Well, Hurricane Ida did develop last week, but it turned out to be not very much of a threat to south Florida.  It did become only the fifth hurricane since 1851 to enter the Gulf of Mexico during the month of November.  Of course, it’s quite likely that a number of such hurricanes over the southern Gulf of Mexico were not properly identified prior to modern satellite and reconnaissance.   Still, Ida was a rarity in that it was a late-season Category 2 hurricane in the Gulf, and it looks like it regained hurricane strength very close to southeast Louisiana on Monday, November 9th.

About the time that Ida was making landfall as a tropical storm rapidly transitioning to an extratropical storm, we received word that at least one platform in Mississippi Canyon (an offshore lease area southeast of the mouth of the Mississippi River) had reported hurricane-force winds for a 2-3 hour period before the two wind sensors were blown away.  At that time, around 3PM CST Monday, November 9th, Ida was classified as a tropical storm with 70 mph winds by the National Hurricane Center.  This new data suggested that Ida may have briefly regained hurricane intensity that afternoon, so we asked the person in charge of the platform to take a look at the wind report from Monday afternoon.

The next day, the data arrived in the form of a very long text file.  Observations were taken at one second intervals, so there was plenty of data to sort through.  With the help of my coworker, Joseph Spain, we were able to import the data into MS Excel and create graphics of the wind trace.   The winds were certainly well above hurricane strength, but the data were recorded at a height of 97 meters.   A hurricane’s winds are measured at 10 meters above the surface, and we use 1-minute average wind, not 1-second winds.  We contacted the National Hurricane Center to let them know of our findings, and to get some assistance in properly converting the 97 meter winds down to the 10 meter level.  James Franklin, a forecaster with the NHC was quite helpful.  He was also quite interested in receiving the data, as he and Lixion Avila were just starting to prepare the post-storm report on Ida.  This new data could mean that Ida would be reclassified as a hurricane just southeast of the Louisiana Delta in the final report.

Below is a plot of Ida’s track across the northern Gulf on the 9th of November.  Note the location of the Mississippi Canyon report and notice that at the very same time as the high winds there, a recon plane just happened to drop an instrument package to measure Ida’s vertical wind profile.  The instrument measured a 73 kt (83 mph) surface wind in Ida’s southwest quadrant at the same time that the platform was reporting hurricane force winds.  But this surface wind measurement was discounted by the NHC because flight level winds were only 53 kts (62 mph) at the time.  Generally, winds at the surface are only about 80-90% of flight level winds, so it didn’t make sense to the NHC that surface winds would be higher than flight level winds.  The 73 kt measurement was discounted as a brief wind gust near a squall and Ida was kept a tropical storm on the 3PM CST advisory that afternoon.

We received permission to share the wind data with the NHC a few days ago  While we cannot divulge the owner/operator of the platform, we can post the wind data here.   Like I said, there were 2 sensors atop the platform, both at a height of about 300 feet above the surface of the water.  The first plot is the actual, unreduced 1-second wind trace.   Below that is the same wind trace but reduced to the 10-meter level (88% of the speed at 97 meters).  Finally, we had to take the 1-second reports and construct a trace of 1-minute average winds.    It is this trace that clearly identifies a period of sustained hurricane force (64kt / 74 mph) winds to the west of Ida’s center that Monday afternoon.

It’s unfortunate that the sensor (both of them) failed.  But the trace does indicate the winds to be above hurricane force from about 2059Z (2:59PM CST) to 2126Z (3:26PM CST), before the instrument was blown away.  Looking back at a satellite loop of Ida over the platform, it appears that hurricane-force winds might have lasted another 30-45 minutes at the platform.  But very shortly thereafter, Ida encountered quite strong wind shear.  The wind shear ripped all the newly-formed squalls from its center, beginning a rapid transformation to a non-tropical or extratropical storm before it reached the Alabama coast later that evening.  Wind reports from land-based stations in Ida’s path were only in the 25-35 mph range.  It doesn’t appear that Ida produced even tropical storm sustained winds across Mississippi, Alabama or the Florida Panhandle.

With just under 10 days left in the 2009 hurricane season, I think it’s safe to say that the season is over.  Strong wind shear dominates the tropics, and that wind shear will only be increasing through the end of the month.   The 2009 hurricane season will go down as a very quiet one, with the possible exception of a small footnote about Hurricane Ida in the Gulf of Mexico in November.

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