Thursday, August 26, 2010

Hurricane Danielle, Tropical Storm Earl, and the Atlantic Blobette: August 26, Update A

Hurricane Danielle: 
Danielle is certainly a lovely looking cat 2 (range: 96-110mph) storm at the moment, with a beautiful clear eye and lots of lovely swirls and curls. She's currently centered at 25.2N, 57W and is moving NW at 15mph. The good news is that since yesterday, she has indeed been moving slightly to the east of the center of the cone so it looks like her center should pass nicely east of Bermuda. But the storm diameter (at the moment) is somewhere around 300 miles, so they may get outerband stuff - hopefully only clouds and a nice breeze... kite-flying weather perhaps? The forecast track has her continuing NW until tomorrow evening, then curving northward and then northeast and out into the Atlantic and away from Bermuda by Saturday.
The forecast also makes her a cat 3 (range: 111-130mph) soon. Currently her official wind speeds are 110mph, so she's essentially a cat 2/cat 3 storm anyway. Cat 3 or higher is classified a major hurricane. As I said a couple of days ago, the vorticity I see in the upper troposphere is something I usually see with a cat 3 storm, and that's what I see at the moment, so I would agree with a potential upgrade. The two things that may help to temper a further increase though are some moderate wind shear (with a small amount of dry air ahead of her) and that the warm 29 deg C water underneath is really only at the surface - beneath the upper few meters of the water column the temperatures are less than 26 deg C.
Her central pressure is estimated to be 968mb. <Science Alert!> Karen M. asked how they know what the central pressure is in a storm when they don't fly planes through or have other direct observations. The answer is that they don't know the exact central pressure. It's usually an estimate calculated from satellite observations, called the Dvorak Technique (named after Vernon Dvorak). Both visible and infrared satellite imagery is used. For the visible, the assumption is made that storms of a certain intensity (and hence pressure and wind speed) have similar features (e.g. one eye, no nose, no ears, a circular body... Picasso painting anyone? ;-)). Infrared satellite images are used to get the temperature of the clouds in the atmosphere and calculate the intensity from that. By looking at the temperature difference between the clouds in the center of the storm (warm) and those surrounding the center (cool) they can come up with an estimate of the intensity which corresponds to an ideal wind speed (max) and central pressure. So for storms that are far away, both wind speed and central pressure are usually estimates and not exact observations (which means that Danielle may already be a cat 3!). The bigger the temperature difference, the more intense a storm and therefore the lower the central pressure. Planes have flown through storms and made direct observations to improve the values wind speed and central pressure corresponding to the intensity calculated from the Dvorak Technique, so these values of pressure and wind speed are usually in the right ballpark, even with storms that are far away. Of course there are all sorts of caveats to this and it's not an exact science. Some variations include needing to take into account the size of the storm, the ocean basin it is in etc. But I won't go into all that now. That's essentially the central pressure business in a nutshell, without going into too much technical detail - I can if you want though, just let me know :-). There is a lot of information out on the internet on the Dvorak Technique if you want to find out the more technical information for yourself. <End of Science Alert!> 
Tropical Storm Earl:  
He's currently at 15.2N, 38.8W moving W at 18mph. His wind speed is estimated to be 45mph, making him a weak Tropical Storm (range: 39-73mph). Water temperatures are a nice 27-28 deg C, so certainly warm enough to sustain him. But poor fellow, he's been suffering from a bout of wind shear and dry air, and at the moment he's looking a bit ragged with not much in the way of convection. However, the vorticity is very good in the lower half of the troposphere, so once he gets over these problems he should pick up intensity. Maybe tomorrow. His central pressure is actually 1003mb - this time I think they based it on an observation from a buoy he passed over (or close to). Seems reasonable to me. He'll continue moving westward for the next day or two before turning NW and following Danielle. I have seen some models forecast Earl becoming a major hurricane as well. I'm not convinced of this yet, and won't be until that vorticity improves in the upper half of the troposphere.
Atlantic Blobette:
So this blobette is actually mostly beyond my crystal ball (i.e. not on my maps and charts and stuff, where 'stuff' is technical jargon for 'more maps and charts'), but it's on the NHC site with a 20% probability of developing into a storm in the next 2 days. The reason I mention it is because of the experimental long-term forecast models. I think I mentioned earlier this year that I'd be watching to see how they performed. So far, they are looking promising - at least one predicted Danielle 10 days before she became a storm. It also predicted Earl, and it also thinks this blobette will develop in a day or two. I thought I'd mention it now so we can all watch and see. The blobette is currently way out yonder, near the Cape Verde Islands. There is a little bit of lower-troposphere vorticity, but earlier today there was a lot of convection.
And finally:
A joke for Steve B. on Bermuda (and others of course) from Bill H. (thanks):


A string walks into a bar and asks for a drink.  The bartender says: "We don't serve strings."  The string goes to a table and musses up his hair.  He walks back to the bar and asks for a drink.  The bartender says: "Hey, didn't I tell you that we don't serve strings?  Are you a string?  " 


The string says: "No, I'm afraid not."

(clue: last two words are a verbal pun - I'll write it out tomorrow) :-) 
Until tomorrow!
Toodle pip!
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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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