Jamaica disturbance getting better organized
A broad 1005 mb low pressure area centered over the island of Jamaica continues to become better organized. There has been an increase in intensity and areal coverage of the deep convection this afternoon, and some upper-level cirrus clouds moving northwards away from the storm show the beginnings of what may be an upper-level outflow channel starting to develop. An upper level anti-cyclone is beginning to develop on top, and the wind shear continues to drop--down to 5 - 10 knots. This low enough to support continued organization. The presence of Jamaica so close to where the center is trying to form will probably not pose a significant problem for the storm, 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 I believe that a tropical depression is likely by Saturday or Sunday. Steering currents are very weak, 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.
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.
Cape Verdes tropical disturbance
A tropical disturbance about 500 miles southwest of the Cape Verde Islands continues to slowly improve in organization. Visibile satellite imagery and QuikSCAT satellite winds suggest that a surface circulation may be forming here, and some modest upper-level outflow has developed to the north. The system is headed towards an area of low wind shear, which may allow some further development over the next few days as it tracks west-northwest over the open ocean. Tropical storms developing this far east in mid-October are very rarely a threat to the Caribbean or North America.
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!
I'll have an update tonight by 10pm if the Jamaica system continues to develop. I'll post my follow-up to the Katrina winds posting I made yesterday when I get it done. This is a complicated subject, and it's taking me a long time to do the write-up, sorry for the delay!