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)


Hurricane:

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.

Eye:

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

Eyewall:

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:

Aletta
Bud
Carlotta
Daniel
Emilia
Fabio
Gilma
Hector
Ileana
John
Kristy
Lane
Miriam
Norman
Olivia
Paul
Rosa
Sergio
Tara
Vicente
Willa
Xavier
Yolanda
Zeke



2012 Atlantic:

Alberto
Beryl
Chris
Debby
Ernesto
Florence
Gordon
Helene
Isaac
Joyce
Kirk
Leslie
Michael
Nadine
Oscar
Patty
Rafael
Sandy
Tony
Valerie
William

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.

Rebecca  

The 2012 Summer and Hurricane Outlook

Hi, it's Rebecca here with another blog installment. This entry will look at some of the parameters that will shape the Summer of 2012; it will also touch on the upcoming hurricane season. I will try to keep it general and also leave the math out of it. I will just mention percentages along with a general overview of where things stand and look to be heading.  However before I get into the Summer outlook. I want to look back on the Winter of 2011-2012 and what the jet stream is.

What is the Jet Stream:

I've touched on jet streams in past blog post, however, I will give a brief explanation on what the jet stream is.

 Jet streams are bands of strong westerly wind around the 300 mb level of the upper atmosphere. This would place it above 20,000 feet, or between 6 and 9 miles above the Earth's surface. The jet marks where the strongest temperature contrast can be found at the surface. Jet streams are formed by the temperature differences between two air masses. These are the  warm and cold air masses, along with areas of high and low pressure you hear about.

The  polar jet stream is the one that has the most impact on our weather in the U.S. It marks the divide between cold arctic air and mild mid-latitude air. Because its position is a result of temperature contrasts, it's position shifts throughout the year, depending on the season. In the U.S. the jet pushes weather systems from west to east.

The winter of 2011-2012:

I'm sure most of you will agree that the past winter was quite unusual. There were quite a few underlying reasons for this. However, the primary reason was the position of the polar jet stream. Below are a couple of images of the jet. The first image shows how the polar jet behaves on average during the winter. The second shows how the polar jet more or less looked like during the winter of 2011-2012; outside a couple of dips south, it more or less stayed in this configuration for most of the winter.

                                                                                  Image 1

   

                                                                                   Image 2

The reason I'm showing you this is to show the impact the jet has on any season. As for winter 2011-2012, the pattern of the jet, locked the cold air up in Canada. Therefore, the lower 48 saw the 4th warmest winter on record.

The jet stream during the summer:


Meteorological Summer is almost here. During mid to late spring temperature and precipitation patterns start to get locked in. As I said above, the jet plays a very important role in how the summer season will behave. During a typical summer the polar jet will move back to the north. Image 3 shows how the jet looks on average during the summer.  

                                                                                     Image 3

During the summer the temperature difference is less extreme between the air masses and the jet stream is weaker.  When the jet is in this configuration, the warm air surges northward. During the summer, the jet orientation often prevents the large scale synoptic storm systems we see during the winter. As a result, our rain in the Northeast is often the result of short-waves and cold fronts. This is why the summer months are normally drier than the other seasons of the year.

La Nina is dead, but El Nino is stirring:

El Nino is defined as warmer than normal sea-surface temperatures (SSTs) in the tropical Pacific ocean.

La Nina is the opposite with colder than average Pacific SST's.

A large change in the ocean temperature patterns is underway in the equatorial waters of the eastern Pacific Ocean, The SST has warmed to the point that in April, La Nina was officially declared to be over. Presently the SST's in the Pacific are near average. This would  place us into a El Niño/La Niña-Southern Oscillation (ENSO)  neutral state. Many are basing their summer outlooks on the idea of the ENSO being/staying neutral.  However there are model trends that are showing the ENSO heading into El Nino. Based on everything I've seen, I feel we will be ENSO neutral for the rest May into the first part of June. Then for the rest of June onward we will under El Nino conditions. Who is right will have huge impact on how the summer and hurricane season will play out.  I think the summer will feature a weak El Nino perhaps becoming a bit stronger heading into next winter. Now that I've set the stage let's move on to the Summer outlook.    

The Summer of 2012:

My summer outlook will be based on the idea of a weak  ENSO along with the  recent temperature/precipitation trends across the Northeast.  Before I begin, it's important to say.....a seasonal outlook is just an educated guess ...and as we saw with my winter outlook....they can go wrong. This is not a forecast per say. Instead it's an attempt to see how the general temperature/precipitation should go. It will not talk about individual areas/municipalities. Also, it will not go into detail about the severe threat over the summer.

