22 May 2019

Storm surge: the deadliest part of hurricanes

Storm surge resulting in severe coastal flooding during Sandy (2012). Photo: Master Sgt. Mark C. Olsen/U.S. Air Force
Hurricanes are categorized only by the peak sustained wind found somewhere in the storm... that is the metric used in the Saffir-Simpson scale.  That category rating unfortunately tells you nothing about how large the storm is, how much rain it could produce, or how much storm surge it could generate*.
(* The original scale did link the wind scale with a surge depth, but that was abandoned in 2010 after numerous storms demonstrated that the correlation between the two isn't very good.)

Perhaps surprisingly, about 3/4 of hurricane-related fatalities in the United States are due to storm surge (1/2) and rain (1/4), while the wind accounts for just 1/12!

The definition of storm surge from the National Hurricane Center is:
An abnormal rise in sea level accompanying a hurricane or other intense storm, and whose height is the difference between the observed level of the sea surface and the level that would have occurred in the absence of the cyclone. Storm surge is usually estimated by subtracting the normal or astronomic high tide from the observed storm tide.

In other words, storm surge is the water that gets bulldozed onto land by a hurricane's wind.  It can cause water levels to rise gradually, abruptly, by a little bit, or by over 20 feet... it all depends on the size, speed, direction, and intensity of the storm, the topography of the land, and the bathymetry of the nearby ocean. Because of all of these variables, it is challenging to predict the extent and depth of storm surge far in advance.  Furthermore, storm surge is not just limited to the immediate coast; it can travel into bays, rivers, and canals.

But when the storm surge arrives can make a big difference in its flooding potential. Will the peak storm surge arrive at low tide or high tide? Will it last long enough to span multiple high tides? The combination of the storm surge and the regular astronomical tide is called the STORM TIDE, and that's the water level you experience.

I have three examples here to illustrate this: Wilma (2005), Irma (2017), and Sandy (2012).

The Wilma case is taken from Virginia Key (Miami).  At its closest, Wilma's center passed about 60 miles to the northwest of this location, but the wind field was large. In this tide chart spanning one day, the regular astronomical tides are shown by the blue line, the observed water level is shown by the purple line, and the difference between them (the "residual") is therefore the storm's contribution, or storm surge.  The storm surge was about four feet, but fortunately, it arrived abruptly right at low tide!  A few hours later, there was an "anti-surge", where winds essentially blew the water away from land.

The Irma case is again from Virginia Key (Miami).  Like the Wilma tide chart, this one spans one day, but you quickly notice that it looks quite different. Irma's storm surge didn't come or go abruptly, but rather very gradually.  At its closest approach, Irma's center passed 95 miles to the west, and it too was a large storm.  In this situation, the storm surge was also about four feet, but it hovered in the 3-4-foot range for hours during high tide, producing a peak storm tide that was 1.1 feet higher than Wilma's. 

Finally, the Sandy case is taken from Bergen Point (New York).  This one is a little trickier, but illustrates the timing issue. First thing: this chart spans five days rather than one... Sandy's wind field was enormous. The regular astronomical tides are shown by the dark blue line, the observed water level is shown by the red line, and the storm surge is the green line (ignore the dashed ones for now). At its closest approach, Sandy's center passed 80 miles south of this location. The first thing you notice is that the peak 9.4-foot storm surge arrived at high tide on the evening of October 29.  This unfortunate timing maximized the amount of flooding that would occur. 

But with the exact same storm and track, I shifted the landfall time to be 5.3 hours earlier than what really happened to minimize the peak storm tide... the result is 3.3 feet lower!  This hypothetical scenario is illustrated by the dashed lines.

Clearly, storm surge must be taken seriously.  Storm surge, not wind, is the aspect of hurricanes that often defines evacuation zones and priorities. Sandy had Category 1 winds when it created the 9.4-foot storm surge and inundated parts of New York City, so the "it's only a Category 1" mentality needs to change in favor of thinking about all of the hazards -- not just the wind.  There's more to the story than the category!

21 May 2019

Andrea kicks off the 2019 Atlantic hurricane season early

Subtropical Storm Andrea has formed southwest of Bermuda... you can find information about it as well as what it means to be "subtropical" in today's update on the Washington Post's Capital Weather Gang blog:

Andrea kicks off the 2019 Atlantic hurricane season early

20 May 2019

17 May 2019

"Cone of Uncertainty" Update and Refresher

Anyone who lives on a hurricane-prone coast or even watches television is familiar with the infamous "cone of uncertainty" produced by the National Hurricane Center.  It begins as a point at the current position of a tropical cyclone and expands to show the potential position in five days. It is called the "cone of uncertainty" because the further out in time you go, the more uncertain the forecast becomes... and it tends to look like a cone!

