Kasim Awan

Had the low been a little further north or the Iberian high less intense it would be a much different story.

Yes but inaccuracies can't make lows appear out of thin air.

Yes increasingly likely snow free South of say Derby on the mainland next 7-8 days, could be changes yet though.

We could end up very lucky or very unlucky depending on the positioning of lows. There is the potential here for a widespread, heavy snow event given the instability around and placement of cold air. However, on some of the runs shown this morning most of the troughs which develop miss the mainland so equally the potential is there for a dry cold spell. So there is no guarantee that "troughs will pop up everywhere in the flow", this is not quite correct, as we need optimal synoptics for troughs to develop. The GEFS does bring that low mid next week significantly further North, so a big snow event from that is still on the cards. Other chances come from the low moving south east Monday into Tuesday and early signs suggesting a trough could develop to the West of Ireland later next week. These are our main chances next week. Original post: https://community.netweather.tv/topic/99760-model-output-discussion-colder-but-how-cold-and-for-how-long/?do=findComment&comment=5008099

The convective NW flow is becoming less likely now as most output has a trough sliding down from the NW with a low moving up from the South. In the past these instances have struggled to produce much in NW England, as we are usually in an area of higher pressure between the two systems. The trough would ofcourse be more problematic than the NW flow with issues from placement and core warming raising their hands. But also a nice risk reward scenario. So we're better off without the trough in my opinion. If the trough doesn't form then the flow will be a straight NWrly with movement from NNW to WNW. If the trough forms it will prevent any streamers from forming. The potential NW flow looks great for snow to almost all elevations with significant streamer activity. Probably bouts of 3-7cm in the region with a localized 10cm in the NW flow.

To me it looks like models are trying to prolong the cold spell with height rises to our SE, such as on the GEM this morning. The energy coming out of Canada also looks to be tilted on a NW/SE axis so wouldn't take much to disrupt East and produce an easterly by day 8/9.

Again some short term upgrades in terms of convective snow showers mostly confined to West Wales / Scotland / Northern Ireland Monday with scattered 2-7cm for around 50% of areas close to coast and 1-4cm on immediate coast. Locally 5-10cm Northern Scotland & elevated parts of West Wales. There is a split in output regarding snow for next Monday / Tuesday in that Irish sea development into NW England / NW Midlands is dependent on no trough forming to the NW / pushing inland. Models which generate this trough significantly reduce Irish sea development vs a clean NW flow with no trough. In the trough instance a fairly widespread 2-15cm in the affected area due to the slow moving component on the slow down phase of the trough cycle, however some marginal warming is possible in the trough core. The streamer potential in the instance of a weaker trough is very significant into NW England with 12-16 hours of optimal conditions over the Irish sea.

Ooo I was right Not great intensity though, only produces 1-3cm along the coast.

ECM is now pushing heavy snow into M4 corridor south. Edit: (It might not, just guessing based on what 120H looks like.).

The low would have drastically dropped surface and air temperatures and a more northerly track would help strengthen blocking to our North. So in my opinion it is a negative if the low misses us. If models show a lack of snowfall, if they verified there wouldn't likely be any snowfall. Inaccuracies in model precipitation predictions are due to the output being incorrect, not the models inaccurately resolving precipitation for the synoptics shown. So we need to see more active troughs / more unstable flow to really increase the snow risk. In the last 48 hours the mainland UK has been indicated to sit between two active troughs, one into Scotland and one into France. In the center there would be an absolute meteorological requirement for higher pressure to form, producing a lack of instability. So if we are placed between these two systems then you can bank on very dry weather. So we need to see a shift in one of these lows early next week. This would only happen with a significant northwards or southwards shift in the pattern. What happens later next week isn't worth discussing until we've got through this first bit of uncertainty in output.

Depends if the trough scuppers the convective flow let's see what the UKV shows.

