Sunday, August 26, 2012

Tropical Storm Isaac: August 26, Update B

Still a fine British summer day in my part of Tampa Bay - cloudy, sporadic drizzles of light rain, slightly breezy, warm enough not to need gloves. ;-) Meanwhile in the mid-Keys, Lisa T. said at 5.10pm that “the squalls on Friday were worse than what came through today. It is blustery out now but no rain.”

I spent this lollygagging sort of day backing up my computer (and a cheer erupts from the computer manager gallery!) because over the past few weeks my machine has complained at all the work I make it do and is slowly but surely heading towards a break-down. It’s probably not a bad thing to do before a storm, but I’m <ahem> sure <ahem> that you all back-up regularly anyway, don’t you?

Tropical Storm Isaac’s convection improved after my last update, primarily over the lower Keys. There is still some strong convection in the Gulf, just off the Keys, but the planes found that he hasn’t strengthened so the official winds have been decreased to 60mph, central pressure 992mb. This makes him a mid-strength Tropical Storm (range: 39-73mph). He continued more westward than the earlier forecast track and is still in the Straits of Florida, about 40 miles southwest of Key West. His center is officially at 24.2N, 82.3W, moving WNW at 16mph.

He still has wind shear and dry air that are inhibiting him, but he is now over the deep warm waters I mentioned earlier which would explain the increase in his convection that you can see in the latest infra-red satellite image:

He is still interacting with Cuba, but less so than earlier. Consequently his circulation has improved at all levels of the troposphere. With these conflicting impacts on his intensity and development, I think a mid-to-strong Tropical Storm is a very reasonable thing to expect at the moment.  The NHC have downgraded his future intensity forecast and keep him as a Tropical Storm until tomorrow afternoon, when he gets upgraded to a cat 1. I agree with this downgrade and slower intensification… he won’t be over the deep warm water because the Loop Current is only in the southern Gulf at the moment, it looks the wind shear will persist all the way into the northern Gulf, and the upper troposphere is not very conducive for development. In fact, on the current forecast track (which takes him into Louisiana/Mississippi region on Wednesday), there’s a chance that he won’t intensify to a hurricane at all – if he does, I’m still thinking in the weak-to-mid cat 1/strong Tropical Storm range.

BUT… there is one thing I don’t fully agree with at the moment: the track and forward speed. The high has developed across the northern Gulf. He will either slow down, or move west (currently more likely)… so that track may shift even farther west.

For those of you who are looking at the storm surge, go to tidesonline (, click on the state maps link on the left side of the page, click on the state you want, and then the location. For example, at the moment in St. Petersburg the water level is actually about 1 foot below mean low level! This is because the storm winds are, essentially, pushing water out of Tampa Bay instead of onshore. Here is the latest data from St. Petersburg as an example:

The top graph shows the water level. In this graph, the red line is the actual observed sea level, the blue line is what the predicted water level would be because of the tides, and the green line is the difference between the two. The height of the green line gives you the storm surge level (-1 ft at the moment). The graph below that is wind speed (still increasing in St. Pete) and direction. Below that is air pressure (which you can see is still dropping), and below that is air and sea surface temperature. Wind speed is still less than 15 knots (about 17mph) (1knot = 1.15 mph) so we don’t have Tropical Storm force winds up here yet.

Must go… it’s wine-o-clock! I’ll be back talking about dessert as usual… ;-)


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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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