The Snow Lovers Appreciation Society Winter 2014/15 #MOD NOTE #1

emmetlego wrote: »

Don't go too far… Some of the models starting to show high latitude blocking… Could be in for an early taste of winter if they come to fruition.

Time for another n00bish question from Hatrick then Does northern blocking other than blocking over Greenland have any relevance to our chances of getting snow?

Also, which models are you looking at? I'm still hoping Meteociel might have fixed the UKMO in time for ramping season, although it's not looking good

I promised a final version of my winter forecast and this is it (copied from a post on Net-weather but with Irish details added) ... sorry it is really just a bit longer restatement of the preview but it does add a few details ...

MTC winter outlook for 2014-15

Readers may already know that I base seasonal forecasts on "index values" from past data that my research identifies as being partial analogies to the coming season. Although my research base is mainly the UK CET daily and monthly temperature data base, I consider likely variations introduced by climate and circulation for the more specific Ireland forecasts.

I have tried to assess all the information offered in various other sources that I find credible, and to some extent I note a general overlap of conclusions made from different approaches, which inspires a bit of confidence. At the same time, I feel subjectively that almost anything could happen this winter as things seem to be in somewhat of a fine balance between a mild zonal set-up and the onset of high-variability stationary if not necessarily retrograde type blocking patterns.

There is bound to be some similarity to 2013-14 around the hemisphere if only because some of the large-scale drivers have not changed all that much, and early indications suggest that the North Atlantic may persist in being active. Very cold air seems poised to invade North America about where it did last winter, and Siberian blocking is primed by early snow cover and the position of ridge and trough features over the northwest Pacific sector.

So I should begin by saying that winter 2014-15 may not seem entirely different from last winter to many, and there could be rather frequent wind storm events in Britain and Ireland. What's different is perhaps that they won't come almost non-stop after a certain point (last winter it was 26 Dec IIRC) but in occasional bursts separated by blocking sequences and less stormy northerly flow (although active enough).

This set-up should mean that winter 2014-15 will not be generally as mild as last winter in Britain and Ireland, but on average around the long-term mean with approximately equal mixture of cold and mild spells. And the difference between them may be quite large, giving rise to a forecast of notable variability, a statistic that can be measured and compared to other winters. History buffs may recall the complete turnaround from late Dec 1986 into mid Jan 1987, or the contrast between 23-29 Jan and 30 Jan-5 Feb in 1969. That sort of high variability could be on tap this winter.

The colder portions of the winter, from my index value research, would appear to be roughly once every month, but more specifically, mid-November, early December, early part of mid-January and possibly a longer cold spell near end of February into March -- these are favoured to be the cold spells and any one of them could generate a snowfall event or two. That snowfall is not likely to hang around very long in a highly variable regime, which may introduce snowmelt flooding into the mix of possible events. For Ireland, the snow in the November cold spell(s) may be generally more noteworthy in higher parts of the north and the southwest, as flow over the Irish Sea may be either too northerly, or slightly too mild, but we can't rule out perhaps one good fall of Irish Sea streamer snow near Dublin. Same comment applies to early December (a spell that I think will be separate rather than an extension of the November spell). Later into the winter, more equal chances for snow will exist in north and east as each successive outbreak becomes a bit more "retrograde" and less northerly.

The milder spells which you can gather from the above timing will be at various points in early and late November, late December, second half of January and possibly first half of February, may contain some windy and stormy outbreaks, but some (the late Dec and late Jan portions) may involve southerly flow around blocking highs to the east and southeast, and those periods could be quite inactive (and prone to dense fog inland too).

What we have in total is a menu of almost all possible winter weather types that I believe will mount up a campaign of noticeable variety giving the impression of a winter with no dominant theme or as people might say, "it just can't make up its mind" but if the late cold is a strong easterly outbreak (the earlier ones are more likely to be northerlies) then it may become the biggest theme of the season, at least as broadly defined, bearing in mind that many believe that March is a spring month of course (despite 2013).

