Tropical Storm Erin forms in eastern Atlantic, Caribbean disturbance heads into Yucatan

While a tropical disturbance near the Yucatan peninsula struggles to organize, the one near the Cape Verde islands in eastern Atlantic strengthens into the season’s fifth named storm: Erin.

Tropical Storm Erin

At 11 p.m. EDT last night, the National Hurricane Center upgraded the disturbance near the Cape Verde islands to Tropical Depression 5 based on satellite observations.  Since then, it has continued to get better organized, and at 7 a.m. this morning it was upgraded to Tropical Storm Erin.  So, we are witness to the very rare occurrence of tropical storm warnings being issued for parts of the Cape Verde islands!

Visible satellite image of Erin... it is just 530 miles off the African coast, or 2360 miles east of Martinique in the Lesser Antilles. (CIRA/RAMMB)

Visible satellite image of Erin… it is just 530 miles off the African coast, or 2360 miles east of Martinique in the Lesser Antilles. (CIRA/RAMMB)

If we look at all known storms (going back to 1851) of tropical storm status within 100 miles of Erin’s current location during the entire month of August, we get a short list of just nine.

Since the storm came off of Africa rather far north, its chances of recurving before reaching land are quite high, as the map of the previous nine analogs shows.  Of the nine storms plotted, only one reached the U.S.: the 1893 “Sea Islands Hurricane”.  It also formed on August 15.

Tracks of all known cyclones that were tropical storms within 100 miles of Erin's current position during August.  (NOAA)

Tracks of all known cyclones that were tropical storms within 100 miles of Erin’s current position during August. (NOAA)

That leads me to the forecast.  Large-scale conditions should remain quite favorable for additional intensification over the next couple of days… that includes warm ocean temperatures, low vertical wind shear, and moist unstable air surrounding the storm. It currently has 40 mph peak sustained winds.  The National Hurricane Center predicts its peak winds will increase to 60 mph in 48 hours.

Then by the end of the weekend, models are in good agreement that environmental conditions will all become less favorable for the storm.  However, they disagree on how the storm will respond; some maintain it as a tropical storm while others indicate it will dissipate into an open wave.

As I described before, storms of different intensities are steered by different layers of the atmosphere, with weaker storms following low-to-mid level winds and stronger storms responding to a much deeper layer.  We can see this behavior in the model track forecasts for Erin below.

Model guidance for Erin over the next 5 days.  The southern grouping generally weaken or dissipate the storm, while the northern grouping keeps it stronger. When you look at plots like this, it's very important to recognize that not all models are created equal.  (NCAR)

Model guidance for Erin over the next 5 days. The southern grouping generally weaken or dissipate the storm, while the northern grouping keeps it stronger. When you look at plots like this, it’s very important to recognize that not all models are created equal. (NCAR)

At any rate, Erin won’t be affecting anyone in the near future, but these “Cape Verde” storms have a notorious history.

Yucatan disturbance

Closer to home, the disturbance in the western Caribbean that I mentioned yesterday (see “Tropical Atlantic waking up: two potential storm threats”) has changed little in organization.  It is still a broad messy area of thunderstorms, and today, its chances for formation are even smaller because it’s about to pass over the Yucatan peninsula.  Also, if you refer to the first figure in yesterday’s update, the storm track (purple arrow) that’s verifying is the southernmost one.

This morning, the center of the disturbance is admittedly not easy to spot in a satellite image.  It’s actually right by the Mexico/Belize border on the southern end of the huge swath of cloudiness and thunderstorms associated with the larger-scale tropical wave (marked with a white dot in the image below).

Enhanced infrared satellite image of the Yucatan disturbance.  (NASA)

Enhanced infrared satellite image of the Yucatan disturbance. (NASA)

It’s hard to say what its future will be once it enters into the Gulf of Mexico, because there might not be much left to look at.  If it does manage to re-organize, it will take a while, so shouldn’t become anything more than a tropical storm.  Of course, once a storm is in the Gulf, it has to make landfall somewhere, and right now, the model consensus is north toward Louisiana, but if it dissipates, a more westward track into Mexico or Texas is likely.

The timing should be around Saturday evening into Sunday morning, but it will just be an anti-climatic increase in rain and thunderstorms.  Over the next five days, areas along and to the right of the path can expect 4”+, with locally higher amounts.  Given the huge uncertainty in the track, there is also large uncertainty in the rainfall forecast.

Five-day rainfall forecast, valid Thursday morning through Tuesday morning.  (NOAA)

Five-day rainfall forecast, valid Thursday morning through Tuesday morning. (NOAA)

Once this system is back over the water tomorrow, we’ll have a better idea of its future track and intensity… right now, there are too many significant unknowns.

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

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