Dean in the eastern Caribbean

August 17th, 2007 |

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The big weather headline for the next week — for North America at least — will surely be Hurricane Dean, a category 3 hurricane over the far eastern Caribbean Sea (see the CIMSS Tropical Cyclones site for the latest information on Hurricane Dean). Earlier today (Friday), Dean entered the Caribbean passing north of St. Lucia and south of Martinique.

The color-enhanced water vapor loop above (shown at 3-hour intervals) shows Dean approaching the northern Windward Islands of the Carribean. The thunderstorms surrounding the hurricane center are vigorous and tall, with brightness temperatures cooler than 210 K. Sea Surface Temperatures along the storm’s track get progressively warmer. As the storm approached the Caribbean, wind shear values deduced from satellite winds at 0300 UTC, 0600 UTC and 0900 UTC show mainly low values with a jump at 0600 UTC that may have slowed intensification as the storm approached the windward islands. Another factor that may have delayed intensification is the large region of dry air surrounding Dean. This is the large black region in the water vapor imagery that surrounds Dean to the north and west. Analyses of Saharan Air Layers at 0000 UTC 17 August and 1200 UTC 17 August from Meteosat 8 and GOES West show that the dry region in the water vapor imagery likely originated over the Sahara. Air masses rich in Saharan dust are known to suppress hurricane formation. Satellite data can be used to detect such dust. Satellite data from the 12 micrometer channel is compared to satellite data in the 11 micrometer channel. Where the (12-11) value is positive (that is, where the 11-micrometer brightness temperature is colder than the 12-micrometer value), dust is likely present. Both Meteosat-8 (over the Equator at 0 W) and GOES-11 (GOES-West, over the equator at 135 W) have a 12-micrometer sensor. Unfortunately, from the perspective of dust detection over the Atlantic, GOES-12 (GOES-East, over the Equator at 75 W) does not.

Now that Dean has entered the Caribbean, the dry air indicated in the water vapor imagery has eroded. Little Saharan dust is detected. Wind shear values are low, and heat content in the water is high. There is little to prevent Dean from becoming a very dangerous storm in the western Caribbean.

The visible image at 1745 UTC 17 August shows the category-3 storm (110-knot winds at the surface based on 125-knot flight-level winds). Strong thunderstorms are present in the bands that spiral out from the visible eye. The color-enhanced infrared imagery from the same time shows outflow in all quadrants. Evacuation of mass through upper-level outflow is expected if central pressure falls are occurring.

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