Everyone always wants to know about the Christmas-New Years period in particular, and there I would say that the chances are fairly good that there might be a little active stormy weather just before Christmas, not particularly cold, followed by mildish settled weather and ending up stormy again near New Years with the risk of it turning a lot colder soon afterwards.

Some heavy snowfalls could develop in November and early December in regions that are exposed to northerly flows off the Atlantic Ocean, Irish Sea or North Sea. There may also be top up snowfalls in brief northeasterly flows then. That part of the winter, depending on how severe it is, may give rise to talk about "here comes a really brutal winter" but keep in mind that the pattern should be variable enough to melt that snow and end the dominance of the cold air masses for the time being.

A particularly interesting part of the winter is likely to come around 4-6 January, which is about when I would expect the most severe storms of the winter. These might be mostly wind and rain at first but could end up with heavy snow.

For North America, I am expecting a similar wild ride with high variability. Some exceptionally mild weather is likely given the state of the Pacific (a raging hothouse at the moment, the current monthly anomaly value at Seattle WA is +3.4 C and this has been the case all through spring, summer and autumn 2014). The arctic is looking active enough too, for this early stage of the developing winter season, so I expect continual battles for domination of the eastern and central regions between much above and much below normal air masses. This often leads to one or two massive east coast storms when deeper troughs become available to harness the potential energy in such a situation (and really this is almost the default climate anyway). The western Atlantic may be under intermittent ridge and even cut off blocking high regimes, but these may be either too weak or too distant from Britain and Ireland to allow for a dominant cyclonic pattern further east, there may be a mean shallow trough partially linked to cut off low pressure near the Azores. This will just contribute to the general variability as a pattern like that can alternatively pump in mild air and storms, or collapse to allow for northerly outbreaks.

Eastern Asia may have another of their increasingly frequent (in recent years) severe winters with heavy snowfall. That may in fact become the main story of the winter globally.

In such a variable pattern, I am rather reluctant to put too much emphasis on CET values but for later testing, here's what I would expect: NOV 5.0, DEC 4.4, JAN 4.5, FEB 3.3, MAR 4.5. (Irish readers note, our own IMT index is similar if slightly higher on average in the winter, so would add 0.5 to all those and the following comments too). It can be seen there that March is the coldest month relative to normal and February the coldest in absolute terms. Some 30-day period in later Feb and early March could average as low as 2.5. Some other 30-day period in midwinter could be as mild as 6.0. I would expect precip to be generally above normal in the range of 125-150 per cent of normal values with little variation regionally as different regimes produce different outcomes. Everyone will get their share of the surplus rain and snow. And at least one windstorm event is likely to qualify as exceptionally severe.

(got your own predictions? I will be running that winter forecast contest again, look for it in November).

So, in simple terms, over the next four months we can expect frequent ramping threads, with outbreaks of bitter disappointment, and the occasional squally row?

M.T. Cranium wrote: »

I promised a final version of my winter forecast and this is it...

Broadsheet'd...

Thanks MT for the long range Winter prediction!

It sure is, look at the long fetch of winds from across the pole!

West Atlantic / Greenland blocking is less defined on more recent model runs, but the fact that it's sporadically appearing in FI so early in the year is surely a good sign?

Also MT, why are you predicting early January for particularly bad storms?

It's a bit too early to be looking for blocking unless its for the purpose of pattern matching for stuff like the OPI. Still another month and a bit of autumn to go. :pac:

Hint at something cooler/ more seasonal in the medium range?

Ridiculous cross polar flow there. Love it. Would be even better at the end of November, but I'll take it!

What was the OPI index number for 2009? (just curious)

I mentioned early January as more likely to be stormy on the basis of the research model indicating a combination of rapid temperature decrease and high energy peaks. A somewhat weaker but similar combination appeared near the end of January and first two days of February. Since I am not looking for a complete return to last winter's parade of deep lows, I think we will get them in isolated sequences of two or three with rather long calmer spells separating them. This should at least give people a break from them, including us -- the weather forum seemed to be on perpetual storm watch from Christmas to mid-February last winter, I was looking back through the forecasts for those six or seven weeks and we had alerts out almost every day.