3 way race?
There look like 3 outcomes late next week. Cold spell, cold snap or it stays milder! The computer models are having difficulty pinning down the pattern which develops across the UK and Western Europe.
I’m still favouring a cold snap with the most wintry conditions in the north. A cold snap I’d define as lasting a few days.
Nonetheless, I’d say the chances of a cold spell have increased, as have the chances of it remaining milder, especially in the south.
So to conclude, it’s now looking like a 3 way race, with cold snap remaining the favourite.
Sat, 30 Nov 2013 08:20:08
Winter 2013/14 forecast
The winter forecast covering December, January and February will be online soon and then remain available until the end of February when the spring forecast takes its place. Two of the most common questions I’m asked are 1) how accurate are the seasonal forecasts 2) how they are produced.
1) As a general rule seasonal forecasts for the UK display a low skill level; in other words they aren’t considered to be very accurate. Many people will say they are no better than a guess. I’ll leave you to make your mind up on how TheWeatherOutlook’s (TWO) seasonal forecasts compare to those issued by other agencies. Please remember on TWO they are issued freely on a take them or leave them basis and no formal verification of them has been done.
2) This is easier to answer. If you’ve read my Winter 2013/14 initial thoughts you’ll already have a pretty good idea. Inputs into the forecast include pattern matching, teleconnections, sea surface temperatures, solar activity, stratospheric temperature forecasts and Siberian snow cover. But there’s another important consideration which is often overlooked, particularly for winter forecasts: Persistence. By this I mean that colder winters tend to (not always) come in clusters with milder winters also grouped together. The 1980s brought some notably cold winters and the 1990s and early 2000s some very mild ones. Since 2008 we seem to have flipped back into a colder cluster.
Finally, some of you may be wondering whether the TWO winter forecast has even been written yet! The answer is yes it has and it’s sitting in readable format 100% complete on one of the TWO development servers right now. Nevertheless, until it is published I will be keeping a close eye on how the weather is expected to develop in the short and medium term and if necessary tweak the winter forecast. As usual the forecast will be concise and easy to read. I don’t want to take up too much of your day! Hope you enjoy reading the winter forecast but please remember what I said above about seasonal forecasts in the UK displaying a low skill level.
The link below will currently take you to the TWO seasonal forecast. This is currently for autumn but will soon switch to winter.
TWO seasonal forecast
Thu, 28 Nov 2013 21:53:42
Change on the horizon – UPDATE 2
It’s still looking like we’ll see a change in the weather next week with the computer model output consistently suggesting a colder north westerly air stream developing across the UK for a while. As I’ve said in the last couple of updates the potency and length of the colder snap isn’t certain yet. Yesterday the computer model runs tended to back off a little bringing in a more benign cold snap, but today a few of the runs are again showing something more potent. Possibly with a risk of snow coming quite a long way south if they’re correct. That’s a big if at this range.
Those of you who like the cold and prospect of snow will enjoy this morning’s GFS run which I expect will be one of the coldest runs to appear today. By Saturday 7th December it shows 850hPA temperatures of -10C or lower across all of the UK apart from the south west. This is the air temperature at about 1500m and -10C or lower indicates a very cold air mass for the UK and often leads to heavy snow showers and accumulations. Here’s the chart:
This will be at the colder end of the range of possible outcomes for late next week and we may well end up with something less dramatic than the chart shows. Nonetheless, the GFS computer model in my view usually predicts this type of pattern very well, so something like it probably will develop. The caveat is it tends to moderate the coldness of the air mass and the longevity of these arctic incursions as their arrival time approaches.
In summary a cold snap looks likely, but I’ll need to see a lot more evidence before I’m convinced we’ll end up with one as potent as this morning’s GFS run suggests.
Wed, 27 Nov 2013 11:30:45
Change on the horizon – UPDATE
Yesterday’s Buzz post which you can read immediately below is still pretty much inline with my thinking after looking through the latest medium range computer model output. It still looks as though a north westerly of sorts is likely to develop for a time next week, potentially bringing colder air south. But there’s very little sign of this turning into a prolonged cold spell, with a 2 or 3 day cold snap looking much more likely. I’m never sure of the definition, but I always think of a cold spell as 5 days or more and anything less a cold snap.
