Model Output Discussion - 28th November - 4th December

[bluearmy]

both naefs and ecm ens mean spreads both have a shortwave off the nw coast of scotland end next week as pressure also tends to build from the sw. given the possiblitlies of some stormy conditions at the end of the weekend, it looks as though the wind is indeed going to be the story over the next 10 days

[MisfitDog3]

Interesting situation developing for the weekend! I'm not surprised the media forecasts aren't going beyond Thurs/Fri at the moment. Just out of interest, and with particular reference to todays events, are there any areas on the website which specifically deal/discussion of effects as cold fronts move through? I love the drama of a good cold front, temp drop, strong winds etc.

[Milhouse]

No sig changes in the overnight runs, with wet, windy and even occasional stormy weather being the main features. Also looks like we should see a couple of chilly days, but overall temps look like being pretty close to average, perhaps rising into the mild category again for most later next week if as expected, winds swing back towards the southwest.

A fair assesment there. 3 or 4 days of blustery showers rattling in on a brisk northwesterly, but there should also be some good sunny spells inbetween, which should lead to overnight frosts but nothing too severe. Temperatures will definately be in the cold category but tending to rise later on as high pressure nudges in from the SW.

[shedhead]

The 06 does offer up the prospect of a few colder days, but with the main one next Monday still almost a week away it's to early to assume the 144hr will verifty as currently shown, in fact history suggests

the cold will get watered down and shunted further east as the models start to better handle the inevitable shortwave developments as we get closer. If nothing else though, the chasing of a temporary

pocket of -5c 850Hpa air at 6 days hence really does show how dire things have been recently. Surely the only way is up... isn't it??

Edited November 29, 2011 by shedhead

[nick sussex]

The 06 does offer up the prospect of a few colder days, but with the main one next Monday still almost a week away it's to early to assume the 144hr will verifty as currently shown, in fact history suggests

the cold will get watered down and shunted further east as the models start to better handle the inevitable shortwave developments as we get closer. If nothing else though, the chasing of a temporary

pocket of -5c 850Hpa air at 6 days hence really does show how dire things have been recently. Surely the only way is up... isn't it??

Perhaps eventually!

The pattern is likely to flatten out and the Azores high looks like it will be edging ne'wards, it's just a case then of how far north it gets.

We may get a surface inversion for some areas if it can edge far enough ne, it's really a case of how flat the upstream pattern will be come the time.

[shedhead]

Perhaps eventually!

The pattern is likely to flatten out and the Azores high looks like it will be edging ne'wards, it's just a case then of how far north it gets.

We may get a surface inversion for some areas if it can edge far enough ne, it's really a case of how flat the upstream pattern will be come the time.

I was just trying to be a bit positive Nick.... I think most are happy to see a return to average temps and wild weather, with at least the chance of some high level snow at times, but

a week or so of this pattern will probably be more than enough. Looking longer term there does appear to be a pretty strong signal for rising pressure, so as you say perhaps some hope

that it will set up sufficiently far north to give us a chance of some inversion cold, but that looks far less likely than a return to mild SW'erlies as things stand.

[January Snowstorm]

Not a great outlook really.

A 24 hour cold spell is what shows to me next Monday, which as others have said will pbly get watered down closer to the time.

All in all a far cry from the last 2 Winters, sofar

[nick sussex]

I was just trying to be a bit positive Nick.... I think most are happy to see a return to average temps and wild weather, with at least the chance of some high level snow at times, but

a week or so of this pattern will probably be more than enough. Looking longer term there does appear to be a pretty strong signal for rising pressure, so as you say perhaps some hope

that it will set up sufficiently far north to give us a chance of some inversion cold, but that looks far less likely than a return to mild SW'erlies as things stand.

Yes theres not alot we can do about the current state of the stratosphere, this temporary amplification of the upstream pattern is delivering something more interesting for the next week to ten days.

After that theres a solid trend across the outputs to push the Azores high ne, if you can get it far enough north and orientated favourably then you could see a more continental surface feed into more especially southern and se areas.

We'll just have to wait and see what the models do when it comes nearer the time.

[Frosty.]

The latest models are now screaming cold snap for the north from the weekend through to the middle of next week with possible reloads although the 6z doesn't suggest a reload. The cold and wintry snap looks most likely in scotland and similar for n.ireland and n.england, a weaker cold snap for areas further south as the coldest air finds it difficult to dig far enough south. In the next few days there is also a risk of snow for scotland, mainly on hills but then less cold, wet and windy weather sweeping in off the atlantic and then much colder air from the arctic with snow showers for northern britain seems likely to last for 2 or 3 days before a cut off ridge spreads east.

