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Why is Chris strengthening?

For the last few days, Bryan and I (and NHC for that matter) have been skeptical of the development of the former Invest 99L. "Too much shear," we said. "Too much dry air," we said. Clearly, we were wrong. With the latest NHC update at 5:30 PM EDT, Chris has intensified to a moderate tropical storm with sustained winds of about 55 mph. The official forecast takes the storm to hurricane intensity in 3 days. So, this begs the question, what happened? I think there are two separate reasons for underestimation of Chris.

1. Thunderstorm Activity
Warm sea-surface temperatures, greater than 28┬║C (Figure 1), made the lower atmosphere ripe for thunderstorms. Unfortunately, dry, Saharan, dust-laden air at the mid-levels tended to inhibit convection. Usually, this mid-tropospheric dry air stops thunderstorm activity and kills off the incipient cyclone. Chris overcame this because its circulation was unusually strong, The strong circulation led to low-level covergence, which provided the trigger for the thunderstorms. Over time, as the thunderstorms erupted and then subsided, the dry air at the mid-levels was slowly moistened due to the evaporation of the clouds and precipitation produced by the thunderstorms. Finally, once most of the atmosphere was moistened, thunderstorms were able freely to fire. Currently, the dry air is far enough away from the storm that it is no longer being sucked into the center (Figure 2). This is allowing Chris to intensify through normal means.

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Figure 1. Sea-surface temperatures over the Atlantic as of July 29. The red "L" indicates the position of Chris.
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Figure 2. Saharan air layer analysis from UW-CIMSS. The red "L" indicates the position of Chris and the hot colors show where there is dry, dusty air.

2. Upper-level outflow
Most of the time in a tropical cyclone, shear is bad. A hurricane is a warm core system that is vertically upright. Shear acts to tilt the storm, meaning the convection is no longer centered over the middle of the storm. This is not conducive to tropical development. In this case, however, the shear may not have been entirely a bad thing. Certainly, the shear was inhibiting development over the weekend, but it may be one of the reasons that Chris is strengthening now.

Before I explain how shear might have helped Chris, we need to think about how hurricanes lower their central pressure. Atmospheric pressure is simply the weight of the atmosphere per area of the ground. In a high pressure area, there is more atmosphere above you per unit area than there is in a low pressure area. However, I mentioned above that Chris (like all tropical cyclones) had low-level convergence. That would tend to add weight to the low-level atmosphere there. So how does the weight actually decrease? Like the valve stem of a tire, it escapes at the top. More air is moved out of the storm at the top of the troposphere than is added at the bottom. What results is a net loss of atmospheric pressure.

Ok, so back to shear. The shear actually provided Chris a high-powered way to evacuate mass from its center. Looking at the upper-level winds around Chris (Figure 3), we see that winds are quite strong to the southwest of Chris. This is the air that is being removed from the storm. How did it get there? You guessed it, shear. The shear actually provided this "outflow channel" to Chris as it was developing, a channel analogous to the valve stem of a tire. Now, the storm has a way to efficiently remove air from its top. Incidentally, this also allows for stronger convection on the southwestern side of Chris, which is where the most intense thunderstorms are located (Figure 4).

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Figure 3. Upper-level winds in the Atlantic. The red "L" indicates the position of Chris. The red arrows show the outflow channel that is allowing Chris to intensify.
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figure 4. Infrared satellite image of TS Chris. Most of the heavy thunderstorms are located to the southwest of the center of circulation

As for the forecast, it's pretty difficult at this point. None of the global models have yet to pick up on Chris, so it's up to the hurricane-specific models to make the forecast. Shear (Figure 5) is actually fairly low over Chris at the moment (<15 kts). So long as it stays in a low-shear environment, there is no reason Chris can't intensify, since, as noted above, SSTs are warm and the dry air is sufficiently far away. The GFDL briefly takes Chris up to hurricane strength and the SHIPS models approach it (Figure 6), but their initial conditions are too weak. The offical NHC forecast has it becoming a hurricane in 72 hours. I tend to agree with their forecast, again provided that Chris stays away from shear. In Figure 3, I pointed out an upper-level low over the Bahamas. If Chris moves toward the upper-level low, shear will increase and Chris will probably be blown apart. For the track forecast, the models are quite divergent. Some have tracks directly across Haiti and Cuba, while others recurve it out to sea without threatening the US (Figure 7). For now, I would just take the average of all the models and keep it moving to the west-northwest. Interests in Florida and along the Eastern Seaboard are going to want to start paying attention to Chris in the near future.

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Figure 5. Shear analysis from UW-CIMSS. The yellow contours are values of shear in knots. The red "L" is the location of TS Chris.

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Posted by Adam Moyer | Permalink