Saturday, September 18, 2010

Hurricane Igor, Tropical Storm Julia, ex-storm Karl, and the Atlantic Blobette: September 18, Update A

Here we go. After days and days of waiting, the outer bands and wind from Igor have reached Bermuda. 
Hurricane Igor:
He's still a big storm in spatial extent, but meeting a girl-storm (Julia) softened his 'I'm a tough guy' act so he's now officially a low-to-mid sized cat 2 with winds of 100mph (cat 2 range: 96-110mph) and a central pressure of 945mb. Unfortunately she couldn't persuade him to move away from the path he'd decided to follow so he's still aiming for the Bermuda region. Sigh. He's currently centered at 27.1N, 64.3W and is still moving NW (alas) at 14mph.  Hurricane force winds can be felt up to 90 miles from the center, and Tropical Storm force winds up to 345 miles. He's 360 miles south of Bermuda. You can do the maths. 

Although the wind speed has dropped to a mere 100 mph (!!!) and he's a low-to-mid cat 2, there are still two things that will make him feel stronger. First, he's a big storm. It's one thing to have a small or quick storm pass by, but another to have a large or slow storm because the winds will be hanging out for longer. The other thing to watch out for with Igor is that his convective activity is really rather strong. The temperature of the cloud tops indicate how deep the convection is in the atmosphere, so the colder the cloud tops, the more thunderstorms, heavy rain, etc. If you look at Infra-red satellite images (e.g., the red and dark grey colours indicate cold temperatures and therefore areas where the convection is really strong. Blue colours are warm (I know, I know, it's not quite what you were taught when you were learning your colours as a kid), and generally indicate areas where it's just cloudy, no rain. Yellow and orange are rain - some light, some heavy, dotted with a few thunderstorms here and there. The hurricane category is just based on wind speed, and doesn't really take into consideration the severity of the convection.
On-the-ground conditions update from Steve B. on Bermuda:
3pm local Bermuda time: "we have ominous clouds. we wait in trepidation. novelty turns to nervousness"
8.25pm local time: "just went outside and moved car away from Casurina trees. Now seems high winds (altitude) and breezy below."
9.37pm: "feels like a storm. we'll muddle through."
As the NHC says, conditions will continue to deteriorate on Bermuda and the center won't get to that vicinity for another 36 or so hours... it's going to be a long few days for you guys! Officially he's not forecast to re-strengthen too much more because, as I mentioned yesterday, he's already getting some wind shear to the north and he's also being truncated to the east by Julia. His vorticity is still very strong throughout the troposphere, so really he's probably got the vorticity of a cat 3 or higher storm, but Julia is really messing things up for him. I don't know how the next couple of days will play out given their interaction, but personally I'd prefer to see that vorticity come down a bit before I say that he's going to be a cat 2 or less.
If you are reading this on the listserve, I've attached a GOES satellite image that shows Igor and Julia. They look like two halves of the same storm, with a big gap in between where the winds are all sorts of squirly (a non-technical term ;-)) and there isn't any convection. If you are reading this on the blog site, I'm going to try and add a picture. My first ever! After all these years of words only! This could usher in a new era! Yes... I might finally reach the advanced technical capabilities of bloggers from circa 2002! How thrilling! (umm... don't hold your breath waiting though! ;-)). 

Tropical Storm Julia:
She's now officially got winds of 50mph and a central pressure of 998mb, making her a weak Tropical Storm (range: 39-73mph). Like Igor, although wind speeds are low there is still a lot of convection and her vorticity is still strong in the lowest half of the troposphere - much too strong for a normal low-level Tropical Storm, but things are a little murky because of Igor. She's moving NNW at 20mph, and is located at about 30.6N, 51.8W.   
Ex-storm Karl:
Karl dissipated over the mountains of central Mexico today after dumping a lot of rain. There is a lot of convection in the western Gulf of Mexico, which may or may not be his final swan-song. I don't know because I didn't see his demise and subsequent data because I was away from a computer all day today (trying to avoid stepping on a wild poisonous snake that was sunning itself on a well-walked path!!! Yes, in the UK! I know, it's an unbelivable tale because I mention sun and UK in consecutive sentences... ;-)) Ummm... so, yes, storm. I don't know if the convection in the Gulf is connected to the vorcity signature from Karl or not. But the convection in qustion is right along the western edge of the Gulf and should move west over land before it develops much. In this I agree with the NHC. It will dump a lot of rain on Texas and Mexico (because that's just what they need after the soaking they got from Karl). Anyway, this is my last entry on Karl.
Atlantic Blobette:
Yup, another one. This entry is just a heads-up. There is some circulation in the lowest levels of the troposphere but it's not very well structured at the moment.  There is also some convection with this blobette. Next name is Lisa, but I'm not going to mention this again until it does something worth mentioning.
That's it for now. Be safe in Bermuda!!

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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 was there and was going to "run away, run away" (Monty Python), I'll let you know.

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