The Enforcer

The 18 Jan 2013 had such a good approach, fronts coming in from the south-west, even though the Low had originated from the north-west. What that and your examples have in common, is that the High concedes ground by moving eastwards rather than southwards, so it can influence the wind direction. Low res models at T+120 have been continually over-estimating snowfall amounts on every NW slider scenario since then, because at that range they are under-estimating the impact the mild sector has on dew points. The draw from the cold continent in our examples keeps the dew points down and if this was being projected now then the snow totals would be more realistic.

I've been rooting about, so I can compare the current fax with the last time a NW-based slider (as opposed to a channel low type slider) produced snow rather than rain here: Now: Then: There needs to be an awful lot more south-westward correction to get to the 18th Jan scenario where there is no spoiler mild sector. In particular, check out the direction of the isobars ahead of the fronts. That's what I'm talking about. Consequently, I'm currently focusing on Friday night's "hidden" trough, which is at least closer to becoming a reality:

I had a discussion with a couple of members about this yesterday and then looked the archive chart up earlier - 11/12th Jan, but slider came in from WSW rather than NW and met polar continental air that was pushing in at the same time from the ENE which meant fronts were negatively tilted and why it was successful in these parts. All the sliders since then have approached from the NW and had a positive frontal tilt resulting in rain. Jan 2013 was the last accumulation that lasted a day (Feb 2015 conjured up a trough which gave an overnight covering, but it had melted by lunchtime).

No, it was definitely after the last significantly cold winter 2010/11 and was definitely a January. I think I recall Feb 2007 though.

I thought that one approached from the south-west and tracked pretty much due east. If it is the snow event I think it is, I seem to recall it happening on a Friday and it gave a reasonable covering here too. It's always been from the north-west since then and a failure every time due to inadequate dew points.

I agree entirely, the likelihood of further changes at the 8-day range seems very high and possibly not favourable ones. I am not keen on north-west to south-east sliders anyway. As shown, it looks to me that there is too big a mild sector with 850s widely higher than -5C on the charts that Frosty posted. I suspect that the dew points would be too high as well. The required westward corrections then could mean the snowy part of the system missing most or all of the U.K. A slider attack from the south-west would be much lower risk. The GFS/UKMO have a more snowy forecast for day 5, so I would prefer the next ECM run to provide stronger support for those at that point, rather than continuing to randomly generate/drop random snow events at the latter half of its run.

Another day where the BBC/Metoffice nowcast inspires no confidence. When does the contract change?

Just like they are not forecasting heavy rain for about half of the UK this morning.

The BBC forecast is wrong again this morning. Even the nowcast says it's supposed to be overcast, but dry and then brightening up. However, it's been pouring with rain.

Another atrocious rain forecast from BBC. Website said no rain here until 4pm, but it started raining here just after 11am. Cue another unforecasted soaking.

Took a direct hit from that one. Genuine 'purple' rain on the radar and hail on the ground. BBC forecast for this morning was woeful. Rain was supposed to have cleared first thing and their map was still showing the rain band over north-east coastal counties even when I got up to find it belting down outside. The rain was still there and was persistently heavy until 11am. Why don't they update their nowcasts? The remainder of the day was correct - sunshine and heavy blustery showers culminating in the aforementioned storm cell.

All of these models have 'nowcasts' that currently grossly underplay the extent and intensity of the current rain band moving up from the South:

BBC are not helping their cause by not updating their website graphics since yesterday evening. This shows no rain over the UK at 7pm tomorrow, which is in contrast to the 7:30pm forecast which has rain arriving here by 2pm. However, having checked Hi-Res models, this is an outlier scenario: Arome - 6pm Arpege - 4pm NMM - 1am Euro 4 - 8pm

This has all the trappings so far of another poor short range forecast by the BBC/Met Office. No storms, rain had cleared here by 9am, and no sun since then and a gusting cold wind. Where is the warm sunshine promised in this morning's forecast at around 9:30am?

Ah yes the "poor short-range forecast" strikes again.

