Friday, August 24, 2012

Tropical Storm Isaac and Tropical Storm Joyce: August 23, Update B

I’d like to take a moment to reflect on the immortal and wise words etched on the cover of a little book I’ve read called The Hitchhiker’s Guide to the Galaxy: “Don’t Panic”.  I heard things today that make it sound like people in Tampa Bay are expecting a direct hit by a cat 5 storm. Dearie me. Have a cup of tea and don’t watch so much scary TV (I heard from Vickie that some reporter said today that “A hurricane could be certainly on its way to Tampa”…)!  If that doesn’t work, move on to chocolate, or wine and cheese (hmm.. cheese J). It’s good to be prepared – I have my hurricane supplies (ice cream, wine, water, batteries, candles, sunscreen, bug-repellant, ice cream, wine) – but no need to stress before we know what’s what, right?

Tropical Storm Isaac
No matter how hard I try, I can’t find the center of circulation where the NHC keeps trying to tell me it is. At the moment they say it is at 16.7N, 68.7W and the storm is moving WNW at 18mph. They have kept moving his center southwards relative to their previous forecast during the day. Isaac does have a very poorly defined center, but it still looks like it is south of the official estimate. I am going to assume they can see something that I can’t see because I only have night-time images at the moment. If their center is correct, it means that the convection is mostly in the southwest quadrant of the system.

I agree with the WNW movement though. The pressure fields have changed slightly, which now allow him to move in that WNW direction. They have slowly shifted his 5-day forecast track to the west, which takes him over Cuba, away from Florida and into the Gulf of Mexico. Now landfall is projected to be in the northern Gulf (AL,MS, panhandle of Florida) on Tuesday afternoon.  I’m surprised that they haven’t put any warnings out for Jamaica… even on a WNW track, it looks like it will pass close by. I think this westward shift in the long-term track is reasonable, but I wouldn’t be surprised if he skirted the southern side of Hispaniola and passed between Jamaica and Cuba in the short term.  

His intensity has taken a bit of a hit since my earlier post. I really think he was stronger than a weak 40mph TS earlier, but he has dropped back down to something like the official 45mph speed they currently have (central pressure 1002mb). From this satellite image you can see that his structure and convection is not as good as it was earlier (although it is improving again):

The decrease in intensity was because although the sea surface temperature is the same (around 28 deg C), it looks like he passed over a region where only the upper 50m of the water was warmer than 26 deg C. On top of that, there continues to be some wind shear that is impacting his outer edges - you can still see this in the satellite image because the east side is quite messy and the clouds stream off to the east.

The NHC have downgraded his intensity forecast (because his intensity decreased, suggesting he was stronger than 40mph at some point) so he will no longer be a hurricane in the Caribbean. I think this is quite reasonable if he heads towards Hispaniola, making landfall at the border of Dominican Republic/Haiti. Although he has the intensifying factors of the warm water there is also some wind shear and dry air, and as he approaches Hispaniola he will interact with land too. If he doesn’t take that path and stays over water longer then of course he may intensify further. Although they say he will become a hurricane in the Gulf, this is not certain – it really depends on his track in the Caribbean. I’d take the 3-5 day part of the forecast with a big lump of rock salt.   

A NOAA plane has made more detailed observations over a larger area around Isaac now and those data will be fed into the models which (from the NHC) “SHOULD IMPROVE THE ACCURACY OF THEIR FORECASTS FOR THE NEXT ADVISORY CYCLE.” I look forward to seeing that in the morning.    

I heard from Tom on St. Thomas this morning (just before 10am): “wind shifting. now coming from southeast. expecting things to get more intense for a few hours. not too much rain yet but that is usually near center or at tail. power still on”. I assume it all went ok, although it looks like they are still getting a bit of rain.

Tropical Storm Joyce
She did take a more northerly path than the forecast, and in doing so, she went right into some wind shear and some dry and dusty air, which resulted in a spectacular collapse in her convection. This is now being followed by a decrease in her circulation (although she still has some in the lower half of the troposphere). They downgraded her to a Tropical Depression. She is officially at 16.3N, 43.7W, moving WNW at 14mph. Winds are 35mph, central pressure 1008mb. Because she still has some circulation, it is worth keeping an eye on her until she has completely dissipated. It looks to me like she is at around 20N, moving NW. The official track is more NW than WNW, and does take her to Bermuda by Tuesday, but the satellite image shows that her center is already to the east of this official track, so even if she is moving NW, she’ll be east of Bermuda. Maybe I’m missing something because it’s night so I’ll see if it looks any different by daylight, otherwise I’m not sure why they are so far off on the track.

More tomorrow of course, which looks like it will be another crazy packed day. Silly storms. Sigh. ;-)

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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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