Nick B

The term 'Indian Summer' originated, as far as I am aware, in North America, but is now commonly used everywhere. The US National Weather Service refers to it as a period of warmth with sunny and clear days with temperatures exceeding 70°F (21°C), usually in late September to mid-November, when a sharp frost has already occurred. I know we're in the UK, so this may not happen quite as frequently as in the continental land mass of North America, but I feel it is correct to see our current period not as an Indian Summer, rather an extension of our lovely summery weather (beyond the 31st August cut-off for meteorological summer). Once a reasonably large part of the country has had at least a ground frost if not a sharp air frost, then any warm weather following could be correctly termed an Indian Summer. Just my own opinion, because I feel it is meant to be an unusual and rather special event, particularly if we experience it in this part of the world. Relative warmth in early Autumn before frosts have occurred is pretty common, just as cold in March is, even if we haven't been particularly spoiled in recent years...

From that chart it would be the next day many might be interested in. However, caution required. Model discussion is always better than expectation this far out. Remember what is being shown is a simulated version of what could happen, not what we actually get (it may end up being worse/better or should I say drier/more precipitous?!).

I think their credibility would be in much worse trouble if they engaged in this type of speculation with the public. There is good reason to hold back on what may be one out of a large number of potential outcomes when it affects so many people in different ways. A lot more certainty is required for public broadcast of severe conditions. As to their credibility, so far when all is said and done I think they have done rather well this winter. For the upcoming spell of colder weather, given how slight changes of placement of the high to the north of the UK could have implication between cold and largely dry (currently modelled as by far the more likely scenario) and at the other extreme, persistent snow showers of varying intensity yet to be established (the outside bet), why would you expect a professional forecasting organisation to mention any more this far out? Ian F has not entirely ruled out far snowier and more severe weather (because it can and does change this far out) but he has laid out quite neatly why at the moment they think the risk is low. So, the high being further north is surely one of the key features some on here will be seeking, allowing the deeper cold air aloft to waft over our shores in a brisk easterly. Will we start to see this reflected in the models though?

Fantastic pics. Anyone who has tried taking pics of waves will know they never come out looking as big as they are in real life. These looked big enough anyway so must have been stupendous. Great job and excellent commentary.

Agreed, by then could just be adding to the Euro LP area further south, except of course by that timeframe GFS will revert to type and send everything NE.

If you mean undercut from the east! That LP in the Atlantic is going nowhere, maybe towards the Azores... that'll do nicely.

The timeframe he has tweeted has given him a pretty big margin for error, though, hasn't it? In terms of that prognosis, were the last ECM forecast to verify, it would probably be reasonably accurate if you're taking the '7 days' bit into account, though perhaps not over 14 days. 7 days doesn't quite get us to the back end of next week. Also to be fair to him, he has higlighted 'could'. If anything, he is again stressing the uncertainty and disagreement between the models.

The question is, were that chart to verify what happens next. Notice the greater instability is on the southern side, the shortwaves and (daughter) LP areas brought in to being by the parent low above, which there shows two main 'centres'. When you watch this type of synoptic evolving, the spawned LP's south will generally shoot through much more rapidly, I imagine they can be perceived as being 'coupled' with the parent low in the physics sense. The key question is therefore, if the angle of attack of the daughter lows is SEerly enough, whether the block is strong enough to hold back the sliding of the parent LP. The parent LP under such circumstances can be held up, the daughters slips south east more rapidly and you then have a case of falling heights in Europe via the Biscay area and a strengthening block moving west carrying with it the much colder air. Indeed the smoother flow on the top side is a benefit, what you don't want to see is any shortwaves on the underside directing too much energy north of east, both holding up the SErly flow of the underside of the Parent LP and also potentially pushing back the block too much.

If your dewpoint stays at 0 or very close and the uppers are below -3 or so, then in heavy precipitation, snow is certainly a good possibility due to evaporative cooling (I guess you probably know this!). Witnessed this in Northampton a few years ago with about 7-8 cms in just over an hour or so. Phenomenal.

When the models are chopping and changing as much as they are at the moment (due to SSW?) is there any point at all in trying to pinpoint areas of precipitation at this range? I mean, sure, do it if that floats you boat, but I would happily engage on a 100-1 bet with anyone that all areas of precipitation don't happen exactly the way they are shown more than 48 hours out (within correct 3-hour timescales and within, say, 50 miles for where the snow actually falls), because the upstream and the 500mB pattern is so volatile at the moment. Later on, if a less volatile pattern sets up and we get a nice feed of streamers of the NE then the bets are off, then it's Lincs, the NE, E Scotland and Kent etc. I think we're all going to see plenty of the white stuff over the next 4 weeks, by the way, even the far SW, Ireland etc. I think there is even enough potential in this upcoming spell for our little group of islands to be completely obliterated at some point by white pixels on the NH map for snow cover. But I wouldn't try forecasting precipitation in detail beyond 24 hours right now. Maybe I'm just being mean. Have some fun...

Not sure we will necessarily 'know' within a hour. We'll see the next GFS model run but even at this stage it may still be clueless (to use your terminology? - apologies if not ). Especially considering the last NAE vs. GFS 48 hr charts you posted. The difference in shape of the LP in the W Atlantic for me was interesting there, resulting in a very different direction for where the jet would be sent, the NAE far more favourable. Would you really be confident that GFS would suddenly have this modelled correctly, or any other model (I am sorry to say!)? I think things may get a little more messy before they are resolved yet, but as has been pointed out this could result in a big payout for some areas. Gawd knows where at this stage. FI is no more than 48 hrs, I'd say, and accuracy for forecast ideas of precipitation between 12 hrs and radar!

