Will Ike ever strengthen?

Hurricane Ike is a very strange storm. After yesterday evening’s four-hour, 11-millibar pressure drop, Dr. Jeff Masters titled his late-evening post “Ike intensifying explosively,” and predicted a Cat. 3 or 4 hurricane by morning. This seems all the more plausible when the pressure dropped yet another 4 mb in the following four hours. But, as of yet, Ike’s wind speed has not budged. He remains a minimal Category 2 hurricane, with 100 mph winds, despite a minimum central pressure that suggests something two categories stronger. The National Hurricane Center’s 5am EDT discussion summarizes the weirdness:

IKE CONTINUES TO EXHIBIT SOME UNUSUAL STRUCTURAL CHARACTERISTICS.  DROPSONDE AND FLIGHT-LEVEL WIND DATA INDICATE THAT THE INTENSITY IS STILL NEAR [100 MPH]…WHICH IS ANOMALOUSLY LOW FOR THE REPORTED CENTRAL PRESSURE.   THE LATTER VALUE…946 MB…WOULD NORMALLY CORRESPOND TO A BORDERLINE CATEGORY 3/4 HURRICANE.   THE TROPICAL CYCLONE HAS A VERY LARGE WIND FIELD…PARTICULARLY OVER THE NORTHERN SEMICIRCLE AS OBSERVED BY BUOYS AND SHIPS OVER THE NORTHERN GULF.   HOWEVER IT CONTINUES TO HAVE A SMALL INNER CORE WITH AN EYE JUST UNDER 10 N MI IN DIAMETER. THERE HAS BEEN A DOUBLE WIND MAXIMUM…ALTHOUGH HURRICANE HUNTER OBSERVATIONS SUGGEST THAT THE OUTER WIND BAND IS BEGINNING TO CONTRACT.

The “double wind maximum” point is key. Ever since Ike exited Cuba, the tiny size of its inner core has seemingly suggested that it’s on the verge of an eyewall replacement cycle — which, in a normal storm at least, would be expected to result in temporary weakening, followed by strengthening, as the inner eyewall disintegrates and then is replaced by a contacting outer eyewall. But no ERC has yet occurred, and thus the double eyewall structure remains. This seems to be playing a big role in Ike’s stubborn refusal to intensify, despite the very low pressure.

Alan Sullivan sums things up thusly:

This morning hurricane Ike continues to display its unusual structure.  Hurricanes often develop distinctive individual characteristics and retain them through much of their cycle.  Ike has had a tiny core from the time it went through an abrupt eyewall replacement just as it reached eastern Cuba’s coastline.  That feature persisted during two land crossings, and it has persisted in the Gulf.  A much larger ring of hurricane winds surrounds the core at a radius of a hundred miles.  This is weird.

With the wind field so extended, Ike has developed a very low central pressure without a corresponding surface intensity. Winds are still only 100 mph despite pressure at 945 mb, which would support category four strength in a small, tightly wound storm. There is concern that this sprawling system might contract (and thus develop stronger winds) just as it approaches the coast somewhere near or southwest of Galveston. This is why NHC fears a major hurricane at landfall, though Ike does not meet that criterion now.

Sullivan himself is unconvinced that such a last-minute contraction will occur. I don’t presume to know whether it’s likely or unlikely, but I trust the NHC that the odds are high enough — if only because of the basic uncertainty that we don’t really understand what’s happening with Ike — to warrant continued concern about a possible landfall by a Cat. 3 or 4 storm, not “just” a Cat. 1 or 2.

Regardless of our slight disagreement on that issue, Sullivan makes a couple of other really good points. For one, he writes, “I do think [the double eyewall weirdness] may induce exceptional tornado risk when [Ike] moves inland and begins to disintegrate.” In comments here, he emphasizes this point again: “I think it is time to start talking about tornadoes. That odd core structure could be a killer, at a considerable distance inland.”

Also:

The effects of Ike may not be that concentrated, but they will be very widespread. With the prolonged run of onshore winds on northeast Texas coast, several tidal cycles may back up into the marshlands, just as floodwaters from inland rains try to head seaward. This could result in a prolonged and dramatic immersion of low-lying areas, compounded by wind-driven waves.

Sticking with the storm surge theme, “Ubu Roi” at Houblog makes another good point: “That wide wind field may be keeping the current velocity low, but it also is allowing Ike to build up one hell of a dome of water to ride atop.” Indeed.

In other words, even if Sullivan is right that Ike will never escape its current Category 2 limbo, this hurricane is still going to be a pretty big deal. (And if it does finally “tighten” into a Cat. 3/4, it’ll be an even bigger deal, obviously. In Ubu’s words: “If it tightens up, and the wind speeds up in the next 24-36 hours, it would be even worse than if it were tight now.”)

Furthermore, even if landfall is a bit down the coast from the “worst-case” coastline between Freeport and western Galveston Island — as projected by the lastest GFDL run, in a retreat from its earlier doomsday scenario — the impact on the vulnerable coastal regions of the Houston/Galveston area might still be rather severe, given the storm’s large size and odd structure.

All in all, it seems like a rather bad time for local officials in vulnerable coastal areas to be gun-shy about evacuations. Personally, if I lived anywhere on Galveston Island, I’d leave now! (On the other hand, if I lived well inland, in a well-constructed home, and not in a surge zone or flood plane, I would not leave. Run from the water, hide from the wind, folks.)