Penellype's Weather Channel

[Penellype]

Posted by one of the weather people (Simon Cardy) on Twitter yesterday:
Still to be confirmed this week but it looks like it was the driest June on record for these locations: Albemarle, Birmingham, Bristol Filton, Cardiff St Athan, Church Lawford, Herstonceux, London Heathrow, Northolt, Lyneham, Ronaldsway, Wattisham and Wittering.

Certainly it has been a very, very dry month.

[Jimmy]

I dug out my rocket spuds which had copious watering and there were some reasonable ones but nothing to write home about.
Was going to follow with some peas but lack of rain I have delayed that.
Jimmy

[Jimmy]

I think there is something wrong with the weather.
I suggest we try rain dances.
If that fails then we should consider sacrifices.
Jimmy

[Lumpy]

I have a rain dance routine Jimmy.
It does involve looking stupid, feathers, wailing (singing) and a certain amount of physical discomfort.
Normal sacrificial animals were goats, cows or chickens!
When it does rain I’m just going to woddle about naked then tell the police I have ‘issues’

[Lumpy]

p.s where are we going to get a chicken from and more importantly who is going to tell her?

[sandspider]

There are 2 types of weather that are incredibly hard to forecast with any precision - snow (because its incredibly sensitive to small variations in temperature) and showers/thunderstorms because they can be very localized. I have known a shower to produce rain in the front garden but none in the back.

The forecast models don't handle either of these weather types very well and you get huge variations between the models in the predicted locations of showers. Weather apps use the raw data from the models with no interpretation at all from a human forecaster. Therefore if your app uses the GFS and the GFS has put a blob of rain indicating a shower over your location, the app will say rain, when in fact its really only the possibility of a shower somewhere in the area. Other models may show no rain at all in the area that day so other apps which use these models will show a dry day.

Currently what we have is high pressure slap over the country, which is why most places have been very dry. Yesterday the south west had thunderstorms triggered by the presence of a low to the south west - this is still there but moving away, so there may be more showers in these areas, but probably less so than yesterday.

Over the next couple of days pressure will lower very slightly over the whole country, so that instead of being in the centre of the high, we will be in what's called a col. This is the situation on Thursday shown by the Met Office Fax chart:

[ATTACH=CONFIG]81388[/ATTACH]

Looking at this you might think that there are lots of lows about and a cold front and therefore lots of rain. However, what isn't immediately obvious is the 1020 mb ridge of high pressure to the west, because there is a 1019mb area right next to it which is labelled L (very misleading). We are in fact still under the influence of a lot of high pressure, it is just having a slight wobble.

The implications of the wobble and the cold front are that showers might break out in places mid week, with Thursday looking the most likely for the most widespread risk. The cold front is forecast to die out and the high to build again so by Friday we are back to high pressure, hot and dry.

All I can say with regard to Chepstow is that you might get a shower around Thursday. It isn't possible to be any more definite than that in the current setup. The current run of the GFS is hopeful:

[ATTACH=CONFIG]81389[/ATTACH]

But remember, this is only one model run and these are showers, not a large area of rain as depicted on the map. Not everywhere in the area shown will get one, even if the model has got the area correct.

By the way you can tell these are convective showers (caused by rising hot air) because there is no forecast rain at all over the sea. If this was a continuous band of rain it wouldn't matter if it was land or sea, but the sea is relatively cold and the land is hot, so convection only occurs over the land.

Thank you for the detailed write up! That all makes sense. A couple of forecasts are now saying some rain likely today or /and tomorrow. Fingers crossed... I've got too many small trees to water by hand!

[Penellype]

Some places may see some rain today or tomorrow, most likely in the south west. It will very probably be in the form of showers, so not everywhere will get one even in the rainy areas. Anything that happens away from the south west is likely to be very light and patchy, so probably not much use.

Sorry.

[1Bee]

You're not responsible, Pene! You're just keeping us beautifully well-informed.

[Lumpy]

I agree with the above.
Keep up the good work Pen as we are all thankful for it.

[purplekat]

Ventusky is showing some northern rain for Saturday - fingers crossed. Had so many grass fires round here

[Jimmy]

Could all this be a portent of Ragnarok ?
Jimmy

[Penellype]

A few showers today in the south and also over parts of Scotland. With total inevitability one of the showers is slap over Wimbledon!

[sandspider]

Had a bit of a shower today. Not much, but better than nothing. We'd better all get our rain dance outfits on...

[chillithyme]

I'm down south and had a single half drop of rain, cloudy but muggy, no rain, definitely very sticky

[Penellype]

More showers likely in the south and possibly in Wales and along the pennines, depending on which model you look at. GFS is also showing the possibility of showers (probably very patchy) fairly widespread on Sunday.