Colder Next Week – Why & How Long For?…

29 Feb

It just goes to show how unpredictable and varied the weather can be, particularly at medium and long ranges.  At the end of last week it looked as though there was little chance of any return to colder weather, even briefly, with my thoughts highlighting what the models where signalling as we moved into early March, which was predominantly mild.  However, subtle changes to the broader scale weather patterns are now signalling the development of colder conditions, at least for a time, during the latter stages of the weekend and into the early part of next week…

Just to re-cap the last 7 to 10 days and that has been dominated by high pressure to the south of the UK and lower pressure to the north and north-west, which has brought a persistent feed of very mild and moist south-westerly winds into the UK, well that is about to change as one would expect.  The initial changes come about due to a marked change in wind direction and air mass over the weekend;

Using the latest FAX chart for Saturday, you’ll notice that a cold front is forecast to move across the UK and it is this feature that will signal the change to a colder west or north-westerly air mass.  Now, what is of significance here is the air mass to the west of the UK.  Note on the above chart that the 1000-500mb thicknesses are less than 528dam just to the West of Ireland, but more importantly are as low as 510dam further upstream within the mid-Atlantic.  This air mass will be advected into the UK on the westerly air mass during the second half of the weekend…

I’ve discussed theta-e (equivalent potential temperature) before in previous blogs, but just as a recap, theta-e charts are used to highlight different air masses, whether these be warm or cold.  In this instance and as a you can see on the above image, which is for approximately 1200Z on Saturday 3rd of March, the black line is signalling the approximate location of the surface cold front and importantly notice how the deep blue and even pink colours are evident to the west of the UK which signals a cold, if not very cold air mass.  This can also be seen on the 850mb temperature chart;

Notice how 850mb temperatures are below -5C to the east of  the UK and further upstream are below -10C and again this cold air mass is then forecast to be advected into the UK over the latter half of the weekend and into the early part of next week.  This drop in temperature can also be seen very well on the latest ensemble graph for Manchester for example;

For at least 2 or 3 days the 850mb temperatures are at, if not below -5C.  Of interest as well is the signal for some precipitation as well in conjunction with these rather cold 850mb temperatures.  The details of the weather for the early part of next week is uncertain, but at the moment a cold and showery west or north-westerly air mass if forecast to affect the UK during Sunday, Monday and perhaps into Tuesday before stabilising from the west.

So the end result is a marked drop in temperature compared with what we have seen of late.  Maximum temperatures are likely to be no higher than 4C to 6C across northern and western areas of the UK and between 6C and 8C further south and east.  There is likely to be a return to overnight frosts as well from Saturday night and through into next week.  Wintry precipitation is possible, but given the time of year it is likely to be restricted to higher ground of northern and western areas of the UK.

So a change is on the horizon to generally colder, but also sunnier conditions as well, despite some wintry showers at times, a welcomed change for those who want to get rid of the persistent overcast conditions of late…At the moment by the middle of next week and beyond temperatures will be on the rise again, so it is expected to be nothing more than a short cold spell and there is little risk of intense or prolonged cold as predicted by some of the forecast models of late.

Regards to all,

M.

 

 

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