21 August 2019

Chantal forms as active part of hurricane season begins

Chantal, the season's third named storm, formed on Tuesday night in the north-central Atlantic.  As of Wednesday morning, it's a tropical storm with 40 mph sustained winds centered 450 miles south of Newfoundland.


Similar to Barry back in July, Chantal's origins were over land, and it eventually acquired tropical cyclone characteristics once over water long enough.  Last Thursday (15th), the disturbance that would become Chantal was over the Florida panhandle, and it subsequently meandered its way across Georgia, South Carolina, North Carolina, then out over the ocean where it began to take shape.


It is forecast to marginally maintain tropical storm status for a couple days before weakening in the face of dry air, strong vertical wind shear, and cooler water.

So far this season, the southernmost named storm was Barry at about 28°N... nothing has formed or existed in the tropics yet. But we're just now entering the peak of the "Cabo Verde" season, where we closely watch disturbances coming from Africa for development (Cabo Verde is the archipelago off the west coast of Africa). The vast majority of major hurricanes have an African pedigree.

Over the past fifty years, the average date of third named storm formation is August 13, so this is about a week late. Using the same climatology, 84% of the season's ACE (Accumulated Cyclone Energy) occurs after August 20. As of today, 2019 is at just 22% of average for the date. But things should start waking up soon.


Closer to home, there's a disturbance near the Bahamas that has a slim chance of becoming a tropical cyclone and will approach Florida this weekend. If nothing else, it will bring elevated chances of heavy rain to the Florida peninsula starting on Friday.  (Miami has already had its 9th wettest August on record, with 11 days to go!)

The yellow X marks the approximate center of the disturbance today, while the yellow filled "blob" is the area where tropical cyclone formation might occur in the coming five days.


13 July 2019

Barry becomes season's first hurricane as it makes landfall in central Louisiana

On Saturday morning, Barry was upgraded to a Category 1 hurricane (with peak sustained winds of 75mph) just before it made landfall south of Lafayette. Dangerous storm surge of 6-8 feet is occurring east of the center (Morgan City area), and heavy rain is just making its way onshore... fortunately, the storm remained lopsided and the majority of the rain is still south of the center. But it is not over; now that it's onshore, the rain will be too!


Again, the classification of the storm (tropical storm, Category 1 hurricane, etc) *only* pertains to the peak sustained wind speed found somewhere in the storm -- it does not tell you anything about the size of the wind field, the amount of rain it will produce, or the depth and extent of the storm surge.  You can find a few long, updating radar loops covering Hurricane Barry's landfall at http://bmcnoldy.rsmas.miami.edu/tropics/radar/


As you can see, rainfall remains a major concern today and in the coming couple of days as Barry moves inland. The graphic below shows the flask flood risk from Saturday through Tuesday morning:


This event fits within what we expect from climatology: the Gulf of Mexico is a favored region for tropical cyclone formation during July.

Barry is the 8th hurricane to make landfall in Louisiana in the past two decades... the other recent hits include Lili (2002, Cat1), Cindy (2005, Cat1), Katrina (2005, Cat3), Rita (2005, Cat3), Gustav (2008, Cat2), Isaac (2012, Cat1), and Nate (2017, Cat1).

Tracks of the seven hurricanes to make landfall in Louisiana from 1999-2018.
Elsewhere in the Atlantic, an easterly wave left the African coast back on July 8th and continues to maintain a minimal amount of organization. The National Hurricane Center is giving it just a 10% chance of becoming a tropical depression by Thursday. It, in whatever condition, would reach the Lesser Antilles around Monday-Tuesday. At this point, it's very likely to be nothing more than a tropical breeze.



