Penellype's Weather Channel

[Penellype]

Ah, yes, our old "friend" James Madden again. The short answer is we don't know. There is a possibility that very cold weather could arrive around the middle of February. This is MUCH too far away to be making statements that sound like facts, but that has never stopped the newspapers before.

As always, there is more uncertainty the further ahead you go, and this is well illustrated by a couple of recent ensemble charts.

This was the mid day run from the GFS yesterday:



Note the position of the thick green line towards the right hand side of the graph. This is showing very cold upper air temperatures, but there are also rainfall spikes, which would almost certainly be snow. The green line is one of the coldest runs - the average of all the runs (white line) is actually above the 30 year average for the time of year. But it is the green line (operational) that is what appears when you look at the GFS charts on weather websites.

Here is the corresponding precipitation type chart for 14 Feb:



Plenty of snow there, so anyone looking at this particular run of the GFS would be able to say that snow is being forecast, although well into the unreliable timeframe.

However, the latest run of the GFS ensembles looks like this:



Now look where the green line is! However, there are more lines going below the red 30 year average line, indicating that there is still a chance of cold. This is typical of what is happening at the longer range - things look completely different from one run to the next.

As always the outcome is critically dependent on the exact position of high pressure. At this point I have to say that I can't in all honesty see the difference in the charts that I see for these 2 runs on 14th February. Both have high pressure building over Scandinavia, both have winds in from the south east, and both have a green coloured air mass over us. I don't have the experience or knowledge to spot the critical differences.

14 Feb from the cold middy run yesterday:



14 Feb from the milder 6am run today:



It certainly wants watching, but I can't stress enough that at the moment it is too far ahead to make predictions. For the time being we are in an unsettled and milder period, with some potentially very windy weather ahead, and also a good deal of rain. The setup is such that weather fronts could get stuck over the top of us because they are unable to move away to the east due to the large area of high pressure which is still sitting over the continent. The models all have this drifting gradually north to Scandinavia, and the question is whether it will position itself so that it drags the air in from the east or north east (very cold) or the south east (quite cold) or whether the storms in the atlantic manage to suck up high pressure from the south which joins us up with the high over Scandinavia, which would put us back into the cloudy, chilly, frost and fog scenario that we have seen so much of recently.

Nobody knows which of these will happen. It will depend on how the stratospheric warming (which has now occurred and continues to develop) plays out, and there simply isn't enough data for the weathermen to make a call on this. Some think that we could see a sudden "flip" in the model output as the effects of this event unfold. We shall see.

[Jay22]

Thanks again Penellype.

[Penellype]

Finally worked out why I can't correlate the map charts to the ensemble ones... Wetterzentrale was showing the 6am run as available, but was actually showing the chart for yesterday at 6am, which doesn't correlate with today's 6am maps! Today's 6am ensemble run is still not available on Wetterzentrale, but the nearest I can get to it, from TheWeatherOutlook for Newcastle, makes the maps much more understandable!



Once again we are looking at the thick green line, which is again a cold outlier. The helpful red 30 year average line is missing, and the average of the runs is shown in purple here.

Phew - thought I had lost my marbles for a bit then (wouldn't be hard!)

This, from the midnight run, is more what I was expecting to see from the warmer ensemble chart:

[Penellype]

Some of the models are forecasting a small but vigorous area of low pressure approaching from the south west on Friday. The met office have issued a yellow warning of wind for some south western areas, although there is considerable uncertainty about the depth and track of this system at present. Severe weather warnings - Met Office

For a detailed explanation of what is going on and where this is coming from, and some level headed thoughts about the possibilities of cold in the longer term see Matt Hugo's synoptic guidance Synoptic Guidance - Bumper Edition! - Blog by Matt Hugo - Netweather.tv

[Penellype]

Just in case we do end up with a Scandinavian high and a "beast from the east" easterly wind originating in Siberia, here is a video of what the forecast might look like, taken from February 1991 and presented by the late, great Ian McCaskill

https://www.youtube.com/watch?v=Hd6DuebHliY

This is a slim possibility at present and is represented on the ensemble charts by the lines that are well below the red average line.

[Lumpy]

I really hope it is not like the great weather man said - our fit as an ox next door but one neighbour is going to move my raised beds for me on Friday..............................maybe next week then!

[Penellype]

This week is mild, although probably wet and windy at times. The risky timescale is anything from the middle of next week onwards. The latest run of the GFS, which has just finished, has us in the freezer again, although the pattern keeps changing. Similarly the ECMWF wants to turn the winds into the east next week. However, these "operational" runs are not well supported by the ensemble members from either model yet. There is plenty of time for things to change, and I think the uncertainty is increased by the state of the stratosphere, which is almost but not quite producing an official sudden stratospheric warming. If this propagates down into the troposphere where weather takes place, it will increase the chances of high pressure to the north, and this is what the models are going for. It is hard to see whether they would have done this without the stratospheric activity or not, as it seems a little early.

