Cold Spell Prospects and Discussion (January + Early Feb 2012)

http://www.accuweather.com/en/weather-news/pattern-change-winter-coming-snow-cold/59847

Interesting update from accuweather regardIng mid month for the USA



"Rather than just a west to east jet stream across North America with little disturbances in it like we had during much of December, we now have much larger southward dips and northward bulges. "

Meanwhile, for the first time this winter the UKMO is hinting at "slightly" colder but more settled conditions in its monthly outlook for the UK.

"UK Outlook for Thursday 19 Jan 2012 to Thursday 2 Feb 2012:Remaining fairly unsettled in the north at first, with spells of wind and rain, interspersed by brighter but colder conditions with showers, wintry over high ground in particular. Further south, somewhat quieter conditions look more likely, with less in the way of rain. Towards the end of January, high pressure may become more influential, resulting in drier and probably brighter conditions becoming more prevalent across the country. Temperatures for many areas will initially be around or slightly above average, with a trend to slightly colder conditions towards the end of the month, bringing an increased risk of overnight frost and perhaps some fog.
Updated: 1329 on Wed 4 Jan 2012"

ECM and GEFS both indicate a spill south of cold over from the Arctic air during the next 5-7 days over North America with further Arctic surges into Eastern Europe later in the same period. By mid-January or closer to 16-18th it appears that anywhere in the Northern Hemisphere could experience such a 'toppler'.

I think the models, particularly the GEFS, is struggling to handle the expected drop into negative territory of the AO/warming of stratosphere and the possible splitting of the polar vortex. As MTC and others here have said, models can change dramatically very quickly in such a set-up so it may take until early next week before we can say with any great certainty what conditions will be like beyond 15 Jan.

At present I would put down the following confidence markers for likely conditions around 16-18th:
Settled with high pressure nearby, temps around or slightly above average 10%
Settled with high pressure nearby, temps below average 10%
Arctic toppler, wintry showers for some, severe nighttime frosts (3-4 days) - 20%
Strong jetstream, stormy and unsettled in a predominantly SW-W flow 25%
Cold zonal (temps around or slightly below average, with mixed wintry showers in a W-NW flow) 35%

Looking at the GEFS ensembles and there are mixed signals but more of a cooling trend that yesterday. Meanwhile, the AO is still edging towards negative territory while the NAO looks like staying low positive.



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Update from UKMO at 1114 today (favours that cold zonal setup mentioned above):

UK Outlook for Wednesday 11 Jan 2012 to Friday 20 Jan 2012:
Northwestern Britain will see often rather unsettled and windy weather with spells of rain, particularly at first. Remaining areas will also see some rain or drizzle on occasion, but should be largely dry, with some patchy fog developing overnight and lasting into the mornings. Daytime temperatures will be largely mild, although there is an increasing chance of overnight frost towards the weekend. The weekend and following week are expected to see the unsettled weather in the northwest spread gradually southeastwards, with showers in the north of the country turning increasingly wintry. Temperatures should become nearer normal in most areas with overnight frosts expected, and there will be an increasing risk of gales or severe gales towards the far north and northwest of the UK later in the period.

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This appears to be backed up by Simon Keeling of weatheronline.co.uk

"The 12Z run of the GFS model is highlighting the jet stream slipping southwards from Sunday 15th. This puts it over the south of the UK/northern France, placing the UK on the colder side of the jet and reducing temperatures.

It's not a huge freeze but is enough to lower temperatures sufficiently for snow to fall in Scotland and perhaps at times in showers over northern England and Wales. Most of northern Europe (UK, Ireland and Scandinavia) will be into these cooler conditions with heavy snow at times through Scandinavia, and perhaps parts of northern Germany and Poland.

Severe gales at times in northern France, the Low Countries and Germany, perhaps even southern England. I'm not jumping for joy at my forecast being vindicated just yet, but it is good to see models falling inline, although this is only the GFS for the moment."

GEFS and ECM both starting to pick up on the warming event in the strato. Both hinting at a cold zonal setup at present. The below chart would certainly deliver some mixed wintry and windy weather i.e. a real winter storm