Wednesday, November 4, 2009

Eyes Out Towards the Skies

Tropical Depression 11 Forms! - The Hurricane Season is almost over and things had never been more quiet, until this bugger appeared this week. There is a disturbance out there, as many of you know by now, so we urge everybody to keep a vigilant eye on the weather updates. Hopefully it will pass us by.

Tropical Depression Eleven has formed in the Southwestern Caribbean, off the coast of Costa Rica, and appears poised to intensify into Tropical Storm Ida later today. TD 11 has increased its organization and heavy thunderstorm activity this morning, and visible satellite loops clearly show the rotation of TD 11's cloud pattern. The presence of a surface circulation was not evident in this morning's QuikSCAT pass, but the satellite presentation of TD 11 was convincing enough to allow NHC to declare this a tropical depression. QuikSCAT saw top winds in the 25 - 30 mph range this morning, and winds at San Andreas Island, about 80 miles north of the center of TD 11, were easterly at 28 mph at 9am EST.

TD 11 is currently under moderate wind shear, 10 - 15 knots, and shear is expected to remain in the moderate range as long as the storm remains south of 14N latitude (central Nicaragua). Sea Surface Temperatures (SSTs) are 29°C and the Tropical Cyclone Heat Potential is about 40 kJ/cm^2, which is plenty of energy for a hurricane to form, if the center remains over water long enough. There is dry air over the northern Caribbean, but this is too far north to slow down development. A limiting factor for development may be the formation of a tropical disturbance (Invest 96E) 500 miles to its west, over the Eastern Pacific south of Guatemala. If this new disturbance grows strong enough, it may compete with TD 11 for moisture.

The forecast for TD 11
The forecast for TD 11 is highly complex with high uncertainty. Steering currents are weak in the Southwest Caribbean, and TD 11 will move slowly over the next two days. The future steering of TD 11 will strongly depend upon the development and track of the Invest 96E disturbance 500 miles to its west. If 96E develops and tracks northwards towards Guatemala, as suggested by the GFDL model, TD 11 would likely be steered northwards later this week, remaining over water as it approaches the Cayman Islands on Monday. If, on the other hand, 96E moves due west away from 97E, as suggested by the NOGAPS model, 97E might also move due west, over Nicaragua and Costa Rica, and emerge over the Eastern Pacific early next week. Another complicating influence might be the development of an extratropical or subtropical storm in the Gulf of Mexico's Bay of Campeche on Saturday or Sunday. The GFS and ECMWF models are predicting the formation of a low pressure system over the Bay of Campeche this weekend, along the remains of an old cold front. This low is expected to track northwards towards Louisiana, and might act to also pull TD 11 northwards. The exact amount of steering influence this extratropical low and 96E might have on TD 11 depends strongly on how large and intense TD 11 becomes. At present, TD 11 is a very small system, and so is only being affecting by steering influences in its immediate vicinity.

If TD 11 intensifies into a tropical storm in the next three days, as seems likely, the storm will probably tap into moisture from the Pacific Ocean. This moisture will flow over Costa Rica, western Panama, and southern Nicaragua into TD 11's circulation, bringing 3 - 6 inches of rain today through Friday. Heavier rains are likely along the east coast of Nicaragua, where an intense spiral band of rainfall has formed this morning. An Air Force Hurricane Hunter mission is scheduled to investigate TD 11 this afternoon to see if it has become Tropical Storm Ida. (
Dr. Jeff Masters' WunderBlog, Weather Underground)

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