All About The Undercut (and Fast Pigeons)Added : Wednesday at 12:17 I wonder what the land speed record is for a pigeon here in the UK? Swear I just saw one fly past the office at nearly 200mph! Anyway, it's windy out there today as the Northwesterly air starts to filter Southeast, but for this discussion we are going to look at the undercut tomorrow.
You might have heard this used by forecasters today. Saying that "it depends on the undercutting of the cold air" as to whether we see some rather significant snow tomorrow, so let's take a rational look at the situation.
Here is the chart for tomorrow across the UK :-
You can see a rather impressive low pressure area across Southeast England on Thursday around about evening rush hour. This is (or was) a wave depression which runs Eastwards along Southern England.
These features are notoriously difficult to predict for numerical models. Firstly the timing needs to be right as to when it develops and also the location.
That said, the GFS ► predicts it developing around the same time as EURO4. As it moves Eastwards it deepens and develops closed circulation. This is important as when it does this it draws in the colder air to the North of the system :-
You can see the cold air to the North of the wave by midday but it wasn't developed low level circulation at this stage which is why it will fall as rain across Southwest England.
But by the late afternoon the GFS ► develops closed low level circulation and pulls in cold air into the centre of the low which in turn allows the rain to fall as snow to lower levels :-
Before you get your sledge out you need to remember this is a knife edge scenario and some of the models take the system further South and some don't develop closed level circulation which keeps it as rain.
However, interests in the Midlands, East Midlands, Southeast England and in particular East Anglia need to be prepared that this could deliver 5-10cm of laying snow by tomorrow evenings rush hour and worth making preparations just in case.
That said, if it does fall as rain then the colder air feeding South quickly after will turn many road and pavement surfaces to ice and black ice, especially where salt has been washed away.
Confidence on that is much higher, but we hope we have explained why this little feature is so difficult to forecast at the moment. |