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July 2013

Tropical Storm Chantal Threatens Lesser Antilles, Hispaniola, then Bahamas & SE US

   Posted by Levi at 2:34am on July 9, 2013

Invest 95L formed a closed circulation last night and was upgraded to Tropical Storm Chantal. Some slight strengthening has occurred today with thunderstorms persisting near and southeast of the center, which has been intermittently exposed. The reconnaissance aircraft mission this afternoon found flight-level winds in the NE quadrant supportive of 50mph surface winds. However, surface pressures were found to be 1010-1011mb near the center of circulation, with only weak and variable winds in the SW quadrant. This indicates that Chantal is still somewhat disorganized and weak, despite good convective organization on satellite imagery. This is likely due to the rapid trade wind flow in which Chantal is embedded, which makes it difficult to maintain a coherent circulation. While only weakly organized, Chantal is capable of producing the 50mph winds measured by the recon plane due to an already strong pressure gradient on the northern side of the storm due to a strong subtropical ridge to the north.

Chantal’s WNW motion is expected to continue for the next 48 hours as she tracks along the southern periphery of the subtropical ridge. In about 48 hours, all of the models agree that Chantal will likely pass over at least a portion of Hispaniola. Flooding will be a concern for Haiti and the Dominican Republic as a result. By this time, an upper low currently over the Bahamas will be in the vicinity of southern Florida, weakening the western edge of the subtropical ridge and allow Chantal to turn more towards the northwest into the Bahamas, with a reduction in forward speed. In 72-96 hours, a longwave trough will dig into the eastern U.S. and help turn Chantal more towards the north, possibly clearing the Bahamas. In 96-120 hours, all of the models agree that a trough-split will occur, with the base of the longwave trough splitting away over the north gulf coast, allowing ridging to build back in to the northeast of Chantal. This is likely to force the storm back towards the southeastern U.S. coastline near the end of the forecast period. Exactly where this turn occurs will largely depend on the timing of the arrival of both the longwave trough and Chantal herself, and uncertainty is large at this time frame. However, a turn towards the SE US appears likely.

While upper-level winds and warm ocean temperatures support intensification of Chantal, history has shown that storms with high central pressures approaching the lesser Antilles typically continue to struggle after crossing into the Caribbean. Thus, only limited intensification is expected, and the forecast peaks Chantal at 55-60mph before hitting Hispaniola, not quite as strong as the 70mph peak currently forecasted by the NHC. The models all agree that Chantal will have significant interaction with Hispaniola in about 48 hours, which should greatly weaken the system. Thereafter, wind shear may increase due to an upper trough near the northern Bahamas, which may hinder restrengthening of Chantal in the Bahamas. However, as the trough-split occurs over the SE US, the resulting upper low over the north gulf coast will begin backing westward away from Chantal, placing the storm in a region of light upper winds in between the aforementioned upper low to the west and another one to the east. This would be a favorable outflow pattern, and Chantal is expected to restrengthen during this time as it makes a turn towards the SE US coast. Exactly how much strengthening occurs is an uncertain question, as this is still about 5 days away, and the setup then may not be exactly as it is forecasted to be now. Regardless, the Bahamas and SE US should closely monitor Chantal’s progress. Hispaniola should also keep a close eye on Chantal, as serious flooding could result from heavy rains as her circulation passes over the island in about 48 hours.