A "cone of uncertainty" for Hurricane Irma (left) and Hurricane Harvey (right). Both cones are from 2017 and are therefore identical to each other in their construction. 
The size of the cone is fixed for every storm during the entire hurricane season, but the size slowly evolves from year to year. If the storm is moving quickly, the cone will appear more elongated and if the storm is moving slowly, the cone will appear more compact... but it's the exact same cone.  The examples shown above are from Irma (left) and Harvey (right); both storms were in 2017, so both cones are identical.

The cone is updated each year prior to the start of hurricane season, and it almost always shrinks each year too.  Hurricane track forecasts are gradually improving, meaning that in general, there is less uncertainty where a storm will track now than there was a decade ago.  In fact, a two-day forecast now is as accurate as a one-day forecast was a decade ago, and a five-day forecast now is more accurate than a three-day forecast was two decades ago!  The map below shows the new 2019 cone overlaid on the 2014, 2009, and 2004 cones for comparison.

So just how is the size updated each year?  The National Hurricane Center uses its own track forecast errors over the previous five years to calculate a circle at each "lead time" (1 day, 2 days, ... 5 days).  The size of that circle is designed to enclose the position of the storm's center with 2/3 probability, meaning that there's historically a 1/3 chance the storm will track outside the circle at that time.  Lines connecting the various circles complete the shape of the cone. [Note that the 2019 cone size is thus determined from all track errors during the 2014-2018 seasons.]

Since the cone is so widely used yet sometimes misunderstood, here are some key refreshers:
  • The cone does not tell you anything about where impacts will be experienced.  Even for a perfect down-the-middle track forecast, impacts such as strong wind, heavy rain, storm surge, and tornadoes will extend beyond the cone.
  • The cone does not tell you anything about the size of the storm.  Regardless of how strong they are, hurricanes come in a wide range of sizes.  Recently, NHC has added the observed size of the wind field to their cone graphics to help illustrate this (see the Irma and Harvey examples above... the orange and red shading indicates the extent of tropical storm and hurricane force winds at the time the forecast was issued).
  • The cone does not tell you anything about the actual uncertainty associated with the forecast. Since the size of the cone is fixed, it cannot become more narrow or broad to accommodate a more or less predictable environment.
  • Nothing magically happens at the edge of the cone. If a hurricane is approaching and you are scrutinizing each new forecast to see if you are inside the cone or not, you are missing the point of it.  It is arbitrarily chosen to be the 67% historical probability threshold... a 75% probability cone would be larger, and a 50% probability cone would be smaller.
  • If you use the cone graphics from NHC, there is some information about intensity provided. At each forecast point, there is a letter written inside the black dot corresponding to a general intensity range: D (tropical depression), S (tropical storm), H (hurricane), and M (major hurricane (Category 3+)).  But keep in mind that there is uncertainty associated with the intensity forecasts too!
To think about a cone of uncertainty for intensity, consider this: averaged over the past five years (2014-2018), the mean error in a 1-day forecast is +/- 9 mph, the error in a 3-day forecast is +/- 15 mph, and the error in a 5-day forecast is +/- 16 mph.  But there is also a wide range of values that go into those averages, meaning that there is a small probability of a very large error and a small probability of near-zero error.

Other important terms:

Storm Surge Watch: the possibility of life-threatening inundation from rising water moving inland from the shoreline generally within 48 hours.

Storm Surge Warning: the danger of life-threatening inundation from rising water moving inland from the shoreline generally within 36 hours.

Hurricane Watch: sustained winds of 74 mph (64 knots or 119 km/hr) or higher are possible. Because hurricane preparedness activities become difficult once winds reach tropical storm force, the hurricane watch is issued 48 hours in advance of the anticipated onset of tropical-storm-force winds.

Hurricane Warning: sustained winds of 74 mph (64 knots or 119 km/hr) or higher are expected. Because hurricane preparedness activities become difficult once winds reach tropical storm force, the warning is issued 36 hours in advance of the anticipated onset of tropical-storm-force winds.

Tropical Storm Watch: sustained winds of 39-73 mph (34-63 knots or 63-118 km/hr) are possible within the specified area within 48 hours.

Tropical Storm Warning: sustained winds of 39-73 mph (34-63 knots or 63-118 km/hr) are expected somewhere within the specified area within 36 hours.

Strong winds and thunderstorms arrive well before the center of the storm (sometimes a couple days), so when the time comes, be sure to plan and finalize your preparations prior to the expected arrival of tropical storm force winds, not the expected arrival of the center.

And if you missed it earlier, I have some general hurricane information and preparedness tips at http://bmcnoldy.blogspot.com/2019/04/2019-hurricane-season-intro-local.html