Penellype's Weather Channel

[roitelet]

'Twas a tad noisy here last night even the cats stayed in. Calmed down a bit this morning but there are loads of leaves to clear up

[Penellype]

Those in the affected area might be relieved to know that Angus moved on a slightly more southerly track than was predicted. This meant that the strongest winds were over northern and western France and the English Channel rather than over southern England. The storm also formed a "sting jet" similar to the famous 1987 Michael Fish "hurricane" which would have hit somewhere around Portsmouth on the forecast track of the storm, but thankfully avoided the south coast altogether.

Even so there seems to have been some notable damage in places, and as predicted, a lot of rain and some flooding. More rain to come for southern and central areas tomorrow into Tuesday I'm afraid.

[Nicos]

Yup...more southerly...it hit us with full force!
No major damage that I'm aware of....so far!

[Penellype]

About time for an update.

This is the ensemble graph from this morning for York:



The upper air temperature has been quite high for the time of year this week, although that hasn't been terribly apparent on the surface. This is fairly common with high pressure at this time of year. The upper air is mild, but on the surface if the skies are clear at night it can get quite cold. Light winds mean that the layers of air do not mix, so the air at ground level is cooler than that higher up. This is called an inversion, and can lead to fog or low cloud which then keeps the cooler air in during the day as the sun can't warm things up. Its a bit of a catch 22 - if its clear it is cold at night with the risk of frost, if its cloudy it is cold during the day, but less likely to be frosty at night.

As you can see the upper air temperature is due to fall over the weekend and into the start of next week. This corresponds to the high pressure slipping south and east. Currently the winds (such as they are) are coming from an easterly direction, across the relatively warm North Sea. As the high slips south and east the winds move round to the south east. You would think this would be warmer, but at this time of year it isn't, because the air is coming straight off the cold continent without the warmth of the sea. The air will be drier too, which means less cloud, so temperatures at the surface are likely to fall a bit, with frosts becoming more likely in the south as well as the north. It is also likely to become breezier and probably wetter in the north of Scotland.

What happens next is completely uncertain, which is why I have not posted any maps. It is changing radically from one run to the next, with some runs having high pressure building again (the thick green line on this morning's run shows this happening with a corresponding rise in upper air temperature). Other runs are more unsettled with low pressure coming in from the atlantic, giving wetter and windier conditions and much more bumpy temperature graphs. Other runs have high pressure ridging towards Greenland, putting us in a northerly wind with colder air and a possibility of snow. Some of the orange lines show quite a prolonged cold spell.

In a nutshell, beyond about Wednesday they haven't a clue.

[Penellype]

The ensemble graph in the previous post was from this morning's GFS 6am run. The 12pm run is now available and illustrates the uncertainty beautifully. Look what has happened to the thick green line in just 6 hours!



Unlike the 6am run, this is really very cold.

[Penellype]

Be prepared for a frost tonight almost anywhere in the UK. Note that the GFS (which many weather apps use) may be underplaying this somewhat:



Many places here showing as just at freezing as a minimum.

The high resolution Arome model has this:



I am not 100% sure if this is the minimum predicted, but it is quite clear that the Arome is forecasting a good 2-3 degrees colder than the GFS minimum.

The GFS run is more recent, as the Arome has not updated yet, but unless you live on the coast or in the north of Scotland, be prepared for a hard frost tonight.

I will explain why the north of Scotland is warmest in the next post, as this is not what most people would expect!

[Penellype]

The weather at the moment is all about the position of high pressure, which is giving us the current mainly dry, sometimes sunny but often cloudy/foggy/cold weather. The important factor is how the air circulates round a high pressure system. In the northern hemisphere the circulation around a high pressure is always clockwise, that around a low is always anticlockwise. The air flow roughly follows the direction of the isobars, which connect points with the same pressure value, so you can see where the air is coming from by tracing the isobars back. In winter land is colder than sea because water takes longer to cool down. So isobars that cross the sea are likely to carry warmer air than isobars from a similar direction over land. Obviously air coming from the north is colder than that from the south at any time of year. Air coming across water is generally wetter and therefore cloudier than air coming across land, and cloud acts as an insulator, so clear dry air means colder weather at the surface. The temperature of the upper air is also important, but less so if it is not windy.

Today's position is this:



While not exactly from a warm direction, the 1030 isobar is crossing the warm North Sea.

By 9am tomorrow the high has moved subtly south and east, so the 1035 isobar is hardly crossing any sea at all.



Clearly this is only one isobar - we are talking about a situation where there is very little wind to start with (and therefore more likely cold as the layers of air can't mix).

If you look to the north of Scotland and trace the isobars back, not only do they cross a large part of the atlantic, but they also come from quite a long way south. This is why it gets warmer the further north you go.

Subtle differences in the position of the high are going to be crucial in determining the weather over the next few days and this is what is making the forecasting so difficult. The following 2 charts illustrate this rather well:



Predicted mid day temperature for York 4 degrees, overnight low 0.



