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A Few Tropical Thoughts

We still have some time left in the ‘official’ hurricane season. The reason I put quotes around ‘official’ is that tropical activity can occur in any month, and the time classified as hurricane season has shifted over the years. Currently hurricane season is slated to begin June 1, and end November 30.

The chart below produced by the National Hurricane Center is a popular one I have used in various forms during lecture over the years. The importance of the graphic is to understand that it represents how much tropical activity (for the Atlantic basin, including the Gulf of Mexico and the Carribbean Sea), based upon historical evidence, is projected to occur within each time frame over a 100 year cycle.

Number of Storms per 100 Years
Graphic 1: Number of Storms per 100 Years - click for full size

In truth, the ‘exactness’ of historical evidence is highly disputed. This is especially so since satellite technologies were not available until the latter half of the last century. Vanguard 2, the first weather satellite was a failure when it was launched in 1959. TIROS-1 went up in 1960 but only lasted a little over two months. Thus we can not be completely confident of historical evidence.

However the graphic above displays a few simple yet interesting things. Notice the trends of the ‘red colors’ representing both tropical storm and hurricane frequencies. Frequency is a tad higher for the october 1 to October 20 time frame than the same window from August 20 through September 10 (Most often defined as the peak time for hurricane formation), yet the frequency of hurricanes only for the same time period is less. Arguments have persisted over the years of the “10 year cycle,” teleconnective tendencies, influences of global warming (both for reducing and increasing both intensity and frequency), the Madden-Julian oscillation and so forth.

Along those lines I have also been playing with some data from the NCDC and NHC. The first chart below represents how many hurricanes have struck the Continental United States by decade, from 1851 through 2000.

Hurricane Frequencies by Decade 1851-2000
Graphic 2: Hurricane Frequencies by Decade 1851-2000 - click for full size

At first glance the chart may be somewhat confusing for you, but do not dismay. There are 5 ‘series’ (lines) running across the chart. Each represents the frequency of each hurricane category (cat 1 – cat 5 Saffir-Simpson Scale) by decade. I chose the color scheme of red and magenta to represent strong hurricanes (cat 4 and cat 5), with green, yellow, orange for cats 1-3.

Remember this is not how many hurricanes have formed during those timescales, but what officials have determined to be the number of each category of hurricane to impact the continental United States during those years. At first glance it may ‘seem’ that the frequency of overall hurricanes is decreasing over time, but instead it is simply those of a tropical storm or hurricane nature that have been determined to reach land. Which, in itself is an interesting debate.

Since I have been working through some of my previous research on snowfall, rain, tornadoes, and hurricanes, I have also decided to share with you a quick sample of my spatial analysis techniques on Tropical Probabilities. Most specifically the month of October since that is where we are as of today.

Historically the NHC has shared their work on hurricane tracks by month, and I have often found that the techniques I apply to the climatologies of weather phenomena differ slightly to those by government agencies.

For example, below is a graphic representing the spatial tendencies of tropical activity for the month of October, based upon historical data from 1851 through all of 2007.  When we discuss spatial tendencies and probabilities as I mentioned in a previous post about snowfall what we are doing is trying to break down the areas where some event has a probability of occuring in a manner easy for the general public to understand. In this case, based upon analysis of historical evidence, I am highlighting areas through historic tendency that are prone to tropical activity for the month of October. Thus, if some tropical activity (in this case of any form) should form, or exist this month, what are the most probable areas geographically that said tropical activity ‘will’ exist at any time in the month.

Tropical Probabilities for the Month of October
Graphic 3: Tropical Probabilities for the Month of October - click for full size

My probability factors are more tightly bound (mathematically) to areas of higher tendency than those shown below from the NHC, based more upon a generality of occurrence. For example, note the highlighted areas of the Caribbean and western Atlantic compared to the NHC.

NHC October Probabilities
Graphic 4: NHC October Probabilities - click for full size

For the purposes of this blog, the important point is to note that historical evidence shows the probability of tropical activity forming for this month and next still does exist, and I have given you a few things to think about and play with. As always, since HAMweather has a fantastic tropical section you can always stay abreast of the most recent information! Such as Marie wandering around out in the eastern Pacific, or a the new risk areas that often form along the Gulf of Tehuantepec this time of year (most probable area of formation for that basin).



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