nick sussex

If I was still in the UK I’d probably be in that panic or joy mood with this slider low ! Yes these snow events are rare and these set ups are difficult because normally if it’s just rain the stakes are low but because of the narrow margin for snow or rain any shift in track can make a huge difference .

Very quiet in here which often happens when some snow is imminent for certain areas . This slider low must surely go down in NW history as one of the most microscopically analysed , with every jog south or north by a few miles met with panic or joy ! Putting aside this current saga which is drawing to a close we have seen some interesting trends over the last day on both the ECM and GFS trying to develop a ridge near Scandi towards day ten. Looking at the De Bilt ensembles we’re beginning to see a small cluster developing some very cold conditions. There are hints upstream that we could see a bit more amplitude developing after a relaxation of the positive PNA. This would help as it will begin to pull back the PV and send a bit more energy se which you need to develop lower heights over Southern Europe to support any high. It might be that we might need more than one attempt to get that Scandi high but perhaps it might arrive a bit sooner if the models are still slow at picking up the correct MJO signal . Singularity a few pages back made an excellent post regarding that and it does seem that the models forecasts were wrong and weakening the signal. So after an initial suggestion that things might turn milder it’s possible we could see the mainly cold weather extended .

Interesting ! The GFS 12 hrs run is holding the low further south to the east over Eastern Europe which allows the toppling ridge to the west some support. You want low heights to remain there and not get pulled north as in the earlier 06 hrs run.

Anyway putting aside the slider low which has had more attention than the first moon landing! This mornings ECM isn't quite as good as last night because a lobe of the PV runs from north of Scandi back west to the main chunk over ne Canada however the broad theme is the same. NOAA have commented that we might see a relaxation in the upstream pattern past T168hrs which means the strongly positive PNA weakening so this will allow the PV to edge east but its not clear yet how far east it's going to get. Before then we do see attempts to build a ridge to the ne and still too early to rule this out, the outputs generally agree on another nw/n flow for next weekend then a quieter period.

I think a further complication for tomorrow to add to the already Da Vinci Code set up is the warm front ahead of the main set of fronts. Yesterdays fax chart did not have this extra front and it runs ahead before edging back south again later. Its a real mess overall and it might just come down to looking out of your window!

This slider low has turned into Play Your Cards Right! Instead of couples you've got the southern v northern contingent. Each output is met with desperate pleas of higher or lower or in this case go north or go south! Given the timeframes its less likely you'll get any big corrections but still room for some changes. Anyway the next instalment of the slider low saga or better named attention seeking model celebrity will soon be arriving!

I really hope you get to see some snow there in Dorset otherwise I might start a funding page to get you to Lapland! A positive that might help those towards the south coast is the flow being off shore re that low at T72hrs, so less issues than say if the flow was se with dew points lower with that more ne flow.

Very happy with both the ECM and GFS in terms of developing some further interest in the run up to Christmas. The key thing here is to get that trough disruption upstream. It does seem that the outputs over reacted to some eastwards push of the PV as the strongly positive PNA relaxed. However I'd like to see a few more runs and get that key trough disruption within T144hrs. The pattern does look condusive to a Scandi high developing as energy digs se. But that's still way ahead.

Good trends from the GFS and ECM with the troughing disrupting to the west at T168 and T192hrs , energy heading se'wards. A chance we could see our Scandi friend pop up in future runs if this continues.

What we might be seeing is an initial over reaction in the outputs who were trying to prematurely breakdown the west USA ridge. NOAA comments re a full latitude ridge wouldn't correspond to the PV firing across the Atlantic. Whilst you maintain that ridge this acts as a lasoo around the PV , it can only eject so far and will disrupt some energy se.

This is a slider free zone! Putting aside the above, the medium term is beginning to look a bit more promising with the Atlantic onslaught beginning to hit the buffers. Its noticeable that we're seeing some trough disruption in the GFS 12hrs run to the main upstream trough with some energy heading se, you would expect some pressure rise to the east/ne but it looks a bit messy at the moment but the trend is there. The UKMO has backtracked from its woeful morning output with a bit more amplification upstream . Theres still quite a lot of uncertainty with the exact set up re that deep low in the east USA. More amplified there will build a better ridge to the nw but it will take a few more runs before we're sure of the upstream pattern. I'll add this in from NOAA, this would tend to support the more amplified solutions: 00 UTC GEFS/ECMWF ENSEMBLES OFFER A WELL CLUSTERED FORECAST SOLUTION INTO DAY 6 OR SO THAT MAINTAINS MAX WPC CONTINUITY AND FORECAST CONFIDENCE. A FAVORED COMPOSITE SOLUTION LOCKS IN A FULL LATITUDE WRN NORTH AMERICAN RIDGE/ERN NORTH AMERICAN TROUGH PATTERN ALOFT. THE 00 UTC GEFS MAINTAINS THE WRN NOAM RIDGE MORE THAN THE 06 UTC GFS/00 UTC ECMWF ENSEMBLES DAY7/NEXT FRI AND THAT MAY BE A PRUDENT WAY TO GO GIVEN INITIAL AMPLITUDE

Never has phase timing meant so much in terms of snow prospects! You can clearly see why the GFS 06hrs run is further south because the phase with the trough to the east is delayed which allows a bit more se momentum before the turn to the east. Putting aside the slider saga the GFS is best in the medium term with better separation and amplification upstream, the ECM is in the middle and the UKMO is dire, horrible ! We really need that UKMO to be chucked in the cat litter tray tonight, its flat upstream with no chance of developing a ridge to the nw. All I can say is be gone with you and don't darken this model thread again with that output tonight!

