Friday, August 30, 2019

Hurricane Dorian Gray: August 30, Update A

The Picture of Hurricane Dorian Gray continues... 

Officially he is now a weak cat 3 storm with winds of 115mph (cat 3 range: 111 - 129mph), central pressure is 970mb. He's at 24.8N, 70.3W, heading NW at 10mph. 

I would agree with this cat 3 ranking. A few hours ago his eye developed and has been pretty steady since then. The circulation in the upper troposphere has improved and looks like the signal one would expect to see with such a manly hurricane. Also, we now see that he is a little more 'circular' in overall appearance as he was under some light wind shear before, but that has now vanished.

Those of you who are paying attention will notice that his forward speed has slowed down, which means we are now looking at possible landfall in Florida on Tuesday instead of overnight on Sun/Mon - a day delay (and more prep time!): 

A slower hurricane does causes issues for forecasting the track, but also for the intensity. He's heading to the northern Bahamas, now arriving there on Sun/Mon night, which is not good for those islands alas, but will stop him from intensifying as he passes over them. A couple of reasons for this are his interactions with the 'land', but also because the warm water is so shallow around the islands that he doesn't have much energy to pull from for a short period of time. You can see the area marked as the dark blue patch in this image, which shows the depth of the 26 deg C water:

(Image and data from NOAA AOML). 

See that dark blue area around the northern islands in the Bahamas (Grand Bahama and Great Abaco)? Depending on his exact path and how slowly he crosses this area, this may decrease his intensity by a category (or maybe two categories if he's slow). This would be the best case scenario - but for this he needs to move slowly. So that's something to watch out for. 

But after that, we run into a bit of trouble. He is forecast to get even slower as he crosses the Gulf Stream (which you can see as the yellow area in the figure above, where the upper 100-125m of the water column is warmer than 26 deg C), which means he will have plenty of warm water to feed off before getting to Florida, which means he will restrengthen (assuming he lost some steam over the islands). If he stalls, we have a second issue in addition to his intensification, which is that we don't know which way he will go after that (which is why the cone of uncertainty is so huge at the moment). 

<Science Alert!> Why would a storm stall? You have heard of atmospheric high pressure and low pressure, right?  Imagine them as hills and valleys in the atmosphere, except made of air. Now imagine the storm is a ball on this undulating field. If the path ahead is low pressure, it's like a downhill gradient and the ball will happily keep on rolling forward at a decent speed. If the path in front is high pressure, it's an uphill gradient and the ball (storm) will slow down (which is what we are seeing with Dorian), or maybe move in a different direction until it finds an easier path downhill. This is why the track of a storm has so many wriggles (technical jargon ;-)). 

But if the ball gets stuck in a 'dip', it is surrounded by high pressure on all sides, and it becomes difficult for the storm to move in any direction, so it will stop where it is until the pressure fields around it change (which they eventually will do). Of course, as pressure fields change, the longer range track forecast will also change, even though the storm may not have moved far. So not only do forecasters need to predict the track of the storm, but they need to predict the entire surrounding pressure field (which means predicting the entire world's atmospheric pressure field as it is all connected) and how it will change before the storm moves again and that's where computer models really kick in - these are massive undertakings. <End Science Alert!>

That's why the forecast track on days 4 and 5 gets a little wobbly.

More later my friends. Sorry Hurricane Dorian Gray isn't a good one - he's good looking though, just like Oscar Wilde predicted all those years ago! 

Ciao for now,

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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