Nick F

That's a sizeable shift north by 06z GFS for Sunday's slider low compared to the 00z, pulled into line with EC now and may stay that way ... 00z06z Perhaps a little preceding snow before turning to rain for most away from Scotland and far N of England

Here’s the EC46 Week 3 showing hints of +ve heights / ridging to our N and NE. Now that would be a nice present for Xmas for many on here ...

The arctic blast from Friday seems to be taking on an increasingly NWly wind direction compared to previous more Nly runs, so looks like western coastal areas most prone to frequent snow showers Friday and Saturday, with snow showers getting through the Cheshire Gap to the Midlands too. Though by Sunday showers tending to die out in the west and confined to eastern coasts Also of note is pretty decent T850s for a NWly across the board, -10C as far SE as Kent on Saturday morning. Some low maxima at the weekend too, bearing in mind GFS maybe too high by a degree or two. On to the slider lows, no luck with the first one, but second time lucky?

Yep, GFS sticking to its guns on the 06z, the precipitation from the slider system still too far SW to bring any snow though where it meets the cold air. The idea that GFS wants to undercut a block over Iceland is very intriguing, given the Euros don’t, which models will give in to who? The 12z output may or may not give us the answers! Which makes it all the more interesting model viewing.

Mesoscale picture somewhat uncertain beyond T+120 wrt the slider low track late weekend, perhaps in between the GFS and EC maybe the best guidance unless the GFS takes a big deviation north with the low, which is possible given that it has been tending to trend too far south with the jet recently. A look at the 00z EC standard deviation of high res MSLP to the spread of the ens suggests an increased level of uncertainty to the NW near Greenland next weekend where the slider low originates: But the bigger picture is still looking good for episodes of cold or cold to continue as we head to mid-month. The 7-12 (5) day EPS mean indicates the Euro trough still entrenched and mean ridge toward Iceland to the west. Also that +PNA ridge continuing to warm the arctic at the H5 level. So even though there’s uncertainty in the mesoscale, such as slider low track late weekend, there does look to be propensity for ridging to return to our west and NW.

Nice southerly tracking jet in 18z GFS FI, courtesy of high latitude blocking over Russia and Barents Sea extending across Greenland eventually and forcing the jet south. The extended EC/GFS ENS and GFS operational runs to day 15/16 over recent days have shown a distinct lack of interest in strengthening and shifting the tropospheric PV to its normal home over Greenland. Rather, instead, shown +ve heights over the pole, created by the +PNA ridge and a little help later on from +ve heights over Russian too, keeping the amplified status quo with the trop PV fragmented into H5 vortices over N America, Europe and E Asia. Which is encouraging for longer term cold patterns re-occurring or persisting through December. In the meantime, 18z shows an interesting little disturbance running S/SE early Friday, unlikely to verify as this track or timing this far out, but goes to show there will be troughs in the flow of the northerly, which could mean its not the just the usual coastal suspects benefitting from the wishbone effect, but chances of some snow getting inland too. Also fax charts updated today have feature running down the west

Indeed, the frigid deep cold vortex over Canada and N / NE USA will inevitably serve to tighten the thermal gradient and fire up the jet downstream over the N Atlantic, just hope that upper trough covering much of Europe remains stationary and keeps the jet to the west of the UK diving SE towards Iberia. Fortunately EPS mean seems to suggest the standing wave trough over Europe ain't shifting still at day 10 and still some ridging in the means toward Iceland too:

Steep lapse rates, but if that were over the relatively warmer seas it would look much more unstable! FWIW, Norwegian and North Seas look most primed for instability on GFS, bearing in mind polar lows are convectively driven. Obviously PLs can only be forecast or seen to develop 24hr or less out!

Please can we stick to the models and not clutter up the thread with the wrongs and rights of the BBC!

Slider low way way to the west on the 18z, no where near our shores. Still, some wedges of -40c air at 500mb dropping down from Greenland may induce a polar low to the north that could bring more organised snow to northern areas. Eventually the ridge to our west collapses to allow winds to back westerly, but that’s FI and not worth worrying over for now ...

