Atlantic loos like doing it on this run

http://modeles.meteociel.fr/modeles/gfs/runs/2015011812/gfsnh-0-174.png?12

Judgement to be made after seeing the GEFS

Indeed Michael. What happens next weekend and beyond that is still very much open to question at the moment.

Convergence lines in terms of snow potential are like super troughs! Anyway a convergence line is when air converges from two directions. This is partly due I think to the fact the line sits in a local trough but also due to the topography of the land, air is refracted around the coasts, if you get a peninsula you get air refracting in both directions and hence converging. Convergenze zones force air to rise and can generate massive convection.  

For me it is T12

GFS 12z is incredibly frustrating for my neck of the woods.

A decent amount of precipitation comes in Tuesday evening but dewpoints look the wrong side of marginal (just!) Of course, as soon as the precipitation moves away, the 2m temps and dewpoints fall away again. 

A classic maritime, very British problem. ;-) 

In situations like these we need to keep it tight and focus on what is happening in the next 3 to 4 days max. The wild chops and changes after Thursday tells us that FI is 72 hours. Anything after that is a waste of energy. We need to focus on the here and now.

The 12z continues the disruption theme. There are some good opportunities for snow this week from the Midlands northwards. The signal continues for a snow event in southern areas on Thursday morning as cold continental air with lower dp's is advected westwards from the near continent as the low moves into France. 

An excellent description

Meanwhile Joe has highlighted the classic problem we have in this country. One of the reasons I prefer a continental feed is the low dewpoints and with most other airstreams being ostensibly moist it can be a real issue.

Because of the small margins of error I would put FI pretty short 

Extreme Strat warming on the latest run, more extreme than the last one. February could be very Interesting!

http://www.meteociel.fr/modeles/gfse_cartes.php?&ech=336&mode=10&carte=1

I prefer an easterly for here - it seems to cancel out any doubt about dew points/marginality etc, at least in January 2013 when we had an easterly influence we got a lot more snow here than the current spell of dustings, this current Polar maritime air is only fantastic for high up areas in NW England.

Totally on the wrong side of the Arctic to benefit is though, surely? It would just squeeze the PV over to our side. 

A tiny increase in a short section of jet north of Iceland seems to give GFS the excuse it needs to have a more rounded trough that doesn't disrupt as we'd like to see. The run then becomes a total mess as the model struggles to handle a fragmented, untidy jet stream (http://max.nwstatic.co.uk/gfsimages/gfs.20150118/12/198/hgt300.png).

A high probability that the stronger sections, and Atlantic storm systems, are being overcooked as usual - the run could have turned out a lot better out to day 10 than it did. After all, the upstream progression is slower than the 06z... with the Friday trough disruption it could have been quite the looker!

My frustration with the run is moderated somewhat by the run of charts for Wed-Thu giving me heavy snow for some 12 hours or so, resulting in the greatest accumulations in many years... but it doesn't cover a very wide area and could easily shift in location, so I can't afford to go counting on such a thing.

No absence of Friday trough disruption on the 12z UKMO run, but the low then heading SE is further east than the GFS 06z had it, while for some reason a vast col develops to our NE - LP doesn't make inroads there, yet the ridge vacates. Despite this, with the Atlantic westerlies reducing (note the lack of eastward progression by the Atlantic trough days 5-6) and a decent trough SE of the UK, the Siberian High could extend back west again days 7-8.

Really not convinced by how far it shifts east days 4-6 though.

As for that strat. warming days 12-16, it's extreme enough to be a major SSW so the vortex would likely be annihilated rather than just displaced toward us. An interesting signal for sure, needs to reel in another few days for confidence to start rising, but it does tie in well with the MT events I mentioned this afternoon.

Maybe for you, but for most of the NW a properly cold NW'ly is the way to go 🙂 , think Dec 2009 !

I'd be surprised if we don't see a single snowflake in the coming week. I think altitude will definitely help. 

Not sure it works like that once it gets down to the troposphere. At the Stat what you want is an quick intense warming which is shown on the latest GFS. 

Fergie reported earlier today that the UKMO 00z was deemed too progressive by the Met Office, and the UKMO 12z is about the same with the low pressure sliding SE later in the week.

With that in mind, perhaps GME is a good guide as to what they may consider more likely:

I am enjoying viewing this model of late, because you get to see the finer details involved in trough disruption - tiny areas of low pressure than can affect the rain/snow balance (e.g. in the Channel both here: http://modeles.meteociel.fr/modeles/gme/run/gme-0-48.png?18-12 and here: http://modeles.meteociel.fr/modeles/gme/run/gme-0-66.png?18-12).

Go GME! 

Thankyou Quantum, that makes more sense than the Met Office description.  The refraction from the coast would make sense as to why they develop in the Irish sea as there are two coasts to refract from!  Was just interesting to see one on the Fax charts (I notice that it's now gone on the next iteration) as I'd not seen one before.

yes - already mentioned by gavin p on his latest update - doesn't guarantee cold and snow but increases your chance in blocking situations

From a regional perspective... southerners raising eyebrows at the sight of this Channel Low forming later on Wednesday, now present on GME, UKMO, GFS and WRF-NMM output... shown to bring heavy snowfall to the southern half of the central swathe of England BUT this depends on the low forming in that particular location. It is quite a coincidence that, though timing differs, the position and track varies little between the four listed models this evening.

WRF-NMM shows the low forming as the low heights move over The Channel - in fact it begins right on the coast. This reflects the fact that such features tend to owe their development to the enhanced vertical temperature gradient over the waters of The Channel.

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Mountain Orcs eh Michael 

Well it looks like not much happens as for wintry weather on Tuesday but for Wednesday and it's night is when we could get sleet rain and snow particularly in mid and upper levels, it looks marginal with 2 to 4 deg. C being daily max and nights at -1 to - 5 the coldest in the North and Central parts of UK.

Very cold on today Sunday and tomorrow Monday, again on Wednesday and then also on Friday to Saturday.

Winds and Pressure is Low at 995 to 1010 Millibar, and NW winds are also set to control the steering flow of our weather, looks like the Mild air will not be visiting until Saturday night to the midday of following Monday, then again a ridge at 12z with 528 dam air and 0 to -5 at 850 hpa briefly return.  

Active PfJ NW and Northern North Atlantic Areas of SW to NE then SE pushing to UK, and PV return numerous times back across SE Newfoundland SE of Greenland and through to SW South Iceland across to Far West North side Atlantic Sea to ur NW and North, and at times to our West the mild sector briefly cross UK, as we go to 168 to 192 hours ahead at 12z aGFS, with Cold Airmass widely across NW and North Atlantic but with mild sector's brought East after 144 hours for UK, then after that ridge then trough and some Stalled to SE track Low's to West side NW Europe SW Central UK down SE enter to NW France at 168 and 192 on the GFS 12z run.

 UKMO and GFS 12z similar to each other but at 96hrs GFS and must I say UKMO as says Stormchaser oops! early in day a.m. Night from Wednesday Night SE England could see wintry showers sleet and snow with cold NE airflow!!.

A quick glance at the 12z ensemble data suggest a large scatter from the 25th. It will be fascinating to see what the weather will actually be like next weekend (and indeed beyond!)

I thought it was hard to tell where blocking would occur after a warming???

I don't know Gooner. The black arts of SSWs are a bit of a mystery to me. In sure I've heard it said that warming over in Siberia isn't much good for us though?