Watching Tropical Storm Maria (Advisory #34)


Tropical Storm Maria continues to make an approach towards Bermuda. A very close brush, or direct hit is likely to occurr in the next 36 hours as Maria heads almost directly towards the islands.

Forecast for Maria and Bermuda

Maria is expected to strengthen further into a Hurricane overnight, and maintain this strength for its closest approach in Bermuda tomorrow between 9am and 3pm. The Tropical Storm or Hurricane will be close enough to bring tropical storm conditions to the island. However, depending on intensity and track, the storm may also bring hurricane conditions briefly to the island. As a result of this ominous forecast, Tropical Storm Warnings and Hurricane Watches are in place for the islands.

The Bermuda Weather Service is forecasting max sustained winds from Maria to be near 60mph with gusts around 75mph in Bermuda. I think this is a reasonable forecast, although I feel that their gusts are a little conservative – I would say gusts as high as 85mph are possible, considering a fairly large hurricane could be less than 100 miles away.

This forecast translates to:

  • Squally weather including a chance for tornadoes.
  • Moderate tree and vegetation damage; this could have an impact on the electricity supply.
  • Minor-moderate flooding of low areas such as the canal in Pembroke, especially at high tide.
  • Extremely dangerous surf around the islands, possibly including overwash on the causeway and south shore making travel near impossible.

Discussion

Wind shear has decreased this afternoon around Maria, as evidenced by a more even upperlevel outflow pattern, thunderstorms forming and persisting over and near the center of the storm, and a better alignment with mid and surface level centers. This has allowed Maria to strengthen from 50mph this morning, to 65mph this evening. In fact, her slower than anticipated forward speed could keep her in this favorable upper pattern long enough for her to become a category one hurricane.

Hurricane hunters have investigated Maria and have found that she has indeed become more organized and intense, which warranted raising her wind speeds to 65mph. They found flight level winds of 84mph, surface winds of  64mph, and a minimum pressure of 1001mb. However, an 84mph flight level wind typically translates to a 73mph surface wind, but because of Maria’s previous state of disorganization, she probably isn’t organized enough to translate these winds properly to the surface, and it is more likely that the winds of 64mph that were measured, are the accurate winds. The pressure was found near the center of the storm, so an estimate was made using this and a knowledge of the pressure gradients and convection in the storm. This was 999mb.  A 999mb storm typically has 50mph winds, so the pressure gradient must be fairly tight around the storm for it to have 65mph winds.

Tropical Storm Maria has encountered a limited amount of dry air, and is passing over the cool water wake of Hurricane Katia from earlier in the month. The marginally favorable waters of 80-81F won’t hinder strengthening, just limit it. While dry air will hinder it, but there isn’t enough present for it to be a huge factor. Wind shear is expected to increase after the storm passes Bermuda, but by that time, it will also encounter cold water and a lot of dry air, so the storm will be weakening rapidly by that time on Friday.

Maria is headed NNE at 18mph; an increase in that speed is expected, and a turn more towards the NE is also expected. These two forecast changes in Maria’s trajectory are expected to take her dangerously close to Bermuda. Maria is a Cape-Verde type storm unlike Bret, Cindy, Franklin, Gert, and Jose whose origins were from tropical waves and frontal interactions. Therefore, her windfield is much larger and her circulation is tighter. Also, unlike both Igor and Irene, Maria is building a core rather than having one eroded.

Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s