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Field Trip to the National Hurricane Center

posted on Sunday, June 27th, 2010 at 3:08 pm
Senior Hurricane Specialist Dr. Lixion Avila Studies Tropical Storm Alex

Senior Hurricane Specialist Dr. Lixion Avila Studies Tropical Storm Alex

Field trip day! About 100 or so television  meteorologists, including Bob Breck, my fiance Jonathan Myers and I, boarded a bus to the National Hurricane Center in Miami yesterday.

Bob Breck and Dawn Brown, FOX 8 New Orleans, National Hurricane Center

Bob Breck and Dawn Brown, FOX 8 New Orleans, National Hurricane Center

It was exciting for two reasons. One, I’ve never been there. And, two, there was a tropical storm in the Caribbean Sea. Alex doesn’t look like it will be much of a threat to New Orleans when it reemerges in the Bay of Campeche in the southwestern Gulf of Mexico. But, we don’t take anything for granted until a tropical system has dissipated. This was going to give me a chance to discuss the storm with the experts at the hurricane center.

When you enter the NHC, there’s a big media room to brief reporters during hurricane season. They now have the hurricane specialists separated from the media by glass walls. I can’t imagine it before they put in the glass. The hurricane specialists are trying to study all this satellite and storm data, and they have these cameras over their shoulder, studying them! Now, only one or two cameras are allowed in the media room during a land-falling hurricane.

National Hurricane Center

National Hurricane Center

Behind the glass walls, hurricane specialists track tropical disturbances in both the East Pacific and Atlantic Ocean basins. Today there were 3 tropical cyclones. Hurricane Darby and Celia in the East Pacific and Tropical Storm Alex in the Atlantic.

Senior Hurricane Specialist Dr. Lixion Avila was the meteorologist tracking Alex. A Cuban native, he was issuing watches and warnings for Mexico, Belize and Honduras, the countries affected by the storm on Saturday.

There were 3 other meteorologists forecasting marine advisories for shipping in the Atlantic and Pacific Ocean basins. What an enormous area to cover. The hurricane center also has storm surge specialists, researchers, etc. All in all, they don’t have a huge staff. The hurricane specialists appear to be a tight knit group.  It makes sense since they have to work closely together over a period of days while they are watching a storm.

I’ll add a little bit more about my visit to the Hurricane Research Division when I update this tomorrow.

Here’s the latest track forecast for Alex. -Dawn Brown

Alex 5-Day Forecast, Image: NOAA

Alex 5-Day Forecast, Image: NOAA

Less Confusing Saffir-Simpson Scale Released

posted on Friday, February 19th, 2010 at 9:21 am
Hurricane Ike, Image: NOAA

Hurricane Ike, Image: NOAA

When Hurricane Ike slammed into the Texas coastline in September 2008, hurricane forecasters rated it a category 2 hurricane on the Saffir-Simpson scale with maximum sustained winds of 110 miles per hour. The storm surge associated with Ike topped 15-20 feet. If you used the Saffir-Simpson scale as a guide, the storm surge associated with Ike should have been about 6′. (See the graphic below.) The mismatch between the wind speeds generated by the storm and the associated storm surge have led the National Hurricane Center to drop storm surge from the widely used scale.

Old Saffir-Simpson Scale, Image: NOAA

Old Saffir-Simpson Scale, Image: NOAA

The Saffir-Simpson scale was developed in 1971 by engineer Herbert Saffir and National Hurricane Center Director Bob Simpson to help forecasters explain the potential damage from a hurricane to the public. It was mainly designed as a way to explain what damage would occur to structures if a hurricane hit with 75 mile per hour winds, 95 mile per hour winds, 110 mile per hour winds, etc.  After the scale was developed, forecasters realized storm surge was a significant part of the damage from a hurricane, and they tried to come up with the height of the storm surge associated with different wind speeds. Unfortunately, it has led to errors in forecasting and misleading the public. The effect of storm surge on a coastline depends on the strength of the winds, the direction of the storm, the topography of the coastline, and the bathymetry (or topography of the coastline under the water.) Now, individual weather service offices, as well as universities, produce individual storm surge forecasts for each storm depending on the strength of the storm and its track. Here are the guidelines given today by the National Hurricane Center for the now called Saffir-Simpson Hurricane Wind Scale:

Category Winds Effects
One 74-95 mph No real damage to building structures. Damage primarly to unanchored mobile homes, shrubbery, and trees. Also, some coastal road flooding and minor pier damage
Two 96-110 mph Some roofing material, door, and window damage to buildings. Considerable damage to vegetation, mobile homes, and piers. Coastal and low-lying escape routes flood 2-4 hours before arrival of center. Small craft in unprotected anchorages break moorings.
Three 111-130 mph Some structural damage to small residences and utility buildings with a minor amount of curtainwall failures. Mobile homes are destroyed. Flooding near the coast destroys smaller structures with larger structures damaged by floating debris. Terrain continuously lower than 5 feet ASL may be flooded inland 8 miles or more.
Four 131-155 mph More extensive curtainwall failures with some complete roof strucutre failure on small residences. Major erosion of beach. Major damage to lower floors of structures near the shore. Terrain continuously lower than 10 feet ASL may be flooded requiring massive evacuation of residential areas inland as far as 6 miles.
Five greater than 155 mph Complete roof failure on many residences and industrial buildings. Some complete building failures with small utility buildings blown over or away. Major damage to lower floors of all structures located less than 15 feet ASL and within 500 yards of the shoreline. Massive evacuation of residential areas on low ground within 5 to 10 miles of the shoreline may be required.

