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Winter Forecast Recap/Update

LomondSnowstorm

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With December nearly over (and with me having some spare time on my hands for once) I thought it would be a good time to assess my winter forecast so far and give my thoughts on where we're likely to head weatherwise in the New Year.
This was my NH composite forecast for the general height pattern for both the troposphere and stratosphere for December, along with my own adjustments to the composite maps:[quote]
[size=5][b]December[/b][/size]
[url="http://f1.nwstatic.co.uk/forum/uploads/monthly_11_2013/blogentry-9298-0-31045200-1383526045.jpg"][img]http://f1.nwstatic.co.uk/forum/uploads/monthly_11_2013/blogentry-9298-0-31045200-1383526045_thumb.jpg[/img][/url]
For December, the Aleutian Ridge and negative height anomalies over the Western side of the Arctic are the most notable features, but otherwise the anomalies are fairly muted, with a positive anomaly out in the mid Atlantic and actually a slight mean trough over Europe.Half of these winters featured either a Canadian Warming or SSW in the first half of winter, with 8/10 featuring a notably cold stratosphere initially, so this may well be the key to our winter once again. The Stratosphere composite looks like this for December:
with a very strong West Greenland centred vortex, but already there are signs of weakness creeping in from the Eurasian side.
[url="http://f1.nwstatic.co.uk/forum/uploads/monthly_11_2013/blogentry-9298-0-21243900-1383526044.jpg"][img]http://f1.nwstatic.co.uk/forum/uploads/monthly_11_2013/blogentry-9298-0-21243900-1383526044_thumb.jpg[/img][/url]
[b]Adjustments[/b]
Given the SST pattern I’d suggest that the mean height anomaly for February is centred somewhat further west over Greenland, while low heights south of 60 degrees north are likely to be confined to southeast Europe through December.[/quote]
Although the month isn't yet over, the anomaly to the 23rd is likely to be broadly indicative of the final anomaly, albeit with a less emphatically strong negative anomaly for north of the GIN corridor, a less strongly positive anomaly across continental Europe (perhaps neutral to negative for the British Isles into Northern France) and a more neutral anomaly for western and central Russia:
[attachment=200213:compday.jopyVF2xYl.gif]
Bearing in mind the likely movements listed above and also the correction suggested in the original forecast, the December anomaly was actually quite a decent match, picking up on the mean ridge across the mid Atlantic to the southeastern US, a strong Aleutian Ridge, low heights into the central portion of the US and generally low heights directly to our north. As for the stratosphere, bearing in mind positive height anomalies around SIberia over the next few days, the vortex positioning anomaly was also pretty reasonable all things considered:
[attachment=200259:strat anomaly Dec.gif]
So how about the actual text forecast?[quote]
A very zonal period to start the month following a short, sharp Arctic blast at the end of November, with west-northwesterly winds and very limited blocking. Near constant low pressure systems will bring wind and rain throughout, with only brief drier interludes. Temperatures above average in England and Wales but near or even slightly below average for Scotland, where colder upper air temperatures will bring the odd smattering of snow even to lower levels in spite of a lack of frost, with precipitation above average initially everywhere.
From around the 12[sup]th[/sup] onwards things will quieten down, with high pressure building in from the south, bringing a brief spell of very mild southerlies followed by a dry and eventually frosty spell in the run up to Christmas. Temperatures above average everywhere up to the 20[sup]th[/sup] but cooling down towards average from the south as heights transfer northwards, precipitation generally below average away from the far northwest Highlands where they’ll be around average. By Christmas, heights will transfer westwards with an initial spell of rain followed by a genuinely cold northerly blast as low heights dive temporary southeastwards, bringing more widespread snowfall and low temperatures, although with accumulation generally confined to the usual spots (which of course vary depending on the exact wind direction) before the dam breaks and the heights sink once more by month’s end. Temperatures will be generally above average for most of England and Wales, with an initial CET punt of 5.6C, the Scottish mean will sit around average at 3.4C. Precipitation will be marginally above average for all of Scotland and much of northwest England but for southern England and Ireland it will be around or below average. In other words, a fairly typical December.[/quote]
I think, all in all, a decent enough forecast, somewhat underestimating the storminess around the 13th-27th and maybe overestimating the cold potential for Scotland but generally a reasonable account of the month. The CET and Scottish mean temperature punts both look a bit on the low side but moreso for the Scottish Mean, which looks like it'll come in around 5C, while the CET is currently sitting around 6.7C.
So how's the forecast looking for the rest of winter?
Well, after having had a positive Arctic Oscillation for the entire month of December we may be looking at the first hints of a more blocked NH profile with the AO forecast to go negative for the first time this winter:
[img]http://www.cpc.ncep.noaa.gov/products/precip/CWlink/daily_ao_index/ao.sprd2.gif[/img]
We can see how this materialises from the 8-14 day height anomaly forecast chart - positive anomalies across the pole centred most strongly over Alaska with a mean UK trough flanked by positive anomalies over Western Russia and the Azores:
[img]http://www.cpc.ncep.noaa.gov/products/predictions/814day/814day.03.gif[/img]

What does this mean for us? Well, it's a slightly messy setup but ultimately one where a broadly westerly pattern will prevail into the first week of January. However, that doesn't quite tell the whole story, as, if the height rises over the Arctic verify, we will be looking at a generally cool regime with transient blocking both in the Mid Atlantic and over Russia providing temporary cold snaps and battleground snowfalls as the month progresses (albeit the details of this is very sketchy and it's fine line between these, cold zonality and regular zonality). Looking even further out, the ECM 32 dayer along with MOGREPS appear to back up this generally unsettled but with a hint of something more seasonal at times into mid month, and (apparently) even as far as the end of January. In addition, both the MJO and AAM look to be giving a slight helping hand to more amplified solutions and this will also give some encouragement to those looking for snow (the favourable phases at this time of year are the 7-8-1 wheel).
[img]http://www.cpc.ncep.noaa.gov/products/precip/CWlink/MJO/CLIVAR/UKME_phase_23m_small.gif[/img]


My original LRF for January broadly backs this outlook up for the first 20 days but indicates the possibility of major stratospheric disruption occuring around mid January which could potentially usher in a much colder pattern.At the moment, however, while there is some reasonable warming in the 'surf zone' forecast, it doesn't look nearly enough to actually force an SSW - for the first half of January at least. I still feel that we are looking at a winter of two halves (although if the OPI wave forecast is anything to go by we may have to wait a while before we see anything meaningful) and there have been some increasingly encouraging teleconnection forecasts as we head into the New Year, so I'm broadly sticking to the original for now, with the caveat that an SSW, if it occurs at all, is unlikely until the very end of January into February at the earliest.

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