Why Heatwaves Get LongerAdded : Tuesday at 15:20 If you'd looked at the GFS ► a few days ago, then you would have seen a return to cooler air across many parts of the UK through Tuesday, but here we are on Tuesday and the heat is still there, in fact, it's been nudged back a couple of days.
Now, you might think that this is part and parcel of the inherent nature of numerical models, but shifting large warm and cool airmasses once they have been around for a few days is a little more complicated than you might imagine.
There are usually one of two reasons or a combination of both. The first, is that smaller scale low pressure systems develop which drag air back in from the Southeast or East. This has happened a couple of times in the GFS, but the normal reason is due to the feedback error.
For this, you will need to image that the GFS ► (instead of forecasting the weather) is going to make a Chicken Chasseur in its slow cooker.
OK, so the starting conditions are all the ingredients and they all go happily into the pot.
Numerical models rely on the timesteps being accurate and correct in order for the next ones to be correct too. If the GFS ► forecasts rain, then the model expects rain. This means that the next timestep will have some physics based on the evaporation of the rain and subsequent energy release to be correct as well.
In such as complicated scenario as we have across Western Europe, the models are struggling with the showers and this in turn results in the next forecast timestep being a little bit out. Not just that, but each consecutive dry day results in the soil drying which means any new rainfall is absorbed even faster than the model expected.
Back to the slow cooker and at T+9 we are ready to take the dish out and have it with some lovely crusty bread. So, at T+12 we butter the bread, lift the lid and.... oh no! We forgot to add the chicken!
The GFS ► brings back markedly cooler air from the West for the second half of August :-
Not much though we can do about the Chicken sans Chasseur though... Crusty bread anyone?
METEOROLOGIST : MICHELINSTAR |