WHAT THE NHC ERROR CONE MEANS
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WHAT THE NHC ERROR CONE MEANS
Mods, I believe this should be stickyied as this is VERY IMPORTANT
There are numerous myths regarding the cone. The two most common are
1. This is the area in which the storm could move. The cone represents the uncertainty
2. The cone is the area where impacts are expected.
Neither of these is correct. The cone is merely the 66th percentile based upon errors in the last 5 years. It in no way tells any information related to the true forecast uncertainty. In addition, it has nothing to do with the impact area as impacts often extend well away from the cone
Please remember this when discussing if an area is within the cone for Matthew. The actual uncertainty may represent an area larger or smaller than the size of the cone. Do NOT use the cone for any decision making processes
There are numerous myths regarding the cone. The two most common are
1. This is the area in which the storm could move. The cone represents the uncertainty
2. The cone is the area where impacts are expected.
Neither of these is correct. The cone is merely the 66th percentile based upon errors in the last 5 years. It in no way tells any information related to the true forecast uncertainty. In addition, it has nothing to do with the impact area as impacts often extend well away from the cone
Please remember this when discussing if an area is within the cone for Matthew. The actual uncertainty may represent an area larger or smaller than the size of the cone. Do NOT use the cone for any decision making processes
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- wxman57
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Re: What the Cone Means
Many people (most?) have no clue what the cone really means. I keep seeing comments like "the NHC is going to move the cone" this way or that. The NHC doesn't move the cone. The identical cone is always there, no matter how much uncertainty exists. We definitely need to get rid of the cone and use something that would indicate true uncertainty in the track, or that would indicate true potential impacts (like a probability of wind impact). Such a "cone" might change shape with each advisory, depending on current uncertainty. It could be bifurcated (indicating two possible tracks) at times if it is based on models.
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- tropicwatch
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Re: What the Cone Means
From the NHC:
Definition of the NHC Track Forecast Cone
Definition:
The cone represents the probable track of the center of a tropical cyclone, and is formed by enclosing the area swept out by a set of circles (not shown) along the forecast track (at 12, 24, 36 hours, etc). The size of each circle is set so that two-thirds of historical official forecast errors over a 5-year sample fall within the circle. The circle radii defining the cones in 2016 for the Atlantic and eastern North Pacific basins are given in the table below.
One can also examine historical tracks to determine how often the entire 5-day path of a cyclone remains completely within the area of the cone. This is a different perspective that ignores most timing errors. For example, a storm moving very slowly but in the expected direction would still be within the area of the cone, even though the track forecast error could be very large. Based on forecasts over the previous 5 years, the entire track of the tropical cyclone can be expected to remain within the cone roughly 60-70% of the time.
Definition of the NHC Track Forecast Cone
Definition:
The cone represents the probable track of the center of a tropical cyclone, and is formed by enclosing the area swept out by a set of circles (not shown) along the forecast track (at 12, 24, 36 hours, etc). The size of each circle is set so that two-thirds of historical official forecast errors over a 5-year sample fall within the circle. The circle radii defining the cones in 2016 for the Atlantic and eastern North Pacific basins are given in the table below.
One can also examine historical tracks to determine how often the entire 5-day path of a cyclone remains completely within the area of the cone. This is a different perspective that ignores most timing errors. For example, a storm moving very slowly but in the expected direction would still be within the area of the cone, even though the track forecast error could be very large. Based on forecasts over the previous 5 years, the entire track of the tropical cyclone can be expected to remain within the cone roughly 60-70% of the time.
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Tropicwatch
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- HurricaneBelle
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Re: What the Cone Means
wxman57 wrote:We definitely need to get rid of the cone and use something that would indicate true uncertainty in the track, or that would indicate true potential impacts (like a probability of wind impact).
Agreed 100%. I basically cringe whenever I see someone mention the cone as if being "in" or "out" of it has any practical meaning for what conditions they can expect.
But as for "probably of wind impact", the NHC puts out a graphic for this. The problem is it's (relatively) buried on their site. This is the only "cone" people need:
(Note: I'm "hotlinking" this image since whatever the current graphic on Matthew is at the time is the appropriate one to display.)
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Re: What the Cone Means
The NHC PWI is better. However, it has the same issue as the cone given that it is based upon the deterministic forecast. It does not say anything about true uncertainty. It is still based off of historical errors.
I am working on a solution to this issue. However, the solution will likely not be available for free
I am working on a solution to this issue. However, the solution will likely not be available for free
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Re: What the Cone Means
Improvements in accuracy:
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.
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.
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- wxman57
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Re: What the Cone Means
panamatropicwatch wrote:From the NHC:
Definition of the NHC Track Forecast Cone
Definition:
The cone represents the probable track of the center of a tropical cyclone, and is formed by enclosing the area swept out by a set of circles (not shown) along the forecast track (at 12, 24, 36 hours, etc). The size of each circle is set so that two-thirds of historical official forecast errors over a 5-year sample fall within the circle. The circle radii defining the cones in 2016 for the Atlantic and eastern North Pacific basins are given in the table below.
One can also examine historical tracks to determine how often the entire 5-day path of a cyclone remains completely within the area of the cone. This is a different perspective that ignores most timing errors. For example, a storm moving very slowly but in the expected direction would still be within the area of the cone, even though the track forecast error could be very large. Based on forecasts over the previous 5 years, the entire track of the tropical cyclone can be expected to remain within the cone roughly 60-70% of the time.
