Data in katrina.dat is summarized  from
the web page http://www.wunderground.com/hurricane/at200511.asp

The columns represent:

Column 1: Time(GMT)
Column 2: Day of Month (All August 2005)
Column 3: Latitude (N)
Column 4: Longitude (w)
Column 5: winds (mph)
Column 6: pressure (millibars)

For example, the first four rows are:

18  23  23.1   75.1      35    1008     
 0  24  23.4   75.7      35    1007     

The first row indicates that at 18 hours GMT on August 23 2005, 
Katrina was at latitude 23.1 north, and 75.1 west.  Maximum winds
winds were 35 mph, and the pressure was 1008 millibars.

18 hours GMT corresponds to 13 hours Central Daylight Time, i.e. 1pm.
(CDT is the time zone for New Orleans).  CDT is GMT minus 5.
My source for that information
is the web page: http://www.dxing.com/utcgmt.htm

The second row indicates that at 0 hours GMT on August 24 2005, 
(i.e. midnight, or 24-5 = 19 hours CDT, or 7pm on August 23)
Katrina was at latitude 23.4 north, and 75.7 west.  Maximum winds
winds were 35 mph, and the pressure was 1007 millibars.