May Bank Holiday – Thoughts & Analysis

2 May

The first Bank Holiday of May is nearly upon us and no doubt many are looking forward to the long weekend ahead.  Unfortunately the weather won’t deliver an early taste of summer, in fact it’ll deliver what is likely to be a very (very) late taste of winter!…There are some interesting reasons why and these are discussed below.

The broader scale weather patterns are shifting towards a far more blocked pattern.  High pressure is forecast to become slow-moving to the north-west of the UK, whilst low pressure is slow-moving over Scandinavia and primarily due to a weak and disorganised jet stream, something I’ll discuss a little later.  For now though, taking a look at the last synoptic chart for Friday;

As mentioned you can see high pressure dominating to the north-west of the UK with atmospheric pressure forecast to be over 1024mb across the North Atlantic, whilst pressure is lower across the Azores.  As a side note, this synoptic pattern is indicative of a -NAO (North Atlantic Oscillation) and of which, if this were winter the UK would be looking at a potentially noteworthy and prolonged cold spell.  But returning to what is expected in the here and now and the feature of interest is the cold front lying across Ireland and into North England on Friday.  It is behind this cold front that a markedly colder air mass is forecast to filter south from an arctic source.

You can see this more clearly on the latest thete-a forecast chart for Friday.  I have discussed theta-e before, but in summary it is essentially a forecasting tool and variable that allows meteorologists to determine whether an air mass is warm and moist or colder and drier and in this instance, note the deep blue and even pink colours filtering south into the northern half of the UK behind the cold front.  This is clearly indicating a change to a colder and drier air mass from the north;

Now earlier in the blog I highlighted that the broader pattern across the North Atlantic is becoming more blocked with a weak and disorganised jet stream.  Well, and as is often the case, the jet stream is directly related to the expected surface conditions.  So, find below the latest jet stream chart for the weekend and note how there is a ridge in the jet stream within the central North Atlantic in conjunction with the surface high pressure, whilst across the UK there is a trough.  More often than not when the jet stream dives down across the UK and we become located beneath a trough in the jet stream the weather often turns colder and is predominantly unsettled.

 

So what does all this mean in terms of actual weather?;

Clearly combining all the above information the Bank Holiday Weekend across the UK is forecast to be dominated by a particularly cold air mass from a northerly direction.  Some scattered showers are possible in places and given the increasingly cold air mass these may well fall as sleet and snow, particularly above 250m across Scotland and Northern England.  For most though, the weekend does look predominantly dry, particularly by Sunday and Bank Holiday Monday with some bright or sunny spells.  Clearly despite the time of year and the increasingly short nights, the air mass will be cold enough to lead to a risk of some widespread moderate and noteworthy frosts for the time of year.  Using the latest GFS Det model as an example, you can see that many inland areas and particularly rural areas of the UK look set to experience sub-zero Tmin values over the weekend, with perhaps some places experiencing -3C to -5c potentially;

 

So despite the rather cold air mass across the UK, it won’t be all that bad and in any sunshine by day the strength of the sun at this time of year should offset the chilly air mass somewhat and should be pleasant enough.  That being said, any gardeners out there beware of the frosts mind, as they are likely to be quite potent for the time of year.

Whatever you’ve got planned have a good Bank Holiday Weekend…

Regards to all,

M.

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One Response to “May Bank Holiday – Thoughts & Analysis”

  1. Kieron May 2, 2012 at 12:15 pm #

    Matt – very good analysis. I’m going to share that with friends of mine who need to know what the weekend’s going to be like.

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