Model Output Discussion - cold spell to end Autumn

[jvenge]

1 minute ago, Mark Parsons said:

Maximum temps are as they are maximum , it may only be for an hour during the day or not even reach that max forecast , it doesn't necessarily mean all day........to much emphasis on this I think, we have an  evolving synoptic pattern we have been starved of which more interesting than worrying about the temperature........ 

Actually from a model output point of view, they will give a  min value and max value for say 12pm Friday. Sometimes there might be only a degree between the min and max, sometimes it can be up to 4 or 5 or so, but this uncertainly range usually narrows nearer the time. I don't pretend to know the difficulties involved.

Some output will then just give a figure, but this usually just ends being the same as the max figure. Different forecasting services will then put out a figure and that is often why you can see forecasts giving differing temperatures. Some will opt for the min, some the max. Some will just shoot for the middle.

 

[SMU]

& just to add to BAs comparison here is the GFS mean V the ECM op- 

 

so that 20% cluster looks to flat-

 

The slider is taking on the right shallow shape now - historically the Midlands do well here although Dec 81 saw the allignment further south..

S

[N/A]

Still the GEFS 6z looks cold to start, then average, then slightly colder than average again but becoming more unsettled and also quite wet at times for Swansea & Pembroke if these charts were to be correct

[carinthian]

8 minutes ago, s4lancia said:

Cheers C. Any thoughts / clues on the following week yet from the portal?

A couple of weeks ago I booked up the 2 weeks either side of Christmas in the lower Porte Du Soleil, on the back of the impressive start to the season and how things were looking for this period. Normmally a very risky period for skiing in the lower Northern Alps, but seems like the right call now

Hi, a bit out of range for that period but the longer term synoptic charts look very encouraging. J10 provides a very good forecast on the Snow and Mountain thread which he updates regularly and I am sure if you PM him in about 10 days he should give you a detailed snow report or expectancy. Anyway , have a nice snowy Christmas Holiday !

 C

[Frosty.]

Meanwhile, the first snow warnings have been issued which ties in with Gfs 6z which shows plenty of snow showers piling in across western, northern and eastern areas in particular and followed by frontal snow events..it's going to be fun..much colder wintry spell will soon be here!

A cold ramp from frosty?..nah

Edited December 5, 2017 by Frosty.

[Iceaxecrampon]

55 minutes ago, Daniel* said:

One thing for certain somewhere in the country, is going to get a pasting. Place your bets..

The Pennines...........

Reload on Wednesday 

GFS output never dull - it's going to be an interested weekend to start.

[karlos1983]

The mean low 2mT peak around the 16th Dec on the ens. But generally very cold all the way out to D15.

small cluster go mild from the 17th, so will keep an eye out on future runs to see if these gain any further support. Majority stay sub 5c 2m temps. Look at the rogue cold run, very tasty ? 

[Chesil View]

Just now, bluearmy said:

The good news from the eps clusters is that the op cluster (around 20%) is too flat  and north compared to others. The control cluster (45%) is better than the op but still not quite as good as it needs to be but at this stage there is less of a step back from the largest cluster to where we need to be to bring a decent snow event to a large part of the country 

Quite so Blue. As Fergie implied yesterday much to much spread on the track of the sliders to make even a national forecast at the moment let alone a regional one.

[Catacol]

2 hours ago, Froze were the Days said:

Yep nice succinct post from TEITS (welcome back)...that about sums it up currently. I too tend to agree of the flattening of the pattern towards the second half of December.

Balance always good. As an aside, however, action in the pacific is suggesting less of a flat pattern than was believed to be approaching from mid December by many a reliable pundit. Relative AAM is surprisingly pretty high backed up by decent global mountain torque, and there appears to be quite a coherent push of convection from the western pacific that will assist in keeping the MJO in a high enough orbit to impact on poleward energy.

I think we are already seeing the influence of the Canadian trough on the production of systems as per standard Nina climatology that many predicted would happen come December... but the track of these lows may continue to throw some surprises into the mix as the pattern may not flatten as much as in a "normal" Nina state. Add to this suggestions of more trop driven strat disruption and the menu for later in December has altered in substance.

Much to be interested with at the moment - possibly the most interesting combo of near model analysis accompanied by distant teleconnective interpretation for many a year.

Edited December 5, 2017 by Catacol

[Iceaxecrampon]

7 minutes ago, Frosty. said:

Meanwhile, the first snow warnings have been issued which ties in with Gfs 6z which shows plenty of snow showers piling in across western, northern and eastern areas in particular and followed by frontal snow events..it's going to be fun..much colder wintry spell will soon be here!

