Penellype's Weather Channel

[Penellype]

A reminder that things are going to feel a LOT chillier from tomorrow. Daytime temperatures will be 10C or more colder in places as the wind turns into the north west. I think it will be some time before we see temperatures in the 20s again as the ensembles continue to look mostly colder than average for the next week at least.

[Zelenina]

Well so far we've not had winter yet as it's been an annoyingly warm one, not come enough to kill anything unfortunately

We definitely had winter in Slovakia. An unusually long and cold one. Most of the time it was even too cold to snow. It's killed off my huge rosemary bush, which started off as a supermarket plant four years ago and survived every other winter without a bother. I'm hoping it's killed off the Spanish slugs too, along with a few other pests and diseases.

[Penellype]

We definitely had winter in Slovakia. An unusually long and cold one. Most of the time it was even too cold to snow. It's killed off my huge rosemary bush, which started off as a supermarket plant four years ago and survived every other winter without a bother. I'm hoping it's killed off the Spanish slugs too, along with a few other pests and diseases.

Yes, mainland Europe, particularly eastern and south eastern Europe, has had a bitterly cold winter. The UK has escaped this by the skin of its teeth on more than one occasion.

Sorry to hear about the rosemary bush

[Penellype]

Anyone who has woken up to wall-to-wall blue sky and bright sunshine with current temperatures around 7 degrees, as I have here in York, is going to be wondering what all this talk of a drop in temperature is all about. The sun is out, why shouldn't today be like yesterday? The answer is all in the wind direction and air mass (where the air has originated from).

This is yesterday morning's chart:



Follow the isobars back and they originate somewhere in Africa with the wind coming from the south. I confess to not being able to tell without being told exactly which air mass is involved for any particular weather map (more about air masses here Air mass types - Met Office), but it is pretty clear that yesterday's air mass (if you lived south of Scotland) was of tropical origin. I would think it was likely continental tropical, as it was not cloudy and damp.

This morning we have this:



The wind is now coming from north or north west, although the truly arctic air has just missed us. High pressure is now to the west, whereas yesterday it was more to the east. A weak cold front, responsible for a band of cloud but little or no rain, moved south during yesterday afternoon and cleared the south of the UK over night. This introduced a new air mass which at a guess is polar maritime. Therefore although the sun is out and is the same strength as it was yesterday, the air that it warms is continually being fed by colder air from the north and it will not be able to warm up anywhere near as much. However high pressure is still pretty much in charge so most places should remain dry with just the odd shower possible, mainly in the far north.

The next few days will see a series of low pressure systems passing to the north of Scotland on a slightly unusual north west - south east path. Normally they would move south west - north east, but currently they can't because of the ridge of high pressure in the atlantic, so they have to go around the top of it. As these systems move away into Europe they drag the winds round into the north, and we get this:



This is not very wet or very windy, but it is chilly and could be showery or damp at times. Each low is followed by a return to a more westerly flow, but you can see another little low waiting in the wings to repeat the process just to the south of Greenland. Each low dives slightly further south into Europe, bringing its influence closer particularly in the east of the UK, so by midnight on Sunday we have:



Even with this situation most of the wetter weather would be in the northern half of the country, with any rain further south being rather showery due to the continuing influence of the high pressure not very far away. Temperatures would be disappointing though with temperatures in the low to mid teens at best, with frost possible at night and wintery showers over northern hills.

The reason for this change is a gradual build of high pressure over Greenland. This tends to drag the top of the high in the atlantic towards Greenland rather that allowing it to topple towards the UK which is the more "normal" setup. The above charts show that this is not (yet) strong enough to block the low pressure systems, which break through to the south of Greenland. The question is what happens next, and as always we run into uncertainty. The options appear to my untrained eye to be 1. a continuation of the current setup - this seems unlikely as it isn't a "normal" pattern. 2. a continuation of the build of high pressure over Greenland into a proper blocking feature. This would be seriously bad news, as these tend to last a long time bringing cold and often wet weather as the main high becomes centred over Greenland, the southern high retreats south and the lows are forced underneath the Greenland block, straight over the UK. Worst case would be sharp frosts and snow for mid-late April as the block develops, followed by a cold and wet early summer. An example of a blocked summer was the washout in 2007. Option 3, for which my fingers are firmly crossed, is a return to the sort of high pressure weather we have had recently. This could lead to a very pleasant remainder of spring. Some of the long range model runs are hovering around this option, but it does have a ring of too good to be true. The downside is that some places have had a very dry winter and drought would not be out of the question if this pattern continued into the summer. There may be other options but they are beyond what I can see from the information available.

The ensembles show the next 2 low pressure systems well, after which things get a bit lost in the uncertainty of timing:



The rainfall spikes are there, but in general they are small, at least for the next few days. In the later part of the chart some of the runs are again going really very cold but there is a lot of scatter and it is impossible to tell what will happen at this stage.

[Nicos]

Nice one penellype..very interesting reading.
Thank you!

