03 April 2013

Forecast Cone Refresher

We are now closing in on the beginning of the 2013 Atlantic hurricane season, and I wanted to post on a topic that is sometimes confusing to people.  The National Hurricane Center issues graphical forecasts of tropical cyclone tracks, and uses a "forecast cone" (a.k.a. "cone of uncertainty", "cone of death", etc) to denote the area that the track will most likely follow.  But what does the cone MEAN??

Last year's Hurricane Isaac will serve as an example.  Shown below is a forecast made when Isaac had just crossed the Leeward Islands on the morning of August 23.  Active watches and warnings (which I'll describe later in this post) are shown by highlighted coastlines -- red, blue, pink, and yellow.  The current center of the storm is marked with an orange dot.  The 0-3 day forecast cone is shown by the solid white shading, then the 4-5 day forecast cone is shown by the hatched white shading.  The center of the cone is shown by the thick black line, with black dots marking the exact forecast positions from this advisory.  I superimposed the entire observed track on the map with a purple line.


As the graphic states: "the cone contains the probable path of the storm center but does not show the size of the storm.  Hazardous conditions can occur outside of the cone."  To arrive at the size of the cone, NHC's own track forecast errors are averaged over the previous five seasons at each forecast lead time: 12, 24, 36, 48, 72, 96, and 120 hours.  The cone is designed to enclose 2/3 of the historical track forecast errors... meaning that on average, there is still a 1/3 probability that the center of the storm could track outside of the cone. Some scenarios in nature are inherently less predictable than others.

Of course, the center of the storm is just a small part of the story... the actual storm is much much bigger, and effects will definitely be felt far from the center.  The forecast cone is NOT an impacts cone.  If the actual track ended up hugging the eastern periphery of the cone in the Isaac example above, MS, AL, and the FL panhandle would have been quite safe, while all of the FL peninsula, GA, and SC would have been impacted.  Impacts experienced far from the center include inundating storm surge, damaging wind, flooding rain, and sometimes tornadoes.  It is quite important to recognize that the cone is only designed to convey an average uncertainty in the forecast of the storm's center, not the areas that will likely be affected by the storm.

In this enhanced satellite image of Hurricane Katrina (left), the center/eye is the blue circle in the middle of the image, but the storm and its destructive impacts extend hundreds of miles away from the center.  The lifetime wind swath (right) shows the cumulative extent of tropical storm force winds in orange, and hurricane force winds in red. Again, I superimposed the observed track in purple.  This wind swath is fundamentally different from the forecast cone!  (as far as other impacts go, there's also a cumulative rainfall swath available, and a map of Katrina-related tornado reports).


Presently, the same cone size is used for an entire season and for all storms.  The 2013 cone size is determined by the track errors from the 2008-2012 seasons.  As track forecasts improve, the cone gets smaller!  The figure below compares the cone used in 2008 to the cone that will be used in 2013 for an identical [hypothetical] storm and forecast.  This is great progress, as reducing the uncertainty in a forecast reduces the time and money spent on preparations and evacuations.


Finally, I'll also describe watches and warnings.  These are issued for tropical storms (sustained winds less than 74mph) and hurricanes (sustained winds 74mph or greater), so there are four iterations to be aware of: tropical storm watch, tropical storm warning, hurricane watch, and hurricane warning.  The definitions provided by NHC are as follows:

Tropical Storm Watch: sustained winds of 34-63 knots (39-73 mph or 63-118 km/hr) are possible within the specified area within 48 hours in association with a tropical, subtropical, or post-tropical cyclone. 

Tropical Storm Warning: sustained winds of 34-63 knots (39-73 mph or 63-118 km/hr) are expected somewhere within the specified area within 36 hours in association with a tropical, subtropical, or post-tropical cyclone.

Hurricane Watch: sustained winds of 64 knots (74 mph or 119 km/hr) or higher are possible within the specified area in association with a tropical, subtropical, or post-tropical cyclone. 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 64 knots (74 mph or 119 km/hr) or higher are expected somewhere within the specified area in association with a tropical, subtropical, or post-tropical cyclone. 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. The warning can remain in effect when dangerously high water or a combination of dangerously high water and waves continue, even though winds may be less than hurricane force.

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.

If anyone has other general questions or points of confusion, please ask!


No comments:

Post a Comment

Note: Only a member of this blog may post a comment.