Tag Archives: Tropical Storm Irma

Hurricane Irma: Important changes in intensity, changes in track


Note that tropical storm force winds may start hitting southern Florida around 1 or 2 PM today, Saturday, and will reach central Florida by about 8AM Sunday.

The eye of the storm should be abreast southern Florida at around sunup on Sunday. The storm may remain a major hurricane as it moves all along the west coast, reaching south of Tallahasse, still as a major hurricane, Monday morning.

Irma has interacted with Cuba more than previously expected. The storm also seems likely to move farther west than previously expected.

Moving over very warm waters over the next several hours will strengthen the storm. If it does move along the west coast of Florida the focus now shifts to different communities, such as the Cape Coral / Fort Myers area.

As a rule, storm surge risks on the west coast are greater than the east coast. The west coast has a broader shallow shelf, and since the hurricanes rotate counter clockwise, there is a greater chance of a direct hit. Correspondingly, the following storm surge map (most current version HERE), which has inundation of over 9 feet if conditions pertain in a given spot, is what we should be paying attention to:

It also seems to me that the keys are in more danger with this track than with the central track.

Here is a reasonable likely scenario, WHICH IS SUBJECT TO CHANGE based on current consensus.

Over-simply put, there are two things that determine the future location of a hurricane. One is the simple long distance movement of the major air mass that is our lower atmosphere. The hurricane is like a cork floating along in a stream. If the stream flow is steady and straight, the cork will be just down stream from where it is now, moving alongat an easily discernible rate, so the distance over a fixed time interval is easy to calculate.

The other factor is all the other stuff. The complex movement of other low pressure systems and ridges of high pressure, all that. The land and the ocean have their own things going with respect mainly to high and low pressure, so as a hurricane moves from being out at sea to being on or near land, these interactions grow increasingly complex. Over the next several hours, this second factor (everything else) takes over and this is where the prediction gets complicated. How much exactly will the storm change its angle of movement, and exactly at what hour will that occur and how long will it take?

So, the tack above looks pretty solid but it may in face be off by a hundred miles or even more before the storm is abreast of southern Florida. That difference will make all the difference in the world.

Update based on NWS advisory Sept 7, late PM

A Hurricane Warning is in effect for…
* Dominican Republic from Cabo Frances Viejo to the northern border
with Haiti
* Haiti from the northern border with the Dominican Republic to Le
Mole St. Nicholas
* Southeastern Bahamas and the Turks and Caicos Islands
* Cuban provinces of Camaguey, Ciego de Avila, Sancti Spiritus, and
Villa Clara
* Central Bahamas
* Northwestern Bahamas

Maximum sustained winds remain near 175 mph (280 km/h) with higher
gusts. Irma is a category 5 hurricane on the Saffir-Simpson
Hurricane Wind Scale. Some fluctuations in intensity are likely
during the next day or two, but Irma is forecast to remain a
powerful category 4 or 5 hurricane during the next couple of days.

Hurricane-force winds extend outward up to 70 miles (110 km) from
the center, and tropical-storm-force winds extend outward up to 185
miles (295 km).

The combination of a life-threatening storm surge and large breaking
waves will raise water levels ABOVE NORMAL TIDE LEVELS by the
following amounts within the hurricane warning area near and to the
north of the center of Irma. Near the coast, the surge will be
accompanied by large and destructive waves.

Turks and Caicos Islands…15 to 20 ft
Southeastern and central Bahamas…15 to 20 ft
Northwestern Bahamas…5 to 10 ft
Northern coast of the Dominican Republic…3 to 5 ft
Northern coast of Haiti and the Gulf of Gonave…1 to 3 ft
Northern coast of Cuba in the warning area…5 to 10 ft

RAINFALL: Irma is expected to produce the following rain
accumulations through Saturday evening:

Northeast Puerto Rico and the British and U.S. Virgin Islands…
additional 1 to 2 inches
Much of the Bahamas and Turks and Caicos…8 to 12 inches, isolated
20 inches
Andros Island and Bimini, Bahamas…12 to 16 inches, isolated 25
Northern Dominican Republic and northern Haiti…4 to 10 inches,
isolated 15 inches
Southern Dominican Republic and southern Haiti…2 to 5 inches
Eastern and central Cuba…4 to 10 inches, isolated 15 inches
Southeast Florida and the upper Florida Keys…8 to 12 inches,
isolated 20 inches
Lower Florida Keys…2 to 5 inches

Update Sept 7, PM

Everything I said in the last post applies now except the chances of the storm making landfall on the Florida Peninsula, and even the chances of a fairly direct hit on Miami and surrounding areas, is much higher than it was this morning. But we are still talking about something that will happen in the middle of the weekend, and this is still Thursday.