Precipitation:

As most of us are aware, the Northeast is having precipitation issues. In-spite of the recent few weeks of rain, many of us are still experiencing mild drought conditions ( 2.5 to 4 inches below normal, with more than that in spots ). I think the current tread of a few days of rainy weather followed by a week or so of drier weather will continue. Therefore, drought conditions will most likely get a bit worse as the Summer swing into gear. However, I see no reason to fear an extreme drought. That said, some areas will have a bigger drought problem than others.

                                             
                                                         Image 4 Northeast drought conditions

Temperatures:

The lack of prolonged heat in the Northeast has been an issue the last 4 to 6 weeks. This has been because of   blocking over Greenland or northern Canada. I think this pattern will continue into at least the first part of summer. If we don't see a lot of heat before the El Nino starts to get its act together we might not see much in the way heat. I'm not saying there won't be a few days were the temps get very warm. But the kind of heat-waves we've seen over the last few years will be  less common. Any severe weather we see this summer will be during those days where we see a bit of warmth. The storms could involve low level convection and be more cold core in nature

So, in general this summer looks like it will feature slightly above (above normal temperatures for some) Along with a chance for below normal precipitation for a big chunk of the Northeast.

 The 2012 hurricane season:

 

The Atlantic hurricane season is officially from 1 June to 30 November.

A few things I took into consideration,

 

The ENSO pattern, as I said, right now the ENSO is neutral. However, I feel it will become a weak El Nino around mid summer.

Past years, I looked at several years that had very similar setups; the two that matched the closest where 1994 and 2006.

The Forecasted Sea Surface Temps (SST's) in the  Atlantic, Caribbean, and Gulf of Mexico

                                                                         Image 5 sea surface temps

As you can see for most of the SST's for most of the Atlantic, Caribbean, and GOM are near or slightly below normal, Now add in the wind shear from  a weak El Nino, and hurricane development should be below average.

La Nina not only effects areas like the Northeast; it also had a large influence on the last two Atlantic hurricane seasons . La Nina provides  more favorable wind conditions over the regions of the ocean where storms spin up. However, El Nino is just the opposite, the wind patterns are aligned in such a way that the vertical wind shear (change of winds with height) is increased over the Caribbean and Atlantic. The increased wind shear helps disrupt hurricane formation.

The 2010 Atlantic hurricane season was the third most active Atlantic hurricane season on record, with 21 tropical cyclones, 19 tropical storms, 12 hurricanes, and 5 major hurricanes.

2011 was not very far behind.  20 tropical cyclones, 19 tropical storms, 7 hurricanes, and 4 major hurricanes.

2012 and 2011 with their back-to-back 19 named storm seasons were well above average; both years stacked up to  97.9% above the 1950 – 2000 average of 9.6 storms per year.

A few statistics:

 During the past 114 years, no El Nino event has had more than one major hurricane making a  US landfall. 

A hurricane that makes at least one landfall or impacts the coast with hurricane force winds is considered a U.S hurricane. with this in mind, the mean annual number of U.S. hurricanes during El Niño years is 1.04 and 1.61 during neutral years. You can see there is a big difference.

Strike probabilities for this season:

(There is a difference between a yearly strike probability and a climatologically based  probability)

For a large part of the Northeast the stats are,

For one or more named systems making landfall it's 11.8%,  land falling hurricane is 7.0%, and  a major hurricane is 3.3%. Keep in mind this isn't just one state or area. For individual areas the odds go down. For example Maine has a 2.9% chance for a hurricane strike and <0.1% for a major strike.

How I see the numbers stacking up:

10 to 12 named storms

Four to five named storms will obtain hurricane status

One or two of those hurricanes will obtain major hurricane status (CAT 3 or greater).

Remember, while the stats show a much lower chance for a season like last year; it only takes one landfalling hurricane in your region to make things terrible. So  please take heed of any hurricane watches or warnings.

Hurricane Return Periods:

Hurricane return periods are the frequency at which a certain intensity of hurricane can be expected within a given distance of a given location.  For example, a return period of 20 years for a major hurricane means that on average during the previous 100 years, a Category 3 or greater hurricane passed within 58 miles of that location about five times.

I thought some of you would find these maps interesting. So I thought I would throw them on here.

                                                                      Image 6 (hurricane return period)

                                                                  Image 7 (major hurricane return period)

  

  

                                                                            Image 8 (Northeast strikes)

                                                                  Image 9 ( Atalantic basin coastal strikes)


National Hurricane Center


I hope you found this interesting and maybe learned something in the process. As always, feel comfortable to ask me any question you may have, either in the comments or as an email.

Rebecca