Wednesday, August 24, 2011

Hurricane Irene & Atlanic Blob: August 24, Update A

Hurricane Irene is a nice looking storm from the satellite images. About 5 minutes after my last update I guess she got my phone message, so since about 1am EST she’s been moving in a NW direction and picked up a bit of speed (the velocity type, not the drug type!) and has been churning forward at 12mph all day. In the last NHC advisory at 8pm EST, she had winds of 120mph which makes her a mid-sized category 3 storm (range: 111-130mph), and a major hurricane. Central pressure is 954mb.  She’s currently at around 23.5N,  75W, moving NW at 12mph. She went over Crooked Island (twice) and is going between Long Island and Rum Cay.

She’s had an eye on and off for most of the day so I think a strong cat 2-to-cat 3 range is about right. She also has some low level convergence (inflow near the bottom) and upper level divergence (outflow near the top) and both the convection and circulation is looking good of course.

Here is a satellite image of the water vapour over the Northwest Atlantic:
This shows how humid the atmosphere is of course, with the dry parts in brown and the humid parts range from white (cloudy) to a dark pinky-purple-blue colour for heavy rain (I’m sure there’s a name for this colour – magenta maybe?). Anyway, you can see Irene over the Bahamas. I wanted to show you this image for two reason, one of which is to show you what good outflow looks like in a satellite image. It is shown by the white clouds, especially on the northern edge of this storm where the outflow is really nice. To me, it kinda looks like the outer edge of a rotary saw blade, for those of you familiar with such tools (umm, yes, so maybe I do know my tools better than my colours. I’m a card-carrying geeky physicist-type… I like technical things and maths and stuff ;-)). In a category 4 or 5 storm, the outflow would be really nice on all sides. I’m sure you’ve all seen images of hurricanes with good outflow on all sides… well now you know what the edges are indicating.  In Irene, on the east side we have a stream of clouds instead of good outflow. This is because of wind shear… which is picking up a bit and is flowing from out of the northwest. The wind shear looks like it might continue to increase as Irene moves farther NW. This is good because it might help to curb her a bit as she gets closer to the Florida Current and then Gulf Stream, but I would not be surprised if she developed into a category 4 storm. Currently water temperatures are around 30 deg C, with water warmer than 26 deg C in the upper 75m. I think she’ll remain a Category 2 or 3 for the rest of today as she carries on through the Bahamas.

Blistering barnacles Tintin, but she’s a tricky one to keep track of (puns intended). As we expected, she took that NW turn and picked up speed because the high pressure in front her eroded yesterday. Today, unfortunately, it has started to build back up. This is not what I asked for! I am trying to figure out a way to show you this (now that I have the technical abilities of a 5 year old and can upload images to the blog site), but it would take me some time to explain what you are looking at so for now, a thousand words will have to tell a picture.

There are two things to chat about here.

First, the actual pressure fields. The edge of the Bermuda-Azores High Pressure is always fluctuating (pressure fields discussed in some much earlier in the season entry), and unfortunately it is fluctuating right along the US east coast at the moment. Yesterday it was moving out towards the Atlantic, today it is moving in towards the coast. Tomorrow I don’t know yet (time machine is on the blink). Based on these fluctuations and maps, I figure out the direction she will take. For now, it looks like she has room to carry on moving NW, but won’t turn NNW, then N and northeast into the Atlantic as quickly as I had hoped. Bother.

Second, is the forecast track which takes her to North Carolina on Saturday afternoon and then to New York (As A Hurricane) on Sunday, and then to the rest of New England and Canada for Monday. This is the other reason why I wanted to show you the vapour image. If you look to the north and find the Great Lakes, just north of that is a swirlygig. This low pressure system that has been moving east across Canada, and that is what the models are essentially incorporating into her forecast track… this is why they are taking her up along the east coast to New York and beyond (AS A HURRICANE – I hope you guys are also getting ready!).

In my head, both the high pressure and that northern low are factors in her track. What I don’t know is how much emphasis the models give to one over the other, and I don’t have enough information to be able to say what will dominate yet either. In the good old days I know they gave too much weight to these low pressures to the north, but the models have evolved over the last few years as well, so I don’t know if this is the case anymore. This will be an interesting thing to watch (from a scientific/observational point of view). I hope everyone is preparing for a pretty big storm! I’ll be looking for changes in the high pressure tomorrow and the following day.

This is a big storm – both in terms of her size and also in terms of her intensity. Please be getting ready – even if she stays offshore, at the moment category 1 or higher winds are within 60 miles of the center, and tropical storm force winds are estimated to extend out to 230 miles!

Atlantic Blob:
Meanwhile, on the eastern side of the Atlantic, there is another blob. It is beginning to look good – convection is picking up because the Saharan Air Layer is no longer a big factor (boo). It is currently at around 12.5N, 27W, moving WNW at around 10-15mph. There is some circulation in the lower half of the troposphere. There is some wind shear, but I don’t see anything that will stop this from becoming the next Tropical Storm.

That might be it for today – busy day!
More tomorrow (unless I see some amazing new data that changes things).

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DISCLAIMER: These remarks are just what I think/see regarding tropical storms - not the opinion of any organization I represent. If you are making an evacuation decision, please heed your local emergency management and the National Hurricane Center's official forecast and the National Weather Service announcements. This is not an official forecast. If I "run away, run away" (Monty Python), I'll let you know.

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