We could end up very lucky or very unlucky depending on the positioning of lows. There is the potential here for a widespread, heavy snow event given the instability around and placement of cold air. However, on some of the runs shown this morning most of the troughs which develop miss the mainland so equally the potential is there for a dry cold spell. So there is no guarantee that "troughs will pop up everywhere in the flow", this is not quite correct, as we need optimal synoptics for troughs to develop. The GEFS does bring that low mid next week significantly further North, so a big snow event from that is still on the cards. Other chances come from the low moving south east Monday into Tuesday and early signs suggesting a trough could develop to the West of Ireland later next week. These are our main chances next week.

Whilst these are great charts, they show a real lack of the convective wind speeds required for snow showers and low pressure structure required for trough development.

It feels like upto 180h GFS 06z has everything wintry possible but lets us have none of it. I'm hopeful we see changes in the modelled positioning of the lows etc.

The settling snowline was 50m and 5 miles inland.

You are spinning my objective analysis of output into a forecast. And you have just contradicted yourself saying I should have used raw data then in the next paragraph say raw output can't be trusted. The 20cm was objective analysis of one model run and was roughly 4x the base instability values than this Monday which gave 5cm max. I am not an amateur. I said 2-7cm in Kent Monday when most models had 1cm max. Thanks, Kasim.

Plus you are now implying that any analysis of model output is that person making a forecast. So if someone points out the ecm would keep us cold, they're forecasting this as an outcome? That's not how it works. P.s. me pointing out the streamer potential was valid because parts of the South East saw heavy snow. There could also be some validity to my points about issues with a lack of precipitation too. These are valid, supported observations and I am perplexed as to why they are met with so much friction.

No mate, I was saying what the operations were showing which not a prediction of what is going to happen. It's too early to make any prediction so my posts are just speculation / objective analysis of conditions not definitive forecast anyone with any inkling in meteorology would know that. So was the 20cm based on one ecm run was not any sort of prediction. I predicted 2-7cm for West Kent during the event just gone.

Yes I'm not saying it will indefinitely be dry just that the models look dry with a lack of fronts / showers this morning. Not sure what is so bad about that?

Hi Joe - my answer is, slack yes but not too slack - a 20knot wind with some convective energy is perfect, not a low centered over the UK with 5/10 knot winds, the latter will just produce rogue showers and the odd confined trough.

Yes lots to be resolved and we could be very lucky or very unlucky. All part of the chase. If we miss the Icelandic low shunting from the NW around 120h aswell as the low to our South then (we will be unlucky) plus any showers will be limited to the immediate coast with winds maxing out at 5-10knots. So we've got 2 main chances in the next 8 days as far as I see it, and as TillyS says the trend has been somewhat negative in terms of the low over the past 24 hours with a trend southwards, and most populated areas don't really have a convective flow to fall back on for snow streamers etc, away from the North / North West. We can't be three times unlucky, one has to deliver I guess, that would be rather exceptional.

Yeah only Kent and North York Moors got anything significant out of December 2022. Looking at the charts one might think instability would be extensive with features popping up regularly: When in reality the lack of thermal gradient in the slack flow only results in very localized (but intense) features that don't last particularly long either, often only covering one or two counties. For the type of trough development many on here are suggesting is possible, i.e. covering large portions of the UK, a far more unstable flow is required. For example, this flow from November 2005. Or progression of a larger trough / low such as the one from Iceland / to our South West could provide the extent of convection required for more widespread snowfall. Now I'm not suggesting that there wont be any snow chances in the upcoming cold spell. Just that if we end up with a quiet area of low pressure under slack winds then likelihood in building widespread heavy troughs is quite low, as proven by the previous cold spell and December 2022. This is proven by the accumulation charts shown by the GFS/ECM/GEM etc. These are fair in giving an indication of potential and if the slack flow indicated was indicative of heavy trough development then these charts would reflect this and show more extensive snow accumulation. This could still change though. We may still source an active and larger area of low pressure which is certainly possible.

I don't think there was any such streamer in early 2009.

Yes but with a slack flow the coverage of these features will be relatively poor, slack flow also means they will struggle to develop into larger features. December 2022 and November 2023 spring to mind. We need the low from Iceland & the one to our South to make better inroads for anything really significant otherwise a slack flow will just generate a lot of very dry cold weather with snow limited to a few coastal areas.

For 8 days of very cold weather that is very poor snow coverage.