Tuesday, August 18, 2015

Stirrings in the Subtropical Atlantic

It has been a quiet tropical storm season so far this year. As of today there have been only three named storms: T.S. Ana from May 8-11, T.S. Bill from June 16-20, and T.S. Claudette from July 13-14.  This morning Tropical Depression #4 was identified by the National Hurricane Center, and this afternoon it was upgraded to Tropical Storm Danny.

The first three named storms of 2015.

While this season may seem quiet, it is actually running close to normal this year so far. Normally by this date in August there are only three to perhaps four named storms, with one of those hurricane. None of the three storm this year attained hurricane strength.

Climatology of tropical cyclones in the Atlantic Basin. The green lines intersect at about today's date.

NOAA's outlook for the 2015 hurricane season, updated on August 6, maintained its earlier outlook for a below average tropical season. The outlook estimates a 90 percent probability for the following:

    6-10 named storms, which includes the three named storms to date
    1-4 hurricanes
    0-1 major hurricanes
The climatological average for a season is 12 named storms, 6 hurricanes, and 3 major hurricanes.

The outlook for a below-average season is the state of the current oceanic and atmospheric conditions and predicted conditions through the fall. These include the strengthening El NiƱo, which tends to produce strong vertical wind shear and enhanced sinking motion across the tropical Atlantic and Caribbean Sea, both kryptonite to tropical storm development, and cooler than average sea-surface temperatures in the tropical Atlantic.

There were only eight named storms in 2014, and then you have to back to 1997 to find a season with that few storms. The period from 1991 through 1994 were seasons with 8, 7, 8, and 7 storms respectively.  Residents of the Atlantic and Gulf coasts should not get complacent and drawn in to a false sense of security, however. It only takes one storm to cause devastation. Hurricane Andrew in August of 1992 was only one of seven storms that year, and was, at the time of its occurrence, the costliest hurricane in U.S. history. It is now second only to Katrina in 2005 (adjusted to 2010 dollars).

So what is the normal character of the season at this time? The season rapidly spins up to a peak about September 10, with a small secondary peak in mid October.

The typical origins and paths of tropical cyclones in August and September are shown below.

The origins and prevailing tracks of tropical cylones in the Atlantic Basin in August (top) and September (bottom).

Tropical cyclones can originate just about anywhere in the basin from August 21-31, with a band from the west coast of Africa through the subtropical Atlantic to the Caribbean most favored.

Locations of tropical cyclone formation for the period from August 21-31. Data for the Atlantic is from 1851-2009.
Source: National Hurricane Center

These maps and much more information can be found on the National Hurricane Center's Tropical Cyclone Climatology web page.

Tropical Storm Danny is currently expected to attain hurricane strength Thursday afternoon. It will several days before any threat to land can be determined. In the meantime you can follow the National Hurricane Center's outlooks and advisories on the NHC website.

Advisory on Tropical Storm Danny issued at 5:00 p.m. EDT August 18.

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