There’s uncertainty about the potency of the cold plunge but past experience suggests this type of thing more often than not brings snow to higher ground in the north with bright, dry and crisp conditions further south. Occasionally these set-ups do bring the risk of snow all the way down to the south, but usually the cold needs to be embedded for several days and that’s not looking likely to happen at the moment. Nonetheless, it’s well worth keeping a close eye on due to the uncertainty.
What happens beyond next week is also interesting. There are signs of the cold snap opening the door to a more mobile spell of weather. In layman’s terms this means wet and windy conditions interspersed with dry and brighter days. Temperatures close to average with snowfall mostly restricted to higher ground in the north. We’ll see, and this interpretation does go against some of the forecasts for the first half of December which are leaning towards a high pressure dominated set-up probably leading to colder than average conditions.
Tue, 26 Nov 2013 13:05:53
Change on the horizon?
High pressure remains in control of the weather this week but there are signs of a change as we head into December. The next few days will see temperatures ticking up a little as mild air moves over the top of the high pressure and down across the UK, then later in the week we’ll probably see another temporary incursion of colder air returning from the north west with a risk of wintry showers over higher ground in the north. Not particularly cold or mild though with temperatures just gently deviating from the average; quite benign weather for the time of year.
Things could become more interesting next week as there seems to be a chance of a transition back to a more mobile weather pattern for a time during the first half of December. At this stage it looks as though we could end up under a colder north westerly flow later next week, possibly bringing sleet or snow showers to northern regions and dry and chilly conditions further south. There’s little sign of blocking high pressure forming and persisting to the north west, so the colder conditions probably only lasting a few days before a return to average temperatures with more cloud and rain in the north. The sort of thing I have in mind is shown on the GEFS6z Perturbation 14 run, with low pressure close to Scotland pulling down colder air across the UK. The next chart in the sequence (you can freely view on the Chart Viewer page) shows high pressure collapsing over the the UK with the cold flow being cut off.
Does this set the pattern for the winter ahead? There have been some very colourful reports about the coming winter in some of the media suggesting a big freeze, but equally I’ve seen a number of pundits on the web calling a mild winter for western Europe with what to me is a surprisingly high degree of confidence. You’ll be able to freely read the TWO winter forecast covering December, January and February later this week.
Mon, 25 Nov 2013 13:06:26
Winter forecast news and short term
You’ll be able to read the TWO winter forecast later next week for a longer term view but in the shorter term it’s not looking particularly cold. During the week ahead temperatures are likely to pick up for a while as milder air from the Atlantic flows over the top of the high pressure area and down across the UK. The warmer air shows up on the GEFS plot below with the air mass at about 1500m above sea level over the UK becoming about 8C warmer than it currently is. The full increase may not be may not be reflected down at ground level during the days due to increased cloud, but I’d not be surprised to see daytime temperatures to be in double figures by Wednesday or Thursday.
By next weekend another pulse of chillier air will probably be returning, but again it will probably be transient before things warm up again. So a very average outlook in terms of temperature. Precipitation is less certain but there are some tentative indications of pressure falling in early December, so that could mean rain becoming more widespread as a more zonal and mobile pattern returns. On this point I’ll sit on the fence for the moment because I’d like to see more evidence from the computer models before making a call. To an extent the weather seems to be in no man’s land at the moment and with pressure remaining low over the North Pole the odds are probably stacked against cold weather becoming persistent in the UK.
Sat, 23 Nov 2013 17:54:26
High pressure rules the roost
Looks like there’s a lot of dry weather to come in the next couple of weeks, especially in the south with high pressure remaining close to the UK. The GEFS plot below shows the forecast Mean Surface Level Pressure (MSLP) for the next 16 days in southern England. The white line as usual shows the average of all the runs, and after the next couple of days it doesn’t fall below 1020mB. A few on the individual runs do bring in lower pressure, but the signal for a spell of high pressure dominated weather at least in the south quite strong.
What about temperatures? The GEFS plot below shows the forecast maximum temperatures for the next 16 days in southern England. There’s no sign of anything unusually cold or mild, but on balance probably a little below average is the most likely outcome in the south over the period as a whole. However, things become a little uncertain during the last few days of November, largely because some of the runs show milder air from the Atlantic spilling over the top of the high pressure and down across the UK. A reduced frost risk and a lot of murky conditions would be what I’d expect if this is correct.
