Winter Model Discussion - Cold Hunting -12Z 06/01/13

[TEITS]

For the moment all im bothered about is the chart below.

http://cdn.nwstatic.co.uk/ensimages/ens.20130107/06/t850Cambridgeshire.png

The mean was at -4C a few days ago but this has continued to drop and now stands at -8C on the 14th for my region as more ensembles come into line.

[MR EXTREMES]

Bit hard to make sense of the models at the moment. ECM is OK but I can't see that easterly surviving the onslaught from the energy to the NW at +240. The ECM ensembles have also downgraded the big high pressure anomaly over Greenland, quite a bit weaker than the past few.

But looking at the OP runs and the ensembles at just +144 they're all over the place so I don't think I'll even bother beyond then.

Luckily in the nearer time frame there's some possible snow events to talk about, this weekend could be interesting

GFS +120

GFS control, another low

There could several possible snow events as the Atlantic moves against the cold air trying to come from the east and not in far FI either, so there's plenty of interest in the nearer timeframe so I wouldn't get too bothered further out because nobody knows with the models all over the place.

this is a good chart.

[WhiteFox]

I don't see why that should be the case. I'm pretty sure none of the clued up guys on the strat thread have ever said a SSW automatically leads to a wintry spell for the UK. What has been repeatedly stated is that a SSW increases the liklihood of HLB which in turn offers chances for Arctic air to reach lower lattitudes. Whether it hits us or not is another matter.

Indeed. I saw the strat temperature chart for 2009/10 posted a couple of days ago and can't find it (buried as it is amongst much gnashing of teeth and wailing) in one of these threads.

I remember reading in The Times (I think) not long after the January 2010 cold spell had ended about the possibility that winter was not yet done due to a SSW event taking place. The SSW did take place, but nothing prolonged and severe resulted for the UK. If you look at archive charts from mid January 2010 onwards you will see lots of blocking and a largely non-existent PV but never in quite the right place to deliver sustained cold and snow for most of the UK. My recollection of mid-January 2010 through to the end of February is that it was chilly or even cold at times, but once the snow from the early January spell had melted there was no more although we had frequent frosts.

Assuming I'm right about the SSW that January (I think I am, if not then ignore and delete this post!), then take a look through those charts to see how SSW can get us very close to ideal conditions, but does not guarantee.

[Stu_London]

Modelling actually looks a lot more solid this morning. The GFS ensembles on the 6z are pretty tight up to the 13th January. There is reasonable cross model agreement at 120 between GFS and ECM (UKMO is a bit different but is also a bit different at +72 also)

My take on things is that a shallow cold spell is likely starting later this week for 5-6 days - this will be followed by a partial breakdown. After this, and taking into account the MO thinking and the effects we should see after a SSW, there is a chance of a more significant longer lasting severe spell, however usual caveats about the multiple ways these have of going wrong for the UK in it's location mean I wouldn't give it more than about a 35% chance of happening at the moment.

If it is going to happen, we will start to see some severe solutions appear on the modelling after +240, but without any consistency. Ensembles means and medians will provide the best guide.

[Weathizard]

this is a good chart.

Indeed, shows the ECM mean still backs a blocked Atlantic, whether or not we are in the cold or not to me is a big question because it will be about how the high orientates and whether it breaks free from the azores/ridges north. I personally don't for a second believe we'll be back in a zonal pattern any time soon, high pressure no mans land for a while or cold from the east.

I've got that feeling as well that this cold 'snap' won't be as short lived as its being projected at the moment. Just that gut feeling you sometimes get!

[Bobby]

this is a good chart.

Yes it's pretty good just the previous ones were better. Looks like the Atlantic is trying to creep back into the SW by then, too big LP anomaly for my liking. But anyway, well into FI so not point bothering much about that either way.

The NAEFS has less LP over Europe.

Good to see agreement on heights over Greenland

Edited January 7, 2013 by Bobby

[Timmytour]

I don't see why that should be the case. I'm pretty sure none of the clued up guys on the strat thread have ever said a SSW automatically leads to a wintry spell for the UK. What has been repeatedly stated is that a SSW increases the liklihood of HLB which in turn offers chances for Arctic air to reach lower lattitudes. Whether it hits us or not is another matter.

The strat thread has offered us some hope during seemingly endless dross churned out by the models for the last few weeks and importantly I'm amazed by the accuracy of some of the predictions. Hopefully we will get lucky with the cold reaching us but if not, I don't think it should take away, firstly, from the great analysis we have seen on the strat thread and secondly the importance of SWW's on the weather in the northern hemisphere.

Hi NS..... I only think it would be the case cos that's how things seem to roll in here at times....especially when toys are being dislodged from the pram!

Interesting you say the strat thread offered hope when there was none....I don't disagree with this, but in relation to my previous post, from my understanding of things, the models bought in the prospect of a cold spell before any SSW affect filtered down into them.

