Duane S.

The dire late winter period continues but so does the potential for a very late winter cold blast. Not just NCEP, all the MJO forecasts are showing high amplitude 6/7/8 to one degree or another, I remain convinced we are finally going to see proper HLB setting up after mid month. Too late for long lasting lying snow but plenty earlier enough in spring for some very decent snowfalls

Three days later and the trend continues unabated. Seen enough now to be sure that starting the 2nd week - mid March is going to bring the best chance of proper HLB all winter. It maybe late winter but March can produce some memorable events. Marginality can be reduced in coastal areas with cooler seas. Obviously lying snow periods will be reduced by a rapidly warming sun but the sun can also be our best friend as well providing energy as precipitation crosses land. Still a way to go but it's starting to get interesting.

Not an exact science of course but when GEFS constantly shows the pressure rising in Iceland in far FI at >1005 average (with the OP normally much higher) there is often something in it in terms of HLB possibilities. Too early yet but the first time for quite a while this winter I've seen it.

This is rapidly turning into the worst Feb i can remember in terms of genuine possibilities of a decent cold spell. Right from the start of the month the Northern Hemispheric profile was not conducive (despite good signs towards the end of Jan) and so it has remained throughout, almost unwavering in giving us any optimism of a late winter proper cold blast. The rest of Feb can be written off for sure. March can of course still produce and often does but no real signs of that either at present. 64 pages of this thread in 20 days probably says it all...

The OPI has failed and failed in a pretty big way. No glossing over it I'm afraid. There obviously is something to it but nothing in weather forecasting is as one-dimensional as it was, at times, portrayed to being. It's probably more of a case that the factors that generated the very low OPI figure ended up keeping the cap on what would have been another 13/14 winter. It is understanding exactly what those factors were that will be the challenge I guess. All well and good saying it was an anomalous low pressure sat in the Barents Sea that inhibited vertical wave activity and subsequent HLB but why was it not predicted and precisely what role does it play? The computer doesn't yet exist that can come close to working out out how the primary drivers work with each other, couple with the atmosphere (or not) and successfully determine the outcome in terms of long range abilities. The QBO, ENSO, solar influences, MJO, GLAAM, GWO and the PDO surely all play a role at times but how on earth can we tell by what degree and how one affects the other(s), couples with or cancels out. Throw in current tropospheric 'reflectors' such as the AO, NAO and the PNA and I can't say I am optimistic it will ever happen.

Tough viewing continues this morning with little to no hope of any sustained cold for the next 10 days+ However the GFS Control offers us some hope of a late winter cold spell... Very plausible in how it forms and still not to late in the season to bring very cold and snowy weather to many.

ECM showing a huge amount of promise this morning. And as I posted yesterday, GEFS continues to spew out the odd very cold run, both in the mid term and the long.Although in the minority, they are not going away...

Very difficult to see true HLB forming in the next 2 weeks with the current profile (both trop and strat) but whilst a UK high can have the downside of being quite a persistent entity (in terms of meh dull benign etc) as the jet rides over the top pumping it up, once conditions allow again, it can very quickly turn it our favour from an MLB location to a more favourable one. Just for fun though, before the 06z removes them altogether, still a few ''rogue' runs being produced by GEFS to make us drool...

I would completely disagree. There is plenty enough evidence to show a strong vortex at the beginning of winter makes HLB less likely ( at least until the latter stages of winter). Conversely a weakened vortex does make HLB more likely. BUT when combined with other factors/drivers, this propensity for HLB (or not) can be muted or completely over ridden.

Bah, the smallest of dustings for Weymouth. We are just a smidgen too far West, looks great for those in the east of the county though!

Wib, you have been beating the drum for the cold spell to be a brief affair for over a week now, according to you we should be bathing in warm westerlies right now but instead the cold spell had been very good for many and indeed has many days left to run yet. A very decent cold spell by UK standards. A UK high is the most likely next step and will likely lead to inversion with frigid temps at times. I for one will be more than happy to have that for a week and take my chances thereafter. Of course there remains a chance for sure it slips east and stagnates, drawing in TM air but retrogression has to also remain a viable option.

