WhiteFox

Days like today are what really drives my interest in the weather! Just look at that perfect squall line that has formed ahead of the cold front: Anyway, dull and drizzly here with winds gusting to 25mph from SW. Pressure 998.2 falling. Temperature 9.6, DP 7 Rainfall since midnight 1.4mm.

I disagree! What you need to do is tune out the noise. The longwave pattern is identical in the two charts that @West is Best posted. The surface analysis is different because it always will be at that range; smaller scale features are enormously complex to forecast. As far as forecasting goes, IMHO, you can only reliably predict PPN amounts and temperature within a range that broadens as you move further out in time (yes, there are times when the range narrows or widens, generally when a pattern change is occurring). Beyond that, you are simply looking at likelihoods and probabilities; no scientific forecaster will be willing to say if it will rain and what the temperature will be at a particular time on a date more than six days in the future. They may forecast above or below average conditions in a range of categories for a period of time, but I don't think that models going past 6 days were ever intended to be taken literally, but rather as a guide. Forecasts do bust, with this winter being a prime example. But, on the whole, NWP models are pretty good when it comes to forecasting PPN (not type!), wind direction and overall conditions out to the six or seven day range. Going further out, I only consider a forecast to be a bust if a predicted pattern change hasn't happened or an unforeseen change did happen (e.g. mobile to blocked) and anything beyond day 10 is simply a possibility rather than a likelihood. A forecast hasn't failed just because a secondary low appeared on one operational output and then does not on the next: that is what I mean by noise.

Yes. It's often said, but look at the overall pattern: the 500mb forecast was largely correct, but it's incredibly difficult to forecast exact conditions closer to the surface at a longer range. Given the range involved, it's pretty good. If you'd looked at the 500mb forecasts you'd have said that the likelihood for the weather in ten days time was unsettled with a high probability of rain at times. And so it transpires.

I suspect that the GFSp needs more than a little tweaking. I've been following some of the recent storms in the US and its predictions for snow amounts in the north east have been utterly woeful: consistently far too high. To be honest, I haven't paid it much attention, but from the few charts I've seen in here it has a consistent bias towards producing cold and blocked output for Europe. That could be because people only post it when it shows cold, but I get the feeling it does this far more frequently than the live GFS. I've seen a couple of forecasters in the US say that the whole model needs to be deleted and to start again!

Some very good points and the one highlighted is the key. Making predictions based on signals is fine, but it's based on what little we know and always has caveats applied to highlight that they are based on probability and are not definite. I fear that sometimes people choose to ignore the probability side and choose instead to make snide remarks, but that should not out anyone off from making a forecast if they base it on sound analysis. We still have a huge amount to learn and have very little data to work with; onwards and upwards! Just to add, it has seemed likely for quite a while now that high pressure setting close to the UK seems the most likely outcome and nothing has changed my view in that respect. The MJO moving into phase 8 means nothing on its own as I posted a few days ago; until the Pacific pattern changes from the current setup there is little to no chance of anything popping up to our north IMHO. I'm interested to know just why the Pacific has been so locked into the same pattern this winter.

Indeed. If you're not pointing out a route to bitter cold you might as well whistle in the wind... For quite a while it's looked like high pressure builds over or just to east of UK. Chance of some surface cold with a chilly continental flow, or a warmer southerly flow with a chance of an Atlantic incursion. Could even be frost and some fog. What does not look likely is a bitter cold outbreak!

The current modelling seems fairly consistent with the projections for the MJO into phase 7. Given the GFS forecast for phase 8 you wouldn't expect that to start showing on output until towards the end of the runs anyway. If the phase 8 amplitude pans out as forecast on GFS then it would be surprising if we did not see some sort of attempt at ridging towards Greenland. For now, the ECM does not back the GFS at the same sort of amplitude though. For the moment, I still think some sort of Sceuro high seems fairly probable in the medium range.

I think they're forecast to phase into the main low. Some very unstable conditions hence the likelihood of secondary waves/lows forming over the weekend.