A cold ramp from frosty?..nah

I take it they are METO warnings Frosty? - Cumbria only had isolated snow showers forecasted earlier on.

[karyo]

Just now, Catacol said:

Balance always good. As an aside, however, action in the pacific is suggesting less of a flat pattern than was believed to be approaching from mid December by many a reliable pundit. Relative AAM is surprisingly pretty high backed up by decent global mountain torque, and there appears to be quite a coherent push of convection from the western pacific that will assist in keeping the MJO in a high enough orbit to impact on poleward energy.

I think we are already seeing the influence on the Canadian trough on the production of systems as per standard Nina climatology that many predicted would happen come December... but the track of these lows may continue to throw some surprises into the mix as the pattern may not flatten as much as in a "normal" Nina state. Add to this suggestions of more trop driven strat disruption and the menu for later in December has altered in substance.

Much to be interested with at the moment - possibly the most interesting combo of near model analysis accompanied by distant teleconnective interpretation for many a year.

The latest enso update looks promising with an increasing emphasis of the coldest waters in east Pacific while the west is warming up a bit (East based La Nina).

http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/lanina/enso_evolution-status-fcsts-web.pdf

[Matthew.]

3 minutes ago, karyo said:

The latest enso update looks promising with an increasing emphasis of the coldest waters in east Pacific while the west is warming up a bit (East based La Nina).

http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/lanina/enso_evolution-status-fcsts-web.pdf

It just goes to show that we don’t need weak to moderate El Niño for cold weather in winter::

[feb1991blizzard]

1 minute ago, Matthew. said:

It just goes to show that we don’t need weak to moderate El Niño for cold weather in winter::

Some of the most stonking snow events have been with either very weak ENSO or enso neutral, I would far rather have enso neutral than strong ENSO signals either way.

[Weather-history]

Crikey how many occluded fronts!

[Polar Maritime]

NetW-wx in house Model take for Friday.

 

 

[Polar Maritime]

Can we please use the correct threads for meto forecasts.

[Craig84]

Hi everyone.

I've been reading this forum a good few years now and I'd like to say thanks to all the regular posters who contribute to this great place for weather geeks!

Thought I was about time I contributed!

Im terms of potential for later in the month, I'm really encouraged by the 06z GEFS. 80% are attempting to build heights NE by the 20th. Shown by the mean charts below

 

 

Way into FI but it seems a strong signal and one to watch.

 

[Man With Beard]

To add to BA's comments on the clusters - you can see how the tiny difference in heights to the NW at T120 makes an enormous difference by T168:

    

You'd have to say the op is the outlier - all the others have a much better slide. But then it is the op so can't bin it at T120...

By D10 the op is back on board with the pattern, which is to bump up the Atlantic ridge again (very good continuity from previous 5 sets of clusters):

... and back to the one cluster FI, which persists with a shallow area of heights to the north:

It will be very very bad luck if at least one spell of inland snow does not come of these charts - considering we've had a couple of favourable-ish weeks already.

Edited December 5, 2017 by Man With Beard

[karlos1983]

3 minutes ago, ShortWaveHell said:

Does a northern version of that chart exist ?

not to my knowledge SWH sorry. Here's the link I use.

http://www.weathercast.co.uk/services/ensemble-forecast.html?eps=london 

updated twice daily at around 11:30am and 11:30pm 

[Derecho]

So a lot of posts mentioning a slider but what is a slider? and why is it mentioned so much?

Well newbies let me try to explain.

So on Friday we have a very cold North-westerly over the UK, leaving it in a cold air mass.

Milder air over the Atlantic however is moving east towards us, leading us to this point



Coming up against our cold pool, we see a fight put up against the milder air. From here things get really complicated as tiny changes really have a profound impact on UK snow potential. The path the low pressure takes appears now dependent on the strength of the low shown below.



In the first scenario, the low is shallow and can't put up much of a fight against the colder air, therefore sinks much further south and not really displacing the cold air mass much. This would leave the south or the midlands in the firing line for snow.

The right hand depiction shows the low a bit deeper and is therefore able to push against the cold harder. Scotland would be more in the firing line here. As for what I think will develop... well difficult call but the signs are at may cross the centre of the UK. This could very easily change though given its a couple of days away. Temperatures would be a little colder then average still but we would see cold rain for many.

