Tropical Depression Six almost has its name

Tropical Depression Six is nearly a tropical storm, as evidenced by winds of 25-35 knots seen in this morning's QuikSCAT pass (Figure 1). Visible satellite imagery shows an increase in curved low-level rain bands forming, and satellite estimates of the storm's strength already put it at minimal tropical storm strength (40 mph). The computer models all forecast that this storm will most likely pass north of the Lesser Antilles Islands, although that is too far in the future to be confident of this forecast. The long-range GFS model forecast continues to show TD 6 becoming a powerful hurricane that threatens Bermuda, but recurves out to sea well east of the U.S. East Coast. Wind shear over the system is low, about 10 knots, but may increase a bit to 10-15 knots over the next two days. After that, wind shear should die down and a large anti-cyclone build over the storm, potentially allowing it to intensify into a hurricane. The large area of thunderstorms about 400 miles southwest of TD 6, formerly designated "Invest 98L", is still there, and may be slowing down the intensification of TD 6. As TD 6 grows, it should be able to absorb the remnants of 98L.


Figure 1. QuikSCAT satellite winds from Monday morning, September 4 2006. Wind speed and direction are coded according to the standard station model, and are color coded (in knots) according to the color scale at the upper right (10 knots = 11.5 mph). Black winds barbs occur where there is rain, and one cannot trust the wind speeds measured in those areas. Tropical storm force winds (35 knots) are colored red, brown, or purple; one can see one red wind barb in TD 6.

Cape Verdes Islands tropical wave
A strong new tropical wave emerged from the coast of Africa Saturday and is a few hundred miles west-southwest of the Cape Verde Islands. The wave has a closed circulation, which can be seen on QuikSCAT imagery from this morning (Figure 1). The thunderstorm activity associated with the wave is limited and disorganized. The wave is under a modest 10 knots of wind shear, and has some potential for slow development over the next few days.

Caribbean tropical wave
The small tropical wave that moved into the Caribbean yesterday has become much less organized and is no longer a threat to develop. The remains of "Invest 99L" can be seen on the QuikSCAT image from this morning (Figure 1) as a small area of black wind barbs (which denote rain) between South America and Hispaniola. There are a few yellow wind barbs in there, representing winds of 20-25 knots. The wind barbs mostly point the same way, with only a small change in wind direction in the wave. This is the sign of a weak tropical wave. A sharp change in wind direction occurs in strong tropical waves, with this strong wind shift eventually amplifying into a complete circular rotation if the wave develops into a tropical depression.

John is dead
The remains of Hurricane John are about 2/3 of the way up the Baja Peninsula, and will spread heavy rains of up to 1-3 inches into Arizona, New Mexico, and western Texas over the next few days. John destroyed many roads and took the roofs off of at least 150 houses in Mexico's Baja.

Typhoon Ioke
Typhoon Ioke is now barely a typhoon, with top winds of 75 mph. Ioke is caught in a large trough of low pressure that is weakening it and recurving it out to sea. Ioke is not a threat to any land.

Next update
I'll have an update Tuesday morning. Have a good Labor Day, everyone!

Jeff Masters