Sunday, 26 April 2015

Weather guide Monday 27th April to May Day Bank Holiday Monday 2015

Headline: Cool, sunny intervals and showers (possibly wet and windy Wednesday); frosty nights. Mostly dry and bright