A personal and higly amateurish conclusion of mine is that the SSW provides us with the potential of variance....it provides us with opportunities of cold shots which come and go as displcements from the PV get shunted around. In a zonal set-up therefore, I can imagine it being very welcome. However if it had occured in a set-up more akin to 1947 (not that I'm saying we are in one and not that I know whether or not it did)) it might have turned that winter into one less memorable, with only repeating periods of cold weather instead of the sustained kind.

Who knows... we might end up thinking that everything was looking rosy for a good sustained spell of winter until the full imapct of the SSW was felt!

But that's just a consideration...not a belief

[Weather Boy]

I think that there is a misconception that SSW's don't give quick tropospheric results but that is wrong - of course they do. The current pattern forecast is a direct response to the SSW and the vortex splitting will be a direct response as well. I have previously described the scenarios that we can get direct and indirect responses to a SSW and that still holds true.

A direct response can be seen in a vortex split. Imagine chopping a log with an axe. If you connect correctly at the top of the log then you can split it into two with one fell swoop. There is no time lag between the log being hit at the top and splitting at the bottom. In the same way the polar vortex being split will instantly affect the troposphere (displacements are less clear cut but the theory remains the same).

So, that brings us on to the indirect response (which is not immediate) following a SSW. Here we see the destroyed stratospheric vortex having a longer term effect on the tropospheric vortex over the next 6 weeks or so. With no upper strat driver the tropospheric vortex will meander through this period allowing blocking to occur. As the stratospheric vortex slowly reforms at the top of the stratosphere, the mean zonal negative winds will be pushed downwards during the following weeks until the whole stratospheric vortex reforms and it is back to its expected strength. And in that timeframe we have time for the downwelling of the negative mean zonal winds to produce that indirect response.

Thanks for correcting my (mis)understanding of the point, Chio. However, I assume from your silence on the issue that you agree with my central point which is that once you have a disrupted vortex, the presence or absence of a SSW does not make deep cold or snow any more likely than the same situation but without a SSW?

It seems to me that, if you are right, then the form horse is that (going from the models) in the short-medium there will be an easterly which may bring snow, but that may not be particularly long-lasting, perhaps giving us some less cold weather for a time before further fun and games begin after the 20th.

Edited January 7, 2013 by Weather Boy

[Ruzzi]

Indeed, shows the ECM mean still backs a blocked Atlantic, whether or not we are in the cold or not to me is a big question because it will be about how the high orientates and whether it breaks free from the azores/ridges north. I personally don't for a second believe we'll be back in a zonal pattern any time soon, high pressure no mans land for a while or cold from the east.

I've got that feeling as well that this cold 'snap' won't be as short lived as its being projected at the moment. Just that gut feeling you sometimes get!

I agree, the models (especially the gfs) tend to be a bit too progressive with an atlantic breakdown once we get that cold chunk of air established over the British isles and western europe. Anything could happen at the moment though. It's probably the most interesting period of model watching that I can remember.

[Village]

As I said last week, I really think that we are looking at dry or dryer episode now for weeks even months.

The models, all models in my opinion do seem to be showing this for a great deal of the country. With none showing anything zonal.

Therefore, as temperatures fall next week, any precipitation of the solid kind wont amount to much. But as I indicated last week the southeast will be the place for the coldest air and the highest precipitation in the form of showers or frontal as the arctic front slips south down the North Sea.

[chionomaniac]

Thanks for correcting my (mis)understanding of the point, Chio. However, I assume from your silence on the issue that you agree with my central point which is that once you have a disrupted vortex, the presence or absence of a SSW does not make deep cold or snow any more likely than the same situation but without a SSW?

It seems to me that, if you are right, then the form horse is that (going from the models) in the short-medium there will be an easterly which may bring snow, but that may not be particularly long-lasting, perhaps giving us some less cold weather for a time before further fun and games begin after the 20th.

No, that is wrong as well. Because then we would be into the indirect response phase where propagation may or may not occur. There are enough studies to show the NH responses to such events. An d the indirect response could last upto 6+ weeks .

My silence is because I am at work!

Edited January 7, 2013 by chionomaniac

[phil nw.]

We still look on course for a colder spell starting around the weekend after a rather complicated changeover from the current mild setup.This fax for Thurs shows the energy starting to disrupt and undercut the building heights to our north east.

As a result by the weekend we have low pressure further south and with high towards Scandinavia we pick up an easterly flow and this looks like advectiving the colder air towards the UK.

Some decent mean outputs for early next week from the last ECM/GFS runs showing this.

backed up by some colder 2mtr ens graphs

However messy the synoptics may appear over the next few days the 500hPa pattern is heading towards a -AO longer term and we still look on course for the colder conditions to establish further with time.

[UV-RAY]

We still look on course for a colder spell starting around the weekend after a rather complicated changeover from the current mild setup.This fax for Thurs shows the energy starting to disrupt and undercut the building heights to our north east.

As a result by the weekend we have low pressure further south and with high towards Scandinavia we pick up an easterly flow and this looks like advectiving the colder air towards the UK.

