Though there will be no further tropical threats to the Gulf this year, I do see a potential stormy period for the northern Gulf late this month into early December.  Details are not clear yet, but I’m seeing strong indications of a storm system tracking across the Gulf Coast late this month, perhaps between the 28th-30th. The storm center should remain inland, but it could result in strong southerly winds late next week followed by a line of heavy squalls and strong wind gusts as the system moves eastward along the Gulf Coast.  This is a common fall pattern, but it’s a change from recent weeks.

In the very long range, we’re seeing a developing pattern that’s much like December of 1983 and 1989 across northwest Canada and Alaska.  In case you don’t remember, the Gulf Coast was hit by several very cold Arctic fronts in those years, dropping temperatures to the low teens (and lower) down to the Gulf Coast. It’s definitely quite a bit colder than normal across Alaska and northwest Canada right now, so we’ll have to watch out for southward push of some frigid Arctic air around the second or third week of December.  Such fronts can produce very strong winds offshore, along with quite cold temperatures.  In 1983, for example, the air was so cold that snow flurries were seen on offshore rigs off the upper Texas and Louisiana coasts.  I’m not predicting snow offshore, of course, but I do think some very cold air may impact the Gulf toward mid December.

Long-range models continue to forecast a southward dip in the jet stream over the coming week, bringing a series of very cold Canadian fronts southward to the Gulf of Mexico.  The first of these fronts arrives tonight.  These early-season cold fronts can produce quite strong winds offshore.   When cold air blows across the still very warm Gulf of Mexico, the wind can accelerate to speeds greater than tropical storm force with gusts approaching hurricane force.  Tonight’s front won’t produce hurricane force wind gusts, most likely, but it should produce wind speeds offshore above tropical storm force on Saturday.

Many years ago, I learned of a general rule of thumb to predict how strong the winds will get across the northwest Gulf following a frontal passage.   The key is to look at the high pressure center behind the front.    Air pressure can be measured in inches of mercury or millibars.  I know that the more meteorologically-correct term now is “hectopascals”, but I’ll never be converted.  To estimate how strong the sustained winds will be offshore, look at the strength of the high pressure behind the front.  If it’s, say, 1045 millibars, then just remove the 10 from the front and the wind offshore will approach 45 kts.  This method actually works quite well, particularly with these early-season fronts.

With tonight’s front, the high pressure behind it will be 1029-1033 millibars,so we should expect winds offshore in the 30 knot range on Saturday, with gusts as high as 40-45 knots.  Along the east coast of Mexico down to the Bay of Campeche, the winds will be even stronger than that, as the wind tends to accelerate southward along the coast of Mexico.  The blast of cool air and rough seas won’t last long  By Sunday morning, the high center will push out into the Gulf and wind and seas will quiet down quite quickly.

In the long range, both the American GFS model and the European (ECMWF) model have been forecasting a major pattern change across the U.S. which could bring some very cold air southward to the Gulf of Mexico as early as next weekend, around the 21-23 of November.  However, I’m seeing way too much variability from one model run to another to have much confidence in the timing of any big cold air push across the Gulf a week or more from now.   But there is some very, very cold air building in northwest Canada and Alaska now.  At some point over the next 2-3 weeks I do expect some of this cold air to break loose and race southward toward the Gulf.  Such a cold front could produce winds offshore to 40-50 knots, along with very rough seas.

So it’s time to stop looking to the south for the rough weather across the Gulf.  The rough weather will be coming from the north from here on out.

Fortunately for those of us in the U.S., Paloma won’t be any threat.  Cold fronts are moving regularly across the Gulf of Mexico now, far enough south to protect even south Florida from Paloma’s effects.   But it’s going to be a rough next 36 hours across the Cayman Islands, as Paloma will very likely be a major Category 3 hurricane with 115-120 mph winds as it moves right across Grand Cayman Island tonight.  As of noon local time Friday, Grand Cayman Island was reporting winds of 35 mph gusting 45 mph as Paloma’s outer bands move across the island.

Beyond tonight, Paloma will start feeling the influence of that approaching cold front, resulting in a turn to the northeast and inland across eastern Cuba Saturday night/Sunday morning.  Models disagree somewhat on Paloma’s fate beyond Sunday.  One camp accelerates a weakening Paloma out to sea to the northeast.  The other group of models dissipates Paloma near the Bahamas and can’t figure out what to do with Paloma’s remnant low.  Either way, I think it’s unlikely that Paloma would survive as a hurricane to impact the Bahamas after crossing Cuba.  It could produce tropical storm conditions across the central to eastern Bahamas and Turks & Caicos Islands late Sunday and Monday, though.