You've got to bear in mind that the overall pattern is much the same. Compare the 500 heights and you will note that the wave pattern is fairly close between the two. At a more local scale, the small differences in the 500 pattern mean fairly large changes locally at the surface. So, it may seem that the model is jumping around a lot, but on this SE two charts they are fairly consistent.

Excellent satellite picture posted by Marco Petagna on twitter showing the rapid cyclogenesis:

Yes, the Pacific pattern seems to be favouring the cut off upper low over Canada; conflicting signals from what I can see. I think that the EN neutral composite probably sits best as we have not seen the PNA swing towards positive and no Aleutian Low. With the Pacific doing its thing, favouring the low over NE Canada, the Sceuro high seems plausible given the push from the west preventing ridging from building too far north. If the MJO moves into phase 8 (and that GEFS forecast is a huge amplitude) then we may see some forcing on the Pacific pattern which could open the door to Greenland if the Canadian Low pulls away. But, ECM does not favour such a high amplitude for the moment, so perhaps the Canadian low will stay close to its current position leaving the UK in a coldish anticyclonic flow. It's all about the amplitude: it seems that it would take something big to change the Pacific pattern and the GFS seems to be providing it...

It's been clear all winter that Atlantic systems are struggling to push through the UK on a regular basis. I never bought into the high pressure setting up to the south scenario; given a lack of momentum from the west, north-building ridges always seemed a decent possibility. Exactly where the ridge ends up depends very much on the upper low over Canada; nudges slightly east or west will have a large impact on the source of the airmass which we receive.

I've thought for a few days now that surface cold from the continent seems a viable outcome. Without the cut-off upper low moving from Canada there just seems too much pressure upstream to enable heights to build far enough north to deliver a direct easterly feed. In these situations, a cold SE flow is far from out of the question. Surface cold with hints of deeper cold, but nothing like last year I suspect.

Noticed that the River Loddon had burst its banks in Shinfield on Tuesday morning (5th February). Can only assume that this is because of snow melt upstream as the river runs from Basingstoke thorough Swallowfield, Shinfield, Twyford and then into the Thames. Won't take much rain to add to flood risk so worth watching what happens PPN wise over the next few days. Not high enough to cause any damage as it is within its floodplains, but suspect the dry months followed by heavy snow upstream have had a sudden effect.

Yes, that's true. But we're barely into EN+ (still neutral by US definition) and it looks like the Central Pacific region warming so that would possibly imply a different effect from a standard EN+. The PNA has not been typical of EN+, so it leads me to believe that we cannot necessarily expect EN+ analogues to verify.

Yes it is. But, I'd like to be able to look at compound anomalies taking into account QBO, ENSO (both trending rather than developed) and the wildcard that is this year's SSW event. On its own, the MJO chart tells us only that in previous months of February where we have EN+ and an MJO phase 8 amplitude>1 northern blocking is more likely. Nowhere near enough input in order to gain a clear idea of where we could be given the current setup.

Here you go: High amplitude phase 8 in February during an EN+ implies above average height anomalies towards Greenland. Worth mentioning that this does not take into account the fact that we are in a developing EN+ and that it looks likely to be Modoki.

Difficult to tell looking at that chart as it's a bit blurry when I zoom in on my phone, but I think it shows high amplitude phase 8 in around 12 days. Add on a few days for lag and say around 20th February. The forecasts MJO will change, but if it keeps hinting at that sort of amplitude in phase 8 and we don't see any signs of northern blocking close by then goodness knows what it would take!

I'll keep my motions to myself, but looks like I picked a good weekend to be in Glasgow. A drive to Loch Lomond beckons...