Some decent mean outputs for early next week from the last ECM/GFS runs showing this.

backed up by some colder 2mtr ens graphs

However messy the synoptics may appear over the next few days the 500hPa pattern is heading towards a -AO longer term and we still look on course for the colder conditions to establish further with time.

It's a fine line between a cold spell and a cold snap judging by what the models are showing Phil, 4-5 days tops before the Atlantic wins this round. Though that isn't set in stone either, could be a bit of a non event the first bite of the cherry, or a bitter prolonged cold spell. Clear as mud really!

Edited January 7, 2013 by Sceptical Inquirer

[Jaffusmaximus]

We still look on course for a colder spell starting around the weekend after a rather complicated changeover from the current mild setup.This fax for Thurs shows the energy starting to disrupt and undercut the building heights to our north east.

As a result by the weekend we have low pressure further south and with high towards Scandinavia we pick up an easterly flow and this looks like advectiving the colder air towards the UK.

Some decent mean outputs for early next week from the last ECM/GFS runs showing this.

backed up by some colder 2mtr ens graphs

However messy the synoptics may appear over the next few days the 500hPa pattern is heading towards a -AO longer term and we still look on course for the colder conditions to establish further with time.

Messy is good! I like messy! Like dropping a glass bowl to the floor in slow motion all we can do is wait to see where the pieces will fall and settle!

[Tim Bland]

For the moment all im bothered about is the chart below.

http://cdn.nwstatic....bridgeshire.png

The mean was at -4C a few days ago but this has continued to drop and now stands at -8C on the 14th for my region as more ensembles come into line.

Indeed! Very impressive ensembles. Take the London 2m temps for example. Very few giving temps in Central London anything near 5C or above in the medium to long term. good news for snow lovers in that any snow that falls could stick around for a while!

[JP1972]

We still look on course for a colder spell starting around the weekend after a rather complicated changeover from the current mild setup.This fax for Thurs shows the energy starting to disrupt and undercut the building heights to our north east.

As a result by the weekend we have low pressure further south and with high towards Scandinavia we pick up an easterly flow and this looks like advectiving the colder air towards the UK.

Some decent mean outputs for early next week from the last ECM/GFS runs showing this.

backed up by some colder 2mtr ens graphs

However messy the synoptics may appear over the next few days the 500hPa pattern is heading towards a -AO longer term and we still look on course for the colder conditions to establish further with time.

Thanks Phil for this summary.

Exciting times ahead it would seem.

[phil nw.]

Yes the when you think the jet is weakening all the time and it`s a good sign that the Atlantic is losing it`s momentum when you see those fronts stalling and some undercutting at the end of the week.

We have to give the pattern time to setup and it will rake a few days for the fog to clear.

Good Ht anomls still for day 10

Looking longer term hts more towards Iceland but more importantly low Hts further south.

[Frosty.]

Well although the synoptics look messy, the meto currently say next week will be cold with wintry showers and frosty nights, perhaps a risk of something milder at times in western britain, I assume that has something to do with the influence of the azores high/ridge. I noticed the further outlook, on balance looks colder and drier which would indicate a cold and anticyclonic pattern for late jan and early feb, a lot will depend on where the high ends up as to whether we can tap into icy air from the north and east for a sustained period but considering the boring mild weather we have had in the last few weeks, there is now a very good chance we will get the type of weather associated with january, about time too!

[Methuselah]

Everything about 8" East at T+24. Definite downgrade. Winter is over!

[124alex]

Everything about 8" East at T+24. Definite downgrade. Winter is over!

still February!

[snowflakey]

I like to view the gfs ens in Wetter, as it shows the average line, and by the 13th all members are below it.

http://www.wetterzen..._London_ens.png

[Village]

I guess all eyes should go to Poland surface temps on Friday as a guide to what will be in the southeast on Sunday undercutting behind the polar front

[No Balls Like Snow Balls]

I like to view the gfs ens in Wetter, as it shows the average line, and by the 13th all members are below it.

http://www.wetterzen..._London_ens.png

I like to view the GFS Ens whilst wearing a mankini

[Stu_London]

12zs about to roll out.

What we should be looking for.

I think the pattern between now and +120 has firmed up a lot overnight and I wouldn't expect too many changes in that time frame (although you couldn't rule it out) - leading to an easterly of sorts by the weekend with some snow risk, although not too severe at present.

Start of FI now looks to be 120-168 (back where it should be) - in this period we need to be looking for shortwave development and any transfer of heights to favoured places.

Beyond 200 - I'd say a 40% of the GFS operational showing something particularly juicy - in fact I would be willing to bet within the next couple of days the GFS will provide on at least one occasion a stellar FI with widespread long lasting severe winter sypnotics.

[BlueHedgehog074]

Everything about 8" East at T+24. Definite downgrade. Winter is over!

Maybe we'll see a mega huge upgrade by 72 hours with a powerful Easterly of uppers around -25*C in places. ;D

(Edit: Darn it! The uppers didn't even get below 0*C for most places as 72 hours unfolded (lol).

Edited January 7, 2013 by DiagonalRedLine