I have my fingers crossed too for Option 3!

[Jay22]

Option 3 for me too please. Thanks again Penellype, great read!

[Penellype]

The GFS model 6am run has been reading my mind

At the end of the 10 day period which is as far as the other models go, we have this:



The atlantic is firmly blocked off and the blocking high is dragging the wind down from the north over us.

By the end of the run the high has centered itself over Greenland and a succession of lows is starting to track over us:



Notice how much further south the yellow and orange colours have gone. Welcome to option 2.

The other 2 main models are not really agreeing with this at the moment. These are the charts for 20th April:

ECMWF



Here the high pressure over Greenland has gone and the block in the atlantic is starting to topple towards us. The wind has gone into the east at this point, but this is really a continuation of what we have now (option 1), if a little more amplified north-south. I'm not quite sure where this is heading longer term.

GEM (Canadian model)



Again the high over Greenland has gone, and while we are not quite back to where we were this weekend, this isn't looking bad at all.

(In case anyone was wondering, I can't use the Met Office model because the charts I can see only go out to 120 hours - this is 240 hours)

All to play for, it seems.

[Penellype]

Still a lotof uncertainty but the models seem to be firming up on quite a cold snap at the beginning of next week (probably Monday - Wednesday). This isn't snowmageddon, but with high pressure and an unusually cold upper air temperature encroaching from the east, if skies are clear and winds are light you could find temperatures dropping to the -3 or -4 level quite widely.

Keep an eye on the local forecasts and be prepared to bring in anything that won't survive cold weather. If you are thinking of buying bedding plants over easter, my advice is wait a week or 2.

The charts fro Wednesday morning are quite interesting. This is the 500mb chart that I usually show - it doesn't look cold at all:



However looking a little lower in the atmosphere at 850mb (the level shown on the ensemble charts I often post) we have:



Here you can see that the -5 isotherm covers the whole country - rather a lot colder than higher up! This is a case of the cold air undercutting the warmer upper air, with the surface heating not enough to compensate yet.

The forecast temperatures on the ground for the same time look like this:

[Penellype]

Reminder - widespread and potentially damaging frosts on Monday and Tuesday nights. Temperatures could drop to -3 or -4 where skies clear so bring in anything that could be damaged by frost or cover generously with fleece.

[Penellype]

Not looking good today - please don't shoot the messenger!

Some very cold nights on the way this week before a slightly less cold period into the weekend. Unfortunately the weather seems to have got itself into a pattern where the high pressure keeps ridging towards Greenland, and this puts us into a cold northerly air flow.

Saturday gives us a swipe of cold air straight from the north pole:



Temperatures will be struggling to get out of single figures and there is a risk of frost at night. This continues through Sunday, and then a nasty little low pressure system starts to move down from the north on Monday. By Tuesday this is sitting in the north sea, dragging down a strong northerly wind:



This would be thoroughly unpleasant, feeling bitterly cold and probably wet or snowy as well.

On the current run of the GFS the low hangs around in the north sea and near continent, dragging down the winds from the north until the end of the week when they slowly change to a still chilly north west. Beyond this and into the start of May we see an attempt for a more normal atlantic pattern to establish itself failing, and more high pressure ridging towards Greenland at the end of the run.

Most of this is thankfully out of the reliable timeframe, but it is certainly going to be cold at night for the next couple of nights, with Thursday night possibly still cold in the south. Let's hope that the models are wrong after that, otherwise after summer in March and early April, we could have winter in late April and early May!

The ensemble chart for York shows the colder than average temperatures very well. Notice how many of the lines drop below the -5 line (cold enough for snow), and how few of them are above the red 30 year average line at all after 22nd April.



Towards the end of the run there are a few more rainfall spikes along the bottom, but the overall picture is still fairly dry.

[burnie]

10 degrees outside, but that's without the wind chill, snow on the big hills and more tonight, no point getting ahead of myself. Went to a garden centre in Aberdeenshire today, full of bedding plants in full flower, if anyone puts those out this month they'll all be dead before May gets here. Overheard people asking for hanging baskets/flowers in flippin' April, it's still winter up here.

[Penellype]

Its too early for bedding plants here too. I went to get some extra fleece this morning and one of the owners of the local garden centre was saying how they would be covering all of the bedding plants in their polytunnel tonight. They will always say if you buy something that isn't hardy and make sure that you are not putting it straight outside. She was saying how angry it made her that all the supermarkets have loads of bedding plants on display - just a waste of money at the moment unless you can keep them indoors.

[Bigmallly]

I contacted my sister (in Cheshire) earlier on today to wish her a happy birpday. She said they would not be doing anything today as her hubby was on call & may be called out to do some gritting..............

[Potstubsdustbins]

A chap I know who lives in Caithness planted some pea plants out yesterday in the spring sunshine, he woke up this morning to everything .........................covered in snow.

So very nice to have our very own weather service.................Thankyou

[burnie]

Minus 1 inside my greenhouse last night