What about the rest of the season?  There are only 3 more weeks left to the 2008 hurricane season, officially.  While I don’t see any more disturbances moving off the west coast of Africa that might track into the Caribbean Sea, long-range models continue to indicate increased thunderstorms across the southwest Caribbean.  This is particularly true after about the 16th of November when another cold front pulls up stationary in the southwest Caribbean. 

It’s possible another storm may develop in the southwest Caribbean the third or fourth week of November.  However, chances of it impacting the northwest Gulf are close to zero.  Even south Florida should be in the clear by then.  With any luck, Paloma will mark the end of the 2008 hurricane season.

I still do plan to write an end-of-season wrap up here.  But with Paloma still out there and the possible development of one more storm, I think I’ll wait another couple of weeks.

The season totals stand at 15 named storms, 7 of those hurricanes and 4 of those 7 major hurricanes.  It’s been an active year as far as land falling storms.  There was a period of 6 consecutive land falling storms across the U.S. - from Dolly through Ike.  Three  Category 2 hurricanes made landfall on the U.S. Mainland this season - Dolly, Gustav and Ike, causing over $50 billion in damage and considerable loss of life.   There was a tremendous impact across Haiti and the Dominican Republic this season, leaving over 1000 people dead from flooding.    With the coming of November, I think that most areas can breathe a collective sigh of relief.

Those of you in south Florida, breathe half a sigh of relief for now.  I’m still keeping my eye on that southwest Caribbean Sea.  We still need to monitor that area for possible development for another few weeks.  Anything developing down there still has a chance of impacting south Florida.

I’ll be preparing a bit of a summary of the season for next week’s post.

But I’m not ready to declare with certainty that the 2008 season has ended for all locations.  There will be pockets of lower wind shear that pop up from time to time farther to the south across the southwest Caribbean Sea.    Very long-range models are forecasting an increase in thunderstorm activity in the western and southwestern Caribbean Sea during the first and second weeks of November.  So it is possible that another named storm could develop there this season.  Most, if not all, late-season storms that form in the western Caribbean Sea take a track either inland into Mexico or Central America or to the northeast across Cuba, perhaps affecting south Florida and/or the Bahamas before heading out to sea.   I don’t think that any such late-season development would likely threaten the northwest Gulf of Mexico.

On Monday, October 13th, Tropical Depression Thirteen formed in the eastern Caribbean.  After drifting northwestward for a 24 hours, Omar strengthened to a tropical storm and turned northeastward generally toward St. Croix.  By the evening of the 14th, Omar became a Category 1 hurricane with sustained winds of 75-80 mph.  The next day, Omar’s winds increased from 85 mph in the morning to 135 mph by late evening as the center grazed the island of St. Croix.  It’s important to note that Omar was forecast to be a Category 2 hurricane when it passed St.Croix but it was actually a Category 4 hurricane.  This is just another example of why all hurricanes need to be taken very seriously.  They can intensify very quickly, and you cannot rely on our ability to forecast that rapid intensification.

Hurricane Omar’s Path

As I look out across the tropics today, I see the changes that are typical of this time of year.  Taking a look at a graphic of seasonal activity that I had posted earlier this season, I think we’re just about right on schedule at this point:

Typical Seasonal Activity

Looking across the tropics, I see cold fronts beginning to make their way out across the Gulf of Mexico in the coming week.  The Polar jet stream is starting to dig farther south into the Gulf and even into the Caribbean Sea.  This is evident in the “backwards” track that Omar took.  Instead of tracking westward toward the Gulf, Omar moved northeastward.  The reason for such a track is that southward-dipping jet stream.  So is the season over with?

No, I’m not convinced that we won’t see at least one more named storm this year. The place to watch for the rest of the season is going to be the western and southwestern Caribbean Sea.  Tropical Depression Sixteen tried to get going there this past week but ran out of room before moving inland.  A number of the models keep trying to spin up something down there over the next week or two.  Climatology says that the western Caribbean is a common area for late-season storms to form. Late-season storms that form in the western Caribbean typically take one of two tracks.  They either move westward and inland into southern Mexico or Central America or they get picked up by an approaching cold front and track northeastward toward the Florida Peninsula or the Bahamas.

So, those of you in south Florida or the Bahamas that think you’re in the clear for the rest of the season should pay attention at least for another few weeks.