Predicted midday temperature for York 9 degrees, overnight low 4. Although the wind direction for York is from the north west, the upper air temperature is quite warm, and crucially it is windier (isobars closer together). This allows the cool surface air to mix with the warmer upper air. It is also likely to be cloudier, the air having come from over the sea. It would not take a very big shift in the position of the high to turn this much colder, with cooler upper air temperatures and winds from the cold north east.

This is what the corresponding ensemble graph looks like:



You can clearly see the correlations with the warm 1st December chart and the colder 4th December one. There is still a lot of uncertainty into the start of December, and the thick green line goes off on its own into quite a prolonged cold spell from about the 7th. We shall see.

[Penellype]

Be prepared for a frost tonight almost anywhere in the UK. Note that the GFS (which many weather apps use) may be underplaying this somewhat:

[ATTACH=CONFIG]70298[/ATTACH]

Many places here showing as just at freezing as a minimum.

The high resolution Arome model has this:

[ATTACH=CONFIG]70299[/ATTACH]

I am not 100% sure if this is the minimum predicted, but it is quite clear that the Arome is forecasting a good 2-3 degrees colder than the GFS minimum.

The GFS run is more recent, as the Arome has not updated yet, but unless you live on the coast or in the north of Scotland, be prepared for a hard frost tonight.

I will explain why the north of Scotland is warmest in the next post, as this is not what most people would expect!

So who was right?



Chart for last night from UK Minimum Temperature.

I'd say the Arpege (not Arome - sorry, I always get these 2 high res models muddled!) was more accurate than the GFS, although the thin spread of weather stations makes it hard to tell for sure. The minimum temperature here was -2.4.

More of the same for the south tonight, although the north looks a tad less cold:

[Penellype]

A long way off, but a cracking example of how a small shift in the position of the high can lead to a large difference in temperature:



Follow the isobars back from this one and you reach Florida or the Canary Islands. With warm upper air temperatures and relatively close isobars, this would very probably give double figure temperatures but likely a lot of cloud as there is a very long track over the sea.

This sort of high pressure system, a hump of high with lows symmetrically either side of it is called an omega block (after the shape of the Greek letter omega). Omega blocks are notoriously hard to shift, which is why most of the models are predicting that this high will roughly stay put well into December.

[Penellype]

Another cold start with a minimum of -1.5 here. Although there is no surface wind, there are some clouds this morning which are moving quite noticably, showing that the upper level wind direction here has changed to NW.

Anyone interested in a more detailed explanation of the likely cloud amounts, frost etc for the rest of the week might find this article by Matt Hugo illuminating: Synoptic Guidance - Blocked Weather To Continue - Blog by Matt Hugo - Netweather.tv. There is a particularly interesting graph of how temperature and dew point vary with height at the bottom of the article showing why people standing on a mountain top under a clear blue sky can be looking down on a blanket of fog. If the temperature and the dew point (the temperature at which water starts to condense out of the air, which depends on how much moisture there is in the air to start with) are close together you get cloud or fog, if they are wide apart you get no clouds at all.

[Thelma Sanders]

I like Netweather because it usually explains why it has reached the forecast it is giving.

So many things to be considered, in the long term, it really is no wonder that sometimes they get things wrong.

[Penellype]

Winter forecast from the excellent Gavin Partridge can be found here: Winter 2016/17 Weather Forecast. He agrees very much with my own assessment that this year is likely to be a mixture of cold periods and warmer, wetter and windier ones giving a winter that is overall rather colder than the last few and probably drier than average.

In the short term the high pressure looks to be staying with us for the next few days, although with a shift to milder cloudier conditions. After that it looks like we will move into a more westerly pattern, which means milder, wetter and windier than we have seen recently. Model agreement for this is now good up to about 9th December after which it becomes much more uncertain. Several models are still going for a colder period around Christmas, although that is still too far away for any sort of confidence.

Here is the latest ensemble run, as always for York, but it will be similar elsewhere.



Note the reappearance of the rainfall spikes along the bottom from the middle of next week.

[Penellype]

All change from today as the high pressure shifts slightly east and the cold air gives way to warmer air from the atlantic. This is causing problems with fog in places today. Even night time temperatures could be in double figures for the remainder of the week in some places, becoming cooler again at the weekend as low pressure moves in. It doesn't take long for the high to build again, with a return to a milder air flow.

After that it is still anyone's guess what happens with some of the model runs showing the warmer and more westerly pattern continuing over Christmas, possibly becoming stormy, while others show a much colder position with cold and dry weather or (occasionally) the possibility of snow. This would happen if the centre of the high were to move north, pulling the winds in from the east which is potentially very cold.



You can see the spread of possibilities here - there are some very cold green lines later on - no charts are available for this timescale yet.

[vixylix]

Roll on double figures... it is freezing here today!

[Jungle Jane]

It was 10 degrees celsius here today,better than the -3 the other day,so foggy though in Braintree & around,visibility was about 50meters on the road,give yourselves extra time for travelling you can't see what's ahead,the fog isn't lifting here so it's going to be dark & foggy on the road tonight,I always imagine I'm in a horror film when it's like that & it's nice when you survive which is every time,I do get scared if I see someone walking though,but lock your doors they can't get you