Will Katia Go Off?

There's chatter amongst some meteorological e-mail groups about the remarkable extratropical transition of Hurricane Katia forecast by the 0600 UTC GFS.  Extratropical transition is the conversion of a tropical cyclone into an extratropical frontal cyclone.  Check out for yourself how Katia moves poleward, begins to interact with colder air over the North Atlantic, and evolves rapidly into a deep occluded cyclone.

In case you missed it, that is a 928 mb low in the last frame.

Indeed, that is a beast of a storm, but this is a medium range forecast.  In the GFS ensemble, there are a number of members that produce a very robust extratropical cyclone, but a few that are much weaker.  In the spaghetti plot below, the blue isobars identify the members with the deepest cyclones (the control sea level pressure is not plotted, but I've left the control wind speed on for reference).

The bottom line is that this is a storm that bears watching, but there are still a wide range of possibilities.  Time will tell if we see a remarkably low sea level pressure.