The beauty of the 12z GFS and EC ops and ens are that they are displaying a standing / stationary wave pattern over N America and Europe. Stationary highly amplified upper trough digging down over Europe provides continual cold air advection over the UK, so when those secondary lows slide down from the NW, with plenty of moisture and energy wrapped in them, they create the ideal scenario for a big snow event on the eastern and northern flank of the low where they pass and bump into the cold air. Good confidence on a cold northerly incursion arriving by/on Thursday, potential for sufficient undercut of the cold and dry arctic air for some back edge snow in places or maybe widespread as per GFS as the cold front clears SE. Followed by the usual wishbone effect snow showers down windward coasts Friday and Saturday, unless the troughs in the flow bring something more organised. The potential for slider lows really in FI territory for now. But, for fun, 12z EC shows a heavy snow event for Wales and central S England for Sun 10/12 Dec. Can’t show the snow charts but here the ppn chart from EC for 10/12: There is a fine line between being under snow on the cold side of such a secondary low sliding SE or under rain in the mild sector. Or, indeed, staying cold and dry. GFS has the slider low too far west to benefit anyone, so cold and dry away from coastal wintry showers. So easy to get carried away and engrossed in the details of these teasing snowy scenarios that one easily forgets that it is unlikely to verify at 8 days out! Nevetheless, 12z GFS and EC operational runs both show these slider lows running into cold air already over the UK, as does the EPS control too, so perhaps a higher chance this evolution could come off. However, I still have my tin hat ready, just in case it all implodes and ends up like the 12z GEM -with the inevitable toys flying.

Interesting on the 12z GFS how the secondary lows dropping SE from S Greenland are forced away from the UK down the far west by the deep CAA wrapping around the cold vortex over Norway, keeping those mild sectors at bay to the west. This keeps the UK in a more protracted cold spell, maybe with a potential snow event in the west where these secondary lows interact with the CAA.

So, 00z GFS and UKMO looking good for a more protracted Nly, 00z ECM and also GEM keen on a brief Nly quickly toppled from the NW. Potential that the EC is underestimating the cross polar effect on heights of the highly anomalous and poleward extending Pacific blocking ridge (+PNA ridge / -EPO), because if this cross polar ridging builds in enough across Greenland, it will prevent the gyres of the tropospheric PV over Canada building in across Greenland area and promote ridging both from the pole and N Atlantic across this area to our NW. The EC is very keen to spread low heights east across greenland from the Canadian PV, but the EPS mean and control suggest this may kept in check by ridging in the area. Unfortunately, can't easily ignore the ECM, but I don't think we should be resigned to the brief Nly and toppler from the NW as advertised just yet.

12z GFS op wind speed, was off the scale on 8/12 when it has its day-after-tomorrow superstorm, clear outlier for depth then ...

Increasing confidence in another Nly at the end of next week into next weekend. To get the northerly, we need the upper trough over the NW Atlantic during the week to disrupt into a cut-off upper low, which will allow a ridge to build quickly to the west in wake of a ‘trigger low’ scooting across the UK from the west, this will allow a deep northerly flow from the arctic. This trough disruption likely due to the major amplification of a long-wave trough upstream over eastern N America. But, like this weekend, it may be toppled from the NW after a few days, shorter if you believe 12z EC det., given the propensity for the trop PV over Canada to expand eastward toward Greenland and strengthen/flatten the jet downstream over the far N Atlantic at the same time. However, looking at the extended ensemble guidance from EPS and GEFS so far today, there seems to be an appetite for ridging to rebuild / re-strengthen near/over Greenland again. Op highlights a marked +ve height anomaly over the arctic which is a response to a highly amplified PNA ridge along the west coast of N America, extending further and further north and warming up the uppers over the arctic and creating a -AO. This in turn reaching the over side of the pole toward Greenland and pushing cold vortices south into eastern N America and Europe. This increasingly amplified and blocky wave pattern developing upstream, can be linked to the anomalously strong E Asia jet extending out to the NE Pacific weakening, likely a result of E Asia mountain torque event, which will buckle the zonal flow downstream over N America and slow it down, strengthening a west coast PNA ridge and then carving out a deepening long-wave trough over eastern N America. While this amplification of the wave lengths from the Pacific across N America will also likely lead to blocking re-strengthening around the Greenland area again – helped by some cross-polar ridging too. Looking further ahead still into mid-late December, signs that the MJO could be in phase 7, which will further aid high latitude blocking and potential for cold patterns across parts of Europe including UK. So, in conclusion, plenty for coldies to be upbeat about, though no prolonged cold and snowy patterns on the cards for now, protracted zonal and mild conditions in association with a strong PV over Greenland, which so often blight our weather in early winter, seem unlikely for a while IMO. The northern hemispheric pattern looks predisposed to amplification and high latitude blocking as we head through early December.