10 July 2019

A hurricane is forecast to strike Louisiana on Saturday

The National Hurricane Hurricane has initiated advisories on "Potential Tropical Cyclone Two" as of Wednesday morning. It has a very high chance of becoming Tropical Storm Barry, and could even be Hurricane Barry by the time it makes landfall on Saturday. Get the full scoop in today's update, available on the Capital Weather Gang blog:

A hurricane is forecast to strike Louisiana on Saturday


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


02 April 2019

2019 Hurricane Season Intro: Local Edition

Five-day forecast of hurricane wind speed probabilities for Hurricane Irma (2017). (NHC)

Hurricane Basics


Hurricanes are large low pressure systems that generally require warm tropical water to form and strengthen.  They come in a range of sizes, but are classified by their wind speed. Since the early 1970s, the Saffir-Simpson Scale has been used to rate hurricanes by the peak sustained wind speed found somewhere in the storm.

Before becoming a hurricane, there are two stages. When a cluster of thunderstorms becomes persistent and organized enough to have a "closed" circulation in the surface wind, it is designated a tropical depression and given a number.  While these can produce tremendous amounts of rain, they are quite weak in terms of wind speed.  If the sustained winds reach 40 mph, it is upgraded to a tropical storm and given a name. Tropical storms also have the potential for heavy rain, but winds are now strong enough to do serious damage to vegetation (and falling trees or tree limbs can take out power lines and roofs/buildings).  It is unsafe to be outdoors during tropical storm force winds.

If a tropical storm continues to strengthen and sustained winds reach 74 mph, it is upgraded to a hurricane.  From there, the stronger it gets, the higher the category rating it gets.  The scale tops out at Category 5 (157+ mph) because at that point, catastrophic failure of structures and trees has already happened.  Only 2% of named storms in the Atlantic ever reach Category 5 intensity. The most intense Atlantic hurricane on record is Allen in 1980 with 190 mph sustained winds, and the most intense landfalling hurricane in the United States was the 1935 Labor Day Hurricane with 185 mph sustained winds.


The term "major hurricane" refers to any hurricane that is Category 3 or higher on this scale.  It has nothing to do with impacts, damage, or fatalities it may cause. A hurricane of any category can be destructive and deadly.

To illustrate the point about size, the satellite images below show two major hurricanes in the eastern Gulf of Mexico at approximately the same scale and intensity: Charley (2004, left) and Rita (2005, right). While the peak wind speed was roughly the same, the larger a storm is, the more areas and people it will impact if it hits land.


In terms of structure, a well-organized hurricane is characterized by a calm, clear central eye which is surrounded by a thin ring of intense wind called the eyewall. The eyewall is the "business end" of the beast, and contains the highest wind speed on which the category rating is based.  Wind speeds drop noticeably outside of the eyewall, but rainbands spiral outward from there for hundreds of miles and contain violent winds and tornado-producing thunderstorms.  Impacts such as storm surge, tornadoes, and flash flooding can all extend hundreds of miles from the center of the storm.

Radar image of Hurricane Irma (2017).  Although radar images depict rainfall intensity, it correlates well with wind intensity too in many cases. The eye is the central "hole", the intense eyewall is the red-orange ring surrounding the eye, and some rainbands can be seen spiraling out from the eyewall.

Seasonal Climatology


The official Atlantic hurricane season spans June 1 through November 30, and by design, the vast majority of activity is typically confined to those dates (not necessarily all of it).  Within the season, about 85% of the activity normally falls within the core months of August, September, and October.


Although the climatological peak of the season is in early-mid September, the month with the most hurricane impacts in south Florida is October.  June, July, and November have typically been relatively quiet here, while August and September are when the strongest storms have hit.  Also notice that about half of the storms that impact us come from the east (generally Jun-Sep) and half come from the west (generally Oct-Nov).  Hurricanes are big, so we can experience significant impacts even without a direct landfall.

Tracks of hurricanes from 1851-2017 that passed within 100 miles of Miami, grouped into each month of hurricane season.
Miami-Dade County has only been affected five times in the past fifty years, including one major hurricane.  Compare that to the previous fifty years, when the county was hit seventeen times, eleven of which were major hurricanes!  The chart below shows a timeline of hurricane impacts in the county with major hurricanes marked in red, and overlaid in the orange line is the county's blossoming population.

Although 2017's Hurricane Irma counts because it produced hurricane-force winds in far western parts of the county, the metropolitan eastern parts of the county experienced "only" tropical storm conditions.

