Ingrid is born; Humberto and Felix--a sign of climate change?

Tropical Storm Ingrid finally managed to get organized enough last night to earn a name. Ingrid continues to struggle with strong upper-level westerly winds, which are creating about 15 knots of wind shear over the storm. Satellite loops of Ingrid show that the shear is keeping most of Ingrid's heavy thunderstorm activity pushed to the storm's east side. There is some weak upper level outflow to the north, and one low-level spiral band forming on the storm's west side. The NOAA Hurricane Hunters are in the storm flying a research mission, and have measured top surface winds of 59 mph with their SFMR instrument.


Figure 1. Microwave image of Ingrid at 7:28am EDT. Ingrid has just one clump of heavy thunderstorm activity near the center, and one spiral bands forming on the west side. Image credit: Navy Research Lab, Monterey.

Wind shear is forecast to remain below 15 knots over Ingrid until Saturday afternoon, then steadily rise to values near 30 knots. Ingrid will probably increase in strength over the next 24 hours, then weaken. It is possible that the shear will destroy Ingrid by the time the storm makes its closest approach to the Lesser Antilles Islands on Tuesday. The exact track of Ingrid as it approaches the islands depends on how strong the storm gets over the next 24 hours--a stronger storm will move more to the northwest, since it will "feel" upper level winds that will pull it farther north. As a result, the 5-day track forecast for Ingrid has higher uncertainty than usual, and it is not a sure thing that Ingrid will miss the Lesser Antilles. The HWRF, GFDL, and SHIPS intensity models all forecast that Ingrid will survive the shear and will be a weak tropical storm when making its closest approach past the islands.

If Ingrid does survive past Tuesday, it may encounter a region of lower wind shear 6-10 days from now that will allow it to re-intensify. A large ridge of high pressure is forecast to dominate the region late next week, which should force Ingrid on a west to northwest track towards the U.S. Depending upon how far north the storm is at that point, it may represent a threat to either Bermuda or the U.S. East Coast. The GFS model forecasts that the next major trough of low pressure capable of recurving Ingrid out to sea will not arrive until Saturday September 22. Stay tuned.

Humberto
The surprise hurricane, Humberto, turned out not to be the flood-making threat originally feared. While the storm was able to dump 10-14 inches of rain over Jefferson County, Texas, Humberto's rains have mostly been under seven inches (Figure 2). Only 1-2 more inches are expected along its path today in Mississippi, Alabama, and Tennessee. All of these states are suffering severe drought, so Humberto is a welcome visitor.


Figure 2. Total rainfall from Humberto as estimated by radar.

Humberto, Felix, and Dean--a sign of climate change?
Many people have asked me if the fact that we've had two record-breaking rapidly intensifying storms this year--Felix and now Humberto--imply that climate change might be affecting Atlantic hurricanes for the worse. It's also very odd that we've had eight Category 5 hurricanes in the past five years, and two landfalling Category 5 hurricanes this year. That's a lot of Cat 5 activity. So, let's look at the facts and see what we can learn.

Intensification rates
Reliable record keeping of intensification rates of Atlantic hurricanes began in 1970, when regular satellite coverage became available. Since 1970, Hurricane Humberto holds the record for fastest intensification from first advisory issued to hurricane strength--18 hours. (Actually, Humberto did the feat in 14 1/4 hours, but this will get rounded off to 18 hours in the final data base, which stores points every six hours). There have been six storms that accomplished the feat in 24 hours--Hurricane Florence of 2000, Hurricane Erin of 1995, Hurricane Bonnie of 1992, Hurricane Earl of 1986, Hurricane Kate of 1985, and Hurricane Kendra of 1978.

If one considers instead the fastest intensification time from tropical depression strength to hurricane strength, Humberto has some company--Hurricane Blanche of 1969 did it in 12 hours, and Hurricane Alberto of 1982 did it in 18 hours. However, these storms spent two and three advisories, respectively, at tropical depression strength, and thus spent more time getting their act together than Humberto did. Thus, Humberto is unique in the Atlantic hurricane record.

Since 1970, Hurricane Felix of 2007 holds the record for fastest intensification from the first advisory to a Category 5 hurricane. It took Felix just 54 hours to accomplish the feat. Hurricane Camille of 1969 also took 54 hours to do so, but the first advisory put Camille as a 60 mph tropical storm. It is likely that Camille would have been classified as a tropical depression earlier had reliable satellite imagery been available.

