Weatherdem's Weblog

Bridging climate science, citizens, and policy

Atlantic Tropical Weather Update 8/13/09

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The only development associated with Tropical Depression Two has been negative since yesterday.  I’ll start with the disturbance’s vitals:

Center located near 14.0N, 37.6W; moving W @ 9mph; maximum sustained winds of 30mph; central pressure ~1006mb.

T.D. 2 has wrapped the dry Saharan air than accompanied the storm into the Atlantic into its circulation.  Combined with some fairly strong vertical wind shear, T.D. 2 is currently unable to maintain organized convection around its center.  It has weakened slightly since yesterday and is now expected to remain at its current strength over the next 4-5 days.

T.D. 2 has remained near 14N for three days now, and should remain near that latitude during the next two days before making a slow turn around asubtropical ridge and moving more northwesterly.  Given the time and distances involved, that turn is now expected to take place to the northeast of the Windward Islands.  Since T.D.2’s track has remained so relatively far to the south during its existence, it seems that the GFS and UKMET, as I mentioned yesterday, have offered track forecasts with the most skill.  Now, the GRDL models have joined the other two in keeping the storm further south than before.  The HWRF and NOGAPS models keep T.D. 2 further north.

Every intensity model forecasts a pickup in strength after 2-3 days.  It should be noted the same forecasts showed a steady increase in instensity from yesterday to today, so take them with a large grain of salt.

We’re getting to a point where previous storms in the same location have a higher chance of impacting the Caribbean and the mainland U.S.  Hurricanes Connie in 1955, Chantal in 2001 and even Andrew in 1992 were located at a similar point in the Atlantic in the beginning of their lifecycles.  The vast majority of storms, however, remained far out to sea including Irene in 2005.  I think what they’re picking up on is the increase in SSTs after two days.  No longer will they be marginal for development, they will be sufficient for rapid development.  Wind shear would remain T.D. 2’s nemesis.

Okay, onto the next topic: the tropical wave that moved off of the African shore yesterday.  It remains vigorous, but elongated.  The broad disturbance, Invest-90, is centered about 250 miles SSE of the Cape Verdes Islands.  A 30-50% chance of tropical storm development remains possible in the next 48 hours.

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