There is no law of nature that says we cannot get hit twice in the same season... it has happened six times since records began in 1851. In 1948, we actually got hit twice within two weeks. Then more recently in 2005, Katrina and Wilma produced hurricane conditions in the county two months apart. Though rare, it doesn't hurt to be aware of the possibility.


Property Preparation


As we saw in the abnormally lengthy span between Wilma and Irma, an extended break allowed a messy situation to arise: twelve years of untested tree growth. The prolonged tropical storm force winds associated with Irma were good at finding weak, rotten, top-heavy, or diseased trees and breaking or uprooting them. If another Irma were to occur this year, the debris piles would be noticeably smaller.  But if something stronger comes along (a hurricane of any category), branches and trees that withstood Irma could go.


This is the perfect time to look around your property to see what you can do to minimize damage long before a storm threatens.  That includes random unused outdoor items, and especially trees.  Having your trees properly pruned, thinned, and/or topped (or removed if they're dead or rotten) now can save you and your neighbors damage and power loss during a storm.  If the tree is growing into or over power lines, it's best to report it to FPL and they'll take care of it -- it saves them time to do preventative work rather than repair work.  (If you have a mango, avocado or other tree with fruit on it now, it's best to wait until after fruiting season to trim it.)

Online:
https://app.fpl.com/serviceQuality/SQController?command=welcome&ticketType=TW&requestNumber=0749
Phone:
305-442-8770

If you have a generator, try it out before the start of each hurricane season just to be sure it works. If you have shutters for your windows and doors, make sure they are all accounted for and in good working order.  These would be issues you don't want to discover two days before a hurricane landfall.

Review your insurance policies and coverage (auto, home, windstorm, flood, renters), and make sure you know a number to call after a storm if you need to file a claim.  Keep important documents (including insurance policies) in a safe and dry place or bring them with you if you leave.  A quick call to your agent is easy and can give you peace of mind that you have the coverage you thought you had.  Having a photo/video inventory of your possessions is also a good idea.


Evacuation Zone and Plan


In the event of an intense hurricane approaching, storm surge increasingly becomes a concern.  Storm surge is the rise of water levels due to strong onshore winds "bulldozing" the ocean onto land, and is the #1 killer during hurricanes in the U.S. by far.  Storm surge also does not correlate perfectly with a storm's category rating... it  depends on the size of the storm, forward speed of the storm, coastal topography, offshore bathymetry, and more.  A large slow-moving Category 1 hurricane could generate a larger storm surge than a small Category 3 hurricane, for example.  Remember: "There's more to the story than the category!"


In Miami-Dade County, evacuation zones are defined primarily by storm surge risk (not wind risk), and the zones are not limited to the immediate coast.  The surge of water can travel up canals and rivers.  The zones are crude in shape, and the entire county would never experience the peak surge or evacuation orders.  But knowing your zone letter gives you a general sense of your potential risk from storm surge and evacuation priority... here is a map centered on the northeast portion of the county:


and here is the interactive website if you want to peruse more on your own:
https://mdc.maps.arcgis.com/apps/webappviewer/index.html?id=4919c85a439f40c68d7b3c81c3f44b58
In Broward County, a two-tier evacuation zone system is used and the map is available at http://www.broward.org/Hurricane/Documents/EvacuationMapADACompliant.pdf

If you decide to evacuate, it is a good idea to have a place and plan in mind before the stress of an approaching storm sets in.  Secure your house, shut off all power/gas, bring valuables and important documents with you, and always let friends and neighbors know where you're going and how to contact you.

If you decide to stay and shelter in place, be prepared to be self-sufficient for at least three days, longer if possible. This means enough food, clean water, and medication for every person and pet in the house. Also expect to be without electricity for an extended time (possibly over a week), as well as internet access and even phone service. Communication will likely be a challenge in the immediate aftermath. If you have a portable generator, don't forget to stock up on gas for it.  Spare batteries and basic first aid supplies should also be part of your standard hurricane season checklist.

Hoping for a safe season,
Brian