronger winds are farther away from the center. A Subtropical storm has a much colder core when compared to a tropical storm. Winds must be continuously blowing at 39 miles an hour up to 73 miles an hour.

The image is from Google Earth, and the yellow figure is Tropical Depression #13, the orange to the lower right is tropical storm Katia, and the red in the center near the top is an extratropical storm.

Terminology and vocabulary:

This is the common terms you many hear during the hurricane season.

Tropical Wave:
A system (often a trough) caught in the easterlies that has moved off the African coast.

Tropical Disturbance:
An area of organized thunderstorm activity. They form over waters of around 80 F or higher. They form out of tropical waves or the remnants of a cold front that pushed off the North American coast.

Tropical Depression:
When the disturbance has become better organized and the has developed closed circulation with sustained wind speeds of 25 to 38 mph it's called a depression.

Tropical Storm:

When the tropical cyclone develops wind speeds of 39-73 mph it is classified as a tropical storm (and christened with a name)


When a tropical cyclone develops winds of 74 mph or higher it is called a hurricane. once the winds reach 111 mph or greater, it's considered a major hurricane. one other thing I should mention is, hurricanes that form over the northwest Pacific Ocean are called typhoons. In the South Pacific and Indian Ocean, they're called cyclones.

Cape Verde Hurricane:

A tropical system that develops near the Cape Verde Islands just off the west coast of Africa and becomes a hurricane before reaching the Caribbean Sea.  Cape Verde hurricanes normally last a long time and become quite strong.


 The Center of Circulation in the hurricane, denoted by calm winds and sometimes sunny skies


Normally this is the most powerful part of the storm, it surrounds the eye. The highest sustained winds and heaviest rains are located here.

Eyewall replacement cycle:

This is when the original eyewall falls apart as a new one is forming. A replacement cycle is a process that major hurricanes undertake when they're reorganizing into a stronger storm, it's a sign of a very powerful hurricane.

Concentric Eyewalls:

This is when there are two eyewall present in the same hurricane. It can be caused by rapid intensification or during an eyewall replacement cycle. A hurricane can't intensify when two eyewalls are present.

Tropical Storm Watch:

An announcement that tropical storm conditions (sustained winds of 39 to 73 mph) are possible somewhere within the specified coastal area within 36 hours.

Tropical Storm Warning:

An announcement that tropical storm conditions (sustained winds of 39 to 73 mph) are expected somewhere within the specified coastal area within 36 hours.

Hurricane Watch:

This indicates the possibility that you could experience hurricane conditions within 36 hours.

Hurricane Warning:
This indicates that sustained winds of at least 74 mph are expected within 24 hours.

Extreme Wind Warning:

 Extreme sustained winds of a major hurricane (115 mph or greater), usually associated with the eyewall, are expected to begin within an hour.

Storm Surge:
Tropical storms and hurricanes push sea water in front of them. As the storm nears the coast or makes landfall, this wall of water is pushed ashore.  The height of the surge is dependent on the depth of the coastal waters, tidal cycles, and the velocity of the wind. A category 5 hurricane can produce storm surges  of over 20 feet. However,  Katrina made landfall as a  Category 3 (she had been a Cat 5 a few hours before landfall) produced a surge of 28 feet, that overwhelmed the levee system surrounding New Orleans in 2005.  The highest storm surge ever recorded was 42 feet, this occurred in 1899 in Australia.

Consensus Model:
This is a computer model that   looks at information from numerous models of a variety of types to create its own forecast. Many feel this is the most accurate way to forecast the track of a hurricane.

Spaghetti Models:
There are numerous computer models that aid in the forecasting the track of the storm.  Every one of them is very good. However, these models take into account many variables. So if you're looking at a three to five day or even beyond. The models can have huge variants between them.  When the models forecast tracks are placed on one image, it is often called a spaghetti model. This is based on how each track looks like a piece of spaghetti.

Cone of Uncertainty:
Average error in track forecasting over the past several years is at about 75 miles for 24 hours, meaning the "cone" will be 150 miles across. At the 120-hour forecast, the average error is 300 miles. It is important to realize that sometimes the actual forecast scenario may be more or less accurate than the historical error cone.

Synoptic Track:
This is a weather reconnaissance mission flown to provide vital meteorological information in data sparse ocean areas as a supplement to existing surface, radar, and satellite data. Synoptic flights are a better way define the upper atmosphere and aid in the prediction of tropical cyclone development and movement
Rapid intensification (also called explosive deepening or rapid deepening):
This is when there is a rapid decrease in the central pressure of a tropical storm or hurricane. The National Weather Service states it is a decrease of 42 millibars of pressure in less than 24 hour.  But normally the term is applied to any tropical systems that has seen a drastic decreases in pressure over a short time span.

Fujiwara Effect:
When two tropical cyclones are in close proximity to each other, they rotate cyclonically about each other. Meteorologists call this dance the "Fujiwara Effect".

The Saffir-Simpson Scale:

As we saw in 2007, when the Fujita Tornado Intensity Scale as converted to the Enhanced Fujita Scale. So too has the Saffir-Simpson Scale. It was modified and simplified back in 2010. It will undergo another minor modification this year. They have increased the Cat 4 rating by 1 mph. This was done to make it easier when converting among the various scales. As is the case with the EF scale, this change will not alter any category assignments in the past.

Category 1,  sustained winds of 74-95 mph.
Category 2,  sustained winds of 96-110 mph
Category 3,  sustained winds of 111-129 mph
Category 4,  sustained winds of 130-156 mph
Category 5,  sustained winds of 157 mph or higher

A PDF explaining the change can be found here.

This year's list of hurricane names:

2012  Eastern Pacific:


2012 Atlantic:


Well That's about it. I hope this helps you navigate all the terms and helps you understand the differences between the different kinds of storms. Any question you may have about something covered here or something not covered, I will be more than happy to answer them.


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