Cold Weekend Ahead – Thoughts & Analysis (18-19th Feb 12)

14 Feb

Clearly the start of this current week has seen a change in the broader scale weather patterns than previous weeks and despite the winds being north-westerly this has resulted in milder conditions than of late as the origin of the north-westerly wind hasn’t been from a cold source.  However, there continues to be growing signs for a change to colder conditions as an air mass with more of a polar origin develops over the UK towards the coming weekend and potentially persists into the early part of next week.

Last nights FAX chart at 120hrs covers Saturday and this, at the moment, highlights the expected synoptic pattern quite well;

The development of a minor ridge of high pressure to the west of the UK is currently forecast to combine with low pressure to the north or north-east of the UK to produce an increasingly cold north-westerly air mass and this may well becoming northerly across parts of the north in particular.  Notice on the above image that if you follow the isobars back that the winds and air mass originate from near Svalbard or just to the East of Greenland, clearly a cold source!.

The cold front on Saturday will be the primary feature that signals the introduction of colder air from the north-west.  The timing of this feature is open to question at the moment, but it would seem that process will take place during Saturday itself.  One very useful variable that meteorologists use to determine different air masses and also where fronts will be is the variable theta-e  or equivalent potential temperature at the 850mb level.  In a nutshell this variable highlights regions of differing air masses by colour with blues and pinks highlight cold or very cold conditions and greens, yellows and oranges warmer.  The latest 06Z GFS Det highlights the cold air behind the southward moving cold front very well during Saturday;

Again at the moment it would seem that the cold front will move down across the UK between 06Z and 18Z on Saturday with an increasingly cold air mass following on behind.  Also of note is the predicted drop in 850mb temperatures which can be highly useful to gauge the risk of wintry precipitation, again note how 850mb temperatures do drop below -5C across most areas of the UK towards the end of Saturday;

By the latter stages of Saturday evening and overnight 850mb temperatures across Northern England, Northern Ireland and Scotland are ranging between -7C and -10C which is a clear signal that any precipitation will be wintry and down to quite low levels as well.  Now the wind direction favours Scotland, Ireland, Northern England and perhaps parts of Wales with regards to the regions at greatest risk of showers.  At this time frame it is very difficult to say how many showers or how much precipitation is likely to develop within the north-westerly flow.  However, given the current conditions which are expected then some sleet and snow showers seems likely and where these are most frequent and heaviest overnight into Sunday then some snow accumulations are possible.  At the moment I would say parts of Scotland, Northern Ireland and Northern England are at greatest risk of some accumulating snowfall, but again this is subject to change given further model runs and guidance nearer to the time in question.

Interestingly, the 06Z GFS Det does highlight the above scenario well and using the predicted ‘weather type’ variable you can see that the areas I mentioned above do match the forecast graphic at the moment;

Weather type GFS Sun 19.02.2012 12 GMT

The other issue which may well limit precipitation or at least signal the reduction in showers is if the ridge of high pressure to the west of the UK builds into parts of the UK more quickly by Sunday for example, than any showers could well be killed off completely or at least reduced in intensity or frequency.  So the longer any ridge to the west stays to the west the better.  Just finally after a likely week without any frosts or icy patches, the latest guidance is for a widespread moderate frost on Saturday night and also perhaps on Sunday as well into Monday morning so don’t put the de-ice away just yet.

So in summary a change to colder weather looks like over the weekend and some areas exposed to the north-westerly air mass will likely experience some wintry showers with sleet and snow possible and perhaps with some accumulations as well in places.  This setup does favor more northern and western areas  of the UK and I expect many southern and eastern areas of England will experience nothing more than a cooling trend with a much lower risk of any significant wintry precipitation.

Regards to all,






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