Model Output Discussion - cold spell to end Autumn

[Methuselah]

Even though the details are, at this stage, largely irrelevant, it's looking more and more likely that the eponymous 'building blocks' might end up where we want them:

[shaky]

8 minutes ago, carinthian said:

Evening all. Our latest snow model has updated the amount of fresh snowfall with 33cm expected on Monday for our location in the Alps. As regards to Sundays snowfall prediction  back in the UK, a shift southwards in the higher percentage range . The experts we use think the slider may well be corrected again by UKMO tomorrow before firming up on snow prediction with-in the 72 hour range. Heaviest snowfall could well be on the higher ground to the NE of the triple point associated with the low pressure . So looking at the latest UKMO fax charts , that looks like NE Wales, Southern Pennines, and West Midlands . Some snow could fall to lower elevation later on Sunday night further south as the 250mb jet  sends the slider further south with a undercut of colder air again. A small warm sector , mainly to affect the SW . Looking further ahead they also indicate the second slider could go even further south and west, so Eire and SW England at risk of snow ( currently 20%) but could increase near the time. Mid Month still favour pressure rise to the NW later to NE towards Christmas but that's a long way off and a lot of weather for you guys to enjoy/endure. Good luck for those wanting snow!

You did call this brilliantly yesterday mate and mentioned the fact that the slider might slide further south and include more of the midlands and thats whats happened!!i also fully expect the ukmo to take it further south by tomorrow mornings run!!before that we got the cheshire gap snow situation to deal with!!fingers crossed!!also after looking at the ukmo at 96 hours i think precipitaion would remain as snow across the midlands northern england and east anglia as 850 temps never drop below -3 in these areas and taking into fact surface cold!!

[northwestsnow]

7 minutes ago, shaky said:

You did call this brilliantly yesterday mate and mentioned the fact that the slider might slide further south and include more of the midlands and thats whats happened!!i also fully expect the ukmo to take it further south by tomorrow mornings run!!before that we got the cheshire gap snow situation to deal with!!fingers crossed!!also after looking at the ukmo at 96 hours i think precipitaion would remain as snow across the midlands northern england and east anglia as 850 temps never drop below -3 in these areas and taking into fact surface cold!!

Hmmm dont be too sure shakster- APREGE joins UKMET/GEM in taking the low further north...

middle ground would suit me and you both!!

Edited December 6, 2017 by northwestsnow

[tynevalleysnow]

8 minutes ago, northwestsnow said:

Hmmm dont be too sure shakster- APREGE joins UKMET/GEM in taking the low further north...

middle ground would suit me and you both!!

Aparge looking good for most 

[northwestsnow]

2 minutes ago, tynevalleysnow said:

Aparge looking good for most 

Yes its a brilliant run!!

Better than GFS for more people too

but then again GFS is superb all the way out to the end!!

Edited December 6, 2017 by northwestsnow

[AWD]

1 minute ago, tynevalleysnow said:

Aparge looking good for most 

Try the higher resolution, UK view;

[phil nw.]

As well as temperatures the crucial factor as the precipitation moves in from the sliding low will be the dewpoints.

Here on the nw extra charts for Sunday

on the left dps and surface pressure and on the right expected precip.and pressure.

No problem further north but as the rain band moves across the south and midlands the dp's are on the rise so it may be for low lying areas only a narrow band get's lucky before the colder drier air comes back south as the low moves away.It may change a little as we still await exact tracking but in this marginal setup we can see that we may well struggle to see widespread lying snow- lower down anyway.

The wind flow chart as the low heads into the west shows that the air is sourced from the south-channel.W.France where temperatures are above zero at that time.

It would be a different story if the low approached from a lower latitude with a high cell to the ne this would bring in lower dew points from the continent ie from the se.

It may well be with better ridging in days to come the angle of attack of future Atlantic systems will be better for trough disruption and winds backing to the se but that's for another time.

Edited December 6, 2017 by phil nw.

[nick sussex]

At T96 hrs upstream the UKMO has an 985 low compared to the GFS shallow feature. Because of that deepening this low runs more ne inflating lower heights over Greenland.

This dominos further and will effect the track of the next low into the UK.

NOAA will definitely be commenting on this later as it effects the New England states.

[V for Very Cold]

This Is model watching at its best.... I must just remember to get off the PC and check what’s happening outside! 

 

[Northern Sky]

9 minutes ago, AWD said:

Try the higher resolution, UK view;

That's a BANK for my location. However it all seems up for grabs at the moment and I suspect will remain so until much nearer the time.

[Summer Sun]

I'm guessing GEM is underestimating the 850's?

[Boro Snow]

1 minute ago, Summer Sun said:

I'm guessing GEM is underestimating the 850's?

Absolutely it's a known issue with that model, I don't understand why they cannot resolve the issue, 

[Weathizard]

5 minutes ago, Summer Sun said:

I'm guessing GEM is underestimating the 850's?

The GEM is a dreadful model to be honest, once a few years ago it picked out a pattern before the big 3 and now everyone still uses the poxy thing! 

It can go in the same basket as the good old BOM imo!

Edited December 6, 2017 by Weathizard

[Yarmy]

1 minute ago, Weathizard said:

The GEM is a dreadful model to be honest, once a few years ago it picked out a pattern before the big 3 and now everyone still uses the poxy thing! 

And yet:

The "big 3" is actually ECM/UKMO/JMA with the GFS/GEM neck and neck trailing behind. Of course, on a day to day basis any one of those models could verify better than the others, but over a month the order is clear.

