Copied WM’s nice summary from the MO thread to start a dedicated thread for the projected midweek windstorm, likely to warrant high-end warnings given the concensus of model output:

‘After a rather quiet and dull week on the weather front, next week promises to be a whole lot more interesting. There has been lots of discussion about potential snow and indeed that is still on the cards for some. But the midweek storm is now the main focus of attention I think.

A number of people have suggested the low may end further south. But the current model runs are going very much in the opposite direction and taking the storm further north. This can of course change as there are still several days to go before the storm arrives.

The other point to note about this storm is that it will be undergoing rapid cyclogenesis as it crosses the UK and hence the wind is likely to be the main concern.

Latest GFS run shows this very nicely

As it approaches the UK the low is just starting to undergo rapid development. At this stage it is just at 981mb

http://modeles2.meteociel.fr/modeles_gfs/runs/2018011306/108-21UK.GIF?13-6

6 hours later we have 972mb

http://modeles2.meteociel.fr/modeles_gfs/runs/2018011306/114-21UK.GIF?13-6

6 hours after that the storm peaks at 964mb as it exits into the north sea. A very strong pressure gradient on the western flank

http://modeles2.meteociel.fr/modeles_gfs/runs/2018011306/120-21UK.GIF?13-6

Very strong winds for most of England and Wales during the overnight period

http://modeles2.meteociel.fr/modeles_gfs/runs/2018011306/114-289UK.GIF?13-6

By midday the low is starting to fill as it moves across Germany

http://modeles2.meteociel.fr/modeles_gfs/runs/2018011306/126-21DE.GIF?13-6

Friday could be interesting with some heavy snow in northern areas as a number of little lows develop in the NWerly flow

http://modeles2.meteociel.fr/modeles_gfs/runs/2018011306/144-21UK.GIF?13-6

Heavy snow is also likely on the northern edge of Wednesday night's low. Serious blizzard conditions are likely on the mountains.

The latest 6z ICON run is very disturbing indeed. The low is further north again and also much deeper.

Here is the chart for 6z on Thursday with the low at 955mb off the east coast of Scotland

http://modeles.meteociel.fr/modeles/icon/runs/2018011306/iconeu_uk1-2-120-0.png?13-11

Wind gusts across N Ireland and NW England could reach up to 150kph or more

http://modeles.meteociel.fr/modeles/icon/runs/2018011306/iconeu_uk1-11-117-0.png?13-11

http://modeles.meteociel.fr/modeles/icon/runs/2018011306/iconeu_uk1-11-120-0.png?13-11

Huge amounts of snowfall across most of Scotland and also Cumbria

http://modeles.meteociel.fr/modeles/icon/runs/2018011306/iconeu_uk1-1-114-0.png?13-11

This has the potential to be a very serious weather event. The track and intensity of the low will change from run to run. But if anything like the ICON 6z turns out to verify then I would expect to see multiple amber warnings for wind and snow and possibly a red warning being issued on Wednesday’

The severity and track of these ‘runner’ lows depend on a number of factors, not least how the wave phases with the entrance and exit portions of the 300mb jet. If the phasing is ‘right’ then RACY can occur (>3mb drop in SLP per hour) and these storms historically deliver the most extreme wind speeds. 

Let’s see how this develops.

This is looking like a beastly little storm. Its all relative but I start to pay attention on this chart when sustained wind speeds reach 40+. Presently the peak brings in a WSW'ly in the early hours of Thursday of 45mph. St Judes for reference was 43mph.

http://wxweb.meteostar.com/sample/sample.shtml?text=EGMD

Due to the rapid bombing of this feature as it crosses the country into the low countries I would expect some pretty savage gusts as sting jets propogate down to the surface.

Nasty nasty nasty.

The ICON model provides an interesting chart showing the height of the snow line (most other models only make a 0 degree isotherm available which can be used as a proxy for where precipitation starts falling as snow).

It turns out this snow line chart is very handy for working out the positioning of the surface level fronts. Of course you can get a good sense of where the fronts are from analysing the sea level pressure, precipitation and 850hPa charts. But you can do a pretty good job just by looking at the snow line chart in isolation. The ICON 6z run shows a textbook case of rapid cyclogenesis. 

The T108 chart has the low just starting to occlude and it has a broad warm sector. The centre of the low is close to the triple point of the fronts and the system has created a closed circulation. By T120 the system is rapidly occluding and the warm air is being overridden by the advancing cold front. A classic bent back occlusion feature appears to have already formed by this time with the centre of the low inside this bent back occlusion. Normally it would take longer than 9 hours for a system to develop between these two points. Hence the reference to rapid bombing.

6z WRF model has the system about 75 miles further south than ICON and 15mb higher at 970mb as the system hits the Low Countries. That 15mb makes a big difference as far as the winds are concerned. WRF would see winds gusting to around 70mph possibly a little higher in exposed locations. Whereas ICON would see winds gusting to at least 90mph.

http://modeles.meteociel.fr/modeles/wrfnmm-eur/runs/2018011306/nmm-2-115-0.png?13-13

http://modeles.meteociel.fr/modeles/wrfnmm-eur/runs/2018011306/nmm-2-120-0.png?13-13

http://modeles.meteociel.fr/modeles/wrfnmm-eur/runs/2018011306/nmm-11-115-0.png?13-13

For what its worth the CFS model is picking up this secondary low. Not the most reliable model but it is looking somewhere in-between the ICON and WRF runs.

