Penellype's Weather Channel

[Penellype]

Just 24hrs after I posted the above there are signs of a possible "game changer", but like most things this year it is not what you would normally expect, so how the models will handle it remains to be seen.

My comment about the mostly dry weather continuing was based on the presence of a persistent blocking high pressure and a complete absence of storm/hurricane activity in the north atlantic. None of this has changed. However, the pacific hurricane season is rather more active, and at present there are 2 tropical storms off the west coast of Mexico - Vicente and Willa. Vicente is forecast to remain a tropical storm and move north west up the coast of Mexico. Willa will soon become a hurricane, and is forecast to become a major hurricane by Monday, before hitting Mexico sometime on Wednesday, when it will weaken as it moves inland. The models are picking this storm up as travelling across Mexico and up the eastern seaboard of America.

At present the waters off the eastern seaboard are unusually warm. What the GFS appears to be doing is strengthening the storm as it moves north over the water, then catching it in the jet stream and depositing it over us, where it takes up residence over Scotland, changing our weather from very dry to wet and windy or stormy.

This is all in the long range part of the model (we are talking about the start of November) so it is difficult to check against other models which don't go so far out, but there are signs at the end of the ECMWF run that it is thinking along the same lines.

Slightly nearer is a small change to the forecast for next weekend, starting the northerly flow on Friday and introducing the possibility of more precipitation as the cold air comes in from the north. This would lead to more widespread wintery showers over the weekend with snow possible over high ground and to lower levels in the north. This is unusual but not unprecedented in late October (it happened in 2008). It won't last long as the milder air moves in after the weekend.

[Penellype]

The Daily Mail have published a headline today warning of temperatures of -10 by the weekend. Not sure where they expect these to be - maybe they have taken up residence in Scandinavia or somewhere. To be clear, it is going to get colder, but this is not anything particularly unusual.

This is the minimum temperature forecast chart for the UK over the next 10 days from the GFS model.



There are a couple of -4/-5s in Scotland, but nothing even remotely approaching -10.

One of these days we really are going to have a severely cold spell, and nobody will believe it.

[Penellype]

Quite a spectacular drop in 850mb temperatures shown on the ensemble charts as the cold air floods south on Thursday and Friday.



Plenty of rainfall spikes appearing for next week too, reflective of low pressure and more unsettled weather coming along. The temperatures gradually recover towards normal for the time of year.

[Thelma Sanders]

Yes, not looking forward to the next few days, a really icy blast predicted. I've just been and picked all the dahlia flowers before the plants get blackened.
I was just thinking that it's been a while since we've had a really cold bonfire night, I can remember a few with snow showers over the years, especially when the children were small

[Penellype]

The cold spell arrived as predicted, with snow to quite low levels (including here in York) for a time yesterday morning. Another cold night is likely tonight and the next few days will remain well below average for the time of year, warming up only slowly.

Hurricane Willa didn't turn out to have much effect on our weather, but we now have another storm in the middle of the Atlantic, Oscar. This is forecast to become a hurricane later today. It is likely to move north and then north east towards us, when it will become caught in the jet stream as an ex-tropical storm and bring some tropical origin (warm and wet) air to us, along with gales or severe gales around next weekend.

As is always the case with ex-hurricanes, where we go after that is somewhat uncertain. At present the models are favouring low pressure to remain around the UK with high over the continent. If this is correct it is unlikely that we will see a return of the cold weather in the near future. However things can easily change, and as I keep saying, ex-hurricanes can cause sudden changes in the overall pattern which are hard for the models to predict.

There is also current interest in the stratosphere, where the zonal westerly winds are strong for the time of year, leading to a strong jet stream - this is probably why the low pressures have been able to break down the persistent high pressure we've had all summer. The models see a potential drop in the strength of the zonal winds in the next couple of weeks, and this (if correct) could have consequences lower down in the atmosphere a little later (weaker jet stream). If this leads to high pressure forming to the north, which it might, this could bring back the cold, but the position of any high will be crucial in determining where the cold air goes.

[Penellype]

Oscar arrived on schedule at the weekend, but apart from a breezy day it had little effect away from the north of Scotland. We are now between low pressure to the west and high pressure to the east, dragging up a warm southerly flow.

For a while it looked like the high pressure might win out and keep us fairly mild and dry, but now it seems more likely that the low will gradually take over, and while it should stay mild, it will become rather wetter, especially in the west where some large rainfall totals could well mount up. It will be windy at times too, with particular emphasis on Friday/Saturday when we could get our next named storm, which would be Dierdre. It is too early to say with any confidence exactly where this would be heading, but at present it is forecast to approach from the south west and move north across Ireland, bringing strong winds to most parts of the country (strongest in the west) through Friday. Be prepared to batten down the hatches towards the end of the week.

[Penellype]

Wet and windy weather already in the west will spread eastwards tonight. There are yellow warnings for wind and rain in Ireland, parts of Scotland and Wales and south west England, and an amber warning of rain in south Wales. https://www.metoffice.gov.uk/public/...ate=2018-11-09

Quite why this is not a named storm, since it has resulted in an amber warning, I have no idea, as I thought amber warnings were the trigger for naming storms. Clearly not always...

Thankfully after tonight it will quickly become less windy and rather less wet, although models are divided about quite how wet it will be over the next week or 2. Due to the setup of low pressure to the west and high to the east, the wettest weather will be in the west.

