donsutherland1
09-07-2004, 01:52 PM
Based on an examination of six past sub-920 mb hurricanes for which a large amount of pressure data is available, the odds of Ivan's growing into a sub-920 mb hurricane are low but not zero.
The hurricanes assessed were:
Allen (1980): 899 mb
Gilbert (1988): 888 mb
Hugo (1989): 918 mb
Isabel (2003): 915 mb
Mitch (1998): 906 mb
Opal (1995): 916 mb
All of these hurricanes met the following criteria:
• Had an explosive deepening phase (drop of 26 mb or more over a period of 24 hours or less)
• Saw no increase of more than 5 mb in pressure prior to first dropping below 920 mb after or during the explosive deepening phase
A look at "near-misses" or hurricanes that achieved 920 mb to 925 mb minimum pressures also reveals that such storms typically meet the above criteria for sub-920 mb storms. Only one such "near-miss" hurricane had experienced a pressure increase that exceeded the above criteria.
Those storms examined were:
Andrew (1992): 922 mb
David (1979): 924 mb
Floyd (1999): 921 mb
Gloria (1985): 920 mb
David saw a brief 6 mb rise but only after interaction with the Antilles before resuming its drop to 924 mb.
Thus, if one looks for a sub-925 mb storm, one likely needs to look for the following:
• An explosive deepening phase (drop of 26 mb or more over a period of 24 hours or less)
• No increase of 10 or more mb in pressure prior to first dropping to or below 925 mb after or during the explosive deepening phase
Ivan saw its central pressure increase 21 mb after its explosive deepening phase brought it to 948 mb (a drop of 39 mb in 15 hours). At the same time, this increase in pressure occurred without any interaction with land.
With its initial explosive intensification phase having failed to see a sustained drop in the pressure, against the above criteria, it would appear that Ivan would not become a sub-920 mb hurricane.
However, there may be some data that suggests that Ivan's prospects for strengthening below 920 mb might not been eliminated altogether:
Hurricane Allen saw its central pressure rocket 32 mb and then come back down explosively, but after Allen had initially achieved sub-920 mb status. What happened is that Allen saw two significant pressure increases due to interaction with land:
http://weather.unisys.com/hurricane/atlantic/1980/ALLEN/track.gif
Nevertheless, working against this idea, one could make the argument that Allen was in a sustained explosive deepening phase and it was only the interaction with the land and not the termination of the explosive deepening that led to its increases in pressure.
Another argument for possible strengthening beyond 920 mb would be the small sample size. Unfortunately, large amounts of pressure data are not available for addition sub-920 mb hurricanes such as Camille.
Based on the above information, I believe that the odds of Ivan's growing into a sub-920 mb hurricane are low but not necessarily zero.
It will be interesting to see how things ultimately work out.
The hurricanes assessed were:
Allen (1980): 899 mb
Gilbert (1988): 888 mb
Hugo (1989): 918 mb
Isabel (2003): 915 mb
Mitch (1998): 906 mb
Opal (1995): 916 mb
All of these hurricanes met the following criteria:
• Had an explosive deepening phase (drop of 26 mb or more over a period of 24 hours or less)
• Saw no increase of more than 5 mb in pressure prior to first dropping below 920 mb after or during the explosive deepening phase
A look at "near-misses" or hurricanes that achieved 920 mb to 925 mb minimum pressures also reveals that such storms typically meet the above criteria for sub-920 mb storms. Only one such "near-miss" hurricane had experienced a pressure increase that exceeded the above criteria.
Those storms examined were:
Andrew (1992): 922 mb
David (1979): 924 mb
Floyd (1999): 921 mb
Gloria (1985): 920 mb
David saw a brief 6 mb rise but only after interaction with the Antilles before resuming its drop to 924 mb.
Thus, if one looks for a sub-925 mb storm, one likely needs to look for the following:
• An explosive deepening phase (drop of 26 mb or more over a period of 24 hours or less)
• No increase of 10 or more mb in pressure prior to first dropping to or below 925 mb after or during the explosive deepening phase
Ivan saw its central pressure increase 21 mb after its explosive deepening phase brought it to 948 mb (a drop of 39 mb in 15 hours). At the same time, this increase in pressure occurred without any interaction with land.
With its initial explosive intensification phase having failed to see a sustained drop in the pressure, against the above criteria, it would appear that Ivan would not become a sub-920 mb hurricane.
However, there may be some data that suggests that Ivan's prospects for strengthening below 920 mb might not been eliminated altogether:
Hurricane Allen saw its central pressure rocket 32 mb and then come back down explosively, but after Allen had initially achieved sub-920 mb status. What happened is that Allen saw two significant pressure increases due to interaction with land:
http://weather.unisys.com/hurricane/atlantic/1980/ALLEN/track.gif
Nevertheless, working against this idea, one could make the argument that Allen was in a sustained explosive deepening phase and it was only the interaction with the land and not the termination of the explosive deepening that led to its increases in pressure.
Another argument for possible strengthening beyond 920 mb would be the small sample size. Unfortunately, large amounts of pressure data are not available for addition sub-920 mb hurricanes such as Camille.
Based on the above information, I believe that the odds of Ivan's growing into a sub-920 mb hurricane are low but not necessarily zero.
It will be interesting to see how things ultimately work out.