The Saharan Air Layer and Hurricanes

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

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 et.al., 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

Floods, High Winds, Quiet in the Tropics!

High wind warnings are in effect for Southern California the next couple of days. Warm temperatures, low humidity and highs winds is a bad combination during fire season. Here’s the latest forecast from Los Angeles.

Central Mississippi, Alabama and Georgia continue to be hit by flash flooding. Once again, you can click on ABC Birmingham for the very latest in radar reports, warnings and the current weather forecast from the local television affiliate. I also have a link to the NBC Atlanta Forecast where heavy rain continues this Monday.

Oklahoma today is under a slight risk of severe thunderstorms. Watch FOX Oklahoma City later this afternoon for the latest watches and warnings.

The National Hurricane Center continues to predict no tropical development in the next 48 hours. The one tropical wave I was watching, Invest 98L, is showing a little bit more organization this morning. Currently, it’s just an area of showers and thunderstorms out in the middle of the Atlantic. Dry air over the past couple of days has been getting mixed into the area of disturbed weather, and kept it from developing into a tropical system. No matter what, it won’t affect the US. My previous blog has more details on the dry air that’s been affecting Invest 98L.

-Dawn Brown

A New Invest In the Atlantic

Invest 98L (courtesy NOAA) Click to Enlarge

The National Hurricane Center’s hurricane models are trying to develop a tropical wave out in the Atlantic off the coast of Africa. It is currently called 98L. (You can always click on the image to make it larger.) I think it’s a good chance this system could develop into another tropical system based on warm sea surface temperatures and favorable wind shear where it’s currently located. However, as it continues its westward migration, it will run into a good bit of wind shear,

Wind Shear (Courtesy NOAA - Hurricane Research Division)

and that will cause problems for the developing storm. As I’ve mentioned in the past, global computer models have a tough time forecasting the strengthening of a storm, the track forecast is more reliable. And the track forecast has this system turning north in the middle of the Atlantic, if it does develop. With the current weather pattern,  upper level troughs spinning off the East Coast of the United States, these storms are being turned north before they can come close to the United States this year. Tomorrow, I’ll explain what an upper level trough is… and why it’s helped us out this year!

These troughs have also created a cool summer for a good section of the country.

Dr. Jeff Masters from Weather Underground, looked at the climate data from this past summer, and had some interesting information about our summer.

Here’s the script from his blog:

“A cool August and cool summer for the U.S.
For the contiguous U.S., the average August temperature was 0.6°F below average, making it the 30th coolest August in the 115-year record, according to the National Climatic Data Center. The U.S. as a whole was below normal for the summer period (June – August). A recurring upper level trough held the June – August temperatures down in the central states, where Michigan experienced its fifth coolest summer, Wisconsin, Minnesota, and South Dakota their seventh coolest each, Nebraska its eighth, and Iowa its ninth. In contrast, the temperatures in Florida averaged out to be fourth warmest, while Washington and Texas experienced their eighth and ninth warmest summers, respectively.

U.S. precipitation in August was below average, as the month ranked 28th driest in the 115-year record. Arizona had its fourth driest August, New Mexico its fifth, and it was the eighth driest August for Colorado, Utah and Texas. Arizona observed its third driest summer, while both South Carolina and Georgia had their sixth driest. It was the 8th wettest summer on record in the Northeast.

At the end of August, 13% of the contiguous United States was in moderate-to-exceptional drought. This is a drop from the 19% figure observed at the beginning of the year. These extreme drought regions were exclusively in South to Central Texas. However, significant drought relief occurred in this region the second week of September, when a large area of tropical moisture settled in over the region, bringing heavy rains. About 19 percent of the contiguous U.S. fell in the severely to extremely wet categories in August.”