For comparison, I made a composite image of Hurricane Ike and Tropical Storm Marco (above).  Marco’s cloud mass was only 40-50 miles across.  It has been calculated that Ike was about 80 times larger than Marco.  Marco was so small that it could have fit in the eye of Ike at landfall.  I’ve been discussing our Hurricane Severity Index in recent posts. For comparison, Ike’s HSI was 30 out of a possible 50 points (20 for its size, 10 for its intensity). Marco rated only a 5 on the HSI, with 3 points for its intensity and 2 points for its size.

Such “midget storms” are extremely uncommon in the Atlantic Basin, though they’re often found in the West Pacific.  Just last month, there were three such tiny tropical cyclones in the West Pacific at one time. Here’s a side-by-side comparison of Marco and one of the midget tropical cyclones in the West Pacific last month:

It’s interesting to note that midget tropical cyclone in the image above was estimated to be only a depression with winds of 35 mph.  Unfortunately, there is no reconnaissance available in the Pacific, so most such midget storms are ignored.  Clearly, it was as strong as Marco was (65 mph) or stronger.  Dvorak satellite intensity estimates just don’t work for such small storms. So without reconnaissance (which did fly into Marco) we would have never known that Marco was a strong tropical storm.

I can answer that last question first – yes, there is a better way.  After the 2005 hurricane season, I began working with coworker and meteorologist Bob Weinzapfel to develop the Hurricane Severity Index (HSI), a 50-point scale that considers both a hurricane’s intensity and the size of its wind field.  Ike scored 30 out of a possible 50 points on the Hurricane Severity Index, making its destructive potential on par with some of the great major hurricanes of the past.   More on HSI later.

While most people are familiar with the Saffir-Simpson scale, very few understand that it’s only a wind scale.  A hurricane’s Saffir-Simpson rating is given by the peak sustained (1-minute) wind found anywhere in the hurricane.  That’s it – it’s a wind scale.  But over the years, some have attempted to associate an average storm surge to each category, similar to the table below:

The problem with this is that storm surge is independent of a hurricane’s peak wind speed (its Saffir-Simpson rating).  A hurricane’s Saffir-Simpson rating is not even considered when calculating storm surge.  It’s the size of the wind field that’s more of a factor in determining potential storm surge.  That, and the general topography and bathymetry of the coast near the point of landfall, the forward speed of the hurricane and the angle at which the center crosses the coast.

A hurricane with a larger area of hurricane force or greater winds will transport a larger volume of water into the coast at landfall, producing a larger storm surge than a smaller hurricane.  We learned this lesson in 2005 when Category 3 Hurricane Katrina produced a storm surge that was larger than the surge produced by Category 5 Hurricane Camille in 1969.  Residents of the upper Texas coast from Galveston to Sabine Pass were taught this lesson again by Ike.   Even though downtown Galveston resides behind a 17-foot seawall constructed after the Great 1900 Hurricane (a Category 4), most of the island was submerged by Ike’s storm surge.

It’s clear that not all storms are alike, as the image below demonstrates.  On the left is 2004’s Hurricane Ivan.  On the right is 2005’s Hurricane Dennis.  According to the Saffir-Simpson scale, there’s no difference between them – both had 120-mph winds at landfall.  But the red shaded area represents the coverage of hurricane-force winds for each hurricane.  Ike’s hurricane-force wind field was extremely large compared to that of Dennis.  A larger wind field means not only a greater storm surge at landfall, but also a greater chance that a location in or near the path of the hurricane will be impacted by hurricane-force winds.  The larger wind field also produces much larger waves covering a much greater area.  Ivan produced significant waves (the highest 1/3 of all waves) in the 50-55 foot range across the northern Gulf of Mexico.  Dennis produced waves closer to 25-30 feet.

Back to Hurricane Ike.   Though Ike was a Category 2 hurricane, its wind field was very large – larger than Rita’s, as large as Ivan’s and almost as large as that of Katrina.  This large wind field resulted in a storm surge of between 12 and 20 feet that inundated much of the upper Texas coast on the morning of September 13th.   So much for a Category 2 producing a 6-8 foot storm surge, right?  There has to be a better way (than Saffir-Simpson) of estimating a hurricane’s true destructive potential. There is.   It’s called the Hurricane Severity Index (HSI).

