16 August 2013

Both Atlantic systems weaken

In the past 24 hours, both Erin and the disturbance near the Yucatan have lost a lot of steam.

The Yucatan system is just now about to enter the Gulf of Mexico, but the surface circulation is devoid of deep convection, and a strong upper-level low to its north is introducing hostile vertical wind shear.  In the images below, the left one is an enhanced infrared image which allows you to see the low clouds in yellow and the higher clouds in white... and the right one is a water vapor image, which mostly shows what's going on in the upper atmosphere.  I added basic low-level features in green and upper-level features in red for clarity.

Since it is now reduced to just a low-level circulation, it should track more toward the west-northwest, rather than turning toward the north.  This shifts the longer-range landfall location to Texas or northern Mexico... but, "landfall" makes it sound more significant than it really is.  Wherever it goes, its biggest impact will be heavy rain -- perhaps not even that!

However, if it does head more westward, environmental conditions would improve for the disturbance in a couple days, perhaps allowing it to regain a little organization.  The current "worst case" model (which still isn't TOO bad) is HWRF, which strengthens it into a hurricane and shows a landfall near Brownsville on Monday night.  Other models are less bullish.

Surface wind (shaded contours) and surface pressure (line contours) according to the most recent HWRF model run.  Not a likely solution, but one with a slim possibility, and worth being casually aware of at least.
The key to the evolution of this system will be if it goes more west than north, and if new deep convection can develop and persist near the center.  If not, then it can be written off.

Much further east, Tropical Depression Erin is gradually heading into drier air and cooler waters as expected.  It is also now reduced to a skeletal cloud swirl centered about 400 miles west of the Cape Verde islands.  It would need to hang around for another three days or so until it tracks into a slightly more favorable environment... but still not ideal.  Most models dissipate the storm within the next 3-5 days, and track it northward into the north-central Atlantic graveyard.

Tropical Storm Erin west of the Cape Verde islands.  (NRLMRY)

For a bit of hurricane history, on this date in 1992, the season's fourth tropical depression formed 700 miles west of the Cape Verde islands.  On August 17th, it was upgraded to the season's first tropical storm: Andrew.  Seven days after getting named and significant intensification, the tiny Category 5 hurricane plowed head-on into Florida, just south of Miami.  It is one of just three Category 5 hurricane landfalls on the U.S., and the most recent.  Do you know the other two??

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