Tropical Storm Chantal nearly succumbs to a hostile environment

Tropical Storm Chantal’s appearance on satellite is even more ragged today than it was yesterday. The weakening trend we saw Tuesday has continued, and as I alluded to in my analysis yesterday, the vertical wind shear was just too strong. These large differences in wind speed and direction with height not only act to separate the low-level vortex from the thunderstorms above, they also allow dry air to work its way into the circulation in a process is called “dry air entrainment.”

Visible satellite image of Chantal's remnants from 10:40am EDT today.  The surface center is on the far western edge of the cloud cover. (NASA)

Visible satellite image of Chantal’s remnants from 10:40 a.m. EDT today. The surface center is on the far western edge of the cloud cover. (NASA)

As forecast, the vertical wind shear is now up to near 35 mph, way too strong for a developing tropical cyclone to withstand. However, a reconnaissance aircraft flying through the storm this morning did barely manage to find a closed surface circulation, and as such, Chantal has held on to its tropical storm status.

As of 11 a.m. EDT (see current forecast track, watches and warnings from the National Hurricane Center), Chantal is centered 140 miles south of Port-au-Prince, Haiti, and racing west at about 30 mph. Over the next day, heavy rain will threaten the Dominican Republic, Haiti, Jamaica and Cuba. Winds will be a minor threat in comparison. The center should pass over central Cuba tomorrow afternoon, then approach southern Florida on Friday morning. It’s possible that after an encounter with Cuba in its already tenuous condition there may not be much left to reach Florida.

Over the weekend, Chantal (or its remnants?) is forecast to head north toward northern Florida, Georgia and South Carolina. But again, the rainfall would be a bigger story than the wind.

Aside from the shear and dry air entrainment, Chantal is also nearing Hispaniola and eastern Cuba with their mountain ranges. If you refer back to my post on Monday and scroll down to the tracks of past July storms that were in a similar location to Chantal, you’ll notice that the majority of them end abruptly in the northern Caribbean Sea. This is not a coincidence, as this region is climatologically hostile to tropical cyclones this time of year, largely due to strong wind shear. Clearly, something was indeed different in 2005 when Emily passed through the Caribbean as an extremely intense hurricane.

A side effect of dramatic weakening is that storms get steered by different layers of the atmosphere. Over the past few model runs, forecast tracks for Chantal have shifted more and more west, but at the same time show a weaker and weaker storm. A weak storm with limited coherent deep convection (thunderstorms) gets steered by winds lower in the atmosphere, while a mature intense hurricane with a robust vertical structure gets steered by a deeper layer of winds, and is much more responsive to troughs and ridges and “traditional” steering patterns.

Chart showing the approximate atmospheric environmental steering layers for tropical cyclones of various intensities.  Based on  research by Chris Velden and others at the University of Wisconsin in the early 1990s.

Chart showing the approximate atmospheric environmental steering layers for tropical cyclones of various intensities. Based on research by Chris Velden and others at the University of Wisconsin in the early 1990s. (Brian McNoldy)

Turning our attention 4,000 miles east, the African wave that I mentioned in yesterday’s update remains a feature of interest.

Visible satellite image of the easterly wave near the Cape Verde islands at 6:30 a.m. EDT today. (SSEC)

Visible satellite image of the easterly wave near the Cape Verde islands at 6:30 a.m. EDT today. (SSEC)

Now centered just south of the Cape Verde islands, it still needs a few days to “cook” before it would be considered a tropical cyclone, that is if it continues to develop. Several models do develop it though, so it’s worth keeping an eye on what would be named “Dorian.”

Brian McNoldy is a senior researcher at the University of Miami’s Rosenstiel School of Marine and Atmospheric Science.

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