Look at the pressure change around Greenland - a good 10mB in the right direction. The low off the Eern seaboard helping to start reinforce the GH. In addition the low further south east is directing energy in a much more N/S direction. Basically more amplified.

(11 / 51) x 100 = about 21.6%. Still a healthy enough percentage of around 78.4% not going with a mild solution.

I believe progressive in this sense means a tendency for low pressure systems to continue advancing (usually quickly) in a generally west to east direction across the Atlantic, with a fairly flat (non-meridional) shape to the jet stream. Less progressive means this motion is less pronounced, possibly because of buckling of the jet stream due to blocking highs (possibly HLB or high latitude blocks), allowing for greater potential for us to source colder air masses from the N, E or NE. The sometime used phrase 'overly progressive' can also refer to a synoptic evolution playing out more rapidly than is considered likely, due to a number of reasons. Aside from the obvious chaotic influences we're potentially expecting from the SSW (s!), typical signs to look for to get out of these flat, progressive patterns would be, for instance, large amounts of energy being deflected up the west coast of Greenland from low pressure systems on the eastern seaboard of the US. Such changes 'upstream' are key to the buckles in the jet stream being set up and allow for the building of the blocking HP features we need if we are looking for a sustained cold outbreak as opposed to a 'toppler' (transient cold outbreak usually part of a cold segment in a low pressure system passing through west to east, quickly overthrown by change back to a milder air mass). The placement of the HP areas are therefore fundamental to what we as a group of smallish islands on the wrong side of a large relatively warm body of water then receive, since we need them to be in just the right place to avoid being the recipients of all the heat energy and moisture that 3,000 miles of ocean can unleash on us. Get it slightly wrong and we get mild dross, get it right (chances are usually not favourable, hence the number of addicts on this site) and we can be locked into 2 weeks + of biting cold.

Awoke to heavy snow and a 5cm covering in Hertfordshire this morning. I would post a pic if I knew how...

Nice Halloween ghost in green on this saved chart.

Hmmm, without wishing to be a killjoy, the poetry might warrant its own thread!

Quite right... I was fairly certain you would know this! Still good to see trends like this coming along more often.

Without wishing to put a dampener on this, for those unsure about charts like this, even if they did verify as shown they would be very unlikely to produce anything more interesting than cold rain or sleet for anywhere other than high ground (300m+). Those who've seen charts like this come and go a few times will know that snow wouldn't quite make it because... 1. Temperature at ground level would be unlikely to be quite low enough yet to really deliver. 2. Dewpoints would be very unlikely to stay low enough. 3. Although the 850s shown are a few degrees below zero, it's marginal enough that it really would need to have been sourced from a cold continent in order not to be modified by the time it reached here. Look where the cold(er) air is sourced from... is it genuinely cold enough? 4. It is quite possible that if precipitation were in heavy enough bursts, there could be areas where wet snow fell for short periods of time, but it would be unlikely to stick around for long. 5. A few other factors mentioned in John Holmes' thread/post (I think it might have been called 'What to look for to get snow', but I may be wrong). I stress 'highly unlikely', not impossible, meaning any fairly excitable new followers to these charts may not want to let their hopes be raised too much just yet, based on these charts... unless even more favourable ones come into a much closer timeframe, say 72 hours or probably even less. It is still rather early even for an unusually early winter appearance mid-month. End-October we all know can deliver a pleasing spectacle (2008!) but for snow which will stick more than a day, I wouldn't be looking really seriously until perhaps November. I'll be pleased to be proved wrong by events! Seeing more and more of these charts appear and with colder uppers may be taken as positive signs, but I feel we have a fair bit more autumnal weather to get through first, which is in itself interesting for some.

If you could just bring the earth's spin on its axis to a halt and then send it in the opposite direction, then once everything has calmed down over a few hundred years you might find the prevailing westerlies had become easterlies! Of course, the distribution of heat and cold and also ocean currents would also then be drastically altered, so there'd be no guarantee at all that they would behave similarly but in reverse. It would actually be quite interesting if such a spin reversal could be modelled by computer just to see where we might end up, for fun...

The key word in the model here is 'Mean', I think. Even if accurate, this chart covers quite a long period and during that time there will be fluctuations in exact positioning of blocks, and would allow for some pretty good cold spells for our part of the world... if it were to verify. I would say it would allow for undercutting lows and battleground snow scenarios, especially if a nice big slab of cold air were sitting waiting for any low pressure areas to approach. I would happily take that chart as a summation of January to March.

Remember a gust of 160mph is eight times as powerful as a gust of 80mph (which can blow people off their feet). Basically a gust like that would take you and put you wherever it wants to.

I would say this is one storm where people will definitely need to look out for their elderly neighbours in some locations. Today very strong winds and rain/snow, potentially bringing down power lines, but then tomorrow much colder, still with strong winds and snow. For houses losing windows, roof tiles etc and then left without heating as a consequence of any power cuts, it could become a bit overwhelming for some in blizzard conditions...

350 metres per second for the speed of sound, so if it seemed instantaneous, then, yes, possibly within a few 10's of metres even. Whilst in an ambulance (therefore perfectly safe due to the Faraday cage principle) a few years ago, I had a strike within a few 10's of metres which basically sounded just like a close gunshot. Apparently, lightning isn't very good at killing you if someone else who knows their CPR is close (unless you get whacked by a CG+ I guess, then you'd be toast) but even so, if you were a cat I'd say you'd just had one of your nine right there, buddy. :o

Do you both mean, in that if you have a strong, organized and very active PV, that when an SSW occurs, it really has something substantial to disrupt, therefore the energy is sent further (and perhaps for a longer time period) and will therefore have more of an impact somewhere as a period of much colder weather?