National Hurricane Center and the Likelihood of Hurricanes
Posted by Stuart Staniford on March 14, 2006 - 9:36pm
We do not, however, expect anything close to the U.S. landfalling hurricane activity of 2004.Well, they got that wrong...
This begs the question of how likely were the 2004 and 2005 hurricane seasons given the NHC forecasts? I'm going to argue that they were quite unlikely, suggesting the NHC, at least in recent years, is systematically underestimating the seriousness of the problem.
This is a simple high-school probability argument based on the following data. In December 2003 the NHC predicted a 68% chance of a major hurricane hit on the US (which I'll call a MLH - major landfalling hurricane), while in December 2004, they estimated a 69% chance of a major storm hitting the US coast.
In fact, in 2004 there were three major hits on the US (Charlie, Ivan, and Jeanne), and in 2005 there were four (Dennis, Katrina, Rita, and Wilma).
If we make one additional assumption - that the probability of any subsequent major landfalling storms is independent of the first and each other, then we can compute the probability of the observed outcomes given the forecast.
Specifically, the probability P that they give (the 69% or 68%) is the sum of the probability of getting one MLH, two MLHs, three, etc. We need to know p, the probability of getting exactly one, rather than P, the probability of getting one or more. P = p + p2 + p3 + .... If you remember algebra, and you imagine factoring a p out of the RHS, you should be able to see that P = p(1+P), so p = P/(1+P). To a good enough approximation for this purpose, 68% = 69% = 2/3. Therefore, p = 2/5 in both 2004 and 2005.
So what are the chances of getting 3 or more MLHs in a year? Well, p3+p4+.... That turns out to be about 11%. What are the chances of getting 4 or more MLHs in a year? Well, about 4%. If you want 3 or more in one year and four or more in the other, the chances of that are 4%*11% *2 (the factor of two comes because we would have viewed a reversal of the two years as equally significant). That suggests the overall chances of the 2004 and 2005 seasons, based on the forecasts, are 0.9%. If we had been willing to accept 2 storms one year and 5 another as equally significant (I wouldn't), that would raise it to 1.8%
The usual level of statistical significance is 5%, and the usual level for evidence to be "very significant" is 1%. Thus, the last two hurricane seasons constitute statistically very significant evidence that the forecasts understated the probability of major landfalling hurricanes in the US.
One possible factor is the state of the loop current in the GoM.
Maybe I have just written to much science fiction and Read to many papers on Chaos and how to make the complex simple.
I almost feel as if I know where this is all heading but can't quite put my finger on it.
Consider who is looking right and who is looking wrong....
Sea surface temperatures (SSTs) are higher. The North Atlantic Oscillation (NAO) is positive and there is a strong historical correlation between increased hurricane activity and the NAO in its current state. Combine that with the SST's and I'd say there's little question significant future chances of major hurricanes making landfall in the Gulf over the next few years IMHO.
http://www.ngdc.noaa.gov/paleo/ctl/resource100.html
North Atlantic Data
http://www.ldeo.columbia.edu/NAO/
http://hurricane.atmos.colostate.edu/Forecasts/2005/dec2005/
It's up to 81% this year! That's after 68% and 69% the past two years. So I guess they're learning...
Second, it's probably not right to assume that the hurricanes are independent. I don't remember the whole history, but Katrina and Rita had similar tracks (at least at the end) and came in pretty close to each other. It might well be that once conditions arise that are favorable for a hurricane to hit, that we could be vulnerable to more than one.
Here's one odd thing from their forecasts. They are all like this, but this is the one for 2006:
The thing is, the probabilities don't add up, even though the subregions are geographically distinct. The probability for the whole coastline is 81%, the probability for the western part is 47% and for the eastern part is 64%. Those two parts add up to 111%.What does this mean? Presumably, that the odds are not independent. The chances are not zero for hurricanes to hit both west of Florida and in the eastern part. I wonder if this could shed any light on their imputed probability for more than one hit, if we did some statistical mojo with these numbers?
Letmebefirstletmebefirstletmebefirst
p(X) = probability of event X
E = Hurricane in East, W = Hurricane in West
~X means NOT event X
& means logical OR
| means logical OR
p(E) = 0.64 so p(~E) = 0.36
p(W) = 0.47 so p(~W) = 0.53
so p(~E & ~W) = 0.36 * 0.53 = 0.19
so p(~(~E & ~W)) = p(E|W) = 1 - 0.19 = 0.81
QED. Cigar?