[Penellype]

The models are starting to agree that some sort of cold is likely towards the middle/end of next week. There are far fewer warmer members of the GFS ensembles in the latest run (mid day today):



This is not bitterly cold (yet), although those runs dipping below the -10 line would be. The difficulty lies in the fact that we are on the boundary between high pressure over the cold continent and low pressure over the warm atlantic, and it is a case of which wins the battle. For now the atlantic is in charge, but I think we should be prepared for a significant cold snap towards the end of next week, with the possibility of snow in the mix as well.

This is not armageddon, its winter!

[Penellype]

Time for an update, although things really haven't progressed much further. Please bear in mind that when I use words like "mild" or "cold" these are relative to the normal for the time of year. Therefore those people living on hills who saw a dusting of snow recently might question the term "mild". However, snow on hills is quite normal in February.

The current position looks like this:



The low pressure systems that brought wind and rain (and some snow) to the south and west are slipping away southwards into France. Normally they would move away to the north east, but they can't because of the huge area of high pressure building over Scandinavia. Meanwhile the atlantic is full of low pressure systems, with the purple colours indicating weather that is likely to be mild and stormy. The UK is slap in the middle, in the battleground between the two, and this is what is causing all the forecasting headaches. The feature to the south west, central pressure 985 on this chart, is a vigorous low which is caught up in the jet stream and is about to undergo "explosive cyclogenesis" - this is what the papers would call a weather bomb. Luckily for us, this is going to happen in the atlantic, not over the UK.

This is the situation by midnight on Tuesday:



The low has developed into a huge and vicious storm. The edges of this will bring us wind and rain, but nothing notable. The important thing about this storm is that it sets up warm air advection from the south west. The swirling storm drags warm air from the azores high to the south up to the north, where it will fuel the high over Scandinavia, so that by midnight on Thursday it looks like this:



You can clearly see how a lot of that energy wound up in the storm has effectively been converted to upper air warmth over Scandinavia - the colour has changed from green to yellow, while the storm to the south of Greenland has lost much of its power. Notice also that the winds over us are coming from the south, but there is now a split in their source direction - to the west they are coming from the warm south west, but to the east the air is starting to feed in from the cold continent.

Upto this point there is no real disagreement between the models, it is what happens next that is still doubtful. By Friday we still have a few ensemble members below the -10 line as in the last post, but most are clustered around or even above the red average line. This is mirrored by the different models as well, with significant differences between the outcomes on different runs, sometimes putting us in the freezer and sometimes back to the atlantic regime.

This is how the latest run of the GFS sees it, which to me seems the most probable pattern (although details may vary).

By mid day on Friday the winds have turned into the east, first on the eastern side of the country and then for most of us - how far west this gets is open to question:



This would be cold, with frost at night where skies clear, but as the flow is fairly slack (isobars are not close together) it would not be bitterly cold. This sort of pattern is likely to continue through Saturday, but comes under increasing pressure from the atlantic as another vigorous low pressure (note the purple colours are still there) sets up warm air advection again to the west. The position of this developing ridge is important - a bit further west and it could build into Greenland, turning the winds into the north and setting up a prolonged cold spell - this is what some of the colder ensemble members are probably doing. As this model run has it the ridge collapses over the top of us and the atlantic breaks through.

By midnight on Monday we are, almost literally, back to square one:



The winds are now coming from the west as a low pressure system has moved across the country. This would be coming up against cold air, and could, if conditions were right, lead to snow at first, although this would likely turn to rain as the milder air moved in. Timings are impossible to predict at this range, and bear in mind this is only one possible solution at well beyond the "reliable" timeframe in a very uncertain and difficult weather pattern.

The GFS then wants to keep us in a more atlantic pattern, mild, wet and windy. However, this is the default position of the model, and with the high pressure over Scandinavia still there and the purple colours still strong in the atlantic, battleground UK seems to me to be the most likely outcome once again.

We shall see.

[JustPotteringAbout]

I get my forecaat from a certain bluetop I wouln't dry my manure-clad wellies on.
Consequently the weather is always a pleasant surprise.

[Penellype]

Massive uncertainties still on the likely cold coming up. I'm surprised that it is already as cold as it was last night, to be honest, and was completely caught out by freezing fog here today.

Today's ensemble chart for York looks like this:



You can clearly see the cold coming later in the week - most of the runs are now nearer the -10 line due to the winds turning into the east. There is significant disagreement about the timing and extent of tonight/tomorrow's rainfall (!) and by Saturday its anyone's guess.