[Weathizard]

3 minutes ago, Yarmy said:

And yet:

The "big 3" is actually ECM/UKMO/JMA with the GFS/GEM neck and neck trailing behind. Of course, on a day to day basis any one of those models could verify better than the others, but over a month the order is clear.

Possibly but are these charts on a global scale? I never really understand how you can measure the accuracy of weather models with data as surely it varies, certain models are more accurate for certain areas in the globe ect.

IMO the GEM is utterly useless when it comes to 850's and more generally on our side of the pond.

Edited December 6, 2017 by Weathizard

[ICE COLD]

Well you don't often see the ensembles in such good agreement all the way mostly ? . It's usually spaghetti junction.

Edited December 6, 2017 by ICE COLD

[Summer Sun]

Cold ens tonight with a lengthy period of below average temps the only uncertainty is how much snow we will see in the coming weeks

Black line is the 30 year average for those not familiar with the below chart

[Yarmy]

10 minutes ago, Weathizard said:

Possibly but are these charts on a global scale? I never really understand how you can measure the accuracy of weather models with data as surely it varies, certain models are more accurate for certain areas in the globe ect.

IMO the GEM is utterly useless when it comes to 850's and more generally on our side of the pond.

There's a model bias page you can check for that:

http://www.wpc.ncep.noaa.gov/mdlbias/

You can put in height, temp at various levels, etc. There's nothing really conclusive to my eye. 

[weirpig]

Euro 4 for friday

 

Edited December 6, 2017 by weirpig

[Frosty.]

Tomorrow is the last mild day for a long time according to the GEFS 12z mean which goes much  colder from Friday onwards and stays cold / very cold cyclonic throughout with ice days / frosty / icy nights galore and plenty of opportunities for snow at all levels..really superb!

Edited December 6, 2017 by Frosty.

[lassie23]

35 minutes ago, V for Very Cold said:

This Is model watching at its best.... I must just remember to get off the PC and check what’s happening outside! 

 

When it does snow, people will miss it because they will be in here posting about when it will stop or where the next lot is coming from

[Tamara]

Still coming slowly off a cautious fence

Long term total global atmospheric angular momentum trends can tell you a lot about pattern changes due to the effects the rises and falls in global wind-flows have on jet stream patterns. Total AAM refers to the net balance of easterly and westerly flow in the atmospheric circulation and the outcome tells you which direction the atmosphere is presently headed and which has initiative forward momentum

Latest levels, assisted by a second tranch of eastward moving tropical convection since October has taken total global AAM back to parity, with even a hint of peeping a head above the parapet. This is to be set as disconnect against the negative (easterly) La Nina base state. Its no surprise that the sometimes pausing, but still steady upward trend overall, began from early October and why the pattern has been increasingly amplified over this time

The importance of these feedbacks keeps being stressed repeatedly I know in these posts, but they are very important in terms of recurrence patterns that can give a good insight into how future patterns may play out. On the basis of the seasonal wavelength cycles spoken of again yesterday, the start of the winter (like any other season) is one of those especially important times when trends within the layers of the atmosphere can be especially augmented by natural seasonal wavelength cycles.

Behind the rises in total AAM is continued buoyant AAM tendency as a result of positive torque mechanisms which are programming swathes of amplification poleward. This begins in the tropics c/o of the MJO cycle and propagates to the extra tropics and then from the latter c/o mountain torque into the stratosphere to sustain a destabilised profile.

The momentum being created is starting to assure a cold pattern persisting longer and longer through December and slowly cancelling out cautious notions that the traditional base state might interfere and de-amplify the pattern in the ways described previously.

As a consequence of the atmospheric flux, the Global Wind Oscillation (a representative plot of rising or falling AAM and global windflows) has continued to stretch its Phase 4 amplitude and enlarge the disconnect with La Nina.

In synoptic terms, the implications of this are for an upper flow into Europe that looks likely to continue to be somewhat flat against the Atlantic ridge and troughing to the NE(hence some of the apparent stubbornness for breakaway lows heading from upstream to deconstruct and take optimum southerly most tracks each and every time). But with the upstream +PNA pattern assisting split flow it continues to favour the angle of the jet stream to be on a NW-SE trajectory, its set well for increasing upper cold air in tandem with surface cold to become more and more in situ and increase the possibilities of surprise more widespread snowfalls popping up more and more as time goes by.

This leads to the extended period. In December the NAO has a low relationship in its response behaviour to the state of the polar field (the AO) However this starts to rapidly change as New Year arrives and the relationship, as such, consolidates much more tightly in January. Hence another aspect of this crossroads theme referred to so often.

The feedbacks in process are starting to suggest with a bit better confidence that lagged effects of these amplification programmes will help prime a cold pattern to sustain or recur heading into the New Year. Assuming the atmospheric circulation doesn't deviate much from the present (and in this respect its still necessary to monitor AAM as tropical convection returns to the Indian Ocean as a potential fly in the ointment), then the effects of seasonal wavelengths bring those height rises to the NE into the equation, and the implication of this is to repress the Atlantic ridge, with less mixing of milder air in trough disruption occurring and better and better chances of snowy undercuts.

 

 

Edited December 6, 2017 by Tamara

[shaky]

Look out for a nice wedge of heightd around iceland at 72 hours if u want the low to slide favourably!!

[shaky]

Looks similar to gfs maybe slightly further east!!could look like ukmo this!!

[AWD]

Just now, shaky said:

Looks similar to gfs maybe slightly further east!!could look like ukmo this!!

How can it be like the GFS & UKMO when they are both rather different regarding Sunday?