How Do You Spot Uncertainty In Weather Forecasts?

[PeggySue]

Hi,

My aim in life is to be able to predict 2 or 3 days in advance if I am going to get wet while walking on Dartmoor! We all know that weather forecasts are uncertain but, apart from the ensembles, forecasts don't come with a measure of confidence interval. The NMM model is pretty good at predicting the general regions that will receive precipitation but the extent of the rain and the timing can be wrong in either direction. I'm new to this game but I get the sense that in some cases you can be certain 2 days ahead that a particular weather pattern will hit a given place within a given small window in time but in other cases you can't be sure of the timing or even of the event happening at all.

Does anyone have any tips on how to divine the confidence in precipitation forecasts please? E.g. Look at these charts and if you see these patterns you can be sure that it will rain within two hours of the given time. Or look at these charts and if you see these patterns you can be sure that the event or its timing are anyone's guess!

Many thanks.

[Terminal Moraine]

There are so many potential synoptic situations its difficult to give a definitve answer to your question but as a rule of thumb I'd suggest the following;

If the situation is a very mobile westerly with a succession of depressions moving in from the Atlantic and a strong, flattish jet stream it's possible, by looking at a short succession of charts, to work out the speed of the approaching rain bearing fronts and extrapolate forward to calculate the estimated time of arrival at your location. There's always the occasional situation where a depression speeds up or slows down but generally speaking you could be accurate to within a couple of hours with this set up.

Situations which are decidedly unpredictable are similar to the one we have at present where an area of low pressure is developing off the south west approaches and moving north/north east into colder air. There's little way of telling, at this distance in time, by looking at the charts, just where this low will go and how fast so pinpointing possible rainfall in your area is problematic although things should be clearer by Friday which is only 24 hours ahead of the expected event.

Other problem charts include Atlantic systems moving east towards Britain with a block of high pressure to the east of north east. It's not always certain that the block will give away and allow free eastward progress of the Atlantic air, the fronts could slow, stall, slide south or south east or any combination of those, in these situations it more a case of watching each set of charts as it appears and listening closely to the forecasts.

This is by no means a definitive list and I'm sure other members on here could add more.

[PeggySue]

Hi Terminal Moraine

Many thanks for that. I will start two lists; one of situations to trust and one of situations to be wary of.

Its a pity I haven't been paying more attention in the past!