Here is last night's 18z CFSv2 (1 month) run. The storm is quite well south and only at 985mb

http://modeles.meteociel.fr/modeles/cfs/runs/2018011218/run1m4/cfs-0-126.png?18

Here is today's 0z run. The system is now at 965mb as it crosses the UK. It is still a bit south of the ICON track and less deep. But quite a significant change from the 18z.

http://modeles.meteociel.fr/modeles/cfs/runs/2018011300/run1m1/cfs-0-132.png?00

Last night's 12z was a bit closer to the 0z but still not as deep

http://modeles.meteociel.fr/modeles/cfs/runs/2018011212/run1m3/cfs-0-138.png?12

ICON 12z is a little further south but not much. The system still goes sub 960mb as it exits the UK.

Very stormy conditions for most areas of England and Wales. Snow to rain in northern England but snow all the way in most of central and southern Scotland.

http://modeles.meteociel.fr/modeles/icon/runs/2018011312/iconeu_uk1-2-108-0.png?13-17

http://modeles.meteociel.fr/modeles/icon/runs/2018011312/iconeu_uk1-2-111-0.png?13-17

http://modeles.meteociel.fr/modeles/icon/runs/2018011312/iconeu_uk1-2-117-0.png?13-17

http://modeles.meteociel.fr/modeles/icon/runs/2018011312/iconeu_uk1-11-111-0.png?13-17

http://modeles.meteociel.fr/modeles/icon/runs/2018011312/iconeu_uk1-11-114-0.png?13-17

http://modeles.meteociel.fr/modeles/icon/runs/2018011312/iconeu_uk1-1-108-0.png?13-17

http://modeles.meteociel.fr/modeles/icon/runs/2018011312/iconeu_uk1-1-111-0.png?13-17

GFS is much further south and only gets to 980mb so a very big difference between these two models.

We will get a first look shortly at what ARPEGE makes of this system.

Getting increasingly concerned about my flight from Manchester to Aberdeen on Wednesday lunchtime.

Me too, you might make it!

(I am joking of course! )

Well we now have some serious disagreement about the track of Wednesday night's low pressure.

ARPEGE takes the system way to the north of the UK. 953mb so that's the deepest yet. Very strong winds for Scotland and some parts of N England and N Ireland on this model

http://modeles7.meteociel.fr/modeles/arpege/runs/2018011312/arpegeuk-2-114-0.png?13-18

http://modeles7.meteociel.fr/modeles/arpege/runs/2018011312/arpegeuk-11-113-0.png?13-18

Yes. 12z GFS makes much less of the secondary low. 50-60 mph gusts in the Channel at most on this run.

http://modeles2.meteociel.fr/modeles_gfs/runs/2018011312/114-289UK.GIF?13-12

As I understand it, in these situations the general rule is that the more the LP deepens the further north it tends to track. The output today would seem to confirm that.

12z ICON takes the low across southern Scotland, deepening by 18 mb in 12 hours.

Agreed. That is also my understanding as a general rule of thumb. Will be interesting to see what tonight's T120 fax chart shows given the METO model is not making much of this secondary low at the moment.

Hmm well ECM slightly scuppers that theory.

Low well to the south and very deep!

ECM already has a closed circulation low at T96 of sub 990mb

12z Wednesday (T96) 

https://stream.ecmwf.int/data/gorax-green-009/data/data01/scratch/render-gorax-green-009-95e2cf679cd58ee9b4db4dd119a05a8d-wYW9xJ.png

By T120 it is off to the Low Countries at around 965mb

http://www.meteociel.fr/modeles/ecmwf/runs/2018011312/ECU1-120.GIF?13-0

http://www.meteociel.fr/modeles/ecmwf/runs/2018011312/ECD1-120.GIF?13-0

So not quite as deep as ICON or ARPEGE but much deeper than GFS which takes a similar track to ECM. It is going to be another couple of days I think before the track of this system becomes clear.

Over on the Arctic sites they are thinking that Svalbard will again cop for it and leave us in stiff S Westerlies?

We are at the time of year that any slough off of Greenland deep cold into the Atlantic , or a P.V. slip with an Arctic outpouring like we see over NE USA, does go a bit barmy when it meets with the warmth pushing up from the south over a warm ( relative to the airmass above) ocean?

So taking stock of the 12z runs, at the moment it seems to be the European models vs the American models on this one.

GFS, GEM and NAVGEM are not that interested in making this a major storm

Whereas ECM, ARPEGE and ICON develop a very nasty storm albeit they all disagree with each other about the track.

METO model is sitting on the fence.

Latest CFS run comes down on the side of the European models as it has been doing

http://modeles.meteociel.fr/modeles/cfs/runs/2018011306/run1m2/cfs-0-126.png?06

In contrast the 12z WRF model has moved towards the American models. It was showing a low close to 970mb on earlier runs. But this time it only has 985mb which is closer to GFS. 

http://modeles.meteociel.fr/modeles/wrfnmm-eur/runs/2018011312/nmm-2-112-0.png?13-19

So impossible to draw any conclusions at the moment. There is the possibility of a serious wind storm on Wednesday night but at this point it looks 50/50 as to whether it will be disruptive and impossible to say at the moment where it will hit.

Quick 18z update.

GFS now has the storm further north and much deeper as it exits the east coast. It gets down to around 964mb as it heads across the North Sea. 

ICON 18z is little changed from the 12z and has the storm a bit further north than GFS at 959mb.

T120 fax chart has the storm at 968mb over the German coast at 12z Thursday. That is fairly consistent with what the latest GFS and ECM runs show.

Worrying signs IMO 

I know we'll continue to see track changes but the intensity progged is quite alarming. Let's hope it gets watered down!

 So if the system takes the track currently modelled by the GFS, you would expect rain in Aberdeen from that ? 

As it stands the strongest winds are likely across NI, the NW then exiting the NE of England.