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Posted at 11:00 AM ET, 10/ 6/2010

Otto may form, but tropical season slowing?

By Don Lipman

A look back at this season and others

* Afternoon shower? Full Forecast | Hansen: 2012 to be hottest year *

td17-100610.gif
Infrared satellite image of tropical depression 17. Source: NASA.

Tropical depression (TD17) formed early this morning north of Haiti and the Dominican Republic and could be named tropical storm Otto some time later today. Because it contains some characteristics more typical of storms in the mid-latitudes, it is technically a "subtropical" weather system. Maximum sustained winds are around 35 mph, just shy of tropical storm intensity of 39 mph. Track models unanimously sweep TD17 out to sea, so it's no concern to land areas.

Should TD17 evolve into Otto, seasonal forecasters will have remarkably predicted the season's activity. To date this season, the actual number (14 named storms, 7 hurricanes) of storms is closing in on long range predictions (15-16 named storms,7-8 hurricanes).

Hopefully, activity will diminish this month - the late August and early September peak behind us. But that hasn't always been the case, as long-time residents who remember Hurricane Hazel in mid-October 1954 will attest.

Defying all odds, Hazel took an inland track from the Carolina coastline and accelerated to over 50 mph so that its wind field had little time to diminish by the time it reached the DC area, which was buffeted by 98 mph wind gusts, the highest ever recorded here. I'll revisit Hurricane Hazel in more detail at a later date.

Past seasons

Since about 1950, when hurricanes/tropical storms were first given names, we had never exceeded 21 until 2005, when 28 storms were recorded. Of these, seven became major hurricanes, a record-tying five reached Category 4 and a record four reached Category 5. However, it is reassuring (somewhat) by one estimate that for the 83 years between 1924 and 2007, only two hurricanes have made continental U.S. landfall as a Category 5 storm--Camille and Andrew.

In terms of property damage, the worst hurricane on record, of course, was Katrina in August 2005. Along with her nasty sisters, Wilma (the most intense ever recorded in the Atlantic Basin, with a minimum pressure of 882 mb and 184 mph winds at its peak) and Rita, there was well over $100 billion in damages along the Gulf Coast during that summer. And despite great strides during recent years in reducing hurricane deaths, the storms claimed over 2300 U.S. lives (about 2000 from Katrina alone), second only to the great Galveston Storm of 1900, which resulted in 6000-9000 deaths.

Hurricanes Katrina, Wilma, and Rita developed mainly in the Gulf of Mexico and Caribbean, west of the other primary breeding grounds for monster Atlantic hurricanes: the distant, eastern part of the Atlantic near the Cape Verde Islands, off Africa. There, the trade winds can sometimes whisk an insignificant atmospheric "wave" into the open Atlantic on a westward journey that can last two weeks or more and thousands of miles.

During colonial times, the proximity of many of these storms to the autumnal equinox led early Americans to call them "equinoctial" storms or "line" storms, thought to be a reference to the tropic of cancer, the imaginary "line" near the hurricane breeding zone that encircles the world at approximately 23 ½ degrees N. latitude (it slowly changes position over time). By the way, the word hurricane comes from the Caribbean god of evil, hurican.

Past seasons and the mid-Atlantic

To be sure, our area is affected by these tropical systems on a fairly regular basis, as we experienced last week. But their impact is usually as decaying, moisture-laden tropical storms (wind speeds under 74 miles per hour) or their remnants, rather than hurricanes.

This is not to say that tropical storm rainfall can't cause havoc. It often does. In 1972, for example, a dying Hurricane Agnes was still capable of causing hundreds of millions of dollars in flood damage throughout a wide swath of the mid-Atlantic, and elsewhere. And who can forget Isabel, whose rampage resulted in a $3 billion path of destruction through the mid-Atlantic in 2003. Again, neither of these was even a hurricane upon sweeping through our area.

To better understand why hurricanes, which are really heat engines, rarely visit us at full strength, we need to focus on where their energy comes from (warm water) and where their highest winds are concentrated (the right-hand, [usually the eastern], semi-circle of the storm). (1)

Even if a hurricane were to head straight for us from the South Carolina coastline, it must travel hundreds of miles over land (as Hazel did). Since hurricanes aren't usually fast movers (Hazel was), by the time we would come under its influence, winds would have greatly diminished due to the loss of its fuel source and the effect of land friction (Hazel's winds didn't diminish markedly).

Also, since most hurricanes begin to "recurve" toward the northeast upon reaching our latitude, such a storm usually passes by to the east (Hazel didn't), placing us in its less intense western, or left-hand semi-circle.

(1): As viewed straight ahead from a hurricane's eye, one would experience the highest winds on the right-hand side because there the storm's forward movement is in the same direction as its winds.

(CWG's Jason Samenow contributed to this post)

By Don Lipman  | October 6, 2010; 11:00 AM ET
Categories:  Lipman, Tropical Weather  
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Comments

"...Caribbean god of evil, hurican."

"hurican" should read "Juracán", then translated to Spanish "Huracán", which is most similar to hurricane.

See http://en.wikipedia.org/wiki/Juracan

Couldn't resist commenting on this, being from Puerto Rico and all :)

Posted by: MaxPowerDJ | October 6, 2010 11:26 AM | Report abuse

@MaxPowerDJ

Good point. Noted :)

Posted by: Jason-CapitalWeatherGang | October 6, 2010 12:44 PM | Report abuse

Agreed: TD #17/Otto is primarily "subtropical"...hybrid and without fronts, but comma-shaped rather than circular.

MaxPowerDJ I've seen the spelling "Huracan" or "Hunrakan"; the reference is to the Mayan or Carib wind god. A similar Greco-Egyptian god Typhon or Seth-Typhon provides a source for the word "typhoon"; another source is the Chinese "tai feng" meaning "great wind". The Japanese called one such storm the "Kami-kaze" or "wind of the gods" when it scattered the fleet of invading Khublai Khan...they tried this trick again against our fleet at the end of WW II using suicide aircraft, but failed.

Posted by: Bombo47jea | October 6, 2010 1:39 PM | Report abuse

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