A Later-Season Hurricane

The "official" hurricane season lasts from the beginning of June through the end of November, so we're beginning to near the end of it.  In fact, we're getting to a time of the year when there are fewer hurricanes forming in the Atlantic.  Here's a chart from the National Hurricane Center showing the annual distribution of Atlantic hurricane frequency:

You can see that the number of tropical cyclones typically peaks in mid-September and then trails off.  However, interestingly enough, notice that there does appear to be a secondary maximum (though not nearly as prominent) in mid- to late-October.  Is there some physical reason for this?  I'm not sure.  But, the fact remains that here we are in late October and we have another hurricane to track:  Hurricane Rina.  (Oddly enough, the name "Rina" was selected to replace the previously-retired name "Rita" from this cycle of hurricane names.  I think that sounds a bit too similar, but...it's the way it is...).  Here's a visible satellite image of Rina from early this afternoon:

You can see that Rina is a fairly well-organized storm with a clear eye and that it isn't experiencing much wind shear.  How can you tell that there's not much wind shear going on from this image?  Look at the high-level cirrus clouds coming off of the storm--the wispy, fibrous like clouds on the outer edges of the circulation.  Notice that you can see those clouds pretty much all the way around the storm (maybe not so much on the western side, but they're still mostly surrounding the storm).  If we had strong wind speed shear (that is, winds strongly increasing with height in a direction that's NOT circling the storm), we'd see all of that cirrus "outflow" on one particular side of the storm, since the upper level winds would blow off all the clouds in one direction.  Because there's cirrus pretty much everywhere, we can guess that there's not a lot of vertical wind shear.  That's good for hurricanes, as it allows them to stay organized.

Strength-wise, Rina is a category 2 hurricane, forecast to become a stronger category 3 storm in the next day or so.  Warm sea-surface temperatures in the western Caribbean are helping fuel this storm.  Also, that lack of strong wind shear is allowing the storm to stay together and not get blown apart.

In contrast to the usual paradigm of hurricane forecasting, while we're pretty sure that the hurricane is going to strengthen, we aren't so sure about where the hurricane is going to go.  In a previous blog post, I talked about using model ensembles to forecast tropical cyclone paths. Remember that ensembles involve running multiple models or multiple perturbed versions of the same model to get many different possible forecasts of the future.  We can see how well these forecasts agree to make decisions about where the storm may go.

Well, here's an example of an ensemble forecast of Rina's path from this morning's ECMWF model ensemble.  Each yellow line represents a different ensemble member's forecast of Rina's path:

The black line shows the previous path of the center of the storm and where it is now.  But look at how many different paths the ensemble members are forecasting for this storm!  Some are sending it south into Nicaragua.  Some are bringing it northwest, then abruptly southwest into Belize.  Many are bringing it into the Yucatan Peninsula.  And a few even have it crossing the Yucatan and then curving back northward to hit Florida.  That's a lot of uncertainty in the path of this storm.  The National Hurricane Center's forecast favors the Yucatan path (and you can see that a fair number of the ensemble members above do too).  Here's the NHC's forecast:

The cone of white surrounding the storm indicates their uncertainty in where the hurricane center will be.  The forecasters at the Hurricane CEnter use their expertise and experience with past hurricanes to try to rule out some of the more unlikely forecasts.  As such, you can see here that they really don't believe the models that are taking the hurricane into Nicaragua or Belize.  But there's still a lot of uncertainty in their forecast track.  Furthermore, they haven't ruled out a landfall in Florida yet.  But Rina is a slow-moving storm, and we will have until the end of the week to see what Rina does and where it goes before determining if it will hit the US.