Katrina (August 24, 2005): Its Name is Katrina

GOES-12 Visible imagery of TD-12/Katrina

GOES-12 Infrared (IR4) imagery of TD-12/Katrina

GOES-12 Water Vapor (IR3) imagery of TD-12/Katrina

The westerly wind shear that had been hindering Tropical Depression 12's growth continued to weaken as the new day began. As a result, deep convection was able to ignite and the beginnings of a spiral band became apparent. By 8:00 am EDT (1200 UTC) aircraft reconnaissance data indicated that the maximum sustained winds (MSW) had risen to 35 kt, thus the National Hurricane Center (NHC) upgraded TD-12 to Tropical Storm Katrina, the 11th named storm of the 2005 Atlantic Hurricane season.

Katrina is captured in this true color image from the MODIS unit on the Terra satellite at 11:45 am EDT (1545 UTC), shortly after being declared a tropical storm. At each cross, the MSW in knots are on the top and the central pressure in millibars is on the bottom. The thick segment of the track line shows the storm's track for 8/24/05. Notice how the vast majority of convection is being sheared to the east.

For the first part of the day, the storm was steered primarily by a weakness in the low-level subtropical ridge. This ridge is a semi-permanent structure that fluctuates in size and shape; usually it spans the majority of the Atlantic basin, but occasionally will expand westward over the Southeastern US. Since the ridge is an area of high pressure, tropical cyclones cannot pass through it, thereby trapping the storms at low latitudes. As a storm reaches the edge of the ridge, it can begin tracking around the ridge and into the mid-latitudes. This transition out of the tropics is referred to recurvature. Once poleward of the subtropical ridge, tropical cyclones are typically assaulted by the westerlies and the associated shear. By this point, the storms are accelerating rapidly and begin losing tropical characteristics. Inevitably, they will transition into extratropical cyclones, if they aren't sheared apart or absorbed by a frontal boundary first.

While the concept of the subtropical ridge is relatively simple, nature has a tendency to make things complicated. Although subtropical ridges are often cited in tropical weather discussions in a general sense, they occasionally will develop a weakness and in some cases splinter entirely into multiple ridges. It was one of the weaknesses that set TD-12/Katrina on a generally northward track early on the 24th.

However, as the storm was able to consolidate and build up convection, the overall depth of the storm increased, causing it to be steered by systems at higher levels in the troposphere. Thus, as Katrina deepened, the primary influence on its track shifted from the weakness in the low-level subtropical ridge, to a developing mid- to upper-level ridge over the northern Gulf of Mexico and the southern US. As a result, by the end of the day, Katrina had begun turning westward towards Florida.

These two 85 GHz microwave images of Katrina were captured about 12 hours apart (top: 8:00 am EDT, bottom: 7:45 pm EDT). Early in the day, areas of deep convection are seen almost exclusively to the east and southeast of the center. By evening, wind shear had weakened, allowing convection to develop all around the center, although this convection was clearly weaker. The overall structure improved throughout the day, as evident by the dark blue and light green cloud lines in the evening image. There seemed to be a more robust inflow from the far south and southeast, along with what might be the beginnings of outer rainband forming to the west.