& means logical AND
Oops
However, those variables are available for NHC to incorporate into their predictions. So if you want to argue that the hurricanes are highly non-independent even when conditioned on the NHC forecast, that suggests there might be good features that NHC is not incorporating into their forecasts.
Poisson process, anyone? Or is there a system recovery time between major storms in a small province like Florida? Or did they just run a zillion simulations with random initial conditions and report the frequency of outcomes?
The window of 3 to 6 hours ahead can be shaky at best, and anything longer than that has error factors. Last season those error factors came to full light, when they could not predict the outcomes of several late season storms and you could tell from reading them there was a massive amount of frustration going on.
The computer models available to them are only as good as the data streams they were designed around. Things are changing and they know it is happening, but they just can't tell you for sure what and how!! This makes their job even harder, because so many people now depend on them getting it right, when they the NHC know they can't get it right if the ocean keeps changing and they don't have new modeling programs that predict those changes.
Its like us asking Dave and Stuart to predict the Oil movement in a basin between March 16th at 5 pm and March 18th at 8 pm and tell us where the next blowout will occur before it happens.
Models and programs of a changing world are only so good as the "Past" data used to make them.
FEMA has developed a companion software tool to HAZUS-MH called the HAZUS-MH Risk Assessment Tool (RAT) to help you produce your risk assessment outputs for earthquakes, floods, and hurricanes. This tool was developed as a third-party model to support HAZUS-MH and is used to display the outputs from the HAZUS-MH risk assessment in an easy-to-use format. The RAT pulls natural hazard data, inventory data, and loss estimate data into pre-formatted summary tables and text. These summaries can support the presentation of data to decision-makers and other stakeholders and in your mitigation plan.
bullet Risk Assessment Tool (zip, 19.3 MB)
p(1 or more) = 81%
p(2 or more) = 49%
p(3 or more) = 23%
p(4 or more) = 9%
So I think they would have to say again that the probability of this year being as bad as the previous year would be quite low, even with these larger probabilities.
Just out of curiosity, what would it have to be to get the probability with 4 or more to be at least 50%? This corresponds to a "lambda" of 3.672 which means a probability of 1 or more big storm hitting of 97.5%! I don't know if even the most stringent advocate of a linkage between global warming and hurricanes would say we are in that territory yet. So I suspect that any way you look at it, we had a substantial bit of bad luck these past two years, and are unlikely to be that unlucky again this year.
One possibility, as you suggest, is that they are just exceptionally bad luck, and 2006 and 2007 will mostly return to the trend of gradual worsening that's been going on for decades (in which case the NHC's 81% is a reasonable looking estimate). The other possibility is that we have hit some kind of non-linearity in the way the climate responds to increasing global warming forcing, and we are in a new (and much worse) regime. In that case, our best estimate of lambda should be 3.something and the probability of 1 or more storms should be 90%+. I don't think we have enough information to argue one way or the other very convincingly at this point.
If we are crossing some kind of threshold, there may be some flickering back and forth for a while. On the other hand, if we were to have another season or two like 04/05, I don't think that would leave much room for doubt. It would also help to have some kind of physical explanation for what switch had flipped (I have no idea at present, though changes in the Atlantic circulation seem like an interesting place to start looking given the Bryden et al paper).
The hurricane conveyer belt is that much stronger.
Weeeeee! Let's fire up some more technology!!!
Does anyone know about the current state of SSTs in the Gulf of Mexico? All week long the Minnesota weather forecasts were talking about great moisture flow/energy transfer coming from the gulf. Are SSTs still uncommonly high? If so, is there any link between high SSTs and land-based destructive storms like those of March 11-13?
http://www.nowcoast.noaa.gov/
Set Louisiana (its a GOM state, not a coastal state?)
Set sea surface temp
Then zoom out to see the entire GOM
Link :-
http://www.cpc.ncep.noaa.gov/products/hurricane/atlsst_lastweek.html
Sure does look like the gyre is bringing more warm water back as Bryden's paper suggested to me it would (we discussed this last fall. That makes things nice and warm where the hurricanes get started and then we have that hot spot in the Gulf starting to build up to turn them into monsters before they hit.
Does anyone know if similar maps are available for historical conditions in prior years?
One caution is that the depth of the warm water layer is just as important as the surface temperature. Hurricanes self limit by churning up cold water from below, so to get a really powerful hurricane, it helps to have a deep layer of warm water (or else a shallow sea where's it warm all the way to the bottom).