12z UKMO and GEM not quite so alarmingly deep or slow with the Nly trigger low, tending to track the low away NE into Scandi as per UKMO or NE across N Germany then Baltic as per GEM - where it deepens rapidly.

Wow at the 12z GFS, blizzard/snow storm of the century for northern and western parts of UK next Friday into Saturday! Caveat that GFS tends to overblow depressions at that range, but the fact that it's been showing for a few runs or more now, worth keeping an eye on. The slow-movement of the deep low over the N Sea very unusual and appears to be tied in with the stationary long wave trough over Europe and its removal from the strong NLy jet further west meaning it doesn't have the impetus to move quickly on like normal depressions that move west to east or southwest to northeast. Alarming charts!

In Paddock Wood, Kent outside doing a tree survey and got this lot of snow heading for me, already a few flakes before the main lot

First snow flakes of the season from a shower driving through west Kent earlier this morning, hit a patch of black ice on a wet bend of a country lane too, managed to recover fortunately.

Something for coldies to make sleep easier tonight, the 12z EC det. was a warm outlier toward the end for London

Really think some are rushing too quickly in presuming that a failure in the northerly next week is a dead cert based on 12z EC and 18z GFS. Yes there’s a good possibility it may transpire this way, but I personally would put low confidence in model guidance after T+144. Here’s why: - I have noticed and posted recently about the models perhaps not handling with too much confidence the meandering southern and northern branches that extend east over the Atlantic from a split in the jet off eastern N America. Take early next Wednesday the jet profiles are different in orientation and strength over the N Atlantic sector 12z v 18z, this has a knock-on effect of trough and ridge / surface high and low placement and movement. 18z essentially flatter with the northern arm 12z18z - 12z EC standard deviation charts for ENS and the deterministic run highlight a relatively higher degree of uncertainty to the NW and N of the UK in particular for MSLP and 500mb for mid-next week too So a few reasons at least why to sit on the fence rather than resign to the idea that the 12z EC and 18z GFS ops are correct and say ‘at least its only early December’ The 12z EPS mean does flatten out the ridge in the Atlantic too in the medium to extended range, but this hides the uncertainty beyond t+144 I think tonight.

The meandering southern and northern branches of the split jet stream over N Atlantic sector causing plenty of uncertainty and low confidence with model guidance in the medium range ... because they can’t seem to resolve where to place the ridge between the two branches and where the southern and northern arms meet this side of the Atlantic. GFS blows up lows on the northern arm on the 00z then drops them on the 06z in the same timeframe, and along the southern arm it can’t decide how far east to move the lows toward Europe, combine that with cold arctic air being pushed south from the pole by the -AO height pattern developing. It turns into rather a roller coaster of evolutions on each new run!

As I mentioned last night, the split flow over the N Atlantic isn’t doing any favours wrt to maintaining the N Atlantic block far enough north, increased energy going into the southern extension of the jet over the Atlantic comes with increased troughing between New Foundland and the Azores - which pushes the Mid-Atlantic ridge toward western Europe. The reason for the split flow is the block currently over the Atlantic and a zonal flow upstream over N America which stretches back all the way to E Asia. The flow over N America does amplify in the medium range, as the strong jet over E Asia relaxes to allow the flow to buckle downstream with NA west coast ridge and deepening east coast trough ... the hope for coldies is that this amplification translates downstream to build the N Atlantic block towards Greenland again, allowing another Nly reload. But there are no guarantees on this evolution, particularly if the trop PV over N Canada migrates low heights further east over Greenland -which would mean the jet flattening out over the Atlantic rather than amplify.

I think the trend away from protracted amplification of the upper flow continuing into the medium range stems from the split flow that develops off eastern N America. The southern arm extending east out into the Atlantic tries to recurve NE mid Atlantic which forces the Atlantic block toward Western Europe, while also giving a west-based -NAO. Whilst the northern arm flattens out over the top preventing the cold arctic getting far enough south. The saving grace later in the 18z GFS run and, indeed, the 12z EC at day 10, is that the flow buckles big time over N America as a long wave ridge develops over the west coast, which teleconnects downstream to amplification over the N Atlantic to allow a Nly reload, but not as soon as early next week as them models were advertising before. A lot could change before then, so more sitting on the fence until we see another Nly reload.

A look at the 12z EC operational run against the ensembles for London, suggests no cold outlier at any stage, it warms up a little late next weekend and early Mon 4th, if anything a little too warm compared to the average, before a big drop in temperature with the Nly reload by Tuesday 5th. The average staying 5C or below right out to the 11th Dec for London. Certainly no worries there ...