Penellype's Weather Channel

[Jimmy]

Dropped to -1.5 deg C around midnight in Preston.
Jimmy

[KevinM67]

I hope the cold temperatures have not killed off all your flowers Jimmy !!

[Jimmy]

I hope the cold temperatures have not killed off all your flowers Jimmy !!

Sunday was an excellent day for gardening.
The sweet peas had blown over and only had 1 flower.
So removed them all.
The front flower border had virtually finished so removed them as well.
Not many flower left now (thankfully).
What's left ?
Carrots full of carrot fly, must try a barrier next year.
Parsnips, not tried any yet.
Leeks, my sister has had a few as a birthday present.
Sprouts, Broccoli and Kale still look OK, will try some Kale when I can get out to pick some.
Jimmy

[KevinM67]

Leeks, my sister has had a few as a birthday present.

You spoil her Jimmy !!!

[Penellype]

The reason why the models are struggling is a little clearer. We have another oddly positioned tropical storm (Rina) in the middle of the atlantic, heading our way. Yesterday the models had this passing north of Scotland, today some of them have it moving over Ireland, which could give the south a battering on Friday night. The weather looks set to get colder again over the weekend and/or into the start of next week with the winds again swinging round to the north.

I am not an expert at this by any means, but I've noticed that when the weather gets into a pattern it tends to repeat itself until something happens to stop it. This year there is a tendency for high pressure to build in the atlantic pulling the winds into the north and then topple over us, but each time this happens the ridge seems to get stronger. High pressure also keeps threatening to build over Greenland, and while it hasn't so far done what the models are predicting in the longer term, there is a definite theme emerging. Some of the forecast charts I've seen for the end of November remind me somewhat of this time in 2010, and there are other factors that are not dissimilar too. A cold end to November and start to December would not surprise me at all, although I am NOT saying we will have a repeat of 2010!

[Penellype]

Chilly night tonight in the northern half of the country, more cloud in the south. Cold everywhere tomorrow night with a widespread frost.

Warmer next week (although remember it is November) before more colder weather looks set to arrive on Friday. Models are still indicating high pressure over Greenland towards the last 1/3 of the month, which could, if things are positioned correctly, give us quite a cold week or 2, but too far off to say at the moment.

[Penellype]

Loads of articles from the usual suspects saying we are about to be plunged into a big freeze, with snow arriving by mid week.

Um, away from the high ground of Scotland it looks highly unlikely that there will be any snow before Friday. Colder air is set to move in again on Friday into the weekend, and this could bring wider snow to Scotland and possible snow showers further south.

BUT, and it is a very big but, there is HUGE uncertainty over what happens after the end of this week. Take a look at the 3 latest runs of the GFS ensembles (for York):

Midnight:



6am:



12pm:



The first hing that should strike you is the amount of scatter after 19th (Sunday). This is getting if anything worse run by run. If you can pick out the thick green line, you will see that on the midnight run it is one of the warmest, on the 6am run it gets lost somewhere in the middle (up and down a lot) and on the 12pm run it goes up then down, up again and finishes off as one of the coldest. Very few of the lines go cold at the weekend and stay that way, although there are a couple.

The thing that seems to be playing havoc with the models now is a complete disconnect between the stratosphere (top of the atmosphere) and the troposphere (where weather takes place) over the north pole. normally if the stratosphere is cold the troposphere responds to this by tightening the polar vortex and locking the cold air up in low pressure over the pole. If the stratosphere is warm, the vortex weakens, high pressure builds over the pole and the cold air leaks out in places over the mid latitudes. Currently we have a cold stratosphere, but it isn't getting its message across to the troposphere, which seems to be trying to build a high over the pole and push the cold air down. This is not normal, and the models are therefore completely confused.

It seems very likely to get cold again at the weekend (possibly with sleet or snow showers in places), but take any predictions of what happens after that with a large pinch of salt until nearer the time.

[burnie]

We've had snow at lower levels near us today, at the coast we got rain.

[Penellype]

On the day that the Express produced its annual winter scare story Snow forecast: UK set for 2 months of BLIZZARDS: NOAA issues rare La Nina winter advisory | Weather | News | Express.co.uk , the models are starting to change their minds about the cold:



Which basically means that the forecast has changed from something like this (the coldest green line on the above chart):



to this (the thick blue line on the above chart):



Both the above for 22nd November, the furthest I can go for the individual runs.

The 1st of these is freezing cold, the 2nd is positively mild.

This is what I mean by uncertainty. It may change back again, or it may not.

[Chippy Minton]

That reminds me, must get a pinch of salt in for the drive. Weather is notoriously difficult to predict and lately seems to be fooling around with the modellers. Keep up the good work Penellype!

[Penellype]

Still massive uncertainty in the models about what happens after the weekend, but colder air is definitely arriving through today and will last the weekend. Be prepared for night frosts and chilly days until at least Monday, after that, stick a pin in it for now.