I wonder if the warmer than usual gulf water flowing north along the eastern seaboard has any effect on the jet stream, which last winter dammed arctic air in upper canada, moderating NA climate, and then released it to flow south into western europe, now bereft of the gulf stream...
Western europe might become used to siberian winters...
Maybe things will get better when greenland has no ice left to melt (or, maybe at that point hurricanes transport sufficient fresh water distilled from the gulf to the northeast to stabilize climate in a new mode.) gw means more rain, but of course not evenly distributed...
Good point about a possible link between SSTs and land-based destructive storms. Global warming combined with shrinking polar icecaps means violent storm energy transfers are longer lasting and moving northward causing more tornadoes and more destructive land-falling hurricanes [just a supersized tornado]. Stuart's ocean temp anomaly graph shows a hotspot off the California coast-- maybe CA might see a mild hurricane/tropical storm this year, last tropical storm was in 1939 linked here:
http://www.usatoday.com/weather/whhcalif.htm
If this CA hotspot continues to grow, it could be similar to the GoM thermal hotspot providing energy to jackup storm intensity levels. We will see. I agree with most other posters that the weather models are not incorporating accurate changes as fast as Nature is changing the weather.
Bob Shaw in Phx,AZ Are Humans Smarter than Yeast?
(Ooops, now I've let it slip: I am really old:-)
www.islandnet.com/~see/weather/almanac/arc2003/alm03aug.htm
Sometimes the statement is made that no hurricane has ever hit California. On the face of this, the statement is highly questionable, because how the heck could this be known???? True the cool California current can be expected to dissipate the energy of tropical storms, but there is no good reason to think this current is eternal or even particularly ancient.
Eastern Pacific hurricanes (Let's agree to call them that for now.) are little known because most of the time they are far from land and far from established shipping lanes. What data we do have prior to 1850 is mostly from the logs of whaling ships that were close to the California coast in search of whales. The best compilation of this sketchy data I've seen is at the NOAA office in Honolulu, but so far as I know it is not available online.
BTW, do you like earthquates?
It is true that many CA boomers are finding they can afford to retire if they sell their CA home and move to, say, Phoenix or las vegas, but cannot retire if they stay in CA. So, CA is rewarding those who otherwise were unable to plan for their retirmenent. THose who left earlier may have had to actually save some money...
There are certainly some areas of CA where the quality of life does not seem to be high enough to compensate for the world's best climate, such a LA, but most areas are pretty nice - we still manage to attract quite a few tourists.
The link to the main web page:-
http://www.nhc.noaa.gov/
Archive of hurricane seasons east and west :-
http://www.nhc.noaa.gov/pastall.shtml
Interesting - so (sez Dr. Gray et al) the thermohaline circulation does affect SSTA*, which in turn correlates with intense or major hurricanes....but it's an increase in Atlantic thermohaline circulation that is linked increased Atlantic hurricane intensity & activity (reading from Section 8 now):
Notice the key word "directly." Also, the area they're measuring SSTA* in is basically the North Atlantic south of Greenland and Iceland between Ireland and Newfoundland. So...what does this imply? If global warming suppresses THC as some suspect, will this dampen hurricanes? Or am I reading this wrong and drawing the opposite conclusion from what I should?
Also, the consensus is probably changing based on the past two years. Two years ago there were probably very few who would have said that global warming was already having an important effect on hurricanes. Now with two bad years in the U.S. the consensus is no doubt changing. But science tends to be conservative, even climatology, and the consensus doesn't change overnight.
So I would be skeptical that the average climatologist would endorse the claim that these two intense hurricane seasons were caused by global warming. Scientists don't like to be jerked around by rapidly changing evidence, so they will withhold judgement until a predictable pattern is established. They know that regardless of the underlying factors, hurricanes are unpredictable and any given year is largely a roll of the dice. Hence two bad years will not make them suddenly change their minds.
If we have several more years of intense seasons, the consensus will probably continue to shift towards a global warming link. If we happen to have a couple of quiet years, the linkage will probably look less significant. It all depends on the data and I don't think any respectable scientist would say that confirmation of linkage is in hand.
and flow rates of the major river systems that
flow into the GOM? (especially the Mississippi)
I could be barking up the wrong tree, but I do
wonder if substantial flows of relatively low
density water at high temperature from river
systems contribute to high surface temps in the
GOM. A feedback loop could exist, whereby hotter
weather in the US in summer results in a higher
surface temperatures for the GOM and hence a
higher likelihood of hurricanes.
