Penellype's Weather Channel

[Bramble Slayer]

A fascinating thread. Lovely to have sone context to our weather. Thanks so much for sharing. Having looked in the log shed today I could be in trouble this winter!

[Penellype]

Now that we have arrived at changeover day, I thought it might be interesting to post the updated versions of the same charts, to see how things are progressing. Obviously the first chart is history, but here are the others.

Tonight



The similarity to the chart posted on Saturday for Tuesday night is remarkable. The model has got this spot on, which was to some degree predictable given the close agreement between the individual members of the ensemble (on the bottom graph).
Night frosts are still expected this week.

Saturday morning



Again good agreement with Saturday's chart, although the general air mass is rather less cold (more green than blue especially over western Europe). The low pressure is less deep at this point, and the isobars slightly further apart which would mean less windy, but the generally cool, breezy and showery picture remains.

Saturday 12th



This is why I described the final chart as "fantasy land". Here the model is predicting a return to warmer conditions as a ridge of high pressure moves in. The air moving in around the ridge originates from the south so this would quite definitely not be cold with a risk of snow! Again this is far too far away to be taken seriously.

The ensemble graphs now look like this:



The plunge into colder air today is quite clear, and the individual lines have converged to show the cold likely to remain into next week, with any warm up at the weekend taking us barely back to average before another likely drop next week. The one orange line going off on its own is an "outlier", very likely to be wrong. However uncertainty does increase the further away you get, and anything beyond 5 days should always be treated with caution.

Note that this is not a "big freeze". There will be night frosts where skies clear, but very few of the lines are crossing the -5 isotherm (for York) implying that snow at low levels is unlilkely. There may be some sleety stuff and snow is definitely possible over hills.

The peaks at the bottom of the graph indicate rainfall, which has been notably absent recently away from the north and west. Rain looks likely to be appearing much more frequently from the weekend. In fact the pattern emerging towards the end of this graph is the typical up and down wave in temperatures from the more normal westerly, wet and windy weather associated with alternating high and low pressure systems. This implies a breakdown in the northern blocking that is currently driving the weather, but as westerly is the normal setup, the models do tend to default to it. All of that is too far ahead for any sort of certainty, as is confirmed by the amount of "scatter" (difference) between the lines.

Summary - becoming much colder as a cold front moves south today, with no prospect of a return to above average temperatures in the next week or so. Frosts likely at night, snow possible on hills as wetter conditions arrive for the weekend. After that it looks probable that the first part of next week will continue to be colder than average, but uncertainty increases and by the following weekend it is complete guesswork.

Tonight's likely minimum temperatures



This is the high resolution French Arpege model from Arpege charts

Thursday night is likely to be a little colder.

[Thelma Sanders]

The 3 month outlook from MetO is for colder than average over the next 3 months
http://www.metoffice.gov.uk/binaries...emp-ndj-v2.pdf
and drier
http://www.metoffice.gov.uk/binaries...cip-ndj-v2.pdf

"The 3-month Outlook provides an indication of possible average temperature and rainfall conditions over the UK in the next three months. It is part of a suite of forecasts designed and produced for use by contingency planners, and is one of several tools used in environmental risk planning by a number of different sectors."

[Penellype]

No doubt the tabloids will interpret that as the coldest winter for 100 years with unrelenting blizzards and sub zero temperatures. I've already seen several articles along these lines this autumn.

The reality is that they are only talking about probabilities, and what they are saying (cold and dry is more probable than warm and wet) makes sense in the context of the currently available evidence.

[Potstubsdustbins]

Seems there may be a chance of some night frosts later this week, may pay to keep up to date with the forecast for your area.

[unclefudgly]

Just come back from the lottie, in SW London and frost everywhere, having to pick a fair few pounds of giant Brandywine tomatoes, and transfer all the chillies back home for overwintering. Tomorrow's going to be even worse by all accounts.

I don't think I'll be picking tomatoes come December like I was last year.

[Lumpy]

I have come to one conclusion as far as the weather is concerned - uncertainty is the only thing for sure.