The Saharan Air Layer and Hurricanes

posted on Tuesday, September 29th, 2009 at 5:52 pm
Saharan Dust, Image Credit: NASA

Saharan Dust, Image Credit: NASA

Meteorologists studying hurricanes are still trying to understand the Saharan Air Layer, or SAL, and its affect on hurricanes. On the one hand, scientists believe the SAL triggers the formation of easterly waves, and they know hurricanes form from easterly waves. But, on the other hand, the SAL is composed of hot, dry air. Hurricanes don’t like dry air. Studies have shown this. (Chris Landsea, NOAA) So, after the wave forms, the SAL can help kill the developing storm by injecting dry air into it.

Now, there’s a new question: What is the effect of the dust outbreaks that often occur and travel with the Saharan Air Layer? (The image above captures a dust outbreak in February 2000.)

After the record-breaking 2005 hurricane season, an international group of scientists came together to study these easterly waves coming off the coast of Africa. During the 2006 season, research aircraft based on the Cape Verde Islands, flew into these waves to study their development into tropical cyclones. As I tell you this, you may remember that the 2006 year was a bust for hurricane forecasters-it was an uneventful season. Many blamed a late developing El Nino. These researchers are blaming the SAL and the associated dust outbreak.

In the new issue of the Bulletin of the American Meteorological Society, authors Zipser et al. investigate, “The Saharan Air Layer and the Fate of African Easterly Waves”.

The SAL is composed of at least 3 layers:

1. The top layer: hot, dry air from the Saharan Desert 2. The mid-bottom layer: cool, moist air from the Atlantic coast. 3. Underneath this:  cool, dense marine air near the ocean surface.

Dust is not on my list. That’s because the SAL and the associated windy conditions can cause a dust outbreak, and then the dust will travel with the SAL. But, this is not always the case. The SAL is also associated with two other important features of hurricane forecasting; the easterly wave and the easterly jet. (The jet is an area of strong winds that is shown to negatively impact tropical storm development. – Chris Landsea, NOAA)

The majority of hurricanes in the Atlantic Ocean come from African easterly waves. (Keep in mind that about 60 waves form each year, and only a small number become tropical storms. – Chris Landsea, NOAA) In the article, Zipser et al. writes, African easterly waves “… originate over the African continent, with their initial growth fueled by the temperature contrast between the hot air over the Sahara desert and the relatively cooler, humid air to its south.” This difference in temperatures creates instability or rising air all summer long.  The rising, unstable air creates showers and thunderstorms. The thunderstorms then move off the coast of Africa toward the Atlantic Ocean, and then westward toward the United States, because the predominant wind flow in this area is east to west.

African Easterly Wave, Image Credit: NASA

African Easterly Wave, Image Credit: NASA

So, you have an easterly wave, simply put; an area of  showers and thunderstorms that persists as it is carried along with the predominant wind flow from east to west across the Atlantic Ocean. These are the tropical waves or areas of disturbed weather that TV forecasters are always pointing out on the screen saying, “This is an area of potential tropical development.” National Hurricane Center forecasters usually give these waves a low (less than 30%), medium (30-50%), or high (50% or greater) chance of developing.

In the images to your left, an easterly wave develops on the African continent, begins moving westward off the coast, and then starts to rotate.

In the first image, the big area of white clouds tells you there is a mass of unstable air that is forming showers and thunderstorms. (Really white images on satellite tell you you’re looking at high cloud tops, so most likely you are seeing thunderstorms.)

In the second image (6 hours later), the showers and thunderstorms begin to move westward toward the coast.

In the third image, the area of showers and thunderstorms, or African easterly wave begins to rotate. This tropical wave would later become Hurricane Irene in 2005. It did not affect any land areas.

Now that we’ve talked about what an easterly wave is, and how it is the starting point for a majority of hurricanes, let’s get back to the SAL and it’s impact on easterly waves.

In the year 2006, the year these scientists launched their research, the Saharan Air Layer filled the Atlantic. (It can grow as big as the continental United States.)

The images of the Saharan Air Layer in this blog are similar to what it looked like it 2006. At times, it stretched from the African Coast to the Caribbean Sea. In their research, Zipser, found a couple of items of note. First, global computer models underestimate how dry the air is in the mid-levels in the SAL. Computers estimated the relative humidity is about 40-50% higher than it actually is. Remember, hurricanes do not like dry air. That data supported other papers that show the SAL can inhibit the development of a tropical storm or hurricane.