Quite amazing to see what happens with the low at phase point, you can see the track more se at T48hrs and T54hrs then phase happens with the trough to the east at T60hrs and you get that shift eastwards the low centre deepens slightly to 970 which edges this ne causing the pivot then the shortwave to the sw runs se causing a south push to the sw and like those weighing scales you get a push north towards the east of the UK. For that reason its the areas towards the nw which get into the cold are just far enough west so that the pivot towards the east still keeps that area in the cold. I know I'm making this sound very complicated but it really is!

The jet was a bit more amplified upstream but the issue is the phasing with that trough. I can't remember the last time I saw this type of slider set up that was so complicated.

No comment! You have to bear in mind those outputs are US focused, sometimes they do give a preview of the GFS 18hrs run in the early stages but not always. The slider is very complicated and that's why the BBC are being vague because its a difficult forecast.

Everyone here started off on the net as a likely beginner and trying to understand about this complicated weather in the UK. So you shouldn't feel worried about asking questions and theres lots of different threads and a learning area to help you along the way.

Eh! No those outputs only go upto T84 and T87 hrs. This is just regarding the slider low drama.

The time between GFS runs seems to be very long when you're desperate to find out whats going to happen with this slider low especially if you're in the higher risk area for snow. In the mean time we don't have much new data apart from the more USA focused shorter term outputs. Now these come with a caveat they're focused upstream but do include the jet track out of the ne USA towards southern Greenland. So these are the respective changes from their last outputs. The NAM was included in the NOAA diagnostic short term discussions and between its 12hrs and 18hrs, it has slightly increased the amplitude upstream with a small sw correction in terms of jet track in towards the UK for the slider low. This is supported by the change between the 9am and 3pm SREF which does similar, that's the short range ensemble forecast system. PS I'm not making any predictions for the GFS 18hrs run! So don't shoot the messenger!

Someones bound to get lucky with snow as the slider heads se then remaining on the cold side of that as it phases going east and then as the shortwave to the sw runs east and phases this could bring yet more snow as we see the pivot but where that might be is guesswork at the moment! Of course some areas have a higher margin for error and those at present are.........

At short range the ops would normally take precedence given their higher resolution but its clearly a horrible forecast given this slider is unusual, most we see don't engage troughing so close to the UK and then the shortwave to the sw will run east and cause another complication! Its so finely balanced that an error in phasing with the trough of just a couple of hours could cause big problems with the forecast.

I'm relieved I'm not having to be subjected to the slider emotional rollercoaster! The fact the UK gets so little snow for its latitude often means these possible snow events become even more important which is totally understandable. This slider has really played some terrible mind games with snow lovers in this thread! There are many moving pieces here, as a slider its certainly up there with very complicated. The troughing to the east complicates matters as that's likely to phase with energy from the slider low, as that happens the track will get pulled more east than se so timing is important. Theres another added variable with a shortwave in the Atlantic which could run east and develop and head into France also phasing with the trough to the east which may pivot the snow line so overall its a forecasting nightmare.

Looking at the ECM spreads the main one is to the west sw of the UK upto T144hrs suggesting there’s still room for changes after the first slider. In terms of the longer range the ensembles do seem more zonal although a small cluster suggest something from the east given the wind directions and associated dew points/temps. If the PV does eject eastwards then we’re going to need some energy heading se and not just over the top. Anyway that’s a way out and could still change, lots going on before that.

Right enough of this shameless ramping! I’d advise people remain cautious and not start picking out hats for the snowman till the track of the slider and depth of cold gets within T48 hrs. The same thing happened with the GFS super blizzard which met a tragic end. I know it’s hard and it’s exciting because snow has been so rare in recent winters but I have several counselors off on holiday at the moment and the NW helpline simply won’t cope if it goes pear shaped !

Looks like the snow wars have started! These sliders always end in a fracture in relations between those in the high risk versus low risk area! If the cold can hang on for a couple of weeks hopefully everyone will get to see some snow. Peace!

They prefer the ECM and GFS blended for the upstream pattern in the USA which is better because the UKMO and GEM aren't as good for the UK slider low. Theres also the issue of pressure rises near Iceland which NOAA wouldn't cover but the UKMO because it runs a deep low into the PV sends more energy eastwards and the second possible slider is too far east. In the medium term however the UKMO does re-amplify the upstream pattern so not all bad news from it.