Also, repeating, note that if this storm makes landfall at the southern tip of the Florida peninsula, it is expected to move north very quickly (about twidce as fast as it is moving now) and reach southern Georgia WHILE STILL BEING A HURRICANE. A land hurricane, if you will. Except of course huge parts of it will be handing out over the very warm Atlantic.

I heard a guy on the news today saying the only ticket he could get out of Florida was a one way flight to Memphis. The hurricane will be in the vicinity of Memphis, or at least central Tennessee/Kentucky, Tuesday. It will be a big wet storm at that time, but that is an area that floods so take it seriously.

This might be a good time to read this post if you haven’t already: Hurricane Landfall: What is it and don’t be stupid about it.
Update Sept 7, Mid Day

Storm surge watches have now been issued for South Florida. The experimental storm surge system of NWS suggests storm surges of up to 6 feet or so in a few places. But, the details of where storm surges may happen and how high they might be will probably sharpen a great day between now and Friday late PM.

Mean while, do the same with storm surge information as I asked you to do with the overall Hurricane information below. Recognize how this information is produced and what it means. There will NOT be a storm surge of the maximum amount indicate on the maps in all the areas covered by the map. The way to use the map is this: If you are in a particular spot with a possible storms surge, the map tells you your local worst case scenario, the scenario you should be prepared for even though something less is probably what is going to happen. If you assume less will happen and are washed out to sea and never seen again, don’t come complaining to me, because that is NOT what I said. If you are in a place where it says 6 foot surge, and you get a 3 foot surge, that is normal. Hopefully, though, you are no where hear the south Florida coast, right?

You can see the storm surge maps here.

Update Sept 7 AM

I want you to look at this graphic and understand its meaning:

There is about a 70% chance that the CENTER of Irma (the eye, approximately but not exactly) will remain between the two blue lines between midnight Saturday and midnight Sunday as it first interacts with Florida. In other words, between those two “m” markers. So, you have to start out by imagining the hurricane’s eye in the Gulf over by Cape Coral, and think about that for a moment. Then, you have to imagine the eye out near Freeport and think about that for a moment. With each of these scenarios, understand that the Miami and Miami Beech area is in a bad storm, windy, lots of rain, probably flooding, etc. but not Houston and not Andrew.

Now, look at the black line with the “M” markers on it. As you move from either blue line towards the black line, there is an increasing chance of the center of the storm being there. It is a bell curve, where the high point of the curve holds that black line. So there is a better chance that the center of the storm will be near Miami than Tampa or Freeport.

Now let’ consider the structure of the storm itself. Catastrophic sustained winds and gusts extend out from the center of the storm about the same width ad Florida itself, but as the storm approaches Florida it will weaken. The strongest wind and most severe storm tides are in the front right quadrant. So, if the storm comes on land in Florida along the southwest tip of the peninsula, that front right punch is going to only graze Miami but will obviously be very meaningful in the keys and everglades. If the center of the storm passes to the right of Miami about the same distance, perhaps Grand Bahama is toast, but Miami and Miami Beech is hit hard but it is a mere bad disaster.

What are the chances that the storm will essentially ride this black line all the way? Low. Maybe 10% chance. But if it does, or does something close to that, what happens?

There are three things to know, one of which I’ve already mentioned. That is that the front right is the most important part of the storm. So, if the storm is close to riding the line, but off to the right 10 miles, that is hugely different from if the storm is close to riding the line but off to the left 10 miles. The difference is in the survival of this urban area, potentially.