Normally you would expect an MJO moving into a high amplitude phase 7>8 to increase the probability of northern blocking. This winter doesn't seem to repsect the rulebook though... Anyway, it seems that we need something special from the MJO to make any difference; even a medium amplitude phase 7 implies a much better Atlantic profile than the GFS has been showing. But, MJO forecasts haven't been great this winter; lots of variation in fairly short timeframes. Could it be that the combination of downwelling easterly QBO, developing Modoki EN+ and SSW simply overwhelms any MJO signal? A high amplitude phase 8 would be a good test, so we shall see.

I keep saying it, but it's the treacle effect! Time and again we see models throwing powerful systems towards us, only for them to downgrade once they get within the 5-6 day timeframe. Looks possible this may be repeated which is a little surprising as I thought the ultra cold airmass from the US would push it through this time. There has been a genuine lack of momentum from the Atlantic since last march; kind of reminiscent of the period in the mid-1990s which saw a prolonged dry spell (and some good summers to boot). "If onlys" are meaningless, but if only that PV was somewhere else. Of course, it's all part of one pattern, so it is all linked. Whatever happens, it does not look like we will be stuck under a mild high to the south for any length of time and we seem to keep seeing the jet tracking south.

Ha ha! I have an email group of some family and friends who are interested in the weather where I post my musings. Always very careful to point out what could go wrong and add estimated probabilities. Never tell those who are not interested in the weather until you are 90% sure! It may seem that it would be great to be seen as a sage for forecasting bitterly cold weather weeks out, but your credibility will bust fairly quickly!

The answer to your first question is climatology: on average we have west to east progression of Atlantic systems. Yes, we could see a flip as the NWP models have struggled beyond five days all season. I don't think we will have a locked in period of high pressure over Iberia for an extended period, and there are likely to be some surprises cropping up at short notice. I think the combination of the negative PNA and uncertainty over SSW impacts on the Arctic profile will continue to cause problems. Throw in some bizarre MJO forecasts and the ingredients are there for flip-flopping. I still think that until we see a more favourable Pacific pattern and consequent shifting of the PV from Canada, we are looking at occasional hints of Scandi blocking or small wedges of high pressure.

Input appreciated very much from all on here. Just a note as we head into what *should* be a relatively quieter time before the next cold spell. The extra umph from last week's frigid spell in the upper Midwest looks set to allow the Atlantic to make an appearance this week as the upstream pattern flattens and we see systems start to track towards and across the UK. It's actually much needed PPN down here, so not too many complaints. The question is whether we become stuck in a pattern where there is high pressure to the south and east and low pressure further north and west. Some signs of this in coming days, yes, but I don't think that we will become stuck under a high drawing southerly winds from north Africa a la 1998 for a great period of time. There is a risk, of course, but the general sluggishness of systems as they approach our treacle in the NE Atlantic may spring a few surprises. What of northern blocking? As we were really, I feel. There are some signals conducive to blocking teleconnection-wise but we've seen difficulty overriding the stubborn Canadian PV lobe all season so until we see a chart within 48 hours showing the Canadian lobe exiting the stage we continue to look to thread the eye of the needle for easterlies or brief northerly outbreaks. In theory you'd think that the direction of the MJO would help to support some blocking, but there clearly seem to be some overriding influences. The signal for troughing to our south seems to be receding, but much is dependent on the AO and the overall Arctic profile. The Siberian high seemed keen on heading west for a while, but with the PNA's resolute refusal to turn positive for a prolonged period it seems unlikely that Mr Siberian High will have the resources to visit our shores. We are approaching the time of year when the PV naturally recedes so chances of cold outbreaks may well increase. But, with the Canadian PV clinging on we could end up With a fudge solution where we see some continental surface cold but nothing of any real depth. Always the chance of some colder outbreaks of cold of course, but I cannot see how any real sustained cold arrives without the Pacific pattern changing, and at the moment there do not seem to be many signs of this happening. Now I've posted this, expect the next suite of NWP forecasts to show a positive PNA and northern blocking...

I think it's the inevitable come down following the snow! I'm very happy with what I got: lying snow has been very rare here over the past twenty years and this is the fourth morning this year with snowcover in the back garden. Always a bit sad to see it go though!