Saturday, August 31, 2019

Hurricane Dorian Gray: August 31, Update A

I have a sneaky feeling that a few million people breathed a sigh of relief today… but a few million others went ‘what the...?!?’ Hurricane Dorian Gray, ever a tricky one, is still on track for the Bahamas, but his forecast took a bit of a turn, didn’t it?

He is currently at 26.2N, 74.7W heading W at slowish 8mph. He is a pretty good looking and strong cat 4 storm with winds of 150mph (cat 4 range: 130 – 156mph). He has had a beautiful eye for most of the day:

The circulation in all levels of the troposphere is quite robust, which matches the signal of a cat 3 or 4 storm. I keep mentioning troposphere so before I go any further, let me explain the mysterious ways of our atmosphere… 

<Science Alert!> The troposphere: Our atmosphere is divided into layers – like a delicious trifle or seven-layer dip or lasagna (depending on what country you are from). In each layer the air temperature either increases with height or decreases with height. The troposphere is one of these layers. It is the lowest layer of our atmosphere and extends up from the earth (ground zero if you like) to about 15-16km in the equatorial regions, and to about 8km in the polar regions of the planet. This is the layer of the atmosphere we live and breathe in. All our 'weather' essentially occurs in the troposphere. The troposphere is defined by decreasing air temperature with increasing height. You would know this if you climbed a mountain. Or the easier option, of course, is to just look at pictures of mountains and see the snow at the top (known as the ‘Flat Florida Option’ ;-)). The top of the troposphere is called the tropopause. Strong tropical storms have clouds that reach as high as the tropopause - and in a few very strong cases, they can extend even higher into the next layer up - into the stratosphere, which is the layer of the atmosphere above the troposphere. The stratosphere is defined by air temperature increasing with height. The ozone hole is in the stratosphere. The top of the stratosphere is around 50km height and is marked by the stratopause. And the layer above that is mesosphere, where air temperature decreases with height again until the mesopause. Above that is the thermosphere, then the exosphere... and then we get to space.<End Science Alert!>

(Oooh… three Science Alerts! in three days. Aren’t you the lucky ones? ;-)).

The fact that temperatures decrease with increasing height in the troposphere gives us a clue about how strong the storm and it’s convection (rain, thunder etc.) is. The satellite imagery above is the infrared imagery and shows the cloud top temperatures. Higher cloud tops means a more turbulent atmosphere and stronger convection, but it also means colder cloud top temperatures. In the image above, the red areas correspond to very heavy rain, strong winds, a lot of thunder, tornadoes… basically everything and the kitchen sink. Orange areas are less strong – thundery weather, strong winds, heavy rain. Yellow would be strong winds and rain, green is mostly rain with some wind, and then it gets to light rain/just cloudy where we see blue and gray. 

We also see in the satellite image that he has some clouds streaming off to the west – he is actually experiencing a little wind shear which is keeping him a little in check, but unfortunately not enough for the Bahamas. As he gets closer to the islands, his convection will get disrupted, but it may not be enough and it looks they will be hit by a substantial storm tomorrow and into Monday: 

If the eye goes directly over the islands (worst case scenario for the islands), that will really help to decrease his intensity. But if it doesn’t, they will still experience a very strong storm. 

As for the track beyond Monday… well, that continues to turn to the right. I don’t have access to good enough pressure field data so I would stay on alert if you are in that cone of uncertainty. 

There are a couple of other blobs and blobettes out there, I know. But I’ll save those for a day or two.

Stay safe Bahamas!!

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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