It's Only 200 Miles...Added : Sunday at 11:25 "It's only 200 miles though" came one comment when we were talking about the 6z GFS ► run yesterday. That got us thinking. You see, "Only 200 miles" is relative. After all, if you agreed to meet a friend at the Red Lion pub later and they turned up at the Red Lion two hundred miles away from the Red Lion you went to then that would be annoying.
If you got a text confirmation that your parcel you had been expecting was left in a safe place two hundred miles away from your house complete with a fuzzy picture of it, then you might be a little annoyed to say the least.
But here is the low pressure from the GFS ► for this coming Tuesday :-
You can see the system brushing the South coast as it runs along the Channel. If you take a look at yesterdays 6z run then the system was two hundred miles further North and this was the chart :-
A big difference right? The reason is that the low pressure taps into the colder air moving South. If the low pressure is further North then snow on the leading edge, if it remains to the South then it's a little too far South to tap into the colder air, move the system a hundred miles further South and you wouldn't even have noticed it was there.
As far as global numerical models are concerned, a track error of this scale at T+78 or so is actually rather rare, which is why we tend to look at the other global models and ensembles in order to see where the agreement (or disagreement) actually resides.
For yesterdays run, the GFS ► was only really being backed up by the GEM with the ECMWF ► and ICON keeping the low further South, which in turn the GFS ► has now begrudgingly moved towards.
But, this is a good example of how impressive snow events across the UK tend to be rather knife edge scenarios and how the global models can fluctuate just a little and have impressively different output scenarios as a result.
For the GFS, it's a slight wobble, for everyone else it's a case of your parcel being left in a safe place behind your neighbours recycling bin two hundred miles away in Pontypridd.
METEOROLOGIST : MARSH |