But, what was more interesting was what they witnessed from the dust. While flying through an easterly wave, instruments showed a, “… deep dust layer up to 4 km altitude.” (p. 1148) Researchers noted that this area of high dust concentration was “highly electrically active”, and it made them question whether the dust actually aided in the development of thunderstorms.

Unfortunately, the article has no conclusion regarding the impact of the dust and the development of tropical cyclones.  The authors hope that their data will aide researchers in the future who hope to answer the question conclusively.

-Dawn Brown

Saharan Air Layer, 2001, Image Credit: NOAA

Saharan Air Layer, 2001, Image Credit: NOAA

The Pacific is Up to “N”, More Snow in the Rockies?, Dr. Gray’s New Forecast

posted on Wednesday, September 23rd, 2009 at 10:49 am

Tropical Storm Nora has formed in the Pacific Ocean. It is well away from any landmass, and will not affect the US, including Hawaii. Cold air, especially for this time of the year, is trapped in the Rocky Mountains. More snow is possible there.  Denver forecasters have the latest. Dr. Gray, the renowned hurricane forecaster, whose specialty has been forecasting long range hurricane outlooks, (producing a forecast in December for the following summer) is now forecasting just the next two weeks. Click Read More…

Meet Dr. William Gray, Hurricane Forecaster

posted on Saturday, September 5th, 2009 at 12:26 am

I met Dr. William Gray when he was in New Orleans for the American Meteorological Society annual meeting in January 2008. While we were walking along the infamous site of the 17th Street Canal levee break in Lakeview, a tour bus stopped to point him out. He waved, getting quite a chuckle out of his renown here along the Gulf South for his yearly seasonal hurricane outlooks. In Fort Collins, Colorado, where he lives and does his research, he was always known as the Mayor’s husband. His wife Nancy (now deceased) was the one people were stopping to greet.

He wanted to be famous when he was young, but for slinging a baseball. “I wanted to be a pitcher,” Gray said.

His baseball dreams stunted by a knee injury, Gray graduated from George Washington University with a degree in geography. He was working on his masters when World War II intervened. Like other meteorologists of his generation, he was trained to forecast weather during the war. Gray was stationed in the middle of the Atlantic on the Azores, an island chain 900 miles off the coast of Portugal, providing forecasts for the Trans-Atlantic flights. At that time, there were no satellites and no computers. “I got a lot of good weather experience.”

At the end of the war, Gray decided to continue his career in meteorology under the tutelage of Dr. Herbert Riehl in Chicago, whom Gray calls the most prominent tropical meteorologist of his time. “The new National Hurricane Research Project had just been formed,” and so in 1958, Riehl and Gray began flying into the center of hurricanes. Dr. Gray wrote his Master/PhD thesis from the flight data gathered during these flights into the center of the storms, describing the internal structure of these storms.

In 1961, Dr. Herbert Riehl moved to Chicago, and offered Gray a job in his department at Colorado State University. Still based far from hurricane country, Gray began spending every summer in Florida, chasing hurricanes. But, there was one big problem. “We would go to Florida every year and wonder was this going to be an active season?”

Dr. Gray states the Atlantic Ocean Basin has the largest year to year variability when it comes to tropical cyclones. “Some years there just weren’t many storms, other years a whole lot of storms and the question is we couldn’t tell before the season. It was completely random.” Gray told me.

In the early part of the 1980s, after Dr. Gray had been flying into storms for 3 decades, he noticed a parallel between two data sets he had collected: the formation of El Nino in the Pacific Ocean and the lack of tropical cyclones in the Atlantic Ocean Basin. During an El Nino year, there would be less tropical cyclone activity in the Atlantic Ocean. The term El Nino refers to the periodic warming of the Pacific Ocean off the coast of South America. For years, it’s mainly been the bane of the Western United States, because it can lead to flooding rains and mudslides. But, in the Atlantic, it leads to wind shear, and wind shear is bad for hurricanes. (Wind shear is a change in wind direction or speed with height.)

When Dr. Gray made the discovery back in the 1980s, it didn’t receive much attention because there weren’t very many storms. But in the last 15 years, the Southeastern United States has been hit by one devastating storm after another. And populations across the Eastern United States and Gulf of Mexico began waiting for his predictions and whether it spelled another active year, or a relatively quiet season. Nowadays, most of his research is done by Dr. Phil Klotzbach, the main author on the seasonal outlooks. Their research and outlook forecasts are much more complicated, based on worldwide weather patterns, and ocean temperatures around the globe.

And, he’s got competition. Forecasters at North Carolina State, the Weather Research Center in Houston, and European forecasters all put out a seasonal hurricane forecast, among others.

However, the official hurricane outlook published by NOAA at the beginning of hurricane season still most resembles the forecast parameters discovered by Dr. Gray.

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