Ophelia... Extratropical Transition MasterclassAdded : Friday at 9:11 Right, a full on meteorological discussion today on the process of extratropical transition. There will be plenty of heavy year three met stuff in this one, but by the end we hope you will be able to understand the process of extratropical transition, why ex-Ophelia will undergo it and why the UK simply can't get hurricanes or tropical storms.
This morning we find Ophelia as category two hurricane out in the mid Atlantic :-
An impressive system and she is still well organised and continued to strengthen overnight with cooling cloud tops and a defined eye.
This is the latest forecast track of the system from the GFS ► :-
The thing we are most interested in is the thickness and symmetry of the core of the system from 900mb to around 600mb. This tells us the intensity, structure and most importantly the phase of the system. Here is the 120hr forecast for the core of Ophelia :-
You can see that at the moment Ophelia is in the Symmetric Warm Core area which is what we would expect of a hurricane in the tropical region of the Atlantic. Within 24 hours the forecast is for Ophelia to transition to Asymmetric Warm Core which is where the system becomes extratropical and loses the characteristics quickly of a tropical storm/hurricane.
This is an interesting phase of development and you will be able to see on the charts below what happens during this process.
The first thing that happens is that the core of the strongest winds suddenly spreads out. If you take a look at the winds around Ophelia as a hurricane you'll notice they are concentrated and symmetric around the system :-
But, by Sunday evening, the extratropical transition has occurred and the wind field increases in size :-
This is a common feature and we tend to see the strongest of the winds start to develop on the Equator side of the system due to inflowing trajectories from the upper level jet stream on the North side of the system.
The other thing to notice is the rain associated with a system as it undergoes extratropical transition.
If you check out the rainfall of Ophelia as a hurricane then you'll notice the symmetric and concentrated rainfall around the core of the system in the eyewall :-
But by Saturday, the extratropical transition process is starting to occur and you will see how heavy rain develops to the North of the system. This is called the "Delta Rain" and is caused by outflow of the system towards the North. At this point, Ophelia is developing a warm front and you will be able to see this on satellite images tomorrow as a large cirrus shield develops.
By Sunday and Ophelia is extratropical :-
You'll notice that the heavy core of rain which was in the centre of Ophelia is now on the left hand side of her. Not just that, but as she heads North, the heaviest of the rain continues to stay on the left hand side. Even, by Monday when ex-Ophelia is across Ireland, the heaviest of the rain stays out West :-
The reason for this is due to Ophelia interacting with the upstream trough and the large scale forcing which develops on the right hand side of her. If you see a chart like the one above then you can be sure that extratropical transition has taken place.
The final thing which completes extratropical transition is the mixing of boundary and mid level air. By the time Ophelia reaches us, she will already have developed a cool inflow and will be interacting with the warm conveyor belt resulting in a warm and cold front within the system.
The FAX charts show this process up well :-
Ex-Ophelia is a large and powerful extratropical system and will bring strong winds across Ireland on Monday and Tuesday, but we hope we have explained how extratropical transition occurs on all these systems and it's why in the UK, at our latitudes, the laws of physics reign supreme and it's not possible for Ophelia or any other tropical system to reach us as a hurricane because the very feature which brings them to us i.e jet stream and mid level wind shear are the very things which destroy the system and convert it quickly to extratropical.
One last thing worth mentioning about extratropical transition systems is that they play complete havoc on numerical weather modelling. The forecast skill drops considerably when these features complete extratropical transition.
METEOROLOGIST : MARSH |