Florence is born

Tropical Depression Six got enough heavy thunderstorm activity surrounding its center to be upgraded to Tropical Storm Florence at 11am EDT. Florence doesn't look much like a tropical storm on satellite imagery, with a very broad center and the main thunderstorm activity well removed from the center. Maximum winds of 30 knots (35 mph) were seen on this morning's 4:33am EDT QuikSCAT pass (Figure 1), which are just below tropical storm force. However, the winds in Figure 1 were taken from the low-resolution 25 km QuikSCAT product, and the higher-resolution 12.5 km QuikSCAT winds did show a few areas of tropical storm force winds. These stronger winds were given as justification for upgrading to a tropical storm. Given the disorganization of the storm and marginal tropical storm force winds on QuikSCAT, the system could have just as easily been held as a tropical depression for one more advisory. The 12.5 km QuikSCAT product is noisier and more prone to error than the standard 25 km product, and is not always used by NHC to make a judgement about upgrading to a tropical storm.

Florence has managed to consolidate the two circulation centers it was struggling with yesterday into one large circulation center. It will probably take another day before the winds tighten up around the center and Florence can begin any substantial intensification. Interfering with this process will be about 10-15 knots of shear and some dry air to the north. The shear should lessen by Thursday, potentially allowing Florence to become a hurricane.

The computer models all forecast that Florence will pass north of the Lesser Antilles Islands, although it is too far in the future to be highly confident of this forecast. A complicating factor is the development of a new disturbance about 800 miles to the east-southeast. This new disturbance, officially designated "Invest 91L" this morning by NHC, is close enough to alter both the strength and track of TD 6. Anytime two storms get within 13 degrees of arc of each other (900 miles), the two storms tend to rotate around a common center (the Fujiwhara effect). The computer models do make some allowances for this effect, but are not very good at handling it. For this reason, one should be suspicious of the track and intensity forecasts for Florence and 91L as long as they are so close. The intensities of both storms can also change as a result of the interaction, with both storms intensifying at a slower rate than they otherwise would, or one storm growing at the expense of the other. If the two storms approach within about 7 arc-degrees of each other (480 miles), this is considered the "zone of death" where one cyclone will surely destroy the other. The surviving storm will not be a "superstorm" that has the combined size and strength of the two storms, however.

The long-range GFS model forecast continues to show Florence becoming a powerful hurricane that threatens Bermuda, but recurves out to sea well east of the U.S. East Coast. Again, it is too early to be confident of this forecast. Keep in mind that early model forecasts are often very unreliable. That is because the center is not well established and often relocates, and that subtle difference can make major
track changes. Also, the global models such as the GFS, UKMET, and NOGAPS represent a weak storm as a very diffuse entity, and that causes problems for the global models and the "zoomed in" models like the GFDL that use a global model (the GFS) as their starting points. Be wary of the track forecasts until the system becomes more established. Tomorrow morning we should have a better idea of the models' reliabilty, since Florence should be better established.


Figure 1. QuikSCAT satellite winds from 4:33am EDT Tuesday September 5 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 and need to have 3 long bars and one short bar attached to the end of the "barb"; there is one barb like this on the east side of TD 6, but it is pointing a different direction than the other barbs around it, and is surrounded by rain-contaminated (black) barbs. One should be suspicious of the accuracy of this lone tropical storm force wind barb.


Figure 2. Preliminary model tracks for Invest 91L, a well-organized tropical wave a few hundred miles west of the Cape Verde Islands.

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, and visible satellite imagery from this morning shows an increase in thunderstorm activity on the west side. The wave is over warm water and is under a modest 10 knots of wind shear, and could be Tropical Depression Seven by Wednesday. Due to its more southerly starting position, this system is more likely to be a threat to land than Florence.

Carolinas
North Carolina is still suffering flooding problems from Ernesto. Where Ernesto came ashore at Cape Fear, North Carolina, the North Cape Fear River is at 16.6 feet, and flood stage is only 10 feet. This is the second highest flood on this river; only Hurricane Floyd of 1999 caused a higher flood. With a strong cold front expected to move through tonight and stall offshore, North Carolina will receive another 1-2" of rain that will make flooded areas slow to recover. Once this cold front does stall over the warm Gulf Stream waters, we need to watch the area off the Carolina coast for possible tropical storm development.

Ioke
Ioke continues its slide into oblivion, and is now a mere tropical storm. 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 Wednesday morning, unless there's something interesting to report on this afternoon.

Jeff Masters