Updated: November 5, 2010 6:30 am ETThe calendar may have flipped to November, but we still have the tropics to worry about this week.
Tomas is heading northward in the direction of Haiti and eastern Cuba after affecting the Windward Islands as a hurricane
this past weekend.
See slideshow: Tomas pounds Caribbean
 Tomas’ current status (Click for more maps) |
 Satellite (Click to enlarge & loop) |
You may rightly ask: “Why did Tomas weaken from a hurricane this past weekend to a depression on Wednesday and then become a hurricane again Friday morning?”
The major impediment was wind shear. For a tropical cyclone to intensify, you need thunderstorms to be colocated with an area of low pressure near the surface.
However, as you can see in the satellite image below, the low-level circulation was displaced Monday about 100 miles west of the most vigorous thunderstorms. Thus, the weakening we witnessed Sunday into Monday.
Tuesday and the first part of Wednesday, Tomas continued to struggle to match one area of thunderstorms with one area of low pressure.
Slow reorganization took place beginning late Wednesday and continued on Thursday. By early Friday morning, Tomas had a much more impressive appearance on satellite with deep thunderstorm activity near the center.
Contrast the current satellite image below with that from Monday above. Definitely a more organized Tomas!
Click to animate
Now that Tomas is approaching Haiti, let’s first lay out the vulnerability of one of the world’s poorest nations, still recovering from a disaster earlier this year.
Haiti’s vulnerability
Almost 10 months ago, a magnitude 7.0 earthquake centered just west of the Haitian capital of Port-au-Prince claimed up to an estimated 230,000 lives, destroyed 250,000 homes, and left over a million Haitians homeless.
Haiti’s hills are also largely deforested. According to the U.S. Library of Congress, only an estimated 2% of the country had tree cover as of 1988.
In 2008, flooding and mudslides from four tropical cyclones…Fay, Gustav, Hanna, and Ike…claimed almost 800 lives in Haiti.
Thus heavy rain can be as much a danger to Haiti as a hurricane’s high winds. Flash flooding can be more severe in hilly terrain anywhere, with gravity’s help. Now, imagine heavy rain falling on hills devoid of trees. Severe erosion and mudslides are an unfortunate fact of life.
As you can see below, even before the catastrophic January earthquake, Haiti had been ravaged by a series of tropical cyclones.
This hurricane season, relatively speaking, Hispaniola has been fortunate. The lion’s share of tropical storms and hurricanes have either tracked northeast or have formed well west of Haiti and the Dominican Republic.
2010 hurricane season tracks-to-date
Click to see the 2010 hurricane season scorecard
But what about Tomas? Let’s dig into the forecast.
A deep dip, or “trough” in the lexicon of meteorologists, is swinging into the eastern Gulf of Mexico and northwestern Caribbean Sea, steering Tomas north-northeastward toward Hispaniola. Incidentally, this trough is keeping Tomas away from the mainland U.S.
Here are some quick forecast snippets regarding Tomas:
– The center of Tomas should pass between or over far western Haiti and far eastern Cuba on Friday as a hurricane.
– Heavy rains will spread into Haiti and the Dominican Republic. Tomas has the potential to dump 5-10″ of rain, with local amounts over 15″ in Hispaniola. The threat of life-threatening flash floods and mudslides is very high.
– Tomas could bring hurricane-force winds to portions of the Turks & Caicos and southeast Bahamas later Friday and Friday night.
– Despite Tomas tracking well to the north, trailing bands of locally heavy rain may persist over parts of Hispaniola, possibly spreading into Puerto Rico this weekend.
 Projected path (Click to enlarge) |
 Model tracks (Click to enlarge) |