Why The Jet MattersAdded : Wednesday at 10:06 This mornings 00z GFS ► run has given us a great couple of charts to show you why the system which develops tonight isn't quite as potent as other ones which develop further down the model run.
Using the latest 00z GFS ► jet stream model and the surface charts you should be able to see why the jet stream isn't as conducive to the development of tonights system and why the one which appears later in the run is.
First of all take a look at the jet stream forecast for later today :-
Whoah! That's a lot of red Metcheck! True, but take note of the direction of the jet and the core of the winds. You'll see that the jet is virtually straight, the sharp eyed of you will notice that there is a very slight bend to it in the form of a ridge. Not just that, but the core of the jet is across Scotland with near 300mph winds.
How take a look at the surface chart for near enough the same time :-
You can see the low pressure system here, but it's South of the core of the jet. Not just that, but with little cyclonic curve in the jet it means that the development of the system is that much slower.
Now take a look at the jet stream for the weekend after next :-
Here you can see the jet stream screaming across the Atlantic with a dip to the South across the Bay of Biscay then heads North towards the UK. This pattern shows us two things. In the area to the West of the UK we have a trough which brings in cyclonic vorticity at the base of it (just South of Ireland). Not just that, but the exit of the jet is across the UK which means that any system underneath this feature has two things happening to it in the upper air. First, spin is created within the system (PVA or Positive Vorticity) and secondly the air is removed quicker from the top than the bottom (Upper Air Divergence). It means that anything underneath this will deepen.
If we look at the surface pressure charts for the same time this is confirmed :-
So, two low pressure systems, but it's all down to the placement and curvature of the jet stream aloft which dictates why one develops slowly, whilst the other develops into a potent storm.
METEOROLOGIST : MARSH |