Time to talk hurricane season!
Today is June 1, which means it’s the start of the Atlantic hurricane season (which runs through Nov. 30). By now, you may have seen the famous Colorado State University Atlantic hurricane season outlook calling for an average season. But wait – didn’t they say back in December that it would be slightly above average? A couple of months ago Accuweather released a graphic suggesting more activity for hurricanes that affect land northward of the Outer Banks and less activity from Charleston southward. The Weather Research Center has hurricane outlook graphics that give the Carolinas a 60% chance of a landfalling tropical storm or hurricane in 2009 – and a 70% chance in 2011! The National Hurricane Center says it’ll be an average season, but doesn’t say anything about the number of named storms that might make landfall in the US. If you’re finding yourself slightly confused about where all these numbers come from, then congratulations, welcome to the world of hurricane outlooks.
Even though hurricane season doesn’t start until today, if you’ve been really on your toes, you’ll remember that we started hearing estimates of hurricane numbers and landfall probabilities all the way back in November. I think that the first thing to establish on these outlooks is that they are uncertain, intended to give more of an overall idea of possible risk. They are NOT intended to be decision-making aids – i.e., it looks like an average year, but that doesn’t excuse you from putting together a hurricane kit with food and essential supplies. Probably the most widely known hurricane outlooks come from the Tropical Meteorology Project at Colorado State University, issued by Dr. Phil Klotzbach and Dr. William Gray. They probably state it best about why these forecasts get issued: “to satisfy the curiosity of the general public and to bring attention to the hurricane problem” (Klotzbach and Gray 2009, p.2).
So with that caveat in place, how can they make these predictions? Climatologists look at decades of past data to analyze how the atmospheric conditions led to tropical storm and hurricane patterns. They use this information to develop predictors that are related to tropical storm formation; some hurricane forecasts also develop predictors of certain landfall patterns. In this April’s Klotzbach/Gray forecast, predictors included sea surface temperatures, sea level pressure, and horizontal winds at various levels of the atmosphere.
NOAA’s Climate Prediction Center explains that in general, for 2009, there are several factors that will have a direct impact on how many tropical storms and hurricanes form. First, we are in the middle of an active period that began in 2005, related to atmospheric patterns that change on the scale of decades. These patterns lead to a wetter western Africa, so tropical waves move off of the African coast into an area of warmer Atlantic sea surface temperatures and little wind shear. With lots of moist energy and weaker winds (meaning that storms don’t get blown apart before they can organize), conditions are ripe for tropical storms to form. It seems like this would lead to an active season, but there are other factors in play. We are in a neutral phase of the El Niño Southern Oscillation (ENSO), an ocean and atmospheric pattern linked to eastern Pacific Ocean sea surface temperatures near the equator off the coast of Peru. Neutral ENSO years favor average hurricane seasons, but some computer models are forecasting that we will move into a weak El Niño phase later in the summer. During El Niño years, a pool of warm water moves eastward in the Pacific Ocean, and this shifts atmospheric circulation patterns in the ways that make it harder for tropical storms and hurricanes to form in the Atlantic Ocean. Finally, the sea surface temperatures in the eastern Atlantic Ocean off of the African coast are actually cooler than normal, providing storms with much less moist energy to keep them going.
Graphic source: http://www.soest.hawaii.edu/MET/Enso/peu/2005_4th/ENSO_Cycle.htm
None of this says anything about whether hurricanes will make landfall in the Carolinas this season. Some academic groups and private companies develop landfall probabilities based on statistical analyses of all the hurricanes and tropical storms on record that have had an impact on the land. NOAA doesn’t issue landfall probabilities, mainly because the odds of any individual storm striking land depends mostly on the weather conditions at the time of that storm, not on the average of past weather conditions. In other words, if a storm is sitting off of the Carolina coasts in August, its movements will be controlled by things like the high and low pressure systems we’re experiencing at the time.
This brings up a very important point: that predicting an average hurricane season does not (I repeat, NOT) give you license to skimp on their hurricane preparations. No matter the statistical averages, we only need one storm in the wrong place to see devastating impacts in North and South Carolina. Just look back to 1992, a below average season where only six named tropical storms and hurricanes formed. One of those was Hurricane Andrew, which formed in mid-August. It raked the Bahamas, then struck southeastern Florida as a category 4 storm, THEN moved through the Gulf of Mexico to make its final landfall in Louisiana as a category 3 hurricane. The time for you to prepare for hurricane season is now, not when the storm is bearing down on us. Check your local newspapers and county emergency management offices for hurricane guides, and be sure to restock that hurricane kit. Personally, that’s on my list to do next weekend.
