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[June 20] PTC3 to Bring Copious Rainfall to North Gulf Coast; Bret Moving Through the SE Caribbean
Potential Tropical Cyclone Three is gradually becoming better organized in the central Gulf of Mexico. The surface circulation has become well-defined, and convection is beginning to take on a curved-band appearance to the NE of the center. However, strong southwesterly wind shear continues to keep the storm lopsided, with the majority of wind and rain occurring well north and east of the center. Rainfall has already begun along the north gulf coast, and will continue over the next couple of days. This heavy rain will be the primary impact of PTC3 along much of the gulf coast. Due to the aforementioned shear, PTC3 is not expected to strengthen much over the next couple days, but will likely acquire sufficient convective organization to be classified as a subtropical or tropical cyclone.
The system is currently interacting and partially merging with a mid-level low over the NW Gulf of Mexico. This low tilts NW with height, and the upper-level low remains just off the upper Texas coast. PTC3 is not expected to move fully beneath the upper low, thus moderate SW shear is expected to remain over the system until landfall along the TX or LA coastline sometime on Thursday. Due to growing proximity of PTC3 to the upper low, convective heating beneath the PV anomaly may cause shear to lessen some just before landfall. Some slight intensification is possible at that time, but the system is not currently expected to have enough time prior to landfall to take full advantage of it. The now robust low-level circulation is expected to become more symmetric, with moisture and potentially tropical storm-force winds wrapping around to the western semicircle. Thus, although most impacts currently remain north and east of the center, by the time of landfall, impacts may extend both to the east and to the west of the storm center when it crosses the coast.
Tropical Storm Bret, after forming SE of Trinidad yesterday, is now moving into the SE Caribbean just off the coast of Venezuela, and maintains and organized appearance, although the effects of southwesterly shear are beginning to manifest. This shear will increase over the next day or so, and is likely to eventually cause the dissipation of Bret in a couple of days. Until that time, however, Bret may bring tropical storm conditions to the southern Caribbean islands, perhaps as far west as Aruba, Bonaire, and Curacao. A tropical storm warning remains in effect for a portion of the Venezuelan coastline and Margarita Island.
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Very close to sandy scenario, in terms of the surface low being right underneath the upper low. Again I ask, is this not strange? I would really appreciate an explanation on why this happens. Thanks Levi
Steve, tropical storms routinely interact with upper lows, but those interactions can differ. Sandy was a very special case, where what we call “phasing” with an upper trough allowed the trough to aid Sandy in a way, rather than destroy it. The majority of the time, upper lows/troughs are detrimental to tropical storms due to the wind shear they produce, but occasionally they can actually help them intensify via processes that we don’t fully understand yet.
Thanks for the explanation.
Levi-
So a more optimal scenario for tropical cyclone development is generally a surface low developing under an upper level high, because then wind shear is minimal….would that be correct?
Great work!
Hal
That is a good rule of thumb – yes. One does not always have to be beneath an upper-level ridge for wind shear to be low, but often that’s a favorable place for a tropical disturbance to be.
Levi, been following you for a few years but now we have a TS on our doorstep in Houston. Can we expect an update video soon? Again, I thoroughly enjoy your work, very detailed and informative!