Second: Note that there is an M on the track south of Miami, and another one north quite a distance. The lower M corresponds to sustained winds of about 150mph, the upper M to sustained winds of 120 mph. So, as the storm moves over the area, it will weaken but it will be very strong anyway.

Third: The shape of the coastline, as discussed and depicted below, in an earlier update, means that the storm surge coming up into the Miami Beach area could be very high. The Hurricane Prediction Center has not posted storm surge estimates at this time, but depending on the exact path of the storm there could be many many feet of flooding over a very large area in and around Miami.

Or, again, the storm may be nothing more than a bad day for the Magic City. Remember that. We still do not know.

Beyond this, there is other complex bad news. Even if Irma scrapes the Atlantic coast of Florida, it may remain a hurricane all the way up to the Carolinas. If it misses Florida, it may go north and then hit the Carolinas with more force. Or it could go into the gulf, or miss everything. Still too early to tell, but Florida looks to be in some sort of trouble.

Remember, mainly, that despite what the news agencies are saying, there is currently no valid prediction of what Irma will do over the weekend. So, if it does something that you were not expecting, that’s you. You don’t get to say “but they said yada yada” because they are saying nothing more specific than the vague and rambling things I said above.

Update Sept 6, late AM

Just hours after the NWS five day put Irma dead in the middle of Florida’s south coast, the newest estimate is quite different. I point this out to underscore what I’ve been saying all along (and I hope you have not been ignoring): Five days out is too far to be accurate. Look:

Update Sept 6, AM

I keep seeing news phrases like “Irma has Puerto Rico in her sights.” Puerto Rico will have some very bad weather, but the main path of the storm has never crossed the island, and there is probably a less than 10% chance of a direct hit there, or less. The small islands of the northern Leewards have been hit directly or close to it, and more are in trouble. It seems most likely that the first larger body of land Irma might strike is Cuba, which has is almost entirely within the cone of probability, but south of the central line of expected movement. If the storm interacts a lot with Cuba, or turns into the island nation, that will be bad for Cuba, and Irma will weaken considerably. Meanwhile, the Bahamas and islands between Irma and the Bahamas are likely to get hit hard.

The more accurate three day forecast puts Irma as a Major Hurricane between Cuba and the Bahamas, or hitting Cuba or the Bahamas. The much less reliable five day forecast has Irma then curving north and striking the very tip of Florida and moving, as a major hurricane, onto the peninsula.

However, as a target, Florida is small compared to the cone of uncertainty Look:

Given this, it is distinctly possible that Irma will pass to the right (east) of Florida and head up the Atlantic. If that happens, it may then strike the mainland somewhere else, or not. Or, given this probability map, it is possible that Irma will pass to the left (west, gulf-side) of Florida, and go into the Gulf, then it will be pretty much impossible for the hurricane to not hit something.

The fact that the middle of this forecast graph is centered on Florida does not mean that it is more likely that the storm will hit Florida head on than not, given the wide margin of error and the relative narrowness of the state from this angle. I am emphasizing this because I don’t want to hear any bellyaching later if Irma skips Florida and hits Louisiana or South Carolina or something.

Either way, Irma is expected to be making whatever northerly turn it is going to make during the night time hours between Saturday and Sunday, and the current (but too far out to be certain of) projection is that Irma will be a Category 4 hurricane at that time. By Sunday night, Irma will have hit Florida, dead on, or not, and is expected to be a Category 3 hurricane on land and inland a ways, if that happens. In other words, worst case scenario includes southern Florida suffering a major hurricane for over 24 hours straight.

It has been said over and over again that the real risk of death in a hurricane is inland flooding that often happens after the hurricane has come ashore, and not so much from the coastal flooding and the winds. I’m not going to exactly dispute that but I want to complexify it a bit. If we count all the hurricane dead in the US, the most people who have died of hurricane effects were killed in the coastal storm surge of one single storm. That is not an outlier in the classic sense (i.e., a number that is so out of whack that something must be wrong with the number). It is a real number but an extreme one, however, and meaningful averages should probably ignore it.