I agree entirely that trying to correlate with
data that is 50 years old is futile when it is
clear that we are in uncharted territory when
it comes to climate.
Perhaps the NHC guys are under instructions
from the Bush administration not to mention that
terrible phrase anthropogenic global warming.
Probably, like most organisations, they have a
set of rules that determine how things are
done and it is only when the discrepencies
become outrageous that anyone has the courage
to change the system.
But in fairness, I do recall reading a
prediction (can't remeber where) released around
June of 2005 that indicated 180% of normal
hurricane activity for the 2005 season.
See US Realtime Daily Streamflow Conditions at the USGS site,
http://waterdata.usgs.gov/nwis/rt
New Orleans Map here,
http://mapserver2.esri.com/cgi-bin/hazard.adol?z=&cgd=&c=New+Orleans&st=Louisiana&cd=g&s=0
I have not done my reading of the Down time info they put up after the season (NHC), but I will bet you most of them are not looking forward to giving predictions this season. To many of their own day to day storm watch predictions failed.
You can check their forecasts from 1999 onwards by clicking on Atlantic hurricanes 2006. They predict this year will be worse than last year which they predicted was going to be worse than 2004.
Link :-
http://forecast.mssl.ucl.ac.uk/
Note that some countries, such as my beloved Jamaica, do rather well (especially for a poor and densely populated country) in surviving hurricanes compared to the U.S., where people sit fat dumb and happy along the Gulf Coast. It is very interesting and informative to compare casualties in Jamaica, for example, with those in Louisiana when they are hit by hurricanes of similar size and intensity.
Cuba also does very well in surviving hurricanes.
Yes, I am aware that it helps to have high ground near shorelines, but where is it written that we should live near shorelines--or in places such as New Orleans that are almost certain to be submerged once again in the near future? Cannot we learn from our mistakes? Cannot we learn from the experiences and adaptations of others?
In any event, Mr. Buffett made some choice comments about hurricanes in his recently released 2005 Annual Report for Berkshire Hathaway. He mentioned that reinsurance rates have not gone up enough to compensate for risk and therefore he was cutting back on reinsurance written. He also mentioned the increased incidence of hurricanes and said, while he wasn't sure if global warming was the culprit, he felt like continued caution was in order.
Berkshire paid out over $2 billion in claims related to Rita and Katrina last year but despite this insurance operations were profitable overall.
Also, politically, it is hard to jack up one's insurance rates 300% to 500% or more; it irritates folk and gets the politicoes after you. Best just to get out of the business.
Not for nothing is Buffett (who started with nothing) now the second-richest person in the world. One very smart dude.
At best, humans are intmittently rational animals.
And in political groups humans are seldom at their best.
Batten down the hatches.
(1 - P) = (1 - p) * (1 - p^2) + (1 - p^3) + ...
p + p^2 + p^3 + ... is greater than 1 for any p greater than 0.5, and therefore cannot be a valid probability.
(1 - P) = (1 - p) * (1 - p^2) * (1 - p^3) + ...
(1 - P) = (1 - p) * (1 - p^2) + (1 - p^3) * ...
Stupid cut and paste! :^)
(1 - P) = (1 - p) * (1 - p^2) * (1 - p^3) * ...
A more careful analysis follows. If formation of successive hurricanes is independent (conditioned on the forecast), we would expect the hurricanes to follow a Poisson distribution (eg imagine dividing up the hurricane season into n intervals each with equal probability of forming a major landfalling hurricane that hits the US of t (n intervals of possibly unequal length, but equal probability). The number of hurricanes will be binomial with parameters n and t. Now take n large and t small and you'll get a Poisson with parameter L = nt. The parameter of the Poisson distribution, usually called lambda, is here called L for convenience. The probability of getting exactly i hurricanes, P[i] is then L^i * e^-L/i!. We need to choose L such that P[0] = 1/3 (keeping the same approximation - I think we can assume that the NHC's prediction errors are larger than the difference between 2/3 and 68% or 69%). That gives L = 1.098. Then the probability of getting 3 or more MLHs in a year is 9.9% (versus 11% in the calculation above), and the probability of getting 4 or more is 2.6% (versus 4% before). The combined probability of 3 in one year and four in another is therefore 0.5% versus 0.9%. So the evidence is a little stronger than my sloppy calculation suggested.