Hurricane Ethel of 1960 holds the pre-1970 record for fastest intensification from the first advisory to a hurricane. Ethel strengthened from a 45-mph tropical storm to a 85 mph Category 1 hurricane in just 6 hours. We don't know when Ethel started as a tropical depression, since this was before the satellite era. Ethel also holds the record for quickest intensification from the first advisory to a Category 5 hurricane--it took Ethel just 18 hours. This all occurred while Ethel approached landfall on the Mississippi coast. Luckily, the storm fell apart just as quickly, weakening from a Category 5 hurricane to a tropical storm in the 12 hours prior to landfall.

The case of Ethel has not yet undergone rigorous review by the NHC committee that is quality checking and revising the entire Atlantic hurricane database. It is possible that some revisions may be made to these records, as the storm's wild swings in intensification seem rather extreme. The minimum pressure measured by the Hurricane Hunters in Ethel was 972 mb--more typical of a Category 2 hurricane than a Category 5. Still, there is little question that Ethel intensified remarkably fast, and we would be in big trouble if another Ethel formed by the coast and didn't fall apart before landfall.

Hurricane Wilma of 2005 holds the record for fastest intensification with respect to pressure by an Atlantic hurricane. Wilma's pressure dropped 97 millibars in 24 hours as it went from a tropical storm to a Category 5 hurricane. The previous record holder was Hurricane Gilbert (1988), which dropped 72 mb in 24 hours. Wilma's pressure fell 54 mb over six hours, beating Hurricane Beulah's drop of 38 mb in six hours in 1967, and Wilma's 12-hour pressure fall of 83 mb beat the old 12-hour pressure fall record of 48 mb set by Hurricane Allen in 1980.

No scientist has published a paper linking rapid hurricane intensification rates with global warming. While the cases of Humberto and Felix are certainly unique, the year 1969 also had two storms that were very similar in their intensification rates. A quick look I did at historical intensification rates don't show any noticeable trends, and I think that the rapid intensification rates of Felix, Humberto, and Wilma the past three years are not far enough outside the statistical norms that we need to invoke climate change as an explanation. Still, it does leave one wondering, and climate change could be affecting hurricane intensification rates.

Category 5 records
Since reliable record keeping began in 1944, there have been 27 Category 5 hurricanes. It is possible that a few Cat 4's that should have been Cat 5's were missed, but I'm guessing this number is at most 10% of the total--two or three. Only ten hurricanes have made landfall at Category 5 strength. Two of those landfalls have occurred this year--the only year that has happened. Thus, the fact we've had 20% of all Category 5 landfalls on record in the same year is truly exceptional. The fact that they both occurred in the Western Caribbean back to back is not that surprising, since the Western Caribbean has the very high heat content waters one needs to fuel Category 5 hurricanes, and the landfalls occurred during the peak part of hurricane season.

This year is the fourth year multiple Cat 5's have occurred--see Wikipedia's Category 5 list to see the details. We've now had six Cat 5's in the past three years, and eight in the past five years. Is this an indication climate change is at work? Well, we did have back-to-back years with two Cat 5's each (1960 and 1961), so one can still argue that the Cat 5 activity of recent years is a statistical abnormality. In addition, recent work done studying sediment deposits indicates that intense hurricanes have gone through cycles lasting hundreds or even thousands of years long. Periods of high Category 5 activity similar to that observed the past five years could well have occurred in the distant past. Still, some very good hurricane scientists have begun presenting evidence that climate change may be increasing both the frequency and intensity of hurricanes in the Atlantic. It is possible that climate change may be partially responsible for the recent spate of Cat 5's and rapidly intensifying storms. Climate change is significantly affecting weather patterns worldwide, and must be influencing hurricanes. Unfortunately, we don't have a long enough or high enough quality data record of Atlantic hurricanes to accurately judge how much of an impact this might be. Furthermore, it's not clear why the Atlantic Ocean would be the most strongly affected--we see little evidence that climate change is creating stronger hurricanes in the other ocean basins. But, the events of 2005 and again this year leave me concerned. Eight Cat 5's in five years is an awful lot of severe storms in such a short period. Climate change may be indeed be changing Atlantic hurricanes for the worse.

I'll have an update Saturday.
Jeff Masters