Though the 2008 season began winding down quickly after Ike made landfall almost four weeks ago, it appears that this season still had at least one additional surprise up its sleeve.   Tropical Storm Marco formed in the Bay of Campeche this past Monday.  What’s unusual about Marco is its size, or lack of size.  Marco became the smallest tropical cyclone ever to form in the Atlantic Basin.  With tropical storm force winds extending out only 10 miles from the center at its peak intensity of 65 mph, Marco was dwarfed by even an average-sized hurricane.

For comparison, I made a composite image of Hurricane Ike and Tropical Storm Marco (above).  Marco’s cloud mass was only 40-50 miles across.  It has been calculated that Ike was about 80 times larger than Marco.  Marco was so small that it could have fit in the eye of Ike at landfall.  I’ve been discussing our Hurricane Severity Index in recent posts. For comparison, Ike’s HSI was 30 out of a possible 50 points (20 for its size, 10 for its intensity). Marco rated only a 5 on the HSI, with 3 points for its intensity and 2 points for its size.

Such “midget storms” are extremely uncommon in the Atlantic Basin, though they’re often found in the West Pacific.  Just last month, there were three such tiny tropical cyclones in the West Pacific at one time. Here’s a side-by-side comparison of Marco and one of the midget tropical cyclones in the West Pacific last month:

It’s interesting to note that midget tropical cyclone in the image above was estimated to be only a depression with winds of 35 mph.  Unfortunately, there is no reconnaissance available in the Pacific, so most such midget storms are ignored.  Clearly, it was as strong as Marco was (65 mph) or stronger.  Dvorak satellite intensity estimates just don’t work for such small storms. So without reconnaissance (which did fly into Marco) we would have never known that Marco was a strong tropical storm.

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