AWIPS images of the DMSP and POES AMSU rainfall rates along with the GOES-12 IR channel imagery (above) shows Hurricane Dean as a Category 4 storm in the western Caribbean during the morning on 20 August 2007. Satellite-based rainfall rates within the spiral bands of the hurricane were as high as 35 mm per hour (1.4 inches per hour), and GOES-12 IR brightness temperatures around that time were as cold as -77º C (-107ºF). Dean continued moving westward across the Caribbean, steered by easterly winds along the southern periphery of a deep layer ridge that was centered over the southeastern US. Refer to the CIMSS Tropical Cyclones site for the latest information on Hurricane Dean.

An animation of GOES-12 visible images (above; Java animation) shows the eye of Dean. When viewed using the normal operational GOES image interval of 15 minutes, the low cloud features within the eye appear to be rotating anticyclonically (clockwise). This is an optical illusion, similar to the “strobe effect”: the low clouds in the eye are moving so quickly that their true motion can only be determined by viewing images more frequently than once every 15 minutes.

Fortunately, GOES-12 was placed into Rapid Scan Operations (RSO) mode beginning at 18:55 UTC, so images after that time were available at 5 to 10 minute intervals. The GOES-12 RSO visible image animation (below; Java animation) shows the low cloud features within the eye rotating in the “correct” direction (cyclonically, or counterclockwise).

A visible image from NOAA-18 (below) provides a closer view of the mesovortex cloud features within the eye of Dean. Use this Java image fader applet to fade between the NOAA-18 visible image and the corresponding NOAA-18 IR image.

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An animation of GOES-12 visible images from the CIMSS Tropical Cyclones site (above) shows that the northern eyewall of Hurricane Dean (rated Category 4, with maximum sustained winds of 145 mph) was affecting the southwestern portion of Jamaica on 19 August 2007. A visible image from NOAA-18 (below) gives a closer view, with some hints of mesovortex structure within the eye region.

The NOAA-18 AMSU-B Channel 16 (89 GHz) image (below) revealed a concentric eyewall structure at 16:55 UTC, and aircraft reconnaissance indicated that the strongest winds were associated with the outer eyewall.

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Hurricanes are classified by the Saffir-Simpson scale, which scale ranges from 1 to 5; the strongest storms are cat Vs. Dean intensified into a strong cat IV storm overnight, with a well-developed eye and excellent upper level outflow at all levels. The 1745 UTC GOES-12 visible image from 18 August (above; Animated GIF) continues to show bands of strong thunderstorms within the cirrus canopy of Dean.

The color-enhanced GOES-12 water vapor imagery from 1745 UTC on 18 August (above) continues to show a lack of dry air in and around Dean. Wind shear values are forecast to remain low, and ocean water temperatures in the northwest Caribbean are high. Dean will likely remain a potent storm for at least the next 48-72 hours.

The yellow numbers on the water vapor imagery show the 120-hour, 96-hour, 72-hour, 48-hour and 24-hour forecast locations (all valid at at 1800 UTC 18 August). The 120-hour and 96-hour forecasts were made when Dean was a tropical depression and a minimal tropical storm, respectively. Forecasts made after the storm was well-organized were far more accurate. Dean is forecast to move over Jamaica on Sunday 19 August, and then make landfall very near Cancun. The 72-hour forecast from today places Dean on the north coast of Yucatan, moving into the Gulf of Mexico.

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An animation of GOES-12 visible channel imagery from the CIMSS Tropical Cyclones site (above) reveals an eye forming as Hurricane Dean reached Category 3 strength over the eastern Caribbean Sea on 17 August 2007.

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