Singularity

Localised zonal flow is a thing, west-east wind. Localised zonality isn’t because zonality refers to planetary scale waves (Rossby waves) progressing west-east. I.e the broad troughs that we see within the jet stream. That can never be a local scale event. Anyway back to the models - EPS are about 29 to 22 against GFS but those 22 are all in one cluster… the largest one. Very clarifying… not!

It’s a tale of two secondary lows on the main own to our west, one moving into it from the northwest Wed-Thu then another from the northeast Fri-Sat. GFS gets where it does by developing neither. Could it really be calling that right over UKM & GEM, with the first call being at under 5 days lead time? Seems odds against, although over-development of secondary lows is one of the most common general modelling biases. UKM was exploring not developing the first of the lows until yesterday’s 12z.

You've essentially described the typical response to the MJO traversing phases 5-7 in Dec. That's what EPS currently indicate when allowing for some interference from the Indian Ocean subduing the mean: On which basis, a mild festive period then colder by New Year via Scandinavian high pressure development is highly plausible. The UK will be in the uncertain zone on the NW periphery of where anomalously high heights are encouraged over Europe by MJO phase 5. Usually it turns out mild for most of the nation but this time there will be an exceptionally strong Arctic high complicating matters & keeping cold chances a bit higher for northern parts especially. The fine balance of the situation is well demonstrated when considering the 00z GEM versus 00z ECM, UKM, & GFS. Unlike those three, it didn't develop the low merging into the northern flank of the trough this weekend into the dominant feature by Monday. Just by leaving a bit more room for high pressure to the W & NW of the UK next Tue-Wed, the path of the next approaching Atlantic low was diverted fully south of the UK. It's of course unlikely that GEM will trump the other three at such short lead time but on the other hand, those three are showing an extreme scenario with how that low behaves (it becomes impressively deep for such a compact system), so I don't think it's as unlikely as would normally be the case. Even so, here in the far south of England, I'm bracing for a notably wet week to come (potentially near a whole Dec's worth of rainfall!) with temps mild then either still mild or near average through to at least Friday, which is more or less the current ensemble consensus. Original post: https://community.netweather.tv/topic/98087-model-output-discussion-the-cold-has-arrived/?do=findComment&comment=4770527

You've essentially described the typical response to the MJO traversing phases 5-7 in Dec. That's what EPS currently indicate when allowing for some interference from the Indian Ocean subduing the mean: On which basis, a mild festive period then colder by New Year via Scandinavian high pressure development is highly plausible. The UK will be in the uncertain zone on the NW periphery of where anomalously high heights are encouraged over Europe by MJO phase 5. Usually it turns out mild for most of the nation but this time there will be an exceptionally strong Arctic high complicating matters & keeping cold chances a bit higher for northern parts especially. The fine balance of the situation is well demonstrated when considering the 00z GEM versus 00z ECM, UKM, & GFS. Unlike those three, it didn't develop the low merging into the northern flank of the trough this weekend into the dominant feature by Monday. Just by leaving a bit more room for high pressure to the W & NW of the UK next Tue-Wed, the path of the next approaching Atlantic low was diverted fully south of the UK. It's of course unlikely that GEM will trump the other three at such short lead time but on the other hand, those three are showing an extreme scenario with how that low behaves (it becomes impressively deep for such a compact system), so I don't think it's as unlikely as would normally be the case. Even so, here in the far south of England, I'm bracing for a notably wet week to come (potentially near a whole Dec's worth of rainfall!) with temps mild then either still mild or near average through to at least Friday, which is more or less the current ensemble consensus.

This weekend, just a couple of days from now, a little low by Iceland will interact with the trough from the Atlantic and either: - Be absorbed into the circulation of the bigger low as that races past to its southeast, the whole lot then heading to Scandinavia by Wed or - Avoid being absorbed and then develop into an intense low that holds up the trough across the UK for a day or two while maturing to the N or NW of the UK That 2nd scenario has suddenly become the majority vote of the deterministic runs this evening. It leaves less room for a ridge to our west, allowing the trough to the west of that to cut across and send over a low or two to visit our shores. At such short lead time this would usually be final, but that little low looks very borderline to me, its future highly sensitive to fine details. Hence we still see a fair few colder outcomes in the ensemble sets even as the mean has shifted milder. So who knows, things might look markedly different yet again tomorrow. Or they might not, in which case I’ll frankly enjoy the reduced heating bills for the 4-5 days or however long the milder interlude lasts (I’m really not convinced by runs that keep it going 7+ days). Original post: https://community.netweather.tv/topic/98087-model-output-discussion-the-cold-has-arrived/?do=findComment&comment=4769844