But I bring this up now because it might be that a catastrophic hurricane will come ashore in an area where a lot of people live on low ground. There could be a storms surge that is substantial relative to the topography, and there could be winds strong enough to flatten homes and buildings and, generally, structures previously thought to be storm shelters. In other words, a worst case version of Irma would have a deadly storm surge and a deadly wind, more so than Atlantic hurricanes tend to have. Then, of course, the inland flooding as well, maybe quite substantial.

Storm surges are more extreme when there is focusing terrain, like a bay flanked by high hills leading to a town or city. In such environments, however, the storm itself is likely to suffer attrition from the nearby mountainous topography. In the case of Florida, the good news is that there are not hill flanked embayments, and the bad news is that florida is as flat as a pancake, so the storm will not be reduced from that sort of friction.

There are some potentially very uncomfortable scenarios. Look at a map of the Miami and Everglades area.

Now imagine a counter-clockwise spinning storm, centered close to the very southern tip of the state. Besides sweeping high winds along teh keys, it may also force a storm surge up the lagoonish area west of Key Largo, up into the northern part of Biscayne Bay, and right up into the waterways of Miami. I would not want to be on Dodge Island if that comes through. Also, northern Biscayne Bay could be flooded and the outlets to the south blocked, so any of several smaller low spots or outlets in Miami Beach may volunteer to be the new channel connecting the bay to the sea. That would be locally very very bad.

Miami is thought to be less vulnerable to storm surge than other cities, because the sea deepens fairly quickly off shore, which decreased the amount of surge that is possible. But, this particular hurricane, coming from the south and being large, obviates that benefit because the storm surge could be coming through the zone behind the Keys, which is very very shallow. Even if Miami itself is spared, the areas around the keys themselves, and the everglades, are pretty vulnerable.

Or, Irma may do nothing like this, and go somewhere else. We’ll soon find out.

Update Sept 5, evening

In the fairly current infrared image below, Irma is clearly bearing down on some land masses. The twin (double) blobs south of the eye just being engulfed by read is Guadeloupe, and the largish two islands south of that are Dominca and Martinique. The several tiny dots right in the path of the eye or near it are Antiqua and Barbuda, Monserrat, St Kits & Nevis, and Anguilla. You probably recognize Puerto Rico; the tiny dots in the blue specked area between Puerto Rico and Irma are the British and US Virgin Islands.

These island are on the elbow of an ark that runs from Grenada to the Greater Antilles. This mostly volcanic island arc, rimming a small continental plate, would be a large arm of much more land and much less sea during low sea level stands of the Pleistocene.

(I just thought I’d mention that because you probably want to know some things about these islands that are about to get blottoed.)

It is still too early to predict what Irma will do when it is nearing the end of its path along the Greater Antilles and Leeward Islands, i.e., when it is nearer the US mainland. To give you an idea of how uncertain all this is, the average of all the models had Irma striking Florida at the end of the peninsula, but now, hours later, there are other models that tend to be pretty reliable showing Irma not hitting Florida at all, and rather, striking land in the Carolina region.

What this means is this: If you are in Florida and preparing for a hurricane, continue to prepare. If you are in South Carolina and thought you dodged a bullet on this one, don’t assume that.

The one thing we are 70% sure of: Late PM on Friday, Irma will be somewhere between eastern Cuba and the Bahamas. Probably. But possibly not.

Update Sept 5, mid day

We have been back and forth on Irma all along, with the possibility of a landfall in the US being very uncertain.

I’m here to tell you that it is still uncertain. There are still some models that show the storm curving north and becoming one with the Atlantic.

However, most models are showing Irma hitting the continental US, and of those, Florida seems to be a favorite spot.

Several models indicate that Irma will interact with Cuba before heading north and running into Florida. It will not cross Cuba on the way north, but rather, veer into Cuba from the North and maybe not even make eyewall landfall there. It is hard to say what this will do but likely the storm will weaken before hitting Florida.

Also, generally, reports of the storm’s strength are exaggerated. It is now a very powerful storm and it could become more powerful but there is almost no way Irma will not decrease in strength, down to a Category 3 or even Category 2 before hitting the US, if it hits the US. There are reasons that Atlantic hurricanes are not as big, not as powerful, and not as persistent as many Pacific storm, and those reasons have not applied to the storm in recent days, but will start applying to the storm now and onward.