The ensembles show the uncertainty, with the operational run of the GFS (thick green line) one of the coldest:



Note that there is massive differences in rainfall amounts between the ensemble members too, reflecting conflicting predictions of high and low pressure.

Be aware that at this time of year warm upper air temperatures do not necessarily translate to the surface, as cold air can be trapped underneath an inversion when pressure is high. Therefore although it looks from the graph as though it will be nice and mild at the beginning of next week, everything depends on the position of the high pressure, which defines the direction of flow. In winter the warmest (but cloudiest) direction is south west, west is mild and wet, anything to the north or east is cold, and even south can be cool because the air is originating from the cold continent, not the warm sea.

[Penellype]

Warmer from the west as rain spreads eastwards tonight, then we have a few mild days, becoming increasingly windy and rather wet especially in the north and west.

As is becoming normal now, anything beyond the next 3-4 days is very uncertain, but the models are suggesting a return to colder weather, possibly as early as Thursday, with the possibility of some snow further south than Scotland this time. All still very much up in the air, but I think we are going to see a lot of this chopping and changing, both from the models and from the weather for some time yet.

[Penellype]

A pretty disgusting day in many places today and over night with Met Office warnings of wind, rain and snow for various parts of the UK: https://www.metoffice.gov.uk/public/weather/warnings.

Very wet in the north and west today, with strong and gusty winds throughout England and Wales. Snow in Scotland over night into tomorrow morning, including probably some at low levels. Snow is very hard to forecast, so don't be surprised if some of this falls as rain, but the possibility is there.

Over the next couple of days the very mild southwesterly winds will be replaced by northerly winds (which are bringing the snow to Scotland) as rain slowly clears to the south east. The large and deep low north of Scotland that is responsible for the cold front and the winds isn't moving very fast as it is caught between high pressure over Asia and eastern Europe and a developing ridge in the atlantic which is joining forces with a high over Greenland. The result is that the low gets stuck to our east and we get rather colder, as is clear from the ensemble chart for York:



How long the colder spell lasts is open to question as there is a lot of chopping and changing between runs, but it look set to stay cold over the weekend, with night frost likely if skies clear and the possibility of snow showers over higher ground as well. Models had been forecasting widespread snow even to low levels, but appear to have backed off that for the moment.

The options longer term are for the pattern to remain stuck with us in a showery northerly flow (cold), a collapse of the atlantic ridge over the top of us, joining with the continental high and pulling the winds into the east (cold), or a breakdown of the atlantic high and a return to westerly winds (warmer, wet and windy). The models generally show a gradual progression through these scenarios, but a huge amount of scatter after the weekend, which isn't actually apparent from the chart above. The forecast pressure (again for York) gives a better idea of the uncertainty:



The top chart is the surface temperature forecast - daytime temperatures around 5 degrees and possible frost at night (although not desperately cold), the bottom chart is surface pressure showing the low passing through over night and a rise in pressure over the weekend, followed by uncertainty.

I would post the more familiar charts, but unfortunately TheWeatherOutlook site appears to be down this morning.

[Penellype]

A chilly weekend ahead, with some places already having had some sleet or snow showers. These will continue over night and throughout the weekend, always most frequent in the west. Any accumulations are likely to be confined to hills. Expect frosty nights anywhere where skies clear, with ice on wet surfaces.

Today's chart looks like this:



Cold air is being sucked down from the north with high pressure over Greenland, but there is still high pressure to the south over Spain. This results in the ridge in the atlantic collapsing over us during the weekend and allowing low pressure to move in on Monday, giving a brief milder spell.

However the cold returns with renewed vigour for the remainder of the week at least, as the high builds up into Greenland. This is a colder pattern than we have at the moment - by Wednesday the high over Spain has gone:



This allows a solid ridge to form in the atlantic reaching right up into Greenland. This is not going to collapse over us anywhere near as quickly and is still just about in place by next Sunday:



This really is a very strongly blocked pattern and it could last some time. If this was January it would be bitterly cold, as it is still November we will have frosty nights and daytime temperatures in low single figures, with snow showers a possibility particularly near north facing coasts. The longer this lasts the colder it will get.

The models always want to break this sort of blocking high down and revert to westerly winds by default, so the fact that it is lasting for over a week shows that it means business. The GFS model (above) has support from the ECMWF, which still has us firmly in a northerly next Sunday:



So the message is clear - get ready for an extended spell of colder and possibly wintery weather after Monday, which is likely to last a week or more. This is not going to be a repeat of December 2010, but it will probably be the longest cold spell since March 2013.

The long range CFS model doesn't see the cold lasting this long, breaking the cold pattern down by next weekend and then building the high back over Spain. After that it favours a milder, more westerly flow. I am sceptical about this, I think this cold spell could last a couple of weeks, and it may not be the last one we see, either. The pattern of highs in the atlantic and lows dragging the winds into the north has repeated itself several times already and may well do so again after this breaks down. We shall see.

[Jay22]

Thanks again Penellype, off to hunt out the thermals!