After the 2005 season, Bob and I came up with a new hurricane scale to supplement (or replace) Saffir-Simpson.  What we came up with is an index that not only considers a tropical cyclone’s maximum sustained winds, but also the size of its wind field.  It’s a 50-point scale – 25 points are allotted for the maximum sustained winds, the other 25 points are assigned according to the size of the wind field.  I won’t go into all of the details here, you can read about the HSI in great detail at the two links below:

http://impactweather.com/pdf/hsi_explained.pdf  - a full explanation of HSI

http://impactweather.com/pdf/hsi.pdf  - a full description of how HSI is calculated

We can graphically depict some well-known past hurricanes to see where they rank on the HSI:

On the graphic above, a tropical cyclone’s intensity is represented by the horizontal scale.  A tropical cyclone’s wind field size is represented by the vertical scale.  The top right of the graphic represents very large and powerful Category 5 hurricanes.  The lower left of the graphic would represent relatively small and weak tropical storms.  Yes, even tropical storms are classified with the HSI, as opposed to the Saffir-Simpson scale which only classifies hurricanes.  On the HSI, Ike scored 30 points at landfall  – 10 for its Category 2 winds but 20 out of a possible 25 points for the size of its wind field.  It was those 20 size points which was the reason for Ike’s extremely high and extensive storm surge.

So Ike wasn’t “only a Category 2” hurricane, it was a massive Category 2 hurricane that produced huge waves across the northwest Gulf and a storm surge that was much greater than that of a typical Category 2 hurricane.   But it’s incorrect to say that Ike packed a “Category 4 surge”.  That simply isn’t true.   Storm surge is not a function of a hurricane’s Saffir-Simpson rating.  Don’t mix the two.  Category 2 hurricanes can produce a very large storm surge.  In fact, even if Ike would have lacked that small area of Category 2 winds, it could have produced the same storm surge as a Category 1 hurricane.    Think about that – a Category 1 hurricane could put Galveston Island and much of the southeast Texas coast under water.

Many people will have to rethink the wisdom of declaring “I’ll ride out a Category 1 or 2, but not a Category 3 or higher.”  If you’re in a surge zone then you need to get out for any hurricane, no matter what category.

Over the coming months, we’ll be continuing our work on the Hurricane Severity Index.  The plan is to create scales/guidelines that equate a hurricane’s wind field size with potential storm surge threat or possible rainfall.  Combining the wind field size and intensity will always provide a better estimate of the potential threat from an approaching hurricane.

Typically, as October approaches, we see changes in the jet stream across the Atlantic Basin that affect where tropical cyclones form and where they track.  Cold fronts begin to move out across the Gulf of Mexico, and the threat to the northwest Gulf diminishes.  That’s what we’re seeing now.  But cold fronts can also be a source of tropical development, so the threat to the Gulf of Mexico and the Gulf Coast is not over yet.  Currently, a cold front extends across south Florida to the Bay of Campeche.  Thunderstorms have developed near the trailing end of the front across the Bay of Campeche, a prime location for late-season development.  I don’t think we’ll see any development down there over the next 3-5 days, but it’s a place to keep an eye on.  Don’t forget Opal of 1995, a hurricane that formed in the Bay of Campeche on September 27th, wandered around for a few days, then rapidly strengthened to a Category 4 hurricane as it raced northeastward toward the Florida Panhandle.   So enjoy the quiet now across the Gulf, but keep an eye to the south for possible development over the coming weeks.

Out in the eastern Atlantic, we’re seeing increasing tropical wave activity now.  But it’s getting a bit late for Cape Verde storms to form.  Still, we can’t rule out something forming off the west coast of Africa over the next couple of weeks. The good news is that it would be unlikely for such a late-season Cape Verde storm to reach the Caribbean Sea or the U.S.   I think that we may see another 2-3 named storms before the season ends.    However, it does appear that the worst of the season is over with.

Ike struck Cuba as a powerful Category 4 hurricane but emerged into the Gulf a much weaker Category 1 hurricane with hurricane force winds covering only a few square miles.  Crossing a large land mass like Cuba can significantly change a hurricane’s structure.  Prior to reaching Cuba, Ike was a relatively small but intense hurricane.   Crossing Cuba disrupted Ike’s core significantly.   But that’s both good and bad.  The good part is that Ike never could get its core regenerated before slamming into Texas last Friday night.  This kept Ike from becoming a much more dangerous Category 3 or 4 hurricane.  The bad part is that all of Ike’s energy had to go somewhere once its core was disrupted.  So instead of strengthening, Ike became a much larger hurricane.   Same amount of  energy as before, but that energy was spread out over a much larger area.  This reduces potential structural damage due to winds, but it significantly increases the size and expanse of the storm surge.

