George R. Kasica
METEO 241 Portfolio #4: My Week as Weather Briefer - Day 2 - September 19, 2006

On the second day (click to open) of my week as briefer I decided to switch my focus to hurricane Gordon as at the time hurricane Helene was no threat to land and hurricane Gordon was on course to strike the Azores in the very near future. I felt that this might give us all some practice at trying to accurately predict the track, intensity and time and location of landfall for the storm.

In my initial briefing I used the IR image from the Joint Typhoon Warning Center and the wind probability chart from the National Hurricane Center. The one question I raised in addition to the prediction of the track and intensity was why the wind probabilities seemed to be so low on the above chart.

This question generated a significant amount of discussion as t why they seemed to be so low, and ultimately an answer was provided by Robert Sandberg and confirmed as correct by Steve Seman below:

re: Tropical Weather Briefing #2, Week of September 18
ROBERT SANDBERG - 9/19/2006 10:58 PM
Hey guys, 

I think I might be missing something on the original questions here.  The NHC graphics indicate the probability of sustained hurricane force or tropical storm force winds are based on 1 minute surface wind speed averages depending on track and intensity.  The tropical storm force conditions have a larger radii than the hurricane force winds as you can seen on the cumulative distribution. 

This brings me to a question. Are the hurricane probabilities lower due to the intensity and track or is it due to poor model initialization or combination of both?

CumulativeWindDistribution.gif  

re: Tropical Weather Briefing #2, Week of September 18
Steve Seman - 9/20/2006 12:11 AM
Robert is right.  Gordon is a storm that's barely a category 1 storm (and weakening) as it's moving through the Azores area, and the area of hurricane-force winds extends up to 40 miles from the center.  But, in reality, the areas experiencing hurricane-force sustained winds are pretty small within that larger framework.  That lends itself to lower probabilities.  The probability of tropical storm-force winds is much higher, because they extend out much farther from the center and while it's not entirely certain that Gordon will remain a hurricane for long, it surely will maintain the strength of a tropical storm.

  TS_Wind_Prob_Gordon.gif  

Lesson two to remember: Always understand WHAT you are looking at in terms of a chart and especially the SCALE it is referencing.

From there the discussion turned to the fact that Gordon did not technically make landfall as Prof. Grenci observed and was confirmed by William Burns a short time later.

re: Tropical Weather Briefing #2, Week of September 18
LEE GRENCI - 9/20/2006 07:22 AM
Here's the updated cumulative winds.  Looks like Gordo did not, technically, make landfall.  Close but no cigar.  Am I correct?

  090450P_sm.gif  

re: Tropical Weather Briefing #2, Week of September 18
WILLIAM BURNS - 9/20/2006 09:29 AM
Lee, you are right. It just missed. The eye passed between the islands of Sao Miguel and Santa Maria. 

From the Associated Press (2 hrs 36 mins ago)

"The eye of Gordon moved between the islands of Sao Miguel and Santa Maria, and was rapidly moving east, according to the National Hurricane Center in Miami."

“As it stands right now it will not make land fall,” said Dave Roberts, a hurricane forecaster for the National Hurricane Center. “There are still some winds there. They definitely felt some effect, but the eye didn’t pass on land.”

The full link:  http://www.msnbc.msn.com/id/14907792/

To end the discussion thread (and more or less the lifetime of Gordon as well) Daniel Depodwin brought up the information that the storm was forecast to become an extratropical system in a short time.

re: Tropical Weather Briefing #2, Week of September 18
DANIEL DEPODWIN - 9/20/2006 11:07 AM
The NHC discussion mentions that Gordon is in the process of becoming extraptropical and merging with a cold front:

A STRONG COLD FRONT IS ALREADY VERY NEAR THE CORE
OF THE TROPICAL CYCLONE AND GORDON SHOULD BECOME FULLY
EXTRATROPICAL LATER TODAY.

The 0z CMC (12 hr--see attached) run has a tough time finding a center of circulation for Gordon.  The Vorticity image (upper right) clearly shows the cold front just to the East of the Azores but no sign of the circulation of Gordon.  This shows that the core is basically imbedded in the front at this point.

  CMC0z.gif

Two lessons were learned from today's briefing by me:

  • Always understand WHAT you are looking at in terms of a chart and especially the SCALE it is referencing.
     

  • The models that predict the track of the storm are not perfect and can have a range of error to them as we saw with Gordon narrowly missing making landfall in the Azores.

Now that we've looked at day number two, lets move on to day number three, Wednesday September 20, 2006(click to continue), or click the following link to go back to Monday September 18, 2006.