Jamaica system continues organizing

A broad 1005 mb low pressure area centered just south of the island of Jamaica continues to become better organized tonight. The areal coverage of the deep convection continues to increase, and a very impressive thunderstorm blow-up with very cold cloud tops has appeared just east of the island. Upper-level outflow has now appeared on the west and north sides of the system, and an upper level anti-cyclone has developed on top. Wind shear continues to drop, but more slowly than this afternoon, and is still in the 5 - 10 knot range. All these factors support continued development, and a tropical depression is likely to form here by Sunday. One complicating factor may be the presence of Jamaica so close to where the center is trying to form. This may slow development by half a day or so at most, since Jamaica is a relatively small island.

Global computer models forecast that the shear will continue to decrease over the area Saturday and Sunday, and the disturbance is expected to stay in the central or western Caribbean for at least the next five days, which would give ample time for this system to grow into a hurricane. The latest 18Z (2pm EDT) run of the GFS model is very unimpressed with this system, and keeps it a weak disturbance that gets swept up in a trough and pulled northeastward into the Bahamas. While this track is certainly possible, the GFS is probably underdoing the intensity. The environment this system is embedded in is very favorable for intensification into a hurricane. However, the 18Z run of the GFDL model is definitely overdone; it brings the system up to hurricane strength Saturday night, and then to a major hurricane by mid-week as it slowly tracks towards Honduras or Belize. While the GFDL is bringing this system to hurricane strength too fast, it does have the right idea about this potentially being a major hurricane. I believe that if this system stays in the western Caribbean for the next five days, the chances of it becoming a major hurricane are good. The GFDL had a very similar idea with Hurricane Rita--only it was too fast with its intensity forecast, and Rita ended up being a major hurricane three days after the GFDL originally forecasted this to happen.

It bears repeating that the eventual track of any tropical storm or hurricane that forms is impossible to forecast with any reliability, since steering currents are very weak and a some erratic motion is likely. The UKMET and NOGAPS models favor a track towards Honduras and Belize, while the GFS takes the system northeast across Cuba and the Bahamas. The early track models (i.e., BAM, LBAR and VICBAR models) have not been run yet for this system, but I will post them when they become available. If this system does eventually affect the U.S., the most likely target would be the Florida Keys or southwest Florida, as there are many troughs of low pressure whizzing by that would grab this system and steer it to the northeast once it got far enough north.

Cape Verdes tropical disturbance
A tropical disturbance about 500 miles southwest of the Cape Verde Islands continues to have some potential for slow development as it tracks west-northwest over the open ocean.

New England
New England continues to suffer the onslaught of a very wet stream of tropical air from the southeast that has caused nine straight days of rain. The axis of moisture has shifted slightly eastwards today, finally giving New York City a break from the 6 - 8 inches of rain that has fallen the past two days alone. This tropical onslaught will continue moving northeast over the weekend before exiting northern Maine on Sunday.


Figure 2. Lots of rain in the Northeast the past week, but currently just a few areas of major river flooding, in New Jersey.

Katrina's winds revisited
In my last blog entry on this subject, we discussed the Florida Sun-Sentinel article commenting on new findings that indicate Katrina was only a Category 3 hurricane at first landfall near the mouth of the Mississippi, and a Category 1 hurricane over New Orleans. The article was rather imprecise in its use of the Category system for ranking hurricanes, and I interpreted the article to mean that Katrina was a Category 1 at landfall in Mississippi. Upon re-reading the article, I think what they were trying to say was that Katrina had Category 1 force winds over New Orleans, not that the storm itself was a Category 1. As several of you have pointed out, it is pretty difficult to have a hurricane with a 927 mb pressure (Katrina's pressure at landfall in Mississippi) with just Category 1 winds. Katrina was a least a strong Category 2, and perhaps a weak Category 3 hurricane at landfall in Mississippi. While Katrina did have unusualy high winds aloft compared to surface winds (which NHC noted on one of their discussions during the storm), this difference was not enough to make Katrina a Category 1 hurricane at landfall in Mississippi. Sorry for sowing the confusion!

My next post will be Saturday morning about 11am.

Jeff Masters