Yes the issue that time was that it was off the back of a very mild December and January and first half of February, so that whenever snow did fall it failed to stick. Then there was the customary mild sector with rain, then a cold sector arrived and the precipitation stopped. In the current case, it's not just the uppers that are comfortably cold enough on Saturday, the uppers and dew points over the continent and even over the North Sea itself over which the air is travelling are cold. It has not been a mild winter here and has been very cold over Central Europe, particularly Greece where some of this air is coming from. Computer says rain. I think the overarching theme is: 'if it can rain, it will rain'.

Thanks for this. It is looking more realistic - mostly snow inland on Saturday morning. Well I thought that could be a possible explanation but the less cold air doesn't arrive until Sunday/Monday and the charts are showing snow falling over the continent and the North Sea before and during Saturday.

I picked up on this last night, but didn't get a reply: Why are the dew points not favourable? I am always being told that 850s from an easterly don't need to be as low as a northerly to produce snow (as it happens they are predicted to be significantly lower than the January northerly), because the surface temperatures and dew points coming from a cold continent are lower than when polar maritime air has to cross the Atlantic. On those charts the dew points are lower on the continent and snow is shown to be falling, but this is not having the desired affect. I get that the North Sea would have an impact on the temperatures and dew points close to eastern coastal areas, but surely it can't affect areas hundreds of miles inland? Even the some of the air temperature and dew points directly over the North Sea itself are lower than inland UK and snow is shown as falling there too. The only explanation I can offer is that it is a bad idea to rely on a global low-res model to look at detail instead of one of the higher resolution options. I believe NMM have a 2km uk model. If I am wrong then it seems a pretty fruitless exercise for those already chasing after a reload easterly, when if we achieve another burst of the desired polar continental air that the net result is that precipitation is rain, the same as if the air was coming from any other direction.

I don't get why those surface temperatures are so high for large swathes of the U.K. given the 850s and where the surface flow is coming from. They don't tally with the charts forecasting snow at the same time.

What I don't understand is that I'm always being told that one of the key differences between a northerly and an easterly for the UK is that the 850's in the latter case don't need to be as low for precipitation to fall as snow, because the air is coming off a cold continent rather than having to cross a mild ocean, producing lower dewpoints and surface temperatures in an easterly. From what I can tell the continent is cold on Saturday and the 850's are lower than the recent northerly and yet inexplicably here we are talking about rain again right in the core period of the cold spell. Is this just a case of a global model being a bit imprecise with this type of detail at such range and a more accurate picture would be gleaned from hi-res modelling nearer the time?

T+216: T+6: A lot of similarities, biggest difference is lower heights in the med helping to prop it up the High a little.

I can beat that: Saturday 14th, Arctic air in place, precipitation in arctic flow, rain.

That deals with one end of the spectrum. To deal with the other extreme, my suggestion is that anyone saying "winter is over" could be shipped off to the Spring forum by the mods and the rest of us can join them in March/April.

Lisbon probably has more chance of getting a snowy easterly than London.

I think it's always going to be difficult to get an easterly with the jet tracking north supressing the Scandinavian block southwards unless the jet tracks so far north it goes over the arctic like in 2004 or 2005 I think it was. Most want rid of the UK High, but I have decided I would like it to stay. It is helping Europe to undergo repeated polar continental attacks so that there is even cold air entrenched way to our south. The best scenario for snow would in my view be for the UK High to stick around until the much mooted amplification that seems to be around allows the jet to align so that it approaches from the south-west. Even if the UK High is flabby and simply backs off to the east, this would still entail an approaching Low pressure system dragging in the cold air off the continent over the UK ahead of it. Normally you'd need a easterly/south-easterly flow, but with Spain and Portugal so cold by my reckoning even a straight southerly would suffice. It would then take a lot longer than normal for the atlantic air to override the cold air so a decent snowfall should spread north-eastwards across the UK before it turns milder. If the High holds firmer then a 1980s type battleground with fronts being pushed back would be possible. This is probably an unlikely scenario, but I think I favour it to easterlies which have a high failure rate when consistently modelled and tend to promise 1987 or 1991, but usually deliver much less for one reason or another.