Thursday, September 1, 2011

Ensembles in hurricane track forecasts

Let's look at the tropics today.  As most people are aware by now, Hurricane Katia has formed in the eastern tropical Atlantic and is chugging along westward, forecast to become a major hurricane.  Katia is a Cape Verde type hurricane, which means that it formed just off the coast of Africa (near the Cape Verde Islands).  Because of this, it will go through a long, slow trek across the tropical Atlantic before approaching the Caribbean or the US.

Where is Katia headed?  We can use the NCEP's experimental GFS ensemble to get an idea of possible tracks.  Remember than a model ensemble involves running the same model dozens of times with slightly different initial conditions each time.  We then get several possible outcomes.  If all the outcomes are similar, we have pretty good confidence in the forecast.  However, if they are vastly different, there's low confidence in the forecast.  Here's several tracks from the 20-member GFS ensemble:
10-day track forecasts for Katia from GFS 20-member ensemble.
The current position of the storm is about where the dot that says "OBS" is located--it's a little to the left of the name "Katia".  From that point, every white dot further westward represents the mean location of the storm over all the ensemble members every 48 hours.  So, each subsequent dot to the west is 2-days later.  We can see that the tracks all agree fairly well out to about 6 days, so we have high confidence in our track forecast out to that time.  We start to see some spread after that, but only one of the ensemble members actually brings Katia into the US coast in Maine.  It seems far more likely that Katia will stay out to sea, perhaps impacting Bermuda, but staying away from the US coast.That's still several days away, however.

Another way of graphically looking at ensemble track forecasts is to look at the spread of the ensemble members about the mean.  For instance, say that at 48 hours out you calculated the mean position of the center of the storm from all of your ensemble members.  You could then go to each individual ensemble member model and calculate how far the storm in that particular model was from the mean position.  You can then find the mean error (a rough standard deviation) of the ensemble members from the mean location.

What does this tell us?  If the ensemble members are all doing different things, then the average difference between any one ensemble member and the mean position will be rather large.  But, if all the ensemble members have the storm in about the same place, then the average difference between any one ensemble member and the mean position will be rather small.

We can plot this on maps.  First, we can plot the mean position of the storm out of all the ensemble members at various forecast times.  Then, around each of those points, we can draw a circle whose radius represents the average distance between the position of the storm in each ensemble member and the mean position of the storm.  As uncertainty in the position grows, the circles will get larger.  As circles start overlapping each other, you get the charactaristic "cone" shape we so often see in hurricane forecasts.  Here's an example for the first 168 hours of the Katia track:
GFS Ensemble 168 hour track forecasts and uncertainties.
In the figure above, each white circle with a number in it represents the mean position of the storm in the GFS ensemble. The number tells you what hour of the forecast it is, starting at 6Z this morning.  As time goes on, the colors transition up the rainbow from purple to blue to green to yellow to red.  The colored areas represent the circles drawn around each mean position to show the uncertainty.

You can see that for the first 24 hours of the track forecast, the colored swath formed by those overlapping circles is very small--it's hardly visible.  Remember that small circles means that there is low uncertainty--the ensemble has a good idea that this is going to be the track of the storm.

However, after that, the circles start widening considerably.  By the time we get to the end at 168 hours, the uncertainty has grown quite a bit.  In theory, the ensemble suggests that by 168 hours (that's seven days from now), the center of the storm could be anywhere in the big red circular area at the end of the track.  Even with that large area of uncertainty, you can still clearly see a track emerging in the colored swath.  The ensemble is pretty confident that the storm will continue moving northwestward over the next week or so.

Let's look at another example.  There's currently a disturbance (identified as "93L" by the folks at the Hurricane Center) in the central Gulf of Mexico.
GOES visible image of invest 93L at 1715Z, Sept. 1, 2011.
It may not look like much now, but most of our models are developing that junky-looking area of clouds and storms into a tropical depression and tropical storm in the next few days.  Since this storm is forming in the Gulf, it probably poses a much greater threat to the US than it's looking like Katia will.  What do ensemble track forecasts say about this one?
GFS Ensemble 168 hour track forecasts and uncertainties.
This plot looks like a big mess.  Note that there doesn't seem to be a clear "track" developing like we saw in the Katia case.  The average position of the storm over the next week jumps all over the place in the north-central Gulf.  By seven days out, the average position really doesn't seem to have changed much, and the red "circle of uncertainty" covers pretty much the entire Gulf Coast (except for west Florida).

So what can we take from this graphic?  It gives us a couple of possibilities to watch out for:
  1. There's clearly a lot of uncertainty in the forecast track, if the models suggest that the storm could be anywhere in the northern Gulf over the next several days.  This makes forecasting the track very difficult, and as such it's possible that a lot of people on the Gulf Coast might be put under tropical storm or hurricane watches and yet never really see significant impacts of the storm.  This could be a very difficult storm to predict, and everyone will have to be cautious.
  2. Since, after seven days, the center of the storm really hasn't moved much (and the storm may never even have made landfall in that time, despite being very close to land), this storm may be a very slow mover and stick around for a while.  What does that mean?  Prolonged periods of heavy rain and windy conditions on the Gulf Coast.  The storm winds don't have to be too powerful--it doesn't even have to be a hurricane--for the storm to cause lots of flooding damage if it sticks around for a while and keeps bringing wave after wave of rain to the coast.
As of early this afternoon, the NHC still has this as an invest area in the Gulf--it's not a depression or a tropical storm yet.  However, a hurricane hunter aircraft currently investigating the area has found winds both measured and estimated to be at tropical storm strength.  So, should a closed circulation center be found, this could become tropical storm Lee in the next 24 hours.  We'll have to wait and see.

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