The link for Dr. Jeff Masters Wunderground Blog is on the left side of my screen. I’m sure he’ll have something about Invest 98L later today.
I’ll talk to you tomorrow! -Dawn Brown

I Like the New NHC Homepage

Tropical Storm Marty (courtesy NOAA)

Tropical Storm Marty, left, is losing it’s punch in the Pacific, and steering currents, are turning it away from Baja California. The steering currents in the Pacific Ocean are one of the main reasons the West Coast of the United States is spared from tropical systems. When these storms form in the eastern Pacific Ocean, the storm will start it’s move toward the north, northwest, but eventually it will turn out to sea. (Note, one hurricane computer model, strengthens the storm, and takes it toward Baja, but NHC forecasters are not buying into that.) Read the NHC discussion.

I just noticed something really cool about the National Hurricane Center’s page this year. Last year, they started posting a tropical outlook image on their front page, like the image in this post. As you can see, today, it says, “Tropical Cyclone Activity is Not Expected During the Next 48 Hours.” If there is a tropical system, they will have the icon for it posted on the page, and then usually they have a couple of shaded circles on the page indicated areas of possible development. The circles are either yellow (low chance), orange (medium chance) or red (high chance) of development. This gives the internet user a quick glance at the tropics, and the areas of disturbed weather the NHC forecasters are looking at for development into tropical systems. THIS YEAR, they’ve started giving you the latest stats on the storm right below the name of the storm. For Marty, it looks like this:

8:00 AM PDT Thu Sep 17
Location: 19.7°N 113.1°W
Max sustained: 45 mph
Moving: NNW at 2 mph
Min pressure: 1003 mb

I love this feature. Before, you had to pull up the advisory or discussion, and it was in somewhere in the middle of the discussion. This makes it a lot easier to track these storms on your hurricane map!

I’m working on behind the scenes stuff today, trying to get my KIDS STUFF category updated, and my frequently asked questions link up and running. Hope you have a great day!

-Dawn Brown

National Hurricane Center

Watching the Gulf… Heavy Rain Expected

Heavy Rain Moving to Texas/Louisiana

The National Hurricane Center has now noted that area in the western Gulf of Mexico with the potential for development. Currently it has a less than 30% chance of development in the next two days. The NHC gives tropical waves and disturbed weather either a low, medium or high chance for development.  Over the next few days it’s going to encounter some high wind shear as it remains stationary in the western Gulf. Both Texas and Louisiana are expected to see some high rainfall numbers even if the showers and thunderstorms remain unorganized.

Just because I’ve tracked down all the links to all the TV stations across the US, let’s take a sneak peek at what folks in Corpus Christiare saying about this area of showers and storms! By the way, the link takes you to the weather page, then look for the box that says video forecast. On this website, scroll down past the 7-day forecast.

If you want to see what the global computer models have to say, follow my link for Tracking Hurricanes.

Talk to you tomorrow… Dawn Brown

P.S… I’m not really following Fred anymore. He’s losing steam, and will decay in the next couple of days. Plus, he’s way the heck out in the Eastern Atlantic!

Fred On a Downward Spiral… Heavy Rain in the Gulf

Fred Satellite Image ( Courtesy NOAA)

Hurricane Fred has now weakened to a category 2 hurricane, and will further weaken in the next few days.  Yesterday, I was talking about the wind shear the storm was headed into, and it has already weakened the southern portion of the storm. The storm was never a threat to the US mainland, but now it looks like it will decay faster than expected.

There is a lot of curiosity about some showers and thunderstorms in the western Gulf of Mexico. In the next couple of days, watchful eyes across the Gulf will be on this area of rain and thunderstorms to see if a low level circulation develops.  Watch this forecaster from Houston on the developing low.

No matter if it becomes tropical or not, Texas and Louisiana will get soaked Saturday into Sunday. Ever since Humberto and Claudette’s rapid development in the Gulf, forecasters keep a close eye on these persistent areas of showers and thunderstorms so we’re not caught off guard if it develops into a tropical system, especially since it’s so close to land. In the Gulf of Mexico there’s plenty of warm water, so if a low does develop, it can rapidly. At the same time, the computer models don’t have a great handle on this system.