Nonetheless, it is very likely true that somewhere in the US there will be a serious hurricane, possibly a major hurricane, hitting something.

Meanwhile Irma is starting to impact land and will continue to do so for several days, mainly on islands.

So, here is a reasonably likely scenario for the next several days, in DECREASING order of certainty as you go down the list.

1) Certainly: Irma is at the moment impressing the heck out of weather watchers. It has a very clear and distinct eye with interesting features. The winds are very very high, and it is classed as a Category five hurricane. At this point, Irma is not merely a major hurricane, but rather, a catastrophic hurricane (that’s not an official word, but it is a word that will be used on the Leeward Islands … there is a good chance the name Irma will be retired).

There are hurricane warnings in effect for Antiqua, Barbuda, Anguilla, Monserrat, St. Kitts, the Virgin Islands, Puerto Rico, and other locations. Between Wednesay AM and early morning Thursday, approximately, Irma will bear down on locations between St. Johns and Puerto Rico. It will likely hit the first islands more directly with the center passing north of Puerto Rico, but it will affect everything in that area and this is going to be a disaster. A direct hit on Puerto Rico is in the range of possibilities.

2) Probably: Between perhaps Thursday AM and Friday mid day the storm will weaken slightly. It may pass north of Hispaniola and eastern Cuba, but a direct hit on DR and Cuba is well within the range of possibilities. If so, it will weaken more, if not, it will weaken less.

3) Maybe, maybe not: Between 4 and five days out, so starting in the middle of the weekend, the storm, still weaker but still possibly a major hurricane, will either be interacting with Cuba or staying north of Cuba, and poised to hit, possibly, Florida. Or not.

If you live in Florida, it would be good to assume you need to react to a major hurricane. If you live in the keys, south of Miami, or the Miami area or the southern Gulf area, and this storm hits Florida, you will need to be paying close attention.

By Thursday it should be a lot easier to say if Floridians, or some other group of people on the US Mainland are in trouble. Whatever you can do to be ready for a Hurricane that is non-committal and reasonable, do now, and you should probably have done that at the beginning of the season.

Update Sept 2, PM

Underscoring the futility of making projections of a hurricane’s path beyond five days out, the bulk of the models now show Irma hitting the East Coast or sliding around Florida and hitting the gulf. One puts it through the Louisiana-Mississippi border, one has it heading for Halifax, and then all the others are in between.

What does this mean? In terms of projecting the hurricanes ultimate path or chance of landfall, and location of any landfall, it means nothing yet. However the fact that this category 2 storm is likely to become category 3 storm over the next few days, and then will head in the general direction of land, means, well, that it gets its own blog post (this one) for now, if nothing else.

Update Sept 2 AM

The latest models, still too early to tell but there they are, mostly have Irna going off to sea. A couple have the storm hitting the east coast. None have the storm in the gulf.

Updated Aug 31 Mid Day

Two new developments in expectations about this storm, but all very provisional.

First, the storm is intensifying more quickly than expected, and it is expected to become a very powerful storm, ultimately.

Second, while earlier projections from many models allowed for a very wide range of possible paths, the models, still being worked far too out in time but also converging significantly, are starting to suggest that Irma’s most likely course will be to curve up the Atlantic. This involves the possibility of landfall anywhere from the middle sates north to Canada, or no landfall at all. It is still too early to say, but it is looking unlikely for this storm to go into the gulf, but at least one or two models do allow for that.

Updated Aug 31 AM:

Here’s a tweet that shows the current range of model projections for this storm (Note it is way to early to actually predict this far out, but this gives an idea of the range of possibilities):

The National Huricane Center originally suggested that Irma was going to remain relatively low grade for a while as it crossed the first part of Atlantic. However, it is rapidly intensifying and is already on the verge of being a major hurricane. That was not expected. So, this may be an interesting storm.


I’ve been putting comments on, or links to posts on, the 2017 Atlantic Hurricane Season Here, but I wanted to start a thread on Irma, which just now became a named storm. Irma is way out in the Atlantic, and its formation is so early that the NWS doesn’t have any significant information on it as of this writing. But, it is heading roughly west.

Stay tuned.