[Penellype]

This ensemble chart for York from the latest GFS run really is astonishing:



Apart from a few showers today and a little rain on Wednesday, there is almost no rain in the forecast for the next 2 weeks. This is November, one of the wettest months of the year. The culprit is the ridge of high pressure currently to our east, which is forecast to build over us. The resulting rise in upper air temperatures is clear to see.

Bear in mind a few things. Firstly this is for the eastern side of the country. The west will be wetter than this at first, and later on in the run it is currently expected that the south may see some rain. Secondly, high pressure, clear skies and light winds at this time of year are likely to lead to fog or cloud and drizzle. Therefore although the upper air temperatures are warm, on the surface it may feel damp and chilly. However, if they avoid the fog and cloud, some places in the south could see temperatures of 16-17C later this week.

Later in the run (around next weekend) the models are keen to move the high pressure towards Scandinavia. This results in a gradual lowering of the upper air temperatures - some of the individual ensemble members do this faster than others.

High pressure over Scandinavia pulls our wind direction into the east. At this time of year east is cold and all the forecasters are getting excited. There is definite potential for a cold spell towards the end of the month, and some of the patterns that are appearing look quite hard to escape from if they materialize, so a prolonged cold spell is not out of the question. That is a long time in the future though, but I would expect that as various websites and forecasters release their winter forecasts, the emphasis will be on a colder than average winter rather than a warm one.

I'm not sure - long range forecasting is very difficult. Even the end of this month is too far away to avoid a series of ifs, buts and maybes. It is worth noting that only a week or so ago the ensembles were looking very wet indeed for the middle of November, and temperatures were forecast to be average at best, and now its completely different. The best we can do is be aware of the possibility of an early cold spell and be prepared to protect any vulnerable plants.

[Greenleaves]

Thanks Penellype....I think

[Gillykat]

Thanks Penellype My tenders are gradually getting moved into the greenhouse as I create space.

And no rain on the east coast (where I am) for the next two weeks will do nicely.....give me time to continue digging and tidying the plot before the end of the year as January onwards tends to be pretty yuck

[Penellype]

The colder air is arriving on schedule, with the wind swinging round to the east during the next couple of days. This was well predicted a week in advance by all of the models.

However, what happens next is far from clear cut. A day or 2 ago all the models appeared to be pointing towards an extended cold spell into December, with many runs forecasting cold and mainly dry. In the last couple of days the situation has become a lot more complicated, the reason being the formation of a low pressure system to the south west. Uncertainties arrive as early as Wednesday. How far north this system moves is critical to our weather - if it stays to the south we will remain in a cold easterly wind, with the northern half of the country remaining mostly dry and possible wintry showers in the south and east. The further north it goes the more the air is pulled in from a warmer southerly direction, and the more rain there will be.

The ensemble chart for York shows the uncertainty:



Note the big differences between the coloured lines as to the amount and timing of any rain round 21st. The end of the week is looking much less cold than it has been - the white line was around the -5 mark for the whole of the rest of the chart a couple of days ago. It will still be chilly and feel very cold at times as the easterly wind strengthens as the low moves in.

After next weekend there are huge uncertainties and this all hinges on whether high pressure or low pressure wins the "battle". If you look carefully at the wide scatter of lines you will see that in general the colours of the rainfall spikes match the colours of the warmer runs. These are runs where the low wins and the atlantic flow breaks through bringing milder and unsettled weather. The colder runs are ones where the high moves to the north or west of us bringing in winds from the north or maintaining the easterly flow. With such a huge scatter of vastly different scenarios and uncertainties even in the 5 day range, there is no point in trying to predict what will happen.

The message is to treat app forecasts with caution particularly beyond a couple of days and be aware that the threat of a prolonged and possibly very cold spell has not completely gone away. With the current setup this possibility could return at any time.

[Nicos]

Thanks Pen.
I was wondering what was going on!

I've been watching my local forecast for the coming week and it's changing several times a day!!!
2cm snow...no snow...no rain...one day of rain....several days of rain....

The fact that nighttime temperatures seem to be a steady -1 to -4C this week does however seem to stay the same!

Always nice to have a short cold spell.....

[Penellype]

Slightly more agreement now regarding what happens next. The ensemble chart is starting to resolve into a more typical up and down pattern as low pressure systems with their associated warm and cold fronts move in from the west. These are associated with bands of rain which are becoming better defined as the time approaches:



Basically this means unsettled with periods of wet and windy weather and a mixture of milder muggy days and cooler, showery ones between the wet spells. The odd ensemble member (notably an orange one in this run) is still going for a cold spell, but they are very much outliers at this stage.

Even though this is more of a standard pattern for this time of year (compared to the very unusual patterns that we have had since spring), it is still not really "normal". The jet stream still has large loops in it instead of a slightly wavy flow, and the models want to build high pressure over Scandinavia again later in the run. The ECMWF does this as soon as Thursday:



This is a pattern that we have seen over and over again recently, and although in this run the low pressure wins, keeping us milder, wet and windy, it wouldn't take much adjustment to put us back into the easterly regime that gave us the recent colder weather.

The basic message is after a fairly chilly weekend and start to the week, expect next week to become wet and windy at times from Tuesday/Wednesday with nothing particularly cold, but take any longer term forecasts with a pinch of salt for now.

[Penellype]

A wet and windy week incoming - yellow warnings for wind and rain have been issued for parts of the country for Wednesday and Thursday. See https://www.metoffice.gov.uk/public/weather/warnings for details.

[burnie]

We had a fair bit of snow on the hills, to quite low levels, the wind is bitter coming off the snow.