Caribbean disturbance slowly developing

Caribbean tropical disturbance
The main area of concern today is a tropical disturbance in the Caribbean sea, south of Hispanolia. Surface pressures have started to fall in association with this disturbance, and deep convection has slowly increased the past 12 hours. Wind shear values have increased this morning over the leading (western) portion of the disturbance, and are now about 10 knots, which is only marginally favorable for a tropical depression to form. The trailing (eastern) portion of the disturbance south of Puerto Rico has less shear (5 - 10 knots) over it, and this portion of the disturbance is showing the greatest growth in deep convection this afternoon. The shear is forecast to to remain constant or decrease over the next 48 hours as the disturbance tracks west-northwest at 15 mph. There are currently no signs of a surface circulation, and the disturbance is still relatively small and disorganized, so it is not expected to develop into a tropical depression today. A reconnaissance airplane is scheduled to visit the area on Tuesday.

An upper-level low pressure system over Cuba is forecast to weaken and move northwards during the next three days, which would lessen the shear over the disturbance and steer it more to the northwest, as seen in the early track model forecast from the BAMM model, shown below. I give this disturbance a 60% chance of becoming a tropical storm by Thursday. However, the upper level winds are far from ideal for a tropical storm, and this system may struggle to survive if it does manage to form.



Mid-Atlantic disturbance
A tropical disturbance located about 1200 miles east of the Lesser Antilles remains poorly organized, and is suffering from wind shear imparted by a large upper-level low pressure system to its west. Development of this disturbance is not expected for the next two days.

Elsewhere in the tropics
Long range models show the possibility of more tropical development off the coast of Africa during the week, as well as the Caribbean and Gulf of Mexico. We still have about three weeks remaining of the peak period of hurricane season, and I expect two or three more tropical storms will form between now and mid-October.

Rita
I mentioned in my blog yesterday how Port Arthur got a direct hit by the eye or Rita, but escaped catastropic storm surge damage. The image below, constructed by NOAA's Hurricane Research Division based on data taken by the NOAA hurricane hunter aircraft, confirms that this occurred because the east eyewall of Rita with its powerful southerly winds never blew over the bay Port Arthur lies on. Thus, water from the open ocean was not forced up into the bay by the eyewall's winds. The maximum storm surge hit a very sparsely populated area of the Southwest Louisiana coast. The small town of Cameron (population 2000) was the largest town along this stretch of coast, and suffered damage similar to what was seen in Mississippi from Hurricane Katrina. The winds of the east side of the eyewall made landfall due south of Lake Charles, pushing the worst storm surge up to that city.


Figure 2. Winds of Rita at landfall as measured by the Stepped Frequency Microwave Radiometer (SFMR) instrument on the NOAA hurricane hunter aircraft.

Jeff Masters