Now lets remind ourselves of what the general pattern is



Low pressure zipping from NW to SE, how do the scenarios play out potentially? Most likely options

20%                                                             40%                                                         30%

  

Now all in my opinion are equally plausible, given northwards ridging seems to be a theme so far I'm leaning towards the middle option. However another scenario which may appear (though less likely):

10% (should say cold air from E Greenland)



A less likely option admittedly but a plausible one still, keep an eye out on it, quite similar to the middle option but we get uppers that would be a bit colder.

So in the end a tricky one for forecasters, too early to ramp but too early to dismiss. Lets see what happens.

Edited December 5, 2017 by Quicksilver1989

[Frosty.]

Once all the occluded fronts are out of the way, the fax charts look increasingly gorgeous for coldies as a strong arctic blast is drawn south along with trough lines swinging south in the increasingly bitter, clean arctic air with snow showers becoming frequent and heavy at times across western, northern and then eastern areas with frosts becoming widespread and sharp, especially over the snow fields!..a cold ramp from frosty?..nah mate its what's being indicated

Edited December 5, 2017 by Frosty.

[Methuselah]

11 minutes ago, Frosty. said:

Once all the occluded fronts are out of the way, the fax charts look increasingly gorgeous for coldies as a strong arctic blast is drawn south along with trough lines swinging south in the increasingly bitter, clean arctic air with snow showers becoming frequent and heavy at times across western, northern and then eastern areas with frosts becoming widespread and sharp, especially over the snow fields!..a cold ramp from frosty?..nah mate its what's being indicated

As lovely as those fax charts are, Karl, they beg an awkward question: why are eastern parts not included in the MetO's warnings?

[Paul_1978]

People say this is no 2010, which is quite correct, but surely we are in a prime position for cold and snow over the coming weeks, if perhaps not as severe as that famous winter.

So don't be disheartened. To be honest, if we can get a repeat of the events Dec 1990 and Feb 1991 alone then that would be a great winter!

So whilst I won't be looking for a 2010, I'll certainly be looking for a one off "big" snow event. Surely we've got a higher chance than normal this winter. 

[Man With Beard]

A question being asked on here - do sliders always correct SW? Not always, it isn't as simple as that. 

The general pattern on modelling history is that if a block is in place to the east, then yes, the slider does usually correct further SE than modelled even at D5/D6. The well-worn example of January 2013 exemplifies. Just 24 hours before, the incoming low looks very unlikely to slide - but notice the large area of high pressure to the east. The low to the west hit the block and, aided by extra energy going under the main trough, goes much further SW than one would expect. Famously, the UKMO picked this up first at T120, was considered bizarre at first, but the others then picked it up at T72/T96:

   

But, when there is not a block the the east, the models do not seem to make this error, and may indeed overdo things the other way. A month before in December 2012, I recall the set-up shown below to predicted at D5/D6 as a slider - but eventually the models all backtracked and phased the lows to the east and west and let the mild in to the south:

  

The key difference was - no strong block to the east.

If this is to repeat now, you'd have to say the slider might not correct SW - because we do not have that eastern block

[Barking_Mad]

20 minutes ago, Quicksilver1989 said:

So a lot of posts mentioning a slider but what is a slider? and why is it mentioned so much?

Well newbies let me try to explain.

So on Friday we have a very cold North-westerly over the UK, leaving it in a cold air mass.

Milder air over the Atlantic however is moving east towards us, leading us to this point



Coming up against our cold pool, we see a fight put up against the milder air. From here things get really complicated as tiny changes really have a profound impact on UK snow potential. The path the low pressure takes appears now dependent on the strength of the low shown below.



In the first scenario, the low is shallow and can't put up much of a fight against the colder air, therefore sinks much further south and not really displacing the cold air mass much. This would leave the south or the midlands in the firing line for snow.

The right hand depiction shows the low a bit deeper and is therefore able to push against the cold harder. Scotland would be more in the firing line here. As for what I think will develop... well difficult call but the signs are at may cross the centre of the UK. This could very easily change though given its a couple of days away. Temperatures would be a little colder then average still but we would see cold rain for many.

Now lets remind ourselves of what the general pattern is



Low pressure zipping from NW to SE, how do the scenarios play out potentially? Most likely options

30%                                                             30%                                                         30%

  

Now all in my opinion are equally plausible, given northwards ridging seems to be a theme so far I'm leaning towards the middle option. However another scenario which may appear (though less likely):

10%



A less likely option admittedly but a plausible one still, keep an eye out on it, quite similar to the middle option but we get uppers that would be a bit colder.

So in the end a tricky one for forecasters, too early to ramp but too early to dismiss. Lets see what happens.

Thanks for this. I struggle to get my head round this in a flat top down view as it's just a 1D image. How does all this look if you were to draw it from a side on 2D view?