Kasim Awan

The sounding profile, i.e. freezing level, warm layers, are crucial. And hi res output which nowadays models evaporative cooling well.. HIRLAM for example was very good with the 4th Dec event. I have personally made an initial assessment on Xmas eve snow line, risk etc. In terms of todays output, yes it's good but not incredible. Trough precip would potentially be somewhat marginal in places, even with the lower heights. However, there is time for the output to change it's upstream handling and therefore cool down the uppers. I think uppers usually decrease within 100h due to higher resolution. I would also like to see a weaker low as this would allow better meeting of the air masses, i.e. the cold air into the precip zone helping to produce snow... The air mass meeting discussed in smaller lows also helps trough formation. I fear on the ecm the low is too large for this to be particularly effective... So. The broad scale evolution is full of potential. That potential will be better realised under a few tweaks. Further east for reasons already discussed and as above. All I want for Christmas is a weaker low on a more easterly trajectory!

ECM 240h is a far cry from a breakdown.. Primed to produce immobilized trough energy shunting south east..

If the low size reduces a bit, expect a more favorable synoptic for short wave development. Under the heights evap cooling would easily allow for snow. Don't forget the 528 DAM is covering the UK - the 4th Dec 2020 event only had -2 uppers.

All depends on the size & location of the low. Shortwave central as per a more UK based centre would limit North Sea shower potential and moderate uppers potentially leading to a poorer situation for you. I would prefer a straight northerly run for the immediate east coast anyway.

GFS mean actually suggests the suite somewhat favours this more straight line northerlt solution. Given the ECM is also more easterly in the placement, the GEM is a reasonable middle ground.

GEM also in with stronger wave breaking around Newfoundland. The upstream result is a progression towards the GFS solution.

Freezing level of 100-200 meters inland on that... given nocturnal cooling that's snow all the way. Then factor in potential 850 upgrades. North York Moors hits the jackpot.

Ditto for GFS

It's basically trough disruption tilted on a nw-se axis, helping to feed the cold pool. This likely given a lack of greenland blocking from the model solutions I've seen. From a perspective of surface conditions this isn't bad at all, especially if the amplification is ample to continually immobilize the heights that have shunted south east. P.S., now I know even uttering the word "Z****", regardless of context, is enough to send people on here into a nervous breakdown

I don't think you read / understood my earlier post. Anyway, this is getting tiresome.

Lets see if the UKMO produces better amplification around Newfoundland at 144h. I'm still very much 5050 re the surge in heights through Greenland shown on the gfs...

Sure.. Again I am using a crude description of eddies using a localized zonal index. People always go against the mainstream with adjectives. It doesn't automatically mean the concept is wrong.

There would be no surface inversion in the modelled set up. In normal lapse rates -6 uppers, in a convective set up, would produce surface temps at 150ft of roughly 3C by day & 0C by night and a freezing level of 380m. This would definitely be somewhat marginal for areas below around 400ft/120m. Bear in mind heavy ppn would effectively remove the top-down lapse rate to near 0 eroding the >0c layer. This would produce a lower snow line.. I also want to clear up confusion about the word zonality and the whole concept of it. When I used the word "zonal" this morning, I was not describing a positive nao / westerlies.. I was refering to zonality as an index which is how many refer to it as. So any increase in zonality does not instantly mean roaring westerlies, instead any change in part of the pattern which increaes the % of westerly vectors on the map for the area being described as having a zonal tendency. This is where the distinction between "zonal" and "zonal component" is made. The movement of low pressure out of south west GL for example as modelled on the ECM at day 9-10 is an example of an embedded zonal component in a reversed environment. This is localized zonality,, not "zonal" conditions. I also made no reference to surface conditions, which may even benefit from the greater values projected for the zonal component around Greenland adding in instability to the cold air mass. The ECM/ GEM still have this polar jet injectio, though it has been phased out a little. So still not 100% keen on the GFS Greenland solution but the option is there.

Well, we are seeing model alignment towards a very nice progression.. I personally would like to see improved ridging & tilting of the ridging at Newfoundland on the next few runs at 120-144h for added confidence. Plus the ECM still leaves room for sliding lows which would deepen the cold pool to our east whilst providing means of polar low development.. The stronger Greenland high on the GFS is certainly not a guarantee, however in both cases the trajectory is cold it has to be said

Yep. Coldest air is on the western boundary of the low. Some room for low pressure into Greenland would assist additional northerlies too. A mix of gfs amplification and polar jet signwave would suit me fine.

Lets let the model run first..

There is a significant difference around Newfoundland there, this would have huge downstream / onwards implications. The GFS is the only model to support the stronger ridging here. That's why the gfs solution should not be favoured imo.

Polar zonality on a nw-se with Arctic injections seems most credible then...

There is a difference between a zonal component and complete zonality - the former when associated with a cold spell produces more of toppler type characteristics / progression, rather than a complete cut off from the polar yet, like ecm 12z yesterday. This progression at the day 7-9 timeframe, which is imo favoured, does not guarantee a sudden break down either. I can't see a gfs carbon copy verifying given the more credible output is more sensible in the evolution.

The more easterly low track has been favoured by the majority of output for 2 runs now. I favour a gem+ecm combo more than the gfs, I don't disregard the gfs completely, it's idea of a huge cut otf low which develops into a NErly seems a little "optimistic". Instead, a more zonal component as faboured would do the following: a more easterly low track will drive in colder uppers. This seen very nicely on the GEM. It also means showers would be plentiful. As the ridge is pushed eastwards, the northerly is initiated. This also hinted at on the ECM. This would be better for many than the GFS..

The ec, gem and icon all have a significantly more zonal / westerly component to the cold injection. The GFS has made some considerable progress towards producing this outcome by more realistically dealing with shortwave activity during the ridge pivoting. This is a classic scenario, imho, where the gfs is playing catchup with other output. Hopefully I'm wrong. The "other output" is just as fugitive for cold / snow risk; more a continued nw-se train with temporary colder sectors, than a complete east circulation / deep cold shown on the gfs. Look at the ec clusters and you will see this

Slightly more positive (flatter) tilt to the gfs - coming in line with other output's more zonal cold blast theme Edit: by more zonal I mean more of a nw-se jet than a ne-sw jet as shown on the gfs. I thought zonal was an adjective, describing zonality as a magnitude / index than a definitive westerly is how I think.

Still, the NW-SE jet alignment is an intriguing trend, and would likely bring both cold air and precipitation.

One is 15th Dec 2010 and one is ECM 144H. You decide. Just saying the upstream component has *potential*. No guarantee that this will be realised.

Didn't know we were expecting these models to predict tornadoes Nontheless, ECM looks chilly fot Xmas eve. Already a good deal of cold air to the north so even the modest nortberly produces uppers of -6/-7, though this run has heigher heights so lower shower risk