An area of low pressure is expected to develop in the Gulf of Mexico sometime Wednesday. It will certainly “look” like a tropical system, but it will lack tropical characteristics. Don’t be fooled – it would bring the same impacts either way. The system will eventually lift to the north and rain chances will gradually increase. Strong easterly winds will lead to coastal flooding and high seas. Highest rain totals will likely stay southeast and east of Baton Rouge. The best chance for rain will be Wednesday night and early Thursday.
A Coastal Flood Warning is in effect for portions of Southeast Louisiana and South Mississippi through 7 AM Thursday – this includes areas surrounding Lake Maurepas and Lake Ponchartrain. Minor to moderate coastal flooding is expected in these areas and east-facing shorelines will likely be the most impacted.
A Gale Warning is in effect through Thursday morning for the open Gulf waters east of the Mississippi, including the Breton and Chandeleur. Winds will be out of the east between 25 and 30 knots with higher wind gusts. A Small Craft Advisory is in effect for our area tidal lakes due to winds between 20 and 25 knots.
Passing showers and embedded thunderstorms are expected Wednesday through early Thursday as the low pressure system drifts inland. Most of the rain will stay east of our area.
There has been talk about a center of low pressure in the Gulf – This low will form within the next 24 hours, but it will not be tropical in nature. Currently, an upper-level low is closing off and is triggering showers and storms in the Gulf.
Upper-level divergence will aid in height falls just off the coast of Louisiana, which will promote a surface low to form. This low will form as a baroclinic low – or due to a horizontal temp gradient. Notice the temperature spread in the mid-levels across the low.
Note that tropical cyclones (TCs) are barotropic – having a uniform temp field, such as the tropics near the equator. They also want the center of circulation to be vertically stacked in the atmosphere. This system will be tilted with the upper-low to the northwest of the surface low.
Now this system is not forming as a tropical cyclone, but can it become one? Surface analysis will show a cold and warm front attached, indicating a separation of different air masses. Tropical cyclones are non-frontal. Even if the fronts dissipate, cool sea surface temperatures and high wind shear create hostile environment.
Looking at forecast models, the GFS (and Euro) depicts this low being a cold-core system for its lifespan (A->Z). Tropical cyclones exist as deep warm-cores, or shallow warm-cores for most subtropical cyclones.
So, all this means is that this low will be a baroclinic, cold-core system that will most likely stay non-tropical for its lifespan. Arlene will have to hold off for now. But there will still be some impacts including heavy rain, breezy winds, and coastal flooding.
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