Katrina (August 25, 2005): Florida Feels it First

GOES-12 Visible imagery of Katrina

GOES-12 Infrared (IR4) imagery of Katrina

GOES-12 Water Vapor (IR3) imagery of Katrina

As the third day of Katrina's existence began, it continued to make its fateful turn to the west. In fact, by the end of the day the storm was tracking south of due west. In the afternoon, as it tracked over the northwestern Bahamas it began to feed off the very warm water of the Gulf Stream. This is likely the cause of a burst of deep near the storm's center. By 5:00 pm EDT (2100 UTC) Katrina had intensified into a category one hurricane with maximum sustained winds (MSW) of 70 kt.

Katrina is captured in this true color image from the MODIS unit on the Aqua satellite at 2:00 pm EDT (1800 UTC), shortly before reaching hurricane status. At each bar, 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 8/25/05 until 8:00 pm. Notice how the center is obscured by high clouds and a small dry slot entering from the southwest and curving towards the center.

Less than two hours later, Katrina made its first landfall near the boarder between Miami-Dade County and Broward County with an estimated maximum sustained wind (MSW) speed of 70 kt. As the storm tracked across the extreme southern tip of Florida very little weakening occurred, likely due in large part to the Everglades. Studies have found that tropical cyclones that track over wetlands, particularly the Everglades, do not weaken nearly as rapidly as they do over dry land. As a result, at the end of the day, as Katrina was nearing the Gulf, Doppler radar indicated that its MSW had only dropped to just below the 64 kt threshold for hurricane status.

For the first time in its life, NOAA WSR-88D weather radars were able to observe the storm, providing insight into its internal structure. It was radar imagery that revealed an eye, a structure satellite imagery never picked up on. This eye was present before Katrina became a hurricane and became even better defined after it made landfall.

This is the same MODIS image as above, with reflectivity data from the Miami radar (KAMX) overlaid on top of it. The dry slot is much more pronounced on radar. Furthermore, it is clear that a weak eye has formed, something that was obscured in the MODIS image by dense high clouds.

This loop of  base reflectivity (left) and base radial velocity (right) radar data from the Miami radar (KAMX) spans from 2:00 am EDT to midnight. At the top of each pane is color scale and at the bottom right is the time in EDT. Around the time of landfall, the amount of precipitation in the northern half of the storm decreases dramatically. (Click on the image to view the loop at full size)

As Katrina made its way across the state, the strongest winds and heaviest rains were found south and east of the center. This was odd because generally speaking, the front-right quadrant (the northwest quadrant in this case) is the most intense and the left side being considerably weaker. However, strong northerly wind shear forced the bulk of the convection southward. Much of southern Miami-Dade County received over 10" of rain, while north of the center, in northern Miami-Dade County and Broward County only 2" to 4" of rain were recorded.

The total amount of precipitation from the storm, as estimated by radar, up to midnight. The rainfall was clearly dramatically higher south of the storm center (the yellow line is the storm track) than north of it.