This weekend, just a couple of days from now, a little low by Iceland will interact with the trough from the Atlantic and either: - Be absorbed into the circulation of the bigger low as that races past to its southeast, the whole lot then heading to Scandinavia by Wed or - Avoid being absorbed and then develop into an intense low that holds up the trough across the UK for a day or two while maturing to the N or NW of the UK That 2nd scenario has suddenly become the majority vote of the deterministic runs this evening. It leaves less room for a ridge to our west, allowing the trough to the west of that to cut across and send over a low or two to visit our shores. At such short lead time this would usually be final, but that little low looks very borderline to me, its future highly sensitive to fine details. Hence we still see a fair few colder outcomes in the ensemble sets even as the mean has shifted milder. So who knows, things might look markedly different yet again tomorrow. Or they might not, in which case I’ll frankly enjoy the reduced heating bills for the 4-5 days or however long the milder interlude lasts (I’m really not convinced by runs that keep it going 7+ days).

Not much of a signal from the tropics for the models to work with which doesn’t help matters. A somewhat muddled MJO influence from the Maritime Continent or thereabouts, not driving any big changes in AAM unless a positive mountain torque occurs over East Asia which seems pretty uncertain. Any west-based negative NAO setups are pretty much happenstance arising due to the movement of the powerful Arctic High that others have flagged up. Same goes for central or east based ones, pretty much a raffle at this stage! Original post: https://community.netweather.tv/topic/98087-model-output-discussion-the-cold-has-arrived/?do=findComment&comment=4768288

Not much of a signal from the tropics for the models to work with which doesn’t help matters. A somewhat muddled MJO influence from the Maritime Continent or thereabouts, not driving any big changes in AAM unless a positive mountain torque occurs over East Asia which seems pretty uncertain. Any west-based negative NAO setups are pretty much happenstance arising due to the movement of the powerful Arctic High that others have flagged up. Same goes for central or east based ones, pretty much a raffle at this stage!

The westerly flow is about a 1km or so above the surface so the moors won't be affecting it much. Radar artefacts are creating both false precipitation and false dry zones. However, I think the main issue is loss to evaporation on the way down. So probably at most a dusting (< 1 cm) for affected areas of Dorset & Hants unless rates hold up enough to sufficiently moisten the near-surface air in the face of a NE to NNE feed undercutting it... which seems a long shot!

Something else to bear in mind is that with the surface flow being from the NE and the upper flow W, snowflakes are going to do a fair bit of travelling in the horizontal before reaching the ground. So there will be some lag between the radar detecting precipitation overhead and it being observed at the surface. With that alongside all the anaprop going on, it’s a tough night for radar-based anticipation!

Possible that the models are seeing a slightly faster push from the north than before, offsetting the greater progress of the wave up to this point. Not saying that's definitely the case though!

Indeed this is true for anywhere south of a line westward from roughly Exeter, due to easterly surface flow along the Channel. The flow is forecast to gradually back toward NE then NNE during the coming 24 hours, bringing surface temps back down to permit snowfall away from the coasts.

So the UKV model is not covering itself in glory today...! I'm baffled as to why the model is adjusting in the wrong direction as new observations come in.

It's a shame we don't have radars operating from platforms at sea to give us better quality readings to work with when trying to second-guess the models on what will actually unfold in the coming hours. We also seem to have a blocked radar beam issue going on over a large part of Cornwall. Unclear what's real and what's not. Also possible that some or all of what's being detected falling from the clouds isn't making it all the way down. I wonder if anyone between Camelford and Plymouth is seeing precipitation at the mo?

Strangely the 09z UKV run retracted the precipitation west despite the 06z having been much closer to the reality for 10 am. The impact for tonight goes to show how important the current eastward extent is: So for those along the Dorset and Hampshire coasts seeking a little snow, it's a good thing the 06z was nearest the mark.