If you really want a techy explanation of the different computer model solutions, check out Crown Weather’s blog today.

Also, both Max Mayfield, former NHC director, and Jeff Masters from Weather Underground have some interesting statistics on this year’s hurricane season if you want to check out their blogs.

I’ll talk to you later!

-Dawn Brown

Tropical Storm Fred

UPDATE:  Tropical Storm Fred is on the verge of hurricane status, and may increase in intensity overnight. Reliable computer models continue to keep it in the middle of the Atlantic, far from the US mainland, and at some point degenerating in the 5-day forecast.

Tropical Storm Fred is currently in a healthy environment for growth, and may end up being the next major hurricane of the 2009 season in the next couple of days. Upper air steering currents are expected to keep the storm in the middle of the Atlantic Ocean Basin well away from the United States. This year, upper level troughs traveling across the continental United States have dominated the tropical weather pattern on the East Coast, forcing these storms on a more northerly path once the storm comes close to the US mainland. Most of the models agree on this track for Fred. Click here to track Fred.

While we like to see these upper level troughs send these storms to the middle of the Atlantic, there is another variable that these troughs and associate cold fronts produce. Once the tail end of the cold front moves into the Gulf of Mexico or off the East Coast, often the front stalls and a circulation can develop along the decaying front. This circulation can allow a tropical system to develop. At this time of year, when a cold front stalls over the Gulf or Atlantic near the coastline, meteorologists watch it closely for possible development. At least one model suggests this may happen in the Western Gulf of Mexico in the next 10 days. We will be watching for that. If you are interested in discussing Fred, check out the MYL Forums.

Also, I have Jeff Masters Weather Undergound blog in my left hand menu bar under HURRICANE BLOGS.  In the next few weeks, he’s introducing a series of blogs on storm surge.  Storm surge has been one of the least understood components of a hurricane’s landfall, but it can be the most destructive component along the Gulf of Mexico. More and more researchers are trying to introduce computer models to give viewers an idea of how storm surge will effect their community during a landfalling hurricane. Storm surge is very localized, meaning the impact is different depending on the terrain where you live. Last year, during Hurricane Gustav, meteorologist Mike Koziara at the National Weather Service in New Orleans briefed me on the storm surge model they run. Koziara explained that terrain factors such as railroads, levees, canals, and the  coastline underwater were all included in the storm surge model. If the storm moves 100 miles to the east or west, this model won’t work, because the terrain is completely different. So, it is an exciting prospect that researchers are tackling this aspect of forecasting a landfalling hurricane.

I just got back from a dive trip on San Clemente island in California. What a great experience! Since Fred is alive and well in the Atlantic, I’ll have another update later tonight.

-Dawn Brown

Quiet Weekend… The Coolest Feature On My Site (I think… scroll down)

Quick Update: The low level circulation has disappeared on the East Coast, and now the only concern for NHC forecasters is the wave off the coast of Africa. Long term models do not bring it close to the US.

The tropics are quiet, the wildfires in California are no longer threatening homes and about 50% contained, and we are entering what is supposed to be the busiest part of hurricane season. This is a great Labor Day Weekend.

Taking a look at the infrared satellite imagery (courtesy NOAA) off the coast of Africa, there is one area of disturbed weather the National Hurricane Center has deemed 95L, but it doesn’t look to develop. (The forums are active with chatter, so check out MYL Forums to get in on the discussion.) There’s a second tropical wave right on the coast (see the blow up of reds and yellows right near the coastline?). Computer models have this wave developing in the long term outlook, but then turning north in the middle of the Atlantic. As I say this, computer models have huge problems with forecasting not only a storm’s development, but it’s intensity, especially if the storm hasn’t formed a low level circulation.

Meanwhile, take a little time to get to know myweatherlady.com. In the navigation bar above, there is a section about me, my bookmark links (I save my bookmarks at a website called delicious.com. Not all of my web links are there yet, but it will have a complete list of all the weather websites I use eventually.) Just to the right of the bookmarks, I have contact info. Currently, the only way to contact me is through this website or at my email: dawn@myweatherlady.com.