[Penellype]

A bit milder tonight, except in Scotland, due to more cloud, but don't be fooled - the weekend is going to be cold and raw with a biting northerly wind setting in. Frosts will return from tomorrow night into next week and there may be snow on hills and even sleet to lower levels (or hail and possibly thunder too).

Beyond the 5 day forecast things are very uncertain:



The wide scatter of lines shows that the individual members are not in agreement, but the main operational run (thick green line) is going for a return to milder winds from the west. The other main model, the ECMWF, maintains the cold right through to the following weekend.

This sounds like a major difference, but actually it isn't. What is forecast to happen is that the high pressure currently to our west topples eastwards allowing low pressure to move across the top towards Iceland. This then comes up against the remnants of the high pressure over Scandinavia and drops south.

The GFS model sees this for midnight on Thursday 10th:



and the ECMWF sees this at the same time:



There really isn't much difference at all, but crucially the GFS is seeing the low dropping south through the north sea, whereas the ECM is seeing it drop south through the Irish sea. This makes a HUGE difference to the source of the air across the UK (and therefore the weather and temperature) - from the west in the GFS version and from the east in the ECM version.

This sort of scenario is quite common for the UK, often referred to by forecasters as "battleground UK". It is a great illustration of why weather forecasting here is so difficult, because just a few miles can make all the difference.

Who is right? Well, the ECM is normally the more reliable of the 2 models, although the forecasters are tending to side with the GFS this time. The Met Office does not go out as far as this at this point. For me, its too soon to say but I will watch how this develops with interest.

[Penellype]

Its fair to say that the GFS has done pretty well with this, both models now going for the low to move down the north sea rather than the Irish sea. This means that the weather will become more "westerly", which means slightly warmer, wetter and windier. The ensemble chart now looks like this:



You can see that the white line has been fairly flat and below the red average line. There is a sharp upward curve tomorrow as the warm front associated with the low pressure moves across the country - this arrives here tomorrow night. Fronts are associated with rainfall, which shows on the spikes along the bottom of the graph. Notice how there are far bigger spikes appearing along the bottom than there were a few days ago. The up and down waves in the temperature line are typical of low pressure systems moving in from the west, with bands of rain on warm and cold fronts. The sharp changes in temperature are often associated with strong winds.

The different ensemble members (coloured lines) have the timing slightly different, but all agree on this up and down pattern.

What this means for the next few days is that the currently showery weather (particularly in the east) with colder nights where skies clear will continue for the next day or so - tonight may well be quite cold. As the warm front moves across on Tuesday it will hit cold air and some of the rain will fall initially as snow over high ground. In the north some of the snow or sleet may make it to the surface even at low levels. The met office have a warning of snow for tomorrow and into Wednesday:

Severe weather warnings - Met Office

Even after the front has passed the weather will still feel cold and could be cold enough for frost at night where skies clear - Thursday night in particular still has high pressure to the west and low to the east, keeping us in a somewhat northerly wind:



After this it looks likely that the weather will be unsettled until at least the weekend with bands of rain and fluctuating temperatures. After that the GFS wants to build high pressure to the west again, bringing back the cold, but as you can see from the ensemble chart, the thick green line is one of the coldest runs later on.

I think it is likely that we will see more colder than average periods like the current one before Christmas. The jet stream (roughly the black line between the green and yellow parts of the coloured graphs) is still very up and down rather than flat, and that means much more chance of the cold air coming down over the top of us at times. Predicting the timing of these is quite another matter.

[Jimmy]

Tomorrow will be very much like today.
Seems to work most of the time !
Jimmy

[Penellype]

Tomorrow will be very much like today.
Seems to work most of the time !
Jimmy

There is a lot of truth in that. What I am trying to do here is take some of the guesswork out of things where possible, and also provide a more realistic assessment than that which tends to appear in the papers.

[Lumpy]

The Met Office Facebok page is discussing a 'polar vortex' - oh joy.

[Potstubsdustbins]

I agree with Pen about the papers.

Todays headline 'traffic chaos after blanket of snow, Café has to clear its footpaths'. WOW it was a whole 2"/50mm.