Could we be heading into the freezer in the next couple of weeks? There’s no suggestion of that happening when accepting the computer model output at face value, but very cold air could become embedded across parts of Europe, possibly into western Germany. If that happens and high pressure centres itself further west in the mid-Atlantic we could begin to see more of a continental influence causing temperatures to fall again, especially in the south east.
Thu, 21 Nov 2013 14:05:25
Cold spell – UPDATE 3
Yesterday I said in my Buzz post “At this point I’m slightly more inclined to go for the first option with cold lasting until next week, but then temperatures returning towards the average in early December. ” Today I think the pendulum has swung further towards this outcome. What’s the evidence for this?
In my Buzz post yesterday I discussed the very cold ECM operational run and the suggestion of it leading to a long cold spell, but how this was at odds with the GFE/GEFS computer model. Since then we’ve had 2 more operational runs of the ECM model and both of these have moved closer to the solution which the GFS/GEFS favours. This is high pressure sitting close to the UK for a while and then probably sinking slowly into the continent. What that means for our weather is cold and dry conditions later this week (once we’ve got the low pressure area out of the way by – see the TWO homepage for more info on this) and into next week, but then a gradual warm up in the north west with more changeable conditions developing. The milder air then probably gradually sinking south across the UK as we head into December.
The latest GFS/GEFS plot below shows this pattern and if anything it is more pronounced than it was at this time yesterday. There are still a few very cold runs on the plot, so the outcome can’t be discounted, but on balance the signal for a warm up is quite strong. What could stop this?
The best chance of it remaining cold would be for high pressure to bubble up again in the Atlantic and build towards Greenland, preventing the mildish west or south westerly flow returning to the UK. This could happen, but it looks considerably less likely than a gradual warm up.
Tue, 19 Nov 2013 12:26:09
Cold spell – UPDATE 2
Colder air is moving down from the north and during the next few days I expect many locations in the UK to see their first falling snow of the season. Accumulating snow is also likely, but more especially over higher ground and in the north of the country. The site homepage and forecasts will give you the latest information on the short term outlook, but what about the longer term? There’s reasonably good agreement now between the computer models for below average temperatures to continue into next week with high pressure remaining close to northern Britain. This would mean cold and calm conditions for most of the north with a brisker east or north easterly flow continuing across southern and central regions, probably bringing wintry showers in from the North Sea. Beyond this things become more uncertain.
The American GEFS/GFS computer model shows quite a wide range of possible outcomes by early December, but on balance temperatures look like returning closer to average with rainfall amounts increasing as low pressure starts to influence the weather more. This is definitely one possible scenario but it looks to be somewhat at odds with the European ECM model which is keener on keeping high pressure just to the north of the UK for longer, hence keeping it colder.
The overnight ECM operational was very interesting, showing the potential for a long cold spell to develop across much of western Europe and last well into December. The chart below plots the ECM operational run forecast 500hPa heights over the arctic in 10 days time. Scandinavia, Iceland and Greenland are visible on the bottom half with the UK just off the map. The key thing to note is it shows higher heights (high pressure) building over the arctic, or northern blocking as it’s often called. This displaces the cold arctic air down towards mid-latitude regions such as western Europe and is usually the cause of the longest and severest cold spells in our part of the world. If the ECM operational run is right and this is the outcome in 10 days then I’d expect a sharp and probably quite lengthy cold spell. Certainly more notable than the scenarios most of the GEFS/GFS runs are going for.
In summary colder than average conditions are almost definite to last this week and into next week. What happens in the longer term is open to debate. We could see high pressure gradually slip south east into the continent with milder air returning to northern parts of the UK initially and then slowly spreading south. Alternatively there’s a chance of pressure rising strongly over the arctic, causing a lengthier and probably sharper cold spell. At this point I’m slightly more inclined to go for the first option with cold lasting until next week, but then temperatures returning towards the average in early December. Nonetheless, well worth watching very closely during the next few days because the pendulum could swing more towards the second option.
Mon, 18 Nov 2013 13:27:04