The problem is, the cone doesn't really represent the probable track based upon CURRENT uncertainty. The current track uncertainty could be far greater (like Joaquin) than the average error over the past 5 seasons. And, of course, the cone, as it's drawn now, is currently shrinking each year. This means that more and more of a hurricane's impacts are being felt well outside the cone. Clearly, there needs to be a better way to visualize uncertainty based on current steering patterns (ensembles).
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Re: What the Cone Means
wxman57 wrote:panamatropicwatch wrote:From the NHC:
The problem is, the cone doesn't really represent the probable track based upon CURRENT uncertainty. The current track uncertainty could be far greater (like Joaquin) than the average error over the past 5 seasons. And, of course, the cone, as it's drawn now, is currently shrinking each year. This means that more and more of a hurricane's impacts are being felt well outside the cone. Clearly, there needs to be a better way to visualize uncertainty based on current steering patterns (ensembles).
Agreed-Cone is looked at way too closely. TWC uses it etc
Excellent example of where Hurricane conditions were expected WELL outside of the cone and very near landfall! One must look deeper than the cone for their safety and the NHC should do something about this because TWC and everybody else uses the cone!!!
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- Kazmit
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Re: What the Cone Means
New for this season, the NHC's "standard" cone now includes the wind field of the storm. Note how much wider the wind field is than the cone itself, which shows how being outside of the cone DOES NOT mean the storm's winds will not affect you.
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I am only a tropical weather enthusiast. My predictions are not official and may or may not be backed by sound meteorological data. For official information, please refer to the NHC and NWS products.
I am only a tropical weather enthusiast. My predictions are not official and may or may not be backed by sound meteorological data. For official information, please refer to the NHC and NWS products.
Re: What the Cone Means
Those wind graphics still do not display the risk area of winds. It does say the areas that are forecast to receive impacts, which is an improvement for sure.
However, to display the risk of impacts, you need a dynamic wind speed probability product, which NHC still does not have. The one I am creating is still in development
However, to display the risk of impacts, you need a dynamic wind speed probability product, which NHC still does not have. The one I am creating is still in development
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Re: What the Cone Means
We need a reminder about what the cone means
THE CONE DOES NOT SHOW THE FORECAST UNCERTAINTY NOR DOES IT SHOW THE AREA THAT WILL BE AFFECTED
The cone is strictly historical errors. Nothing more than that. Far too many people continue to misuse the cone
THE CONE DOES NOT SHOW THE FORECAST UNCERTAINTY NOR DOES IT SHOW THE AREA THAT WILL BE AFFECTED
The cone is strictly historical errors. Nothing more than that. Far too many people continue to misuse the cone
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Re: What the Cone Means
Alyono wrote:We need a reminder about what the cone means
THE CONE DOES NOT SHOW THE FORECAST UNCERTAINTY NOR DOES IT SHOW THE AREA THAT WILL BE AFFECTED
The cone is strictly historical errors. Nothing more than that. Far too many people continue to misuse the cone
Thank you for explaining this. You and other professionals would be doing a great public service if you could write this in terms that the average person can understand and then start to get the word out. Maybe local Mets and emergency management groups could be a place to start. As simply an interested member of the public, I would certainly share any such explanation on my FB page. I hear far too many people in FL right now saying "we're out of the cone," and believing it's absolute fact because that cone is quite a distance from the state at the moment. That could be correct, but not for the reasons they think. It's still prudent for people to pay attention.
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- gatorcane
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Re: What the Cone Means
I agree we need a cone that represents the uncertainty at the time the advisory is made. If the models are in good agreement 5 days out, the cone should be smaller to reflect that. Conversely if the models diverge at 5 days out, the cone should be wider accordingly.
But I agree too many people focus on the line and whether they are in the cone or not. Irma is a good example. SE Florida moved out of the cone with line going through the west coast of Florida. A lot of folks in SE Florida were surprised how strong the winds got.
But I agree too many people focus on the line and whether they are in the cone or not. Irma is a good example. SE Florida moved out of the cone with line going through the west coast of Florida. A lot of folks in SE Florida were surprised how strong the winds got.
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Re: What the Cone Means
http://www.nhc.noaa.gov/aboutcone.shtml
The cone represents the probable track of the center of a tropical cyclone, and is formed by enclosing the area swept out by a set of circles (not shown) along the forecast track (at 12, 24, 36 hours, etc). The size of each circle is set so that two-thirds of historical official forecast errors over a 5-year sample fall within the circle. The circle radii defining the cones in 2017 for the Atlantic and eastern North Pacific basins are given in the table below.
The cone represents the probable track of the center of a tropical cyclone, and is formed by enclosing the area swept out by a set of circles (not shown) along the forecast track (at 12, 24, 36 hours, etc). The size of each circle is set so that two-thirds of historical official forecast errors over a 5-year sample fall within the circle. The circle radii defining the cones in 2017 for the Atlantic and eastern North Pacific basins are given in the table below.
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Re: WHAT THE NHC ERROR CONE MEANS
A dynamic representation of the uncertainty is optimal but to do that you need an ensemble distribution of the forecast track at each model run time. The Tropical Storm Risk wind probability maps have a similar issue to the NHC cone, they are constructed from the distribution of NHC track/intensity/quadrant wind radii errors over a prior five year period so can only represent a mean uncertainty. Most of the time the uncertainty will be different. There remains the question of how many ensemble members you need to reliably capture the uncertainty in the dynamical forecast.
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