Winter Model Discussion 18Z 13/1/13 onwards.

[nick sussex]

It's time for one of my paint jobs!

Here is the GFS 06hrs run at 114hrs:

For those reading earlier you would have seen mention of the gap to the north, this is very important in keeping the Atlantic at bay.

You can see circled in black pressure starting to rise as the shortwave has cleared east into Scandi, this allows the high to extend further north and stop energy running over the top aiding the Atlantic.

Now once you get pressure increasing there you add some forcing on the main troughing in the Atlantic, this can then help shear some energy away from the next incoming low with energy heading se'wards into France.

In a sense its an anti clockwise relationship with these all helping to fight back against renewed energy heading east.

Although you can't simply pattern match as each set up has to some effect its own personality you can use historical context and past experience to suggest the likeliest outcome.

Historically it's no surprise that many of the past heaviest snow events occur more towards the south and sw in these types of set up.

You can often correlate longevity of similar set ups in terms of repelling the Atlantic by how far north and east the snow reaches because this is a reflection of the strength of the block.

In a sense its the base marker and signals how likely you are to lock in the cold.

[london-snow]

.. and also Nick the deep troughing over Europe which is going nowhere fast.

This the 06z GEFS H5 anomaly for day 10:

The development of the residual vortex over Hudson Bay should not be looked at in terms of how we might look at a traditional pv which has a strong zonal wind profile behind it. As we know, the vortex has been shattered, and zonal winds reversed or very light. That allows the stratospheric vortex to wind itself but not able to draw the downstream response, which explains why the flow across the Atlantic is running out of puff and disrupting.

Temperatures look to be 3-4C below average according to latest GEFS out to day 12 at least. Interestingly, to reignite the 'which is the better teleconnective index for UK cold' issue, the AO continues negative whilst the NAO positive. Which index is performing better (as a predictive tool) in terms of the UK ?

Still punting for a very cold end of the month Stewart?

If current models are to be believed things could stay cold with temperatures struggling up to the 20th and possibly beyond. So i am thinking very cold - extremely cold... As your thoughts seem to point for a renewed plunge of cold from the north east towards month end?

[Summer Sun]

After today's snow clears away Tuesday and Wednesday will see snow showers along the east at times before attention then turns to the west late on Thursday night / very early Friday as a band of rain pushes in and turns to snow at the same time a band of snow will also affect eastern areas

Cold uppers remain firmly in place

Into the weekend and the wintry weather remains

During Saturday and Sunday the cold air is shown to move east again so those in the west are likely to see a return to rain at this stage

Edited January 14, 2013 by Gavin.

[Wadey]

After today's snow clears away Tuesday and Wednesday will see snow showers along the east at times before attention then turns to the west late on Thursday night / very early Friday as a band of rain pushes in and turns to snow at the same time a band of snow will also affect eastern areas

Cold uppers remain firmly in place

Into the weekend and the wintry weather remains

During Saturday and Sunday the cold air is shown to move east again so those in the west are likely to see a return to rain at this stage

would that low to the west from the 20th bring much snow ? And how cold could it get still learning

[Summer Sun]

Gavin,

Do you think the bands of rain Thursday/Friday will be falling as snow? If so could be a fairly major event, no?

At the moment yes snow looks likely on both the west and eastern sides of the UK, still 4 days to the possible event so many things are likely to change.

I've just had a look at the snow depth charts for 18z Friday and Dartmoor has 12cm of snow, 16cm for parts of Northern Ireland with 7cm to 12cm widely away from the coast, another 5cm to 8cm for parts of the east and 8cm to 17cm for parts of Scotland

[Summer Sun]

would that low to the west from the 20th bring much snow ? And how cold could it get still learning

There could be some big totals by 18z on Sunday away from the south west where milder air will be edging in here

This is based on the 06z run

[phil nw.]

I thought i would just have a look at some of the medium range outputs just to see where we might be heading past the weekend and i picked T192hrs-day 8 from the 00z ECM/NAEFs runs.

For any new members the NAEF`s is the North American Ensembles Forecasts which are the combined outputs from the Canadian(GEM) and the American(GFS) models-they also share this information with the Mexican MO i believe.