You can really see the impact of lower grid resolution in the GEFS there! While the deterministic (op) run can just about differentiate Poole from the sea, GEFS average the two out.

Very slightly! I'm in the same boat, nowcasting is just too engrossing.

Radar Extra just jumped to 9:05. The 'arm' is reaching a little east than UKV modelled for 09:00 but otherwise it's pretty close. UKV appears too showery with the NE flank but the radar might be smoothing out some details while it's still so far offshore. The main uncertainty for today is how far east the precipitation makes it within the westerly upper flow before fizzling out. Forecast models are showing little interest in getting it past western Devon at the most but I've seen this process underestimated enough times past to know that some very light snow can't be ruled out further east. Nothing special for most, just a possible decoration to add to the festive ones. Convergence may boost the rates over the moors, though. Tonight, area of positive vorticity is predicted to run east along the boundary zone - its precise track will determine where any heavier precipitation occurs.

It's now showing 8:05 on the mobile version of the radar but still 5:10 on the browser version... suggests they're getting there but still some issues to resolve. Hopefully it'll be back up within an hour or two!

Of all the days for the radar network to take several hours off! As of 4:50 when it cut out there was somewhat closer proximity precipitation than I’ve seen modelled.

Just realised that if you run through the GFS 12z days 8-14 on the N Hem view, you can see a really neat example of a longwave upper ridge retrograding from the Pacific side to Scandinavia. Such longwave features do in fact behave like that, the big question then being whether the polar jet forces it quickly south or allows it to hold at the mid-high latitudes long enough for cold continental air to work its way across to the UK. Just one of many possible ways in which the highly amplified Arctic patterns might play out during the 2nd half of the month. We might instead see the high on the Pacific side try to link with an Atlantic ridge. Either way, seems to be a more dynamic half to the month coming up, with a higher propensity for milder interludes in the south but also increased scope for collisions and interchanges with cold airmasses. Consequentially, it's not looking as dry overall (which is not saying much where I am - all that's accumulated to date is fog and frost!). Original post: https://community.netweather.tv/topic/98087-model-output-discussion-the-cold-has-arrived/?do=findComment&comment=4765393

I see the Met Office have picked up on the possibility too. In fact I'm surprised just how far east their zone of potential reaches! Models often seem to undercook the amount of precipitation generated by outflow aloft from a frontal boundary, so this might be one of those occasions but with the unusual condition that what falls is snow. Some loss to evaporation on the way down limits the probable amounts at low elevations but I'd take even just a cm or two.

Yesterday was a real let down for me snow-wise in the end, but did deliver an impressive hoar frost. Now my attention has turned to tomorrow, which may yet bring something to parts of our region. Cornwall best placed but I do wonder if counties further east might get involved a bit. What are the chances that snowfall stops 5 miles to my south again?

Just realised that if you run through the GFS 12z days 8-14 on the N Hem view, you can see a really neat example of a longwave upper ridge retrograding from the Pacific side to Scandinavia. Such longwave features do in fact behave like that, the big question then being whether the polar jet forces it quickly south or allows it to hold at the mid-high latitudes long enough for cold continental air to work its way across to the UK. Just one of many possible ways in which the highly amplified Arctic patterns might play out during the 2nd half of the month. We might instead see the high on the Pacific side try to link with an Atlantic ridge. Either way, seems to be a more dynamic half to the month coming up, with a higher propensity for milder interludes in the south but also increased scope for collisions and interchanges with cold airmasses. Consequentially, it's not looking as dry overall (which is not saying much where I am - all that's accumulated to date is fog and frost!).

One of the most prevalent biases in numerical weather prediction modelling is concentrating all positive vorticity into low pressure complexes instead of having some escape along the polar front. This has become evident yet again for next weekend. Occasionally a single low pressure complex does actually happen but more often you see breakaway vorticity which we experience as either waves on a weather front or full-blown lows which on occasion can be pretty intense if they sync well with the polar jet. Going forward I’ll be watching for any trends in the high over mainland W Europe with interest in case there’s some over-extension of lagged response to the recent drop in AAM going on - if there is then the models will correct toward less of a high there.