12 September 2016

Ian, season's ninth named storm, forms in central Atlantic

A tropical wave we've been tracking since it left the African coast on September 6th was upgraded to Tropical Storm Ian on Monday morning.  It is in the central Atlantic, nearly 600 miles east of the Leeward Islands and not forecast to affect land at any point.  This is the first use of the name Ian -- it replaces Igor from 2010 which was retired due to its severe impact in Newfoundland.

As the satellite image above shows, it is in an area of strong vertical wind shear... in this enhancement, the low-level clouds show up in the yellow shades, while the upper-level clouds are white.  The surface center of circulation is completely exposed.  It is a minimal tropical storm now (40mph maximum winds), and is forecast to strengthen just slightly as it heads north, then gradually transition to an extratropical cyclone by the weekend.

There are weather satellites in orbit than routinely measure the surface winds over water, they are called scatterometers. One such satellite passed right over Ian this morning and helped justify the upgrade to a tropical storm.  In this swath of wind vectors, the center is found toward the bottom, with winds circling counter-clockwise around it.

Climatologically, the 8th named storm forms on September 21, so this is a bit ahead of an "average" year (using a 1981-2010 climatology).  If you missed my post about the various climatologies, check out When is the ‘peak’ of hurricane season? It’s more complicated than you think.

However, in terms of ACE, this season is slipping behind, now at about 76% of average for this date.

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