Tropical Storm Irene

Guest Post by Willis Eschenbach

When is a hurricane not a hurricane? Well, when it doesn’t blow 64 knots (33 m/sec, 74 mph), because then it’s only a tropical storm. Inspired by a post over at the Cliff Mass Weather Blog, I’ve been trying to find a single report of sustained hurricane force winds anywhere along Irene’s path at or near landfall … no joy. I knew exaggeration was the order of the day for some folks in the climate debate, but I hadn’t realized that the illness had infected the Weather Service itself.

Figure 1. The path of Tropical Storm Irene over the mainland of the US. Symbols with a yellow center to the black storm symbol indicate a (claimed) hurricane. SOURCE ibiseye

We were fortunate in that we have very good records of the wind speed when Irene made landfall. It went almost directly over the wind recording station at Cape Lookout, at the bottom of Figure 2.

Figure 2. A closeup of Irene’s landfall. There are four wind recording stations in the area, at Beaufort (below the “70” marker at lower left), at Cape Lookout (bottom left) and at Cape Hatteras (upper right). The Onslow Buoy is located offshore, southwest of Cape Lookout.

The wind record at Cape Lookout is quite interesting, as the eye of the hurricane passed right over the anemometer there. Figure 3 shows the wind dropping as the eye went over, coincident with the deep plunge of the barometric pressure to 950 hPa.

Figure 3. TS Irene wind (light blue) and barometric pressure (violet) at Cape Lookout before, during, and after landfall. Green line at the top shows the minimum wind speed for a storm to be classified as a hurricane (64 knots).

Figure 3 shows the classic pattern of a hurricane passing directly overhead. The “eye” of the hurricane has almost no wind, and is at the center of the low pressure area. You can also see the “calm before the storm. But what you can’t see is any trace of hurricane force winds.

Not finding hurricane force winds at the eye, I looked at the other nearby stations as well. The weather station at Cape Hatteras is in the “dangerous semicircle”, the right hand side of the storm track (Fig. 2) where the speed of the storm is added to the speed of the winds circulating around the eye. Beaufort, on the other hand, is in the safer half of the storm, where the speed of the storm is subtracted from the circulating speed of the winds. The Onslow Buoy is also in the safer semicircle, on the left of the storm track in Figure 2. Figure 4 shows those records.

Figure 4. Winds at TS Irene landfall for Cape Lookout, Beaufort, Onslow Offshore Buoy, and Cape Hatteras.

As you can see, although Irene definitely qualifies as a solid tropical storm (winds greater than 35 knots), it does not reach or even really approach the 64-knot threshold for hurricanes. Other than at the eye itself, the winds did not exceed 50 knots, much less reach 64 knots.

After crossing over the land near Cape Hatteras, Irene headed back out to sea again. I thought perhaps it might have picked up steam when it went out over the ocean again. It made a second landfall in Atlantic City and went along the coast to New York.

Figure 5. Second landfall for Irene.The nearest stations to Irene’s track are Costeau (near Mystic Island above Atlantic City), NY Harbor Buoy (outside the mouth of the harbor, in the dangerous semicircle), Sandy Hook (hook shaped peninsula just above Long Branch and central hurricane symbol) and Kings Point (near New Rochelle above New York City). Note that the storm is claimed to be a hurricane until it gets well into New York State.

It appears from an examination of the station data shown below in Figure 6 that it did not pick up strength over the water. By the time Irene reached land a second time, it barely qualified as a tropical storm, much less a hurricane.

Figure 6. Wind speed from Tropical Storm Irene as it made the second landfall.

So, despite looking at Irene before, during, and after both landfalls, there is no hint of a hurricane anywhere. By the time it got to New York the eye of the storm had dissipated, what was left were huge bands of rain clouds.

Is there a moral in this story? Well, I can understand people taking extra precautions, better safe than sorry is a good rule. And I certainly imagine that when the Weather Service re-examines the records, the error will be corrected.

But that doesn’t help in making the decisions. As soon as Irene hit land, it should have been downgraded immediately to a tropical storm. That’s what it was, not a hurricane making landfall but a tropical storm. As far as I can tell, we still haven’t had a hurricane make landfall during Obama’s presidency, a historical oddity.

Individuals and city mayors and the people in charge of the emergency response can call for any level of reaction to storm threats. They may decide an exaggerated response is appropriate.

But they need accurate information to do that, not exaggerated claims. They need the actual facts, the best estimates with no exaggeration on either the high or low side.

In this case, it appears that people got so wrapped up in the question of the winds, and the fear of the winds, that they overlooked what actually made Irene unusual. This was not the wind speed, but the size of the storm. Combined with Irene’s generally slow movement over the ground, Irene’s huge dimensions meant that any given area would get rained on for a really, really long time.

And in turn that meant that the cities and towns along the coast, the ones receiving all of the attention from the fear of high winds and attendant storm surges, weren’t the towns in danger. Unlike the coastal cities, the vast expanses inland were not able to have the rainwater just flow back into the ocean. Inland, the water piled up and overflowed the banks.

And so, because of the overestimation of the wind speeds, our attention was diverted from the real threat. Because of the claimed hurricane-force winds, a storm surge up to eight feet was predicted in New York Harbor. But in the event, the storm surge was barely three feet, a non-event … and meanwhile, New England was getting badly flooded.

So the moral to me is, honesty is the best policy for a National Weather Service. Don’t exaggerate the possible effects to be on the “safe side”, don’t minimize the possible effects. Just give us the best information you have, and let us make up our own minds. As Sergeant Friday used to say … “Just the facts, ma’am” …

w.

NOTE: All wind data is from the NOAA National Buoy Data Center http://www.ndbc.noaa.gov/.