Edited November 29, 2011 by Polar_ Low

[weathe20]

The latest models are now screaming cold snap for the north from the weekend through to the middle of next week with possible reloads although the 6z doesn't suggest a reload. The cold and wintry snap looks most likely in scotland and similar for n.ireland and n.england, a weaker cold snap for areas further south as the coldest air finds it difficult to dig far enough south. In the next few day there is also a risk of snow for scotland, mainly on hills but then less cold, wet and windy weather sweeping in off the atlantic and then much colder air from the arctic with snow showers for northern britain seems likely to last for 2 or 3 days before a cut off ridge spreads east.

cold snap is a good description, so is 'blink and you will miss it'.

[Backtrack]

cold snap is a good description, so is 'blink and you will miss it'.

Hardly 'blink and you will miss it' it's a 3-4 day event..

[chionomaniac]

It is not a cold snap that would concern me presently, but the high likelihood of a damaging short wave off the main depression next Sun/Mon. The conditions are ripe (Jet stream position and strength alongside increasing thermal differential on backend of major depression) for this to occur, though the models may not get a grip until later as shortwaves can be difficult to forecast.

Any shortwave would hit much further south leaving the most populated area of the country in its path. I thought that this period of strong zonal conditions may have come slightly earlier in the month but am not surprised now that they have arrived. How long will they last?

[Frosty.]

cold snap is a good description, so is 'blink and you will miss it'.

it all depends on location, for example in scotland the cold air sweeps in on saturday and doesn't depart until next wed/thurs and with the chance of another swipe of cold air after a milder interlude although the 6z just ends it with a large mild sektor following, possibly a mild outlier for that particular period.

[weathe20]

it all depends on location, for example in scotland the cold air sweeps in on saturday and doesn't depart until next wed/thurs and with the chance of another swipe of cold air after a milder interlude although the 6z just ends it with a large mild sektor following, possibly a mild outlier for that particular period.

The predictied strong winds from that short wave is the interest. Some wintry mix for the north too, but nothing to get excited about, just normal UK winter stuff.

[Frosty.]

The predictied strong winds from that short wave is the interest. Some wintry mix for the north too, but nothing to get excited about, just normal UK winter stuff.

I had forgotton how good normal winter stuff can be after the most boring autumn I can remember. I realise that for most of the uk this cold snap won't amount to anything but the northern half of scotland will have a real taste of wintry weather with snow and high winds as long as the models don't downgrade much.

[shedhead]

The latest models are now screaming cold snap for the north from the weekend through to the middle of next week with possible reloads although the 6z doesn't suggest a reload. The cold and wintry snap looks most likely in scotland and similar for n.ireland and n.england, a weaker cold snap for areas further south as the coldest air finds it difficult to dig far enough south. In the next few days there is also a risk of snow for scotland, mainly on hills but then less cold, wet and windy weather sweeping in off the atlantic and then much colder air from the arctic with snow showers for northern britain seems likely to last for 2 or 3 days before a cut off ridge spreads east.

As has been previously stated, looking ahead beyond T+96hrs and expecting a Pm incursion to be as potent or last the amount of time currently being shown is fraught with danger. What originally looks like a potential 3-4 day spell more often than not get whittled down to a 24-36hr event and in some cases can virtually disappear altogether over England and Wales as shortwaves develop and track east.

Edited November 29, 2011 by shedhead

[nick sussex]

The model bias of the GFS has been noted by forecasters in the USA.

Prediction of southward progression of cold air over done

Model a bit too extreme in temp patterns beyond 84 hours

Precip Type Algorithm off of GFS too eager to depict snow

Although these are generally in relation to the USA the same biases would be evident globally.

[Northern Sky]

The model bias of the GFS has been noted by forecasters in the USA.

Prediction of southward progression of cold air over done

Model a bit too extreme in temp patterns beyond 84 hours

Precip Type Algorithm off of GFS too eager to depict snow

Although these are generally in relation to the USA the same biases would be evident globally.

The tendency of the GFS to overdo the southward progression of colder air is something that has been evident in its output for a long time and many people have commented on it. This tends to be more so in its low resolution output - is this due to an inibility to correctly model the development of shortwaves at this timeframe? I was wondering, given that this seems to be a consistant feature of GFS output, is it something that can be addressed and perhaps rectified, in the programming of the model?

