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June 2013

Bubbling Tropics to Bring Heavy Rain to Florida and SE U.S.

   Posted by Levi at 11:52pm on June 2, 2013

The tropics continue to be active in the vicinity of the Yucatan Peninsula, where a weak monsoonal gyre is developing as expected, promoting moderate thunderstorms over the peninsula and the northwest Caribbean. This gyre appears to consist of a string of lows, oriented SSW to NNE. Surface observations indicate a weak low east of Ciudad del Carmen, Mexico, and another one near Merida, Mexico. A third low may be developing against the pressure gradient just north of the Yucatan Channel under the influence of a mid-level disturbance. This new, northern-most low will likely become the main center of the system as it rotates a bit to the northwest during the next couple days to a position due north of the Yucatan Peninsula. Low pressure will likely consolidate a little more in this location and then begin drifting northeastward on Tuesday into the eastern Gulf of Mexico.

Now that we have a disturbance to track, it is apparent that environmental conditions are not particularly favorable for tropical development. The system is large, and will take days to organize. In addition, the subtropical jetstream is racing over the southern Gulf of Mexico, imparting 30-40kt of shear on the region. This will be lifting closer to the north gulf coast during the coming days under the influence of passing shortwaves to the north and building convection to the south. The resulting pattern aloft will be divergent, supporting low pressure development, but still sheared, favoring a heavily east-weighted storm system. This pattern is typical of early June. Tropical storms have formed under similar conditions in this location, but they are usually weak, with most or all of their convection east and north of the center of circulation. With the CMC now being the only global model depicting a tropical cyclone in the Gulf of Mexico, and the environmental setup being obviously marginal, the probability for significant tropical development is rather low. A broad, sheared system is expected, with gradual deepening as it moves northeastward in a few days, but with winds mainly below tropical storm force.

The future track of this system is likely to be similar to many early-season tropical disturbances in the Gulf of Mexico. The shortwave currently moving into the Mississippi Valley is not expected to be able to capture the low as far south as it is, but it will help it start gaining latitude over the next two days. By Tuesday, the nose of the mid-level sub-equatorial ridge along 20°N over the northern Caribbean is expected to impart a slow northeastward motion to the low towards Florida. On Thursday, a new shortwave trough will move into the Mississippi Valley, and this is expected to accelerate the system northeastwards across the northern Florida Peninsula sometime Thursday Night. Heavy rainfall is expected to be the main threat as a diffluent flow aloft allows a broad area of convection to develop east of the low center, and rainfall amounts may exceed 10 inches in parts of central/southern Florida. After crossing Florida, height rises near Bermuda in the wake of the previous shortwave should keep the system fairly close to the coastline of the southeastern United States. The ECMWF and ECMWF ensembles are in closest agreement with this forecast. Baroclinic influences of the new trough to the northwest of the system will likely enhance rainfall on its northern side, affecting the Georgia and the Carolinas on Friday. The western and northern Bahamas are also expected to continue receiving rainfall on the southeastern flank of the low during this time.

Overall, a broad, messy system is expected to move out of the Gulf of Mexico, across Florida, and up the eastern seaboard of the United States this week. Heavy rainfall, with storm totals in excess of 10 inches in southern/central Florida, is expected to be the main hazard. Chances for development into a tropical storm are slim, and even if development occurs, winds will not be a significant threat.