Category Three Hurricane Irene has just passed over Abaco Islands in the Northern Bahamas, and is generally affecting the northern Bahamas. Widespread damage is reported. Over the weekend Irene promises to be a very significant weather event in the US. In order to understand the events that will take place between today and early next week, it will be necessary to revisit the concept of “landfall,” which we have discussed here in the past.
Here’s the deal. “Landfall” is the moment in time when the eye of a hurricane contacts the land, including any barrier island or mainland. “Landfall” is NOT the arrival of a hurricane. Think of it this way. If you are standing in the middle of the street and a semi truck’s path intersects with your position, and we measure the location of the truck from its center, some 25 feet or so back from the front bumper, and the truck stops too that this reference point falls short of reaching you by three feet, you are still very much squished by the semi truck. The eye of the hurricane can be thought of at its “center” (even though it is not always in the middle) but a hurricane can be hundreds of miles wide. If hurricane force winds are arbitrarily set at 70 mph and above, then the part of the hurricane that is analogous to the semi truck’s front bumper can hit you on land, mess you all up, and move on up the coast without landfall ever happening. In the case of Irene, this zone of 70 mph or greater winds is shown as the darker color on this map, and the tropical storm force winds as the lighter color:
The darker blob is about 80 to 100 miles in diameter, and the tropical storm force zone is about 500 miles in diameter. Convert these to radii: There is a zone of nastiness over 200 miles from the eye, to the left of the eye as it moves north along the US coast. As long as that eye is within 200 miles, coastal residence can expect to have a very serious storm with high surf. When the eye is closer to between 30 and 50 miles off shore, coastal residence can expect hurricane force winds and a very strong tidal surge.
By the way, we are experiencing a new moon. During a new moon or a full moon, the moon is lined up with a line extending from the center of the sun to the center of the earth (and beyond). This causes extremes in tidal forces. You see, the moon’s tidal force is a bulge that is lined up with the moon, so the water is high on the side of the planet facing the moon as well as the side facing away from the moon. Thus approximately two high tides a day. However, this is also true of the sun. If there were no moon, we’d have a small solar tide as well. When the moon is either on the sunny side of the sun-earth line or the far side of the sun earth line, the solar and lunar tides line up and you get more tide.
A hurricane is a low pressure system. That causes a tide to go up near and in the eye.
A hurricane pushes water towards land along its leading edge. So as the hurricane spins counterclockwise, and moves north along the east coast, the shore to the north of the hurricane will experience high water because the wind is pushing the water towards land.
So, during normal high tide periods while Irene is moving north along the coast and still off shore, especially when it is close to land and moving as a category two or greater storm, there will be very high tides along the coast.
A high tide hitting a barrier beach is bad, because barrier beaches are low. A storm surge could actually flow completely over the beach, which would be considered a major form of flooding. Even worse, potentially, is the backwater effect. This is where the storm surge causes lots of water to flow to the lagoons between the barrier beach and the mainland. If during the flooding the channels that normally communicate between the lagoons and the main ocean are blocked (and this is not even a little bit unlikely) then new channels could be formed by the water, built up and trapped behind the barrier beaches, as it seeks a way to return to the sea.
Some of you may remember this book, about a fictional hurricane in Florida that creates a new channel between lagoon and sea right through the location of a condominium built by crooked contractors occupied by partying annoying people with novel-worthy stories … in other words, a run of the mill storm in Florida. But I digress.
The point is, hurricanes that skirt the coast are bad news, and I promise you that the “news” is going to be muddled and messed up and made hard to understand unless those reporting it have been reading my blog and finally understand that “landfall” is NOT when the hurricane gets there.
In fact, this is the worst case scenario with a hurricane like this: No landfall. Instead of achieving landfall and losing much of its energy over the continent, Irene could maintain a greater distance from the shore, keeping her eye over the ocean, and wobble back and forth as she moves north. When she wobbles east, she picks up energy from warm offshore water. When she wobbles west she slams the coast with hurricane force winds and heavy rains, as well as a reasonable storm surge facilitated by constant movement of the leading part of the storm, which will have its winds moving from east to west as the storm spins counterclockwise. In this scenario, the eye stays out over the sea while the storm ravages the Carolina coast, then the storm continues north over more open ocean and does the same thing to Delmarva, then traverses a bit more open ocean and then slams into Connecticut or something.
Having said all that, let’s see what the National Hurricane Prediction Center people are saying about Irene right now. The bottom line is that Irene, a Category Three Hurricane, is now a very strong storm and will affect the US coast starting some time on Saturday, and continue to effect the coast for several days thereafter, with very significant stormy weather running from Florida to the Maritimes in Canada.
It is possible that Irene could become a Category Four hurricane during the next 24 hours. The peak of the storm’s strength will likely be Friday late afternoon Eastern Time.
Irene is currently moving north-northwestwards, and is moving rather slowly, which will allow her to gain additional strength over warm waters. By some time early tomorrow, she will turn due north and by late in the day Friday, while reaching maximum strength will shift north-northeastward from a position well off the Florida-Georgia border.
While earlier predictions put Irene off shore with the eye possibly rounding but not going over the Outer Bank barrier islands, the latest forecast suggests that the eye will actually pass over the mainland through North Carolina. This, however, could change. The best estimate for now is that the eye, following a significant swath of tropical storm force and hurricane force winds, will contact the central South Carolina coast late Saturday PM, and then stick close to the coast, passing by New York city and New Jersey in three or four days.
UPDATE (Thursday 11PM est): The latest information suggests that Irene is gaining energy at a slightly higher rate than previously measured, as she moves over open waters. The overall forecast remains the same but with a few more MPH for the wind speed tomorrow PM and later.