I have to agree the standard of driving today is abysmal but 2" for goodness sake.

A little story. A few years back my lovely MiL passed away just before Christmas and the funeral was between Christmas and New Year. On the day here in Nottingham we had had a few flakes so I took the precaution of ringing the BiL up in Buxton to check, "Aye" he said "We've had a skittering (Don't ask) now't to talk about."

When I arrived in Buxton they had had 6" and the traffic was running normally, after the service in a local church we had to travel up and over the Cat & Fiddle to Macclesfield for the cremation, they had had a good foot or more on the tops.

Result, not once did any of the cars skid, slide or loose traction, yet 2" causes chaos, no carp drivers cause chaos.

[Penellype]

Milder, but also wetter and at times windier for the next few days.



The graph shows a sharp increase in upper level temperatures on Sunday and Monday followed by an equally sharp return to cooler conditions on Tuesday/Wednesday. The temperature rise at the surface won't be as sharp, but will be enough to take most places into double figures on Tuesday.



Tuesday's chart (above) shows the warm air over the country with the wind originating from the south west.

By Thursday the winds are still from the west, but the origin is now from Greenland and the upper air is much cooler. Expect a return to frosts from Thursday night.



Beyond that is into the much less reliable timeframe, but the models are consistently seeing the low pressure sliding south so that by Monday 21st we have something like this:



The wind is now coming from quite a long way north and this would be cold. In addition, the low pressure sitting over the south could be expected to produce quite a lot of snow for the south of the country.

This is a long way off and it may well not happen, but it is well worth keeping an eye on.

[Penellype]

Here we go again *sigh*. According to the Daily Express the UK is going to be colder than antarctica by the end of the week with temperatures of -11C.
Big Freeze coming: Polar weather to hit Britain hard | Weather | News | Daily Express

Reality check...
The actual wording of the article and the video at the beginning are not so bad - they are a reasonable representation of what is probably going to happen. The headline and the accompanying pictures of deep snow etc are totally misleading.

Let's look at the charts



As noted in my previous post, temperatures take a nosedive on Tuesday/Wednesday (this chart is for York, this will happen earlier in the west). The models have seen this coming very well. There is good agreement for a few days of colder than average temperatures, with the upper air temperature hovering around -5, the threshold temperature for snow. As usual after about 5 days the uncertainties start to introduce a lot of scatter, but currently the trend is towards a warmer period next week.

The situation now:



Warm air covers the country with the air flow coming from the south west. You can easily find where the air is coming from by following the isobars (white lines), remembering that air circulates clockwise around high pressure and anticlockwise around low. Temperatures today are mild, in the mid teens here, and with fairly cloudy skies they are forecast to remain above 10 all night. Note the little purple blob (980) to the left of the chart - that is the low that is going to change things around.

Thursday:



The little low has got rather bigger and moved to the north of Scotland. Now if you trace the isobars back you can see they are sucking in air from the north of Greenland and from Scandinavia, both of which are pretty cold. As forecast previously, expect night frosts with this. However, it will NOT be -11, except possibly on the very very highest ground in the north of Scotland! There is also a signal on the ensemble chart for rain on Thursday (bottom of the graph). Given the temperature it is possible that this could fall as snow on high ground. Keep an eye on forecasts nearer the time as snow is exceptionally hard to forecast in marginal conditions.

Saturday:



Looking cold and potentially frosty with light winds and cold upper air temperatures, but again this is nothing spectacular. Once again a developing low to the west is about to change things around.

Sunday:



The low to the west has joined forces with the original low which had stalled over the country. The air is now coming from the south west again, which is warmer, resulting in the sharp rise in temperature shown by the thick green line on the ensemble charts. Most of the individual runs see this happening, but they disagree on the timing. Things could get quite breezy for a while as the low pushes its way up against higher pressure over the continent, squashing the isobars closer together. Whenever you see closely packed isobars, expect strong or gusty winds.

The high pressure over the continent is acting as a block, preventing lows from moving in their usual west to east direction. Where this situation leads after next weekend is very uncertain, but many of the models are starting to forecast another cold spell at the end of November/start of December.