First the mean Ht anomalies and actual mean hts.at 500hPa level

and the actual mean hts

A pretty solid picture of the Atlantic low pressures heading south east as they comes against those heights to our north.The direction and development of the troughing is across Ireland across the south west of the UK and then Europe.

.We can see on the mean 850hPa temps we are on the edge of the cold block which will ebb and flow through this week and into next as that relatively weak Atlantic energy tries to come through but then looks to take the easier route to the south.

These outputs of course are watered down somewhat because they are average results but never the less show what a fascinating pattern we are in.

We can`t relax for one day with the Op runs as they seem to be constantly changing beyond days 4-5 and so the weather for the UK is a nightmare to forecast.

Often in a typical UK Zonal(mild)pattern we can have confidence much further out.

All we can deduce from these is that areas of the UK further north and east will be closer to the coldest air and will have the highest chance of snowfall as any frontal systems come in.

Some mixing out of the cold air will probably happen from time to time but this look at the wider picture tells me currently that we are unlikey to get into a mild setup as we start next week.

A glance at the latest London Ens continue to show low 2mtr temps with the 00z GFS run a mild run.

All in all a very interesting few days to come especially if the models continue to go the way of the 00z UKMO and ECM outlooks.

[Summer Sun]

Do you have a similar chart for Friday/Saturday

18z Friday

18z Saturday

[OldMetMan]

Fascinating model watching today as each grapples with the fate of the cold block.

Rather a sharp temperature contrast today across the occluding front as it slips south, and this process seems to be the pattern several of the models favour in the days to come as approaching fronts slide away SE. The GFS and GEM are showing a partial block collapse at least - but I am amazed at what looks like a pretty weak upper high still shown to be there even by the end of the week.

The 500mb flow and the associated jet is very revealing:

The jet in particular, by its current orientation, suggests that most of the energy will head NE, not E. Indeed, as I suggested yesterday, we could end up with a distinct split stream to our W with fronts occluding out and slipping into France. I am surprised by the UKMet showing a deep LP out in mid-Atlantic with marked troughing over the S of the UK, suggesting a slow-moving occluding front somewhere over England - classic prolonged snow territory this is, well, for some at least. I am surprised because I thought this model would follow the GFS and destroy the block.

The other extreme is shown by the ECM with fronts being kept well back.

The small 500mb high has been gradually shifting E to the N of Scandinavia and if the jet energy does indeed push NE it could well maintain this high or cause a reformation further W, while at the same time, small short waves feed into the Mediterranean upper low, thus keeping the block intact.

At this stage, I would favour the UKMet as being a happy medium so the extent and location of snow would most likely be north of the Midlands. This is a very interesting type of synoptic development to watch unfold, indeed it has been one that has featured in the notorious 1947 winter but with the S of the UK bearing the brunt of the snow.

Fact remains, the upper flow is very distorted and meridional right now. It is tempting to say that maybe heights will rise and link up right from Scandinavia right across the Pole to the Bering Sea and the NE Pacific ridge!! The Canadian vortex is gradually sinking S after all. Some model predictions are showing rising pressure over Greenland, so we just need a stronger HP to our N, plus that all important feed of new cold air coming into Europe.

It's a lot to ask for I know but something looks quite anomalous in the flow right now - something conducive to cold.

[Carlrg]

The models to my very untrained eye look quite similar up to 17/01

http://www.meteociel.fr/cartes_obs/comp_panel.php?mode=0&ech=60&size=2

Then start to go off on their own tangents again.

http://www.meteociel.fr/cartes_obs/comp_panel.php?mode=0&ech=84&size=2

Who'd want to be a forecaster, certainly not me!

[Mucka]

Dismiss GFS 06z it is just an echo of an old pattern, like a fuzzy memory we can't quite place and as such is garbled and out of context with current reality.

GFS 12z will be a big improvement again as will its ensembles and be more in line with ECM.

Expect UKMO to stick to its guns more or less and ECM to show further improvement.

I think JH mentioned a small cluster of ensembles went for a strengthening retrogressing block past day 6/7 and although people may not care to dream that "Holy grail" of wintriness will be achieved I expect to become a strengthening signal.

