Tropical weather analysis - July 2, 2010

Invest 95L

An extratropical low pressure system recently broke off from the unseasonably deep longwave trough that moved only slowly across the United States over the last several days. This low formed over the Big Bend region of Florida, and has recently moved southwest over the northeastern Gulf of Mexico. This low was designated Invest 95L by the National Hurricane Center today. Visible satellite animations as well as long range doppler radar out of Tallahassee, Florida strongly notes the axis of this low pressure area, centered at approximately 29N 84W. Recent buoy observations indicates that the low pressure area associated with this system is probably not yet at the surface, as surface pressures are high, and not significantly falling.

The lowest I pressure I could find was 29.99 inches from Station SGOF1, which is also located at 29N 84W. Interestingly, the buoy observations, while not at all indicating a closed low, are at least indicating that a broad area of low pressure exists in association with this system. This is a potential sign that 95L is attempting to work this low pressure area down to the surface.

95L is currently experiencing 10 kt of northwesterly vertical shear. This shear appears to be induced by the eastern periphery of a building ridge. However, this shear, based on the aforementioned graph from CIMSS, appears to be located above 300 mb, and 95L's cloud tops do not presently reach anywhere near that high into the atmosphere. Hence, I expect this shear to have little effect on the system.

I expect this system to move SW for the next day or two, followed by a gradual bend to the NW or N after that. On this track, the system will have, at most, 72 hours over water before moving back inland over the northern Gulf Coast. Upper-level winds appear to be only marginally conducive at best, with a band of 15 to 20 kt northerly shear forecast to impinge upon the system throughout most of its existence. Hence, development, if any, will be slow to occur. The National Hurricane Center is giving this system a 10% chance of becoming a tropical cyclone within the next 48 hours, and this is a reasonable forecast. Regardless, residents all along the northern Gulf Coast should carefully monitor the progress of this system over the next several days.

Regardless of development, locally heavy rainfall and strong gusty winds will be possible as the system slowly deepens (since at least some minimal deepening is possible, the aforementioned northerly shear notwithstanding). Note by "deepen", however, I do not necessarily mean "become a tropical cyclone". I'm just saying that I don't feel that the northerly shear is going to be strong enough to prevent the system from lowering its surface pressure a little bit over the next couple days, which will in turn generate deeper convection, which will in turn generate heavy rains and strong winds.

The GFS, UKMET, and ECMWF indicate that an area of low pressure will form along this axis while moving across the northern Gulf of Mexico. The CMC also indicates this, but suggests that it will occur on the Atlantic side, after the system traverses Florida. The CMC solution seems ridiculously unlikely, however, given the currently building ridge to the north of the system. Hence, I have discounted it.

Elsewhere

Elsewhere in the tropics, the reliable ECMWF model is forecasting a tropical depression will form across the western Gulf of Mexico on July 8. The precursor to this system is probably a tropical wave currently analyzed near 40W. I will continue to monitor this system very carefully, as the GFS also indicates some sort of low pressure in the area by 168 hours. The ECMWF nailed the genesis of Alex a week in advance, and it has been consistent with this prediction as well.

The NOGAPS also continues to indicate the possibility of a tropical disturbance across the western Caribbean in the next five days. The instigator to this system is probably a tropical wave currently analyzed along northern South America along 70W. Development in the western Caribbean from this wave appears unlikely for now, based on steering patterns, which take it into the East Pacific.