Monday, August 22, 2011

Hurricane Irene: August 22, Update A

As expected, Irene is now a hurricane and the first of the season. She was upgraded overnight as she visited Puerto Rico, where I hear she caused some damage – power loss to over a million people, trees down, flooding etc. No reports of major injuries though (phew).

From the US VIs, Tom reported in at 2.12am with:
“2:12am aug 22. Power still on and uninterrupted since around 7pm last night. raining all night long with heavy wind gusting the dickens out of the vegetation...forget the late mango season. not sure how the rest of the island is faring but the wind part of this storm is still a factor. but, as hurricane and storms go, the worst is facing all of the rain which will impact negatively when it comes 7 inches in a short period.  As a wise old British Virgin Islander was quoted (donkey years ago) as saying about hurricanes and storms..." it's the tail end that stinks"”

And at 11.12am:
“After my 2 am report we fell asleep and got up at 6am to very little wind and mostly light rain(and internet serving tower down with no power). still light rain on and off but we are not free of the systems' rain bearing clouds. Have not heard about the impact on San Juan or Puerto Rico...looks like Irene got stronger as she was leaving us”

From the British Virgin Islands, amongst other things, I read that Sir Richard Branson’s house was completely destroyed (burned) because of lightning and winds from this storm (

Irene has been strengthening slowly and her center is near the Dominican Republic at the moment. As of the last advisory (11am EST) she was at around 19N, 67.5W heading WNW at 13mph. From looking at the satellite images, it looks like her center is now at around 19.1N, 67.7W, but I will put my slight westward estimate down to a cloudy satellite image (puns always intended ;-)), and will quite happily duct tape yesterday’s niggling voice in my head. ;-)  

She’s a weak-to-mid-sized category 1 hurricane (Cat 1 winds range: 74-95mph). Winds are 80mph, central pressure 988mb. Tropical storm force winds extend out to about 160 nmi northeast from the center. It looks like she weakened slightly earlier this morning because both the circulation in the upper troposphere decreased a bit, and also the lower level inflow and upper level outflow (convergence/divergence – that fun & fascinating ‘bit-o-science’ over the weekend J) weakened slightly.  Although the circulation has already improved at the highest levels of the troposphere, I think she will be slow to strengthen today because her outer bands are interacting with Hispaniola.

The forecast has her going north of Hispaniola and through the Bahamas, becoming a major hurricane overnight on Weds evening/Thurs morning. A major hurricane is defined as a category 3 or higher, and has winds stronger than 111mph. Here are the latest conditions that will impact Irene’s intensity:

1. The wind shear is low, and looks like it will remain low, so this won’t inhibit her from strengthening
2. The water temperatures are 29-30 deg C. Water temperatures are warmer than 26 deg C in the upper 75m or so of the water column. That’s a small reduction since yesterday, so for now it will allow her to develop but not as quickly as she could have if she was somewhere else.<
3. She is out of the dry, dusty Saharan Air Layer, so that will no longer inhibit her development.
4. Her vertical structure is looking good, with strong circulation in the lower half of the troposphere, and slowly increasing circulation in the upper troposphere.
5. Finally, and perhaps most critically, the intensity will depend on what her track is. If she is closer to Hispaniola than the current center of cone forecast suggests, then her intensity will not increase as quickly as forecast.

The track is the key to her intensity. Once she gets over the Florida Current and Gulf Stream there will be very little to stop her from growing. The NHC have repeatedly cautioned the reliance of the 4-5 day forecasts (and I would agree with them saying this). Over the last 5 years the average error for the 4 day forecast is about 200 miles, and for the 5 day forecast it is about 250 miles.

The rain and thundery weather is over the DR at the moment, and heading towards Haiti. As usual, Haiti is the most vulnerable at the moment from flooding etc. They will get that later today, unfortunately.

Meanwhile, a small Atlantic blob is trying to develop at around 20N, 33W. Circulation is good in the lowest levels of the troposphere, and poor thing is trying valiantly to get some convection going, but it is surrounded by dry and dusty air.

Here is a satellite map of the Atlantic:

The blob to the left is what’s left of Harvey, the middle one is Irene, and the one to the right is the little Atlantic blob. Pretty cool image, huh? J

Lunchtime over. More later!

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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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