The GFS operational (thick green line) has gone much colder. This now has us in an easterly wind for the foreseeable future. By Thursday 16th (which is as far as the other models go) it has this:



There is a huge area of high pressure to the north, and this is going nowhere fast. Kinks in the isobars imply possible frontal systems too, which could lead to snow. This would be a prolonged, cold and wintery period of weather. Note how much less purple there is in the atlantic compared with yesterday's charts, and there is another vigorous low feeding more warm air advection, further to our west this time.

At the same time the ECMWF has this:



While not warm (because of high pressure leading to probable frost and possible fog), the flow is mostly off the sea rather than the continent, and this would be milder, much milder for the west.

The GEM (Canadian) model, which had us firmly in the freezer yesterday, now has this:



The atlantic is firmly in control here and this would be mild and unsettled, after a possible transition from cold to snow to rain earlier in the week.

So, to summarize, after a spell of wet weather today/tonight/tomorrow, with some strong winds in the north west courtesy of the vigorous low currently in the atlantic, expect things to turn drier and colder for the rest of the working week. After that it is anyone's guess what happens - keep an eye on the forecasts, and be prepared for something a lot colder and more wintery than we have seen for a few years. It may yet not happen, but it is wise to be prepared.

[Penellype]

Things are starting to look a little less uncertain now, at least for the next week or so.

For the next few days we can expect cold easterly winds, overnight frosts and probably a good deal of cloud particularly in the east. There are a few light showers around today and with a sharp frost forecast tonight there is a risk of ice. Light snow flurries are possible for tomorrow and Friday along eastern coasts and these are likely to become somewhat heavier and move further inland on Saturday, with snow possible quite widely into Saturday night. Keep an eye on forecasts for details, which are hard to pin down, but snow is possible to low levels and there could be a few inches in places on higher ground.

The reason for the colder weather is clearly visible on the upper air temperature chart - this chart is for the same level in the atmosphere as the ensemble charts:



This shows that by Friday evening the -10 isotherm (blue colours) is firmly over much of the country as bitterly cold air has moved in from the east. Any precipitation in the blue area would fall as snow even to low levels. The kink in the isobars over the North Sea is the source of the snow forecast for Saturday.

The latest ensemble chart for York looks like this:



Note that there is much less scatter and that all the members are now seeing the upper air temperatures lifting after the weekend with most staying there for a while. The high pressure over Scandinavia is sinking south into Europe rather than building to the north of us, meaning that the flow is from the south east or south rather than from the east. The source of the air moves round from Russia late this week to Greece and then France/Spain, putting an end to the cold weather that will feature for the rest of this week and into the weekend. On this run of the GFS the atlantic starts to have more influence, hence the increased number of rainfall spikes in the later stages of the chart. As I have said before, this needs treating with some caution as westerly, atlantic type weather is the default position for these models.

At the beginning of next week expect the weather to warm up a bit, although this can take some time depending on how cold it has got. Where there is lying snow temperatures are lower and this can take a while to shift. The current GFS run is seeing temperatures into double figures by Tuesday in the south west and by Wednesday for much of England, Ireland and Wales - this may be rather optimistic for areas where snow has settled.

[Penellype]

As usual we have the press hysteria over this weekend's snow. Yes it will be cold, yes there will be snow showers. Yes it probably will reach double digits over Scottish mountains and there will be several inches of snow over some high ground and some snow at lower levels too. It's February.

[Penellype]

Perfect illustration of just how difficult forecasting this sort of weather can be. Some people have had snow and ice over the last 24 hours - this applies specifically to what I am seeing here in York.

Forecast for Friday into Saturday morning:
Bitterly cold continental polar air with the -10 isotherm over the region. Starting below freezing on Friday morning and staying around freezing all day. Cloudy with light snow showers becoming more frequent over night with snow likely to settle as temperatures drop below freezing before rising a little on Saturday morning. Bitterly cold easterly winds.

What actually happened (data from local weather station less than 1 mile away):
Minimum temperature recorded on Friday 1.6C at 8.30am - for most of the night it hovered around 2C.
Maximum temperature recorded on Friday 5.4C at 12.30pm.
Sunny intervals and light snow showers.
Little wind (max gust about 6mph before 6pm, increasing slightly over night). Felt chilly but not "bitterly cold" at around 7pm.
Minimum temperature over night into Saturday 2C at 9pm, current temperature at 9am 4.3C.
3mm overnight rain, sleet or snow (melted). Currently dry but more rain clearly on the way from the radar.
Current wind speed around 5mph with occasional gusts upto 20mph.

I suppose the general gist of the forecast could be said to be vaguely in the right direction, but its nowhere near accurate. If that's the worst this cold snap can throw at me I'm quite happy.

[burnie]

Last night we were looking at a very bright moon in what was almost like daylight, the garden white with a light covering of snow, minus 3 and not a breath of wind. Howling gales, overcast, no snow and raining at plus 8 this morning, certainly not any point trying to grow anything out there just yet.