This could all trend back again, but it shows that we are still on a cliff edge, an edge that until now was just slightly larger than the ensemble perturbations. 

I think the chance of a cold spell is now back at 50:50, and I still think it will go one way or the other.

I doubt it http://modeles.meteociel.fr/modeles/gfs/runs/2016010118/gfs-2-156.png?18 

Ah ballcocks. An IMBY overnight channel storm event if that came off . . . but it is a week away, a long time in forecasting terms:



EDIT: You just beat me to it, Phil. You know me so well.

I was thinking, "which Ian is he on about? Ferguson? Brown? Who?"

Then opened the link.

Good look with that one (if it comes off), Mr Johnstone 

LOL! Look at that arc of rain over north Merseyside/SW Lancs/Manchester 

This is anything but mild.

Very seasonal in the north with spells of snow, a mix of hail, rain and sleet showers in the south.

Must be very marginal for the GFS to resolve snow from this:

Yes, "our" Ian.

Yes chance it will be gone on the next run Ian. But GFS does seem quite persistent plotting a storm near us around this time.

850hpa temps are so misleading, people often just use the -6C benchmark. This is a mistake, -6C is fine under average pressure when the wind is coming from the NW,N or E. But you have to take into account the environment.

1) Low wind speeds: Evaporational cooling will play a role

2) Well below average pressure, 850hpa temperature level is lower so can afford to be warmer.

In these circumstances I'd take 2C off which puts as more like an effective 850hpa of -6C which is cold enough for snow. Warm occluded fronts are even worse, if the wind is from the SE bin 850s entirely and rely on other indicators. 

Here's an interesting chart showing model accuracy since the 1980s.

http://www.emc.ncep.noaa.gov/gmb/STATS/html/aczhist.html

I think it's more like six of one and half a dozen of the other tbh, but after 15 yrs of model watching, I can say for certain that it will go one way or the other 

Indeed.  Why the up and down nature of the graph?  Is that seasonality - i.e. models do better in summer, less so in winter?

Yes, that's my understanding. I think it's because the forecast accuracy decreases in the summer when patterns become more chaotic, due to the generally weaker jet.

When you've got a fax chart like this at t96, then anything is possible beyond:

http://www.weathercharts.org/ukmomslp.htm#t96

No disrespect to some excellent posts these last few days, but that is by far the most interesting graphic I've seen for ages. Two things of note - the gradual upward trend and the persistently superior red line of ECM.

Anything is possible yes, but some things are less likely 😊

A glance at the 850s for London tells me all I need to know - when the mean does but get below -2 the whole run, it's wellies and raincoats and not snowboots that are needed, at least below the M4.

Atlantic domination continues. A quick round up of the early model runs, excluding ECM suggests that the Polar Vortex has got bored of its December home near Greenland and has now moved closer to the south of Iceland for the first part of January.

The result is that the advancing and currently very tasty looking cold pool to our ENE

eventually gets overpowered and pushed away NE'wards from whence it came. The jet powers over the UK with some nasty storm systems and relatively high rainfall totals at times in a very mobile set up.

One positive is that the relocation of the PV means that the endless grey TM airmass of December is shunted further south and we stand the chance of some brighter, cleaner days between systems.

After a potentially cold start to the month, especially in Scotland and the NE with a chance of snowfall

it becomes milder everywhere. The risk of frost becomes very small with time too.

Tough times, especially for southern cold weather fans.

The ECM 0z is trickling out and there is a crumb of comfort such that the PV is aligned further south. This prevents the obliteration of the cold pool over Scandinavia. Height rises over Greenland offers some longer term hope for cold air advection from the NE.

We shall see.

GEFS0z looking very poor for the prospects of cold weather, although the op run turns it colder for a few days. 

Also a height rise from the south looks a distinct possibility and this could cause the rain totals in the north west to stack up again.