[Thundery wintry showers]

I agree with the comments about 3-5 day polar maritime incursions often being reduced to 24-36 hour events, but at the same time, it doesn't mean that the cold zonal interlude will be reduced to a 24-36 hour thing. It does indeed depend on location- as is normal in this type of synoptic setup, the further south you are, the more sensitive your chances are to the specific track of depressions, and therefore the more likely it is that you'll be looking at one or two 24-36 hour blasts. As Damianslaw mentioned in a thread on polar maritime snowfalls, December 1999 began with quite a similar setup, and many lowland parts of Scotland saw lying snow from it on the 3rd-5th.

[A Winter's Tale]

I agree with the comments about 3-5 day polar maritime incursions often being reduced to 24-36 hour events, but at the same time, it doesn't mean that the cold zonal interlude will be reduced to a 24-36 hour thing. It does indeed depend on location- as is normal in this type of synoptic setup, the further south you are, the more sensitive your chances are to the specific track of depressions, and therefore the more likely it is that you'll be looking at one or two 24-36 hour blasts. As Damianslaw mentioned in a thread on polar maritime snowfalls, December 1999 began with quite a similar setup, and many lowland parts of Scotland saw lying snow from it on the 3rd-5th.

This was quite an event on December 4th 1999. I think there will be a few more chances for decent polar maritime incursions and perhaps even beyond the 5th December there could still be some potential. Right now, there is colder air in behind the front that has passed over Ireland with currenty temperatures of 4-6C.

This shouldn't amount to much and late on Thursday and into Friday the potential of having the -5C line below Northern Ireland and Scotland is there with perhaps -6/-7C uppers although there may not be many showers around for any wintry precipitation.

And into the Weekend and Monday there is at the moment indications of a promising incursion but it isn't quite reliable yet. At the moment, I would say that this December has got some similarities with 1999 in some of the synoptics.

[A Winter's Tale]

http://hw.nwstatic.co.uk/gfsimages/gfs.20111129/06/147/h500slp.png

In fact that chart for December 4th 1999 is quite similar to chart for December 5th 2011:

- High Pressure in the Atlantic

- Low Pressure in the Eastern Coasts of the North Sea

- Low pressure in N. Scandinavia

- Wind direction over Scotland where it goes from a Northerly to a NWly.

Edited November 29, 2011 by A Winter's Tale

[weathe20]

The model bias of the GFS has been noted by forecasters in the USA.

Prediction of southward progression of cold air over done

Model a bit too extreme in temp patterns beyond 84 hours

Precip Type Algorithm off of GFS too eager to depict snow

Although these are generally in relation to the USA the same biases would be evident globally.

So it seems the up and coming event for Scotland will probably just fade into nothing as the GFS is being too keen.

[Frosty.]

As has been previously stated, looking ahead beyond T+96hrs and expecting a Pm incursion to be as potent or last the amount of time currently being shown is fraught with danger. What originally looks like a potential 3-4 day spell more often than not get whittled down to a 24-36hr event and in some cases can virtually disappear altogether over England and Wales as shortwaves develop and track east.

At least the models are showing something interesting after weeks of pure dross and the meto with all at their disposal, still predict low ground snow for scotland for a time but mostly on hills and even some snow to hills further south plus frosts so to assume that shortwaves will shunt the cold snap away so quickly just sounds negative, obviously with the current trend it will be more wintry further north, I feel the south will have to wait until we get an Easterly..whenever that may be.

[nick sussex]

So it seems the up and coming event for Scotland will probably just fade into nothing as the GFS is being too keen.

For Scotland I think theres a decent chance that the cold air will get far enough south so snow prospects there are much better than for the rest of the UK.

It's more marginal further south and so its here where any downgrades would make more difference.

We'll just have to wait and see how much amplification there is upstream and what the models do with that shortwave between 120 and 144hrs, for Scotland I'm reasonably optimistic about snow chances.

It's always risky saying too much in here regarding snow chances as this can cause bedlam to break out!

[A Winter's Tale]

For Scotland I think theres a decent chance that the cold air will get far enough south so snow prospects there are much better than for the rest of the UK.

It's more marginal further south and so its here where any downgrades would make more difference.

We'll just have to wait and see how much amplification there is upstream and what the models do with that shortwave between 120 and 144hrs, for Scotland I'm reasonably optimistic about snow chances.

It's always risky saying too much in here regarding snow chances as this can cause bedlam to break out!

Probably the hills in NW Scotland. Nothing further south and lower down in the most populated areas.