Contrary to popular belief, the height of a hurricane’s storm surge has virtually nothing to do with its Saffir-Simpson rating. In fact, nowhere in the calculation of storm surge is a hurricane’s peak wind speed (its Saffir-Simpson rating) even considered.  Those tables that assign a particular storm surge range to a Saffir-Simpson category are worthless and should never be used to estimate storm surge.  Yes, a Category 3 hurricane could produce a 9-12 foot surge under certain conditions, but a Category 3 hurricane could also produce a 3 foot surge or a 30 foot surge.   What’s more significant is the size of a hurricane’s wind field and the coastal topography in the hurricane’s path.   And, as I mentioned above, Ike became a very large hurricane after crossing Cuba.    Not intense, but large.   I will go into more detail about storm surge in a future post.

Two days from landfall (36 hours from impact of tropical storm conditions), I watched in amazement as local officials in Galveston pondered the need for evacuation.  Ike was projected to hit the southwest end of Galveston as a Category 3 hurricane.   If that’s not enough to trigger evacuations, then what is? But Ike was “only a Category 2 hurricane” at the time.  Many people say they’ll ride out a Category 1 or 2 hurricane, but not a major Category 3 hurricane or greater.  These people are uninformed as to the power of even Category 1 hurricane winds and potential storm surge, and they have no idea how quickly a hurricane can strengthen in the hours just prior to landfall.  Ike struck the Houston-Galveston area as a Category 2 hurricane.  Not a major hurricane, so no big deal, right?  Tell that to the residents of the Texas coast whose homes (and lives) were washed away by Ike’s 15-20 foot storm surge.

Yes, Ike was only a Category 2 hurricane at landfall. Friends and neighbors of mine find it hard to believe that a Category 2 hurricane could do so much damage.  Ike was actually beginning to intensify as it reached the Texas coast – not enough to become a Category 3 hurricane, but enough to develop a band of hurricane-force winds in its western eyewall.   I spent about  6 hours in Ike’s western eyewall Friday night, along with much of Houston.   For the most part, most of central and eastern Houston saw strong tropical storm force winds over those 6 hours, but there were a few periods when the wind probably just barely reached sustained hurricane force winds of between 75-80 mph.  But that’s probably the strongest wind the Greater Houston area has seen in 50 years, and the trees took quite a beating.  

The graphic below is from the Hurricane Research Division (HRD) web site.  The contours represent Ike’s wind field at landfall (in knots). I circled Ike’s Category 2 winds in yellow and the Category 1 winds in white.  As you can see in the graphic below, valid 2:30AM CDT Saturday, Sept. 13th, Ike’s Category 2 winds were confined to the beach areas near High Island – east of Galveston.  The Houston area experienced strong tropical storm force winds and some brief Category 1 winds.

Normally, a hurricane weakens immediately upon landfall.  But Ike was different, at least as far as its southwest quadrant of squalls.  As Ike moved northward through Houston, its winds actually increased west and southwest of the center.  By 8:30AM CDT, Ike’s southwest quadrant had winds of 70 kts (80 mph) as seen in the image below.

So, as Ike was passing east of Houston, its winds on the western side were increasing between 2:30AM CDT and 8:30AM CDT.  That means those of us in the fourth largest city in America saw periods of hurricane-force winds for probably the first times in our lives last Friday night. Spending those 6+ hours in Ike’s western eyewall was more than enough excitement for me.  I’ve had neighbors come up and ask me about Ike.  They can’t believe that the winds which ripped their trees out of the ground and deposited them on their roofs were only lower-end Category 1 winds.   Well, that’s a hurricane, folks.  There’s no such thing as a minimal hurricane.

It turns out that the European model didn’t do too badly with Ike. It was only off by a few hundred miles on its 7-day forecast.  Not bad at all, unless it just happened to track right over your head.  I’ve been without power for a week now at my home in southwest Houston.  As the temperature climbed yesterday, I went to Lowe’s and purchased a small window AC unit.  It’s held in with scrap wood and duct tape, but it works.   Let me give you a little advice about purchasing a home generator.  Make sure you get one that has a 6-7 gallon tank and runs all night.  I actually won mine as a door prize at a local hurricane conference 3 years ago.  Worked fine, but the 1 gallon tank runs out of gas every 2-3 hours.  Next year I’ll have a new generator, I guarantee you that.  But at least it kept the fridge cold (and my neighbor’s fridge). 

One final note. There has been a lot of talk about replacing the Saffir-Simpson Hurricane Scale after Category 2 Ike devastated southeast Texas. People are saying that we need a new scale, one that takes into account not only a hurricane’s peak winds but also its size.  We developed just such a scale three years ago after the 2005 hurricane season.  It’s called the Hurricane Severity Index.  Next week, I’ll introduce you to this new scale we’ve developed and we’ll take a look at how Ike rated on the scale compared to some well known hurricanes of the past.