It's not even as though we are currently enduring a prolonged zonal period with no end in sight. Quite the opposite in fact, which makes such whinges even more incomprehensible. For a start some of us have 5 or 6 days of very cold weather yet to come. Thereafter, plenty of options are on the table. Even if we end up with a uk high ( IMHO the most likely possibility right here, right now), what happens thereafter is very very much up for grabs and even THAT doesn't even take us to mid month!?

It's your longitude. Taking precedence over elevation currently.

It's only you singing from your own hymn sheet I'm afraid. The name sort of gives it all away anyways... A 'heading milder' option at D10+ on the model output is always going to be there in any cold spell, even if it transpires I suspect it will be more transient than a pattern change, the best is yet to come.

I can see it has already, quite rightly, been well covered in the last hour or so but this is the best set of GEFS ensembles I have seen in a long long time... Welcome to February folks!!

I posted a few days back that the formation of a polar bear could be a possibility but that was for later on this weekend when we pick up a true Northerly, to get one form in a north west flow makes me doubt whether it can be one in its true sense.That said, it doesn't stop it being of great interest as it makes it way South and East! Edit: polar LOW, not bear, that was Lorenzo's fault for that posting that amusing picture earlier!

It's always nice to see better GEFS support but as Jason says above, in this scenario, I wouldn't read too much into it (yet). Seen it plenty of times before, my money is on them pulling in towards the GFS Op's way of thinking by the weekend. That said, they are there for a reason, so cannot be wholly discounted. So the sooner they do pick up on the signal sniffed out by the higher res ops, the better!

I think the 00z was out on the town with the 18z last night, the diving low pressure system is there this morning, upgraded if anything as it is slightly deeper and a touch further North. Euros doing the same albeit not as such a well defined trough, maybe more realistic. Anyway, it's another GFS op run that has Dorset averaging around -5 850s for 10 days, the control goes further and does it for all 16 days! This is no cold snap... Essentially then short - medium term is looking great. Thereafter potential remains rife. I'll take that!

What the 18z shows is how important that early seperation and driving SE of energy is to sustaining the Northerly and would also help open the door later on to a Continental flow. A great run to conclude a very good day of model output I would say!

What a great post. Any newbies out there would do well to bookmark that and use it for future reference while learning. Put my earlier short response to the same question to shame!

Good grief what is the 18z up to!? Running that not insignificant system SE to our W. Much interest

I think what you are asking is why in particular is a northerly airstream often regards as unstable. An Arctic Maritme air flow in the winter, and to a degree PC and PM air masses, are unstable because as it modifies (I.e. as it moves away from its frigid source towards a warmer UK) it will warm up from the bottom layers and, naturally, rise. What goes up must come down and often it does, quickly, as snow. Worth remembering that the much overused polar low can only truly form in a true AM airmass and one of a decent duration. Will be worth keeping an eye nearer the time for the formation of these rare little beasts from next weekend when a northerly kicks in. No point trying to look outside 12 hours or so as they normally pop out out of nowhere and disappear as quickly as they arrived. Great fun to be underneath one at the time though!

With some places in the Alps looking to get around an astonishing 2 metres in less than a week, it should set the Alps up for next year as well!! :-) As often happens when there is a slow start, the balance is redressed

From zero to hero, the GFS makes the best of this upcoing cold spell with a stonkingly good run all in all. If this were to verify (it won't of course but let's face it, it's intriguing to see it getting shown by an op run), Dec 2010 comparisons will not be so outlandish for what would follow on a few days on from this...

Absolutely. The 850mb temp profile is only one aspect. Here's a chart I wheel out every year to show it is not the be all end all. This fairly innocuous looking chart produced insane amounts of snow for areas in the south and south west, right onto the coast, I can still remember it as a kid. I'm not trying to draw any similarities to the upcoming cold spell, just pointing out not to simply look at the upper temps and assume yay or boo to whether it will snow in your garden. They are a good guide, probably the best single guide but too simplistic to rely on solely