Firstly, and this is something I have been saying across different forums for a couple of days, the models were not getting to grips with the amount of energy likely to push under the block meaning the when the real Atlantic push came it simply went NE collapsing a ridge over the UK and eventually bringing in very unsettled milder weather type by day 7. That is still a possibility but I believe the models are now picking up on how the block will be better aligned to resist the Atlantic and thus we will see the jet buckle and energy undercut keeping much of the UK, especially NE in cold air. Obviously in the boundary area there could be some very heavy falls of snow but others will be screaming because they will see rain. Could be quite messy with the boundary line moving back and forth SW/NE.

The second thing the models are not picking up on and we should see this slowly improve in the next couple of days is the amount the powerful weather systems in the Atlantic will actually disrupt. The models really struggle with this and will want to have the core of the LP's further East and too much energy forcing directly up against the block. The reality will probably be earlier and greater disruption and thus energy being squeezed under the block rather than just barrelling through. This in practice would mean a slow strengthening of a blocking signal to our North and East in the output and slow weakening and Southward correction of Atlantic incursion. It is actually possible that places that are currently projected in mean output to be in the firing line and boundary of air masses with rain/snow 50/50 will actually remain much colder but dry and the snow line is pushed further south.

If the trend is our friend and I have not just been talking through my hat the "holy grail" synoptic will surface more and more in the output.

Of course should the 12z output flip back in favour of Atlantic domination then you just wasted 5 minutes of your life reading that but take comfort in the fact I lost 10 writing it.

[Snowman.]

Mister PV you ok buddy?

If that came off the possibilities after that are endless...

[Weathizard]

Astonishing tug of war going on in the Atlantic at 102, 955 and 965 LP systems trying to use team work to smash down the block. Holding firm at the moment with some energy going underneath. Not anywhere near as progressive as the UkM

[Panayiotis]

On this run, the band of precipitation coming from the SW, has delayed its approach on Friday by a few hours, but when it reaches the UK most of it falls as snow except for the far west..

[Jonan92]

GFS in this run shows High pressures building in the east coast of the US, that's new.

[Wilxy]

PV wants to take a dive into the atlantic ocean, intresting times .

[Panayiotis]

UKMO at 36hrs, shows a small shortwave to the south of Ireland, could re-assert the easterly and bring some precipitation (probably snow)

Edited January 14, 2013 by Panayiotis

[bobbydog]

On this run, the band of precipitation coming from the SW, has delayed its approach on Friday by a few hours, but when it reaches the UK most of it falls as snow except for the far west..

just to add, its also larger, covering a greater area-

[Snowman.]

UKMO at 36hrs, shows a small shortwave to the south of Ireland, could re-assert the easterly and bring some precipitation (probably snow)

When it comes too 36 Hours on that chart I its likely!

[Weathizard]

Atlantic smashes through at 120, I'm pretty sure this is caused by the Jet not playing ball. Can't look at the jet charts for somereason they crash my phone.

High to our North showing signs of retrogression to Greenland area though. Overall compared to the 6z a step in the right direction I feel, despite the GFS blowing up that Atlantic low like a baloon.

Edited January 14, 2013 by Weathizard

[Cheese Rice]

South West England/Wales looks like the place to be up to about Saturday with a number of lows making inroads effectively hitting a brick wall. We will be looking at increasingly deep snowfall.

I'm pleased it's there as the South West generally does the worst for snow in the UK.

Edited January 14, 2013 by Cheese Rice

[Panayiotis]

UKMO at 48hrs is very cold with a easterly coming all the way from Russia/Asia and very similar to its previous output, could we see the same undercut later in the run though?

Edited January 14, 2013 by Panayiotis

[Panayiotis]

Very nice again to the UKMO, shows another shortwave to SW with strong ESE at 96hrs, could see some hefty snow showers in the west from that, however for it to show something similar to its 00z it a very encouraging sign!

[karlos1983]

Surely the HP Building off the Eastern Seaboard is a good thing?

[Jonan92]

GFS in this run shows High pressures building in the east coast of the US, that's new.

Because of that High the low moves to the east and the green area moves towars Greenland

Because of a low in the east cost of the US, the high starts to rise and breaking the jet