Monday, August 28, 2017

Tropical cyclones pose multiple hazards

The slow-developing situation involving Hurricane Harvey has caused me to contemplate just what our weather forecast message has to be in order to be effective in mitigating the threats.  As I write this, the death toll is less than 10 people.  Every casualty is a tragedy but things clearly could have been a lot worse.  However, that isn't what I want to consider here.

There's been an effort in the last few years to try to help people understand that there are three different primary hazards posed by landfalling tropical cyclones

1.  the very strong winds near the eye,
2.  the "storm surge" of ocean water pushed onto the land by the storm's winds and low pressure, and
3.  the extreme amounts of rain.

Some hurricanes also produce tornadoes in their outer rainbands.  This event (Harvey) was forecast pretty well.  The rapid intensification was anticipated but it likely exceeded most expectations when Harvey became a Cat-4 hurricane (on the Saffir-Simpson scale of hurricane intensity) just as it was making landfall.  Evidently, the storm surge was not a very big factor in Harvey.  What was really well-anticipated was the extreme rainfall once Harvey came on shore.  This was the highlighted threat in most of the forecasts by both public and private sector forecasters, days in advance.  Arguably, in this event, it was the most important hazard, and it seems it will have become a rainfall event that, if not unprecedented, is on the extreme end of such hazards.

Every hurricane making landfall will offer some combination of these three main hazards.  Each of them carries with it a certain potential for damage and loss of life.  The actual evolution of those hazards in a particular event depends on a several factors that govern the dangers from storm winds, storm surge, and heavy rainfall.  What the storm will actually produce in terms of damage and casualties is not necessarily easy to predict with high confidence, but Harvey's main threat was forecast to be the heavy rainfall;  that was the gist of the forecasts, and that's pretty much how Harvey has evolved.

Our challenge as forecasters is to try to anticipate the hazard level for each of the three elements and seek to pass that information on to the users of our forecasts so they can make informed decisions.  In this event, as is common, some people did well in preparing themselves for the anticipated hazards, but others, not so much.  A big challenge to our forecasting community is to try to convey to the public that the windspeeds, which are the basis for assigning the Saffir-Simpson intensity category for the storm, are not always the main threat from the storm.  A weakening tropical storm may not pose much of a wind or storm surge threat but it still can be quite capable of producing copious rainfall, with its associated potential for both damage and casualties.  Although the wind threat is declining, the storm can remain seriously dangerous!

A big challenge with the threat from heavy rain is the question of what people can do to prevent becoming victims of flooding.  Obviously, flood damage is unavoidable if you live in a location vulnerable to flooding during a heavy rain episode - you can't move your home out of danger.  Many people have little or no knowledge of the actual flood threat in their particular location.  Even more challenging is what people in an area subject to flooding in extreme rainfall can actually do to protect their lives.  Evacuating the area might be effective in saving lives, but when people delay an evacuation decision to the 11th hour, then the roads will be choked and people trapped in their cars can be killed by rising floodwaters.  Some people may not be able to evacuate under any circumstances.  If you're going to evacuate to be safe, do so early or not at all!  If you choose to ride it out at home, you should have a plan regarding what to do if the waters rise high enough that even your roof is no longer safe from the rising water.  Not much you can do in that case if you don't have a boat!

Flooding is one of those hazards that if you find out you're not safe in your location, it may not be safe anywhere within miles of where you are.  By the time you figure out your life is in danger, there likely will be few, if any, good options.  Protecting yourself from flooding is not the same as protecting yourself from wind or storm surge.  What you need to do for your own safety depends on the nature of the threat, and most people have no idea how to deal with rising floodwaters.  This was the case with the awful tragedy of Katrina (which evolved very differently from Harvey) and it resulted in an awful death toll.

Frankly, most people are blissfully ignorant about how to use weather forecast information to help them make good choices.  That ignorance means they don't have the information they need to maintain situation awareness in what might be a fast-changing situation.  To rely exclusively on your own personal judgement in a complex weather scenario is to risk your life and those of others around you.  You may think you have a good bead on how to handle the situation, but you stand a good chance of being tragically wrong.

I don't believe in some magic bullet we might put into our broadcasts of weather information to cause everyone to make the right, life-saving decisions.  Essentially, everyone's situation is different - what works in one household may well fail in a different household.  We meteorologists agonize over the wording in our broadcasts of weather information but I don't think the answer to potential weather disasters lies in some magical turn of phrase.  Dealing with weather hazards involves (a) inevitable uncertainty about how the weather will actually evolve, (b) recognizing the threat early enough to take effective actions, and (c) knowing what actions will be effective in your own personal situation.  If you choose to be safe instead of sorry, you may waste some time and effort needlessly at times, but you'll be alive and well should the situation ever become life-threatening.  The choice is yours, actually.  You need to accept responsibility for your own safety and that of your friends and loved ones - learn and be prepared before the threat appears on the horizon!

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