Al eerder plaatste ik de opvallende vooruitzichten van MetOffice (Engelse KNMI). Gisteren hebben zij een persbericht uitgegeven over hun opvallende vooruitzichten. Na het persbericht heb ik nog de verwachting zelf opgenomen in dit bericht.
Vertaling in hoofdlijnen. Zij geven aan dat het opvalt dat er veel onzekerheid is. Daarmee bedoelen ze of het of wordt behoorlijk zacht of behoorlijk koud. Alsof er geen middenweg is. Voor begin februari zijn er twee scenario's (gebaseerd op ENS, zelfde methodiek als EC met haar leden). Zwart-wit verdeling. Als verklaring geven zij de ontwikkelingen in de stratosfeer (!),nl een flinke stijging in de temperatuur, en dat heeft geleid tot ontwikkelingen op lagere hoogtes (zij noemen daarbij voorbeeld in het noorden van de Pacific). Dit is een complexe interactie, waarbij de modellen steeds er anders mee omgaan. Bekend uit de literatuur dat bij een SSW (oostelijke wind op 60N, 10hpa hoogte) de verwachtingshorizon nog lager is. Vaak zie je dan zeer verschillende uitkomsten.
Tot zover de vertaling.
Opvallend dat MetOffice dit zo duidelijk uitlegt en een verband legt met de stratosfeer. In mijn beleving wordt het dan ook steeds belangrijker om de ontwikkelingen die daar plaatsvinden te monitoren. Dat dit lastig is, nieuw gebied is duidelijk, maar dat maakt het wat mij betreft wel zo interessant. Een stap voorwaarts in de ontwikkeling van onze meteorologische kennis. Deze winter is wat mij betreft ook een mooi vb hoe belangrijk de stratosfeer is. D.w.z. de extreem koude stratosfeer die ook verwacht kon worden, deed in grote mate vermoeden dat we een flink positieve NAO/AO zouden krijgen, heftige poolwervel waarbij de stroming zo sterk was dat elke rug geen schijn van kans had. Dat is geen wijsheid achteraf, maar al vooraf, en zeker ook tijdens. Ook werd duidelijk dat met een opwarming er meer kansen zouden komen. Een heel verschil hoe de virtuele weerkaarten nu zijn en een tijdje terug.
Two stories for the start of February
19 01 2012
Anyone looking at our current 16-30 day UK forecast (for the first half of February) will have noticed that uncertainty is the theme – much more so than usual, even when looking this far ahead.
The outlook for this period is finely balanced, with two scenarios looking equally possible: the first being a continuation of the changeable and relatively mild conditions we’ve seen so far this winter; the second being a shift to a period of much colder weather.
Whilst two different outcomes are possible, it does look as though the general pattern that establishes itself by the start of February could last for quite a while, perhaps even until the middle of the month.
However, it is not possible to say which of the two scenarios is more likely due to an unusual amount of uncertainty in the forecast.
For these longer range outlooks, we use sophisticated techniques to understand the uncertainties – called ensemble forecasts.
In an ensemble forecast we run a forecast model many times from very slightly different starting conditions. The range of different outcomes gives us a measure of how confident or uncertain we should be in the overall forecast.
Currently the ensembles for the start of February onwards show a fairly even split that does not clearly favour either the mild or the cold option – and there are relatively few solutions in between.
There is a possible explanation for this relatively rare situation. In recent days there has been an increase in temperatures in the high atmosphere (the stratosphere) over the northern hemisphere. This change seems to have been driven in part by marked changes in the weather patterns lower down in the atmosphere – over the north Pacific for example.
Research, developed in part by the Met Office, suggests that changes such as this high up in our atmosphere can – in time – go on to affect weather patterns at surface-level. However, this is a new and very complex area of meteorology where we are still developing our understanding of all the mechanisms involved.
It is possible this is why we are seeing the volatility in the current forecast, as different model runs handle the interactions differently.
Rest assured, whatever happens we’ll keep you up to date with the latest information through our forecasts and weather warnings – so as soon as we know, you’ll know.
http://metofficenews.wordpress.com/
UK Outlook for Tuesday 24 Jan 2012 to Thursday 2 Feb 2012:
The beginning of the period looks set to remain generally unsettled and similar to recent weeks with largely cloudy and mild periods accompanied by outbreaks of persistent rain separated by brighter and colder conditions, with frequent showers and a risk of overnight frosts. Some of the showers are also likely to be wintry over higher ground, particularly in the north of the UK. There then remains considerable uncertainty regarding the weather during the last few days of January and into February. The slightly more probable scenario at present is that conditions will remain similar to those described above. However there is also a risk of colder weather becoming established across the UK, with snow in places and widespread overnight frosts.
Updated: 1204 on Thu 19 Jan 2012
UK Outlook for Friday 3 Feb 2012 to Friday 17 Feb 2012:
The forecast for the first half of February is very uncertain. As with the day 6 to 15 forecast, there appear to be two main scenarios, each equally probable, but which are very different. It is also fairly likely that the prevailing weather type at the end of January will continue into February. The first scenario consists of a changeable westerly or southwesterly type, with rain at times (amounts greatest in the west), and with temperatures noticeably above average for early February, with only occasional frosts. The alternative scenario is that very much colder weather, with winds from an easterly or northeasterly quarter, will prevail well into February, bringing widespread frosts, and snow to some areas. In this scenario it would be the east that was most vulnerable to snowfall.
Updated: 1200 on Thu 19 Jan 2012
Quote selectie