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Tropical Update

In the previous article I was discussing what was then Hurricane Irene and her progression towards the East Coast of the United States. For a quick meteorological summary of Irene from that point onwards, after pounding the Bahamas where she is reported to have damaged as much as 90% of housing on one of the islands, Hurricane Irene moved towards the East Coast. The trackers were out in full force on the online forums and each little wobble on satellite imagery was causing someone to jump to unnecessary conclusions about a change in forecast track. On Friday the 26th August, Irene was heading up the East Coast towards the Outerbanks of North Carolina.

Satellite blackouts begin to occur in the early morning hours around this time of year and when the satellite came back up Irene had evidently weakened a lot since the previous satellite image. The weakening was obviously due primarily to dry air which was being pulled in from the South East states of the US into the SW quad of the circulation.

When she made landfall on the outerbanks though, she began to clearly intensify as was visible on radar where a developing eyewall could be seen, something that was lacking previously. This intensification process did not last too long though and she began weakening as she headed towards water again. She would never really recover from this point on and the weakening process had begun. Damage in the Outerbanks was limited primarily to flooding, some surge and minor structural damage and downed trees. The surge that occurred was much less fierce than what the SLOSH models were suggesting, this likely due to the quick weakening that occurred prior to landfall. Irene did manage to bring some decent strength winds to the OBX though with a 120mph gust being reported, Irene may well have been borderline Category 2 hurricane at a point during landfall, though the winds were very isolated.

Moving up to the NE coast she began pounding areas in Philly, New Jersey and New York. Again storm surge experienced in these areas, while still occurring, was much less than models had forecast. By the time Irene made landfall in New York the winds being reported were only moderate Tropical Storm strength winds of around 40-50mph. And the whole East Coast was definitely spared from what could have been a much more disastrous hurricane. The most wind damage associated with this storm were actually from tornadoes which occurred in the outer bands of the hurricane.

The media as usual, clearly overhyped this storm from the beginning and while Irene did have potential to be a disaster, it was clear from late Friday into early Saturday already that it wasn’t going to happen that way, yet media outlets kept running with the angle of “Potentially catastrophic storm”. This is dangerous in that it will lead to complacency in future storms and ultimately end up in the loss of lives.

From a meteorological point of view the storm was extremely interesting, the models had always shown that it was going to be a large system but in many cases it’s merely the outflow of a hurricane that makes it look large, with Irene on the other hand, she had an extremely large area of tropical storm force winds associated with her. Because of this large area of circulation, she had a large area where she had to disperse her energy to. Her pressures were around 950mb for most of her landfalls, this is a pressure that is usually associated with high end category 3 hurricanes, but due to the large circulation it meant that the wind response to the lower pressure was much less visible. Smaller systems are able to have stronger winds in many cases because of the pressure gradient, remember that it’s not about the pressure measurement but rather about how tightly those isobars are packed. To reach a category five status, Irene would have needed pressures around 915mb or less.

Katia’s Future

Just this morning Invest 92L which is located just west of the Cape Verde islands was designated as Tropical Depression 12. This storm is forecast to become Tropical Storm Katia over the next couple of days as it heads generally WNW. This is likely another storm that the US will need to keep an eye on. The usual rule of thumb is in these cases, the stronger they get early on, the more chance they have of being a fish (no landfall).  And while development is forecast to be aggressive with the NHC forecasting a 100mph hurricane in a few days, the storm is currently under some shear which should keep future Katia’s strength down a bit for the first day or two, though it will then begin to move into more favourable conditions. Models are for the general consensus for a WNW movement and then a slight bend back west in 5 days, after that it could be anywhere from the Gulf of Mexico to Canada or even out to sea. Days are early with this system and I have a feeling it’s going to be forcing me to get no sleep because of compulsive tracking.

If Tropical Depression 12 does indeed form into Tropical Storm Katia, this will be the 11th named storm of the year, and there’s still a good month of peak hurricane season left.

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