Thursday, September 23, 2010

Tropical Depression Lisa & Tropical Depression 15: September 23, Update A

Gollygeewhiz! Some of these storms are like boomerangs this year! We have the remnants of Julia in the middle of the Atlantic. She's been trying to stage a come-back tour but although the circulation has improved, her convection could do with a make-over. She's around 32N, 37W and is just bugging the fishies really. And then there is Lisa...
Tropical Depression Lisa:
The ultimate boomerang storm! Lisa formed near the Cape Verde Islands, headed west/northwest for a few days lulling the residents of the Cape Verde Islands into a false sense of security and sunshine, and has now returned to drop off some rain she forgot to leave the last time she was in the area. But at least she's been downgraded to a Tropical Depression because of that low pressure front to the northeast that I mentioned yesterday. She's around 17.5N, 28.9W and moving N at hardly any speed at all (2mph). Winds are 35mph and central pressure is 1002mb. Vorticity is still really good in the lowest half of the troposphere, but the convection just isn't there right now. The NHC think she might get back to being a Tropical Storm, but she won't be very strong and will stay in the Atlantic - I see nothing to disagree with in that assessment.
Tropical Depression 15:
This is the former Caribbean Blob. It was upgraded a few hours ago and is currently centered at 13.9N, 76.2W and is moving W at 15mph. Winds are 35mph and the central pressure is 1007mb. Convection has been really strong over much of the Caribbean because of this blob. I sure hope no-one is trying to get a tan on the deck of a cruise ship in the Caribbean this week! The vorticity (circulation) has slowly improved over the lowest half of the troposphere, but there's nothing higher up (which is a good thing). Although the official direction of movement is West, I think it's moving WNW and has started to move away from the South American landmass. I also think the forecast track on the NHC page is reasonable until a point. I think that it will continue to head WNW/NW towards the Nicaragua/Honduras region, and then possibly on to Belize and Mexico. It continues to experience a bit of wind shear, which is still why the development is slow. However water temperatures are rather warm at around 28-30 deg C and the warm waters of over 26.5 deg C can be found in the upper 75-100m. As it gets closer to Central America it will be closer to the deepest warm water in the ocean and, depending on it's proximity to land and how much wind shear there is, we might see it blossom like it's brother, Karl. The forecast track gets interesting after Monday (when it is over the Yucatan) - the forecast shows it doing a right turn and heading north. Although this looks odd, it is not unusual for a late-season storm. We saw it in 2005 with Hurricane Wilma, which ended up making landfall in southwestern Florida and crossing the state. It's really too soon for me to tell but just so you know a possible reason behind such a sharp turn - <weather science alert> it's because a low front is forecast to come down from the northwest, and will whisk the storm away along its frontal boundary and towards the northeast. A forecast track like this depends on three things:
1. If there is such a front,
2. if that front is strong enough relative to the storm to have an impact, and
3. where that front is when it meets the storm. 
If the front moves faster than forecast, the sharp bend could occur sooner. If it is slower, a weak storm could dissipate over land and not be affected at all. Or a stronger storm could get farther north before turning.  <end of weather science alert>
Until tomorrow my friends!
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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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