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Posted at 11:29 AM ET, 08/21/2012

Tropical depression 9 forms, tropical storm Isaac likely soon; warnings for eastern Caribbean

Enhanced infrared image of Tropical Depression 9 at 9:15 a.m. EDT. (NOAA)
The disturbance that has been making its way across the Atlantic since August 16 finally became sufficiently organized to become tropical depression 9 (TD9) at 5 a.m. EDT. TD9 is likely to become tropical storm Isaac later today and perhaps a hurricane within two days. It poses a near-term landfall threat in the eastern and northern Caribbean and potentially the U.S. Gulf and/or Atlantic coasts early next week.

As of 11 a.m. EDT, the intensity estimate is 35 mph with a 29.74“ (1007mb) central pressure. Since midnight, strong thunderstorms have persistently been developing near the center of the circulation, which not only is a criteria for earning the upgrade to a depression, it’s also a foundation for further intensification.

TD9 is located about 400 miles east of the central Lesser Antilles and heading due west at a fairly speedy 20 mph. On this path, the center is forecast to reach those islands by midday tomorrow, but outer rainbands and gusty winds will begin affecting them before then.

Tropical storm warnings are in effect for some of the Leeward Islands (shaded in blue below), and a tropical storm watch is in effect for the remainder of the Leeward Islands as well as the Virgin Islands and Puerto Rico (shaded in yellow below). You should always look for the latest forecast, watches, and warnings at the National Hurricane Center website.

National Hurricane Center track forecast and tropical storm watches (yellow) and warnings (blue) (National Hurricane Center)
The track forecast is of course of great interest to everyone along its potential path. The steering winds that govern the motion of tropical cyclones often aren’t the same for storms of different intensities. Weak storms are typically shallow; that is, the bulk of their structure exists at low levels in the atmosphere... as such, weak storms are steered by lower-level winds.

Steering currents for weak (top) and strong (bottom) storms as of 8 a.m. EDT today. The location of TD9 is marked by the red L. Speeds are in knots... multiply by 1.15 to get mph. UW/CIMSS.
As a storm gets stronger, the depth of the atmosphere responsible for steering the storm increases. To determine the steering currents that will govern a particular storm (at a particular time), winds are averaged over a given depth in the atmosphere. Two extreme examples are given to the right: the top figure shows the present steering flow for very weak systems like TD9, and the bottom figure shows the flow for an extremely strong hurricane.

The weaker flow (top) takes TD9 on a more southern track - almost due west. The stronger flow (bottom) turns it more towards the north.

The present westward motion is being driven by a large mid-level ridge (high pressure area) to the storm’s north. According to this morning’s run of the GFS global model, that ridge is forecast to weaken by Saturday, allowing the storm to gain some latitude and move north of due west.

As the ridge breaks down further and moves off to the east, the storm would get steered even further north, traveling over Cuba on Sunday. This continues into Monday, and the storm intensifies quite dramatically north of Cuba as it heads for southern Florida early next week.

Th south Florida landfall is one scenario, so shouldn’t be taken too seriously (yet), but also shouldn’t be ignored. It’s worth noting that this general picture has been painted on several recent runs.

Related: Could hurricane wreak havoc at Republican convention? Will it be Isaac?

But drastically different outcomes remain possible. For example, if the storm begins to turn northward a little sooner, it would easily brush by Florida and head up to the Carolinas or even miss the coastline altogether. If it takes a little longer to turn north, the storm would could bypass Florida and enter the Gulf of Mexico.

It is simply too far out to say which of these scenarios will verify, but it is something to be aware of and to monitor very closely. It’s never a bad idea to have emergency plans and materials ready, so when a storm does come, you’re prepared.

The plot below shows track forecasts from a large number of models (some simple, some sophisticated), and as I explained in my post yesterday, this is a valuable technique for assessing the model uncertainty or confidence... the less spread there is among the tracks, the more confident we might be in those models.

Track forecasts from a variety of models over the next 5-7 days.
The intensity guidance (below) is mostly in agreement that this should become the season’s forth hurricane this week. The official NHC forecast is shown in the red line labeled ‘OFCI’.

5-day intensity forecasts from several models. NCAR/TCGP.
Once this reaches sustained winds of 40mph, it would be upgraded to Tropical Storm Isaac. Yesterday, I received a comment on my post about the notorious history of “I” storms lately. I’ll copy my reply here for everyone:

Since 2001, several ‘I’ storms have been retired: Iris, Isidore, Isabel, Ivan, Ike, Igor, and Irene. Nothing magical about the letter, but more a function of the time of year we typically reach the 9th named storm: right when the Cape Verde season is ripe and long-lived major hurricanes are most common.

There are two other areas of interest in the Atlantic today again, but are being overshadowed by the potential of TD9. While they are both candidates for new tropical depressions in the coming days, neither will affect land soon. One is in the west-central Gulf of Mexico, and the other is in the far eastern Atlantic.

By Brian McNoldy  |  11:29 AM ET, 08/21/2012

Categories:  Latest, Tropical Weather

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