Monday, September 09, 2019

Post-Tropical Cyclone Dorian Gray and Tropical Storm Gabrielle: September 8, Update A

I had a couple of days off because Dorian Gray was behaving pretty much as expected (I've read the book) so I took advantage of the time and caught up on current global political events by watching the new Dark Crystal. ;-)

Post-Tropical Dorian Gray
Meanwhile, over the last 2 days we all saw Dorian Gray zip up the US east coast and on to Canada as a very blustery post-tropical/extratropical storm with hurricane-strength winds. His remains are currently at around 52.1N, 53.4W, heading ENE at 24mph. He's still got Tropical Storm force winds of 60mph and a low central pressure of 980mb, but he's definitely winding down.

He's actually not been a Tropical Storm for a couple of days - he began transitioning into an Extratropical Storm, or, as the NHC called him, a Post-Tropical Storm, and has now fully transitioned. Uh-oh... I feel a <Science Alert!> coming on....

<Science Alert!> A quick overview of storm systems... 

Extratropical Storm: These usually form or exist in the extratropics (i.e. north of 30 deg N or south of 30 deg S) and have cold air at their core. A cold air mass meets a warm air mass, and as as the warm air rises (because it is lighter than the cold air), it releases potential energy that results in these storm systems or fronts. Because warm air rises, a low pressure is formed which is why these are also called low pressure systems. Because these storms are formed by the collision of air masses these systems can occur over land or water, and occur frequently in the winter. They are often associated with snowstorms/blizzards and, in the US, these are also Nor'easters. Before around 2010, Tropical Storms that had moved far enough north and had the same characteristics of an Extratropical Storm (or merged with a low pressure front) used to be called (quite sensibly) Extratropical Storms. 

Post-Tropical Storms: These are Tropical Storms that take on the meteorological characteristics of Extratropical Storms, so really, they are Extratropical Storms. The NHC changed the name from 'Extratropical Storm' to 'Post-Tropical Storm' around 2010 in a concerted effort to make things more confusing and less scientific. Sigh. I'll get over it one of these years. 

Tropical Storm: These usually form in the tropics and have warm air at their core. The energy source for these differ from Extratropical Storms. These storms form over water only and the energy source is latent heat. Warm water evaporates into the air. As the rising warm moisture-laden air in the center reaches colder altitudes in the atmosphere, the water vapour condenses to form clouds and latent heat is released. The heaviest rains and winds are, as you know, in the bands closest to the center. No fronts are associated with these storms (although 'waves' in the atmosphere are) - which makes it difficult to determine too far ahead of time when a storm will develop. Tropical Storms are named when they have a closed circulation and wind speeds are greater than 39mph. Before that, a closed circulation with wind speeds between 17-39mph are called Tropical Depressions.  

Subtropical Storm: These contain some characteristics of both extratropical and tropical systems. For example, imagine an extratropical storm moving over warm water. Now that storm begins to get some energy from latent heat as well, and the cold air in the center (near the surface) is replaced by warm air, so the storm core can change from cold to warm. The heaviest rains and winds in this case are not near the center though. Like a tropical system, a subtropical storm is when the winds are greater than 39 mph (34 knots). If the winds are less than that, it is a subtropical depression. 
<End Science Alert!>

As I didn't want you to be still reading this next week, I didn't get into Potential Tropical Storms (a nomenclature that the NHC added just 2 years ago!), Could-Be-Tropical Storms, If-You-Squint-Tropical Storms, or You-Must-Be-Joking-Tropical Storms (please note: the last three don't currently exist as formal naming convention for storms, but I'm sure will be added over the next 4-5 years). 

But back to Dorian Gray... thank goodness he is almost gone! It's been only one week since Dorian arrived at Abaco and the images and reports from there and Grand Bahama are terrible. There is still not a full accounting of the loss of life. The destruction he caused there and beyond, and also in the VIs and Caribbean prior to that, means that we will never see another Hurricane Dorian - this is a name that will be retired after this season.

This is my last update on Dorian! 

Tropical Storm Gabrielle
Our little Gabrielle is still hanging out in the Atlantic and is currently at 36.5N, 49.3W, heading N at 15mph. The forecast has her turning to the NE, which seems about right. Heading over to the UK for a cup of tea and jaffa cakes seems like a sensible thing to do. 

She's currently a strong Tropical Storm with winds of 65mph (TS range: 39-73mph) and an estimated central pressure of 995mb. The circulation is pretty good in the lower half of the troposphere, but the signal isn't quite as well developed in the upper troposphere yet, so I think this is about right - she's definitely a Tropical Storm and not a Hurricane at the moment. 
(imagery from Tropical Tidbits as the NOAA site has been taking too long to download!). 

The upper ~75m of the water column is warmer than 26 deg C, but as she gets closer to 40 deg N, this will decrease and she'll be over colder water which will inhibit her development. But before then, it looks like the wind shear drops down a little so there is a small chance for her to get a bit stronger - but then wind shear picks up again so even if she gets to hurricane strength, it doesn't look like it will be for long. 

That's it for this lovely day. More tomorrow (or possibly the next day unless someone is in the middle of the Atlantic...?). 
Toodle pip, 

Twitter: jyovianstorm
These remarks are just what I think/see regarding tropical storms. If you are making an evacuation decision, please heed your local emergency management and the National Hurricane Center's official forecast. This is not an official forecast.

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