Hurricane Dean has is being maintained as a weak category 2 hurricane with winds of 100 mph and a central pressure of 976 mb, but this may be generous. Judging by satellite imagery, Dean has been weakening and is looking much more ragged than he did this morning. While there is strong outflow in all quadrants, the circulation itself does not look to healthy.
Not only is Dean's general circulation looking much more ragged, but his eye has also completely disappeared. Despite my utmost effort, I was unable to locate a significant eyewall signature in any of the infrared, water vapor, or microwave imagery. There appears to only be a central dense overcast area.
Why exactly Dean is having so much trouble seems to be the question of the hour. The most obvious culprit that I can come up with is the broad area of dry air to the northwest of Dean.
Like a '49er looking for gold, Hurricane Dean is moving westward at a very rapid clip, topping 25 mph at last report. This should enable Dean to get out of the fairly dry air it's in now and move into the Caribbean quickly where factors are more favorable for strengthening. Despite the dry air, it seems as if Dean isn't going to be held down for long. The computer models are quite clear expecting Dean to strengthen. While not all models can properly resolve Dean's intensity due to limited spatial resolution, they nearly all capture the strengthening trend that we expect.
Two other important factors to look at when examining hurricane strengthening is Sea Surface Temperatures and Tropical Cyclone Heat Potential. Hurricanes need warm water to strengthen and the warmer the water, the stronger a storm can be. Higher than 26-27 degrees Celsius is best.
Tropical Cyclone Heat Potential or TCHP is a slightly more sophisticated way of determining how much "hurricane fuel" the ocean has. TCHP is measured in heat energy per sea water surface area (kilo Joules per square centimeter). In layman's terms, it is the amount of heat available to a hurricane above the minimum theoretical threshold for development. Zero TCHP would barely sustain a Category 1 hurricane, so any number above that is additional energy available for intensification.
The eastern Caribbean is often a quiet zone for hurricane activity. Due to oceanic and large-scale atmospheric influences, hurricanes often struggle in this area. However, the central and western Caribbean often act as supercharging fuel for hurricane intensification. It is in this area that I expect Dean to take off.
The following three images are the forecast track for Hurricane Dean, the TCHP for the region, and the SST's for the region. Note that the hurricane is passing over an area with both high SST and high TCHP which makes for an atmosphere very conducive to strengthening. Click to enlarge.
As you can see, Dean is moving over some very favorable areas which should lead to significant strengthening as soon as the dry air gets out of the way.
A plethora of Hurricane and Tropical Storm Watches and Warnings have been posted for the Leeward Islands on the far eastern edge of the Caribbean Sea. With Dean predicted to strengthen further, interests in most of the Caribbean should take close note of this storm and make preparations as needed. Most recently, watches are now active in Puerto Rico and build west.
For further charting information and satellite shots, see the Hurricane Dean charts page. Click any image to get a larger view.