On the left hand side, there is a list of all my blogs organized by categories. Currently, the categories I have are HOME (listing all the blogs), FORECAST TODAY! (to help forecasters who move around the country adjust to their new gig), FORUMS, and TV WEATHER (my blogging about the business of TV Weather, etc.). I want to add a category for KIDS as well as for Freqently Asked Questions. I’m working on those as well as continuing to add links and blogs to the categories I just listed. Below the categories, you will find links to the best hurricane blogs from around the country (My favorites are Jeff Masters from Weather Underground, and Surfline.com forecasters–but all are extremely valuable so you don’t miss something in your forecasting), as well as other weather bloggers below that.

Meanwhile, on the right hand side, you can keep up with me through twitter, check out the latest comments on my website, go visit my t-shirt shop(there’s stuff for kids, pets and people there). Scroll down to the GET YOUR LOCAL TV FORECAST! link. I think this is the COOLEST feature on my website. When you go to this link, you will find a list of all the TV stations across the country listed by state and town. Click on the town you live in or the town you are visiting, and you will find the local weather guy or gal giving the forecast. I love it! Check out David Bernard at the CBS in Miami, Jackie Johnson at KCAL in LA, or Steven Douglas in Fairbanks, Alaska. (I just randomly chose him.)

I will be gone for the next 36 hours on a scuba diving trip, so I will not update on Monday.  To track hurricanes while I’m gone, visit the hurricane blogs, or go to the weblinks I’ve set up under Tracking Hurricanes.

-Dawn Brown

Tips for the Gulf South TV Forecaster

Moving to a new TV market is tough. I’m putting together some of the most important websites you’ll need to hit the ground running. Good Luck! Since the Gulf South is driven in a large part by the tropics during the summertime, you will have many days of frustation before you begin to learn the patterns and the different small scale boundaries that can develop.  Severe weather develops rapidly, especially in the fall, winter and spring. Even a summertime thunderstorm can cause flooding in the streets. Of course snow is not a common occurence, but if you get all the right ingredients, you will have a miracle snow day during a cold winter.  

 Looking at the radar and satellite, (if your shop doesn’t have what you need, go to the National Weather Service Eastern Region Headquarters and Aviation Weather) to get a general idea of the weather pattern.

Use your synoptic scale models (large scale models) to get storm placement, flow patterns, and 850 mb temperatures, and precipitation amounts (this is available at Unisys Weather), and make a short-term forecast. Look at atmospheric forecast soundings  site, especially if thunderstorms are expected, to determine severity. Make sure you visit the Storm Prediction Center to see if you are in a risk category for thunderstorms.

Generally, your local NWS office can give you the risk of inland flooding due to repeated storms, but here’s a link to the Hydrometeorological Prediction Center – Inland Flooding in case that is a concern during repeated storm events or a tropical storm has moved inland.

Tides are extremely important for coastal communities, here’s a link to Tide Predictions for Gulf States. Marine and boating forecasters also generally use buoy data. NOAA Buoys Gulf of Mexico

Make an extended 7-day forecast using the latest and most complete suite of extended model runs (e.g. the GFS), available at NCEP Model Analyses and Forecasts.

Also, you’ll need a temperature forecast to compare all of your graphical data against for a highs/lows forecast.  The best place to find MOS (Model Output Statistics) is Current NWS MOS Forecast Products.

When it comes to tropical weather, which can also often be a player, things can get even more interesting, here’s a link to my blog on Tracking Hurricanes.

Oh! One added note, sea fog can develop in the wintertime along the Gulf South. The NWS Office in New Orleans has a decision tree Sea Fog Forecasting to help you forecast a sea fog event in the wintertime. It’s fascinating!

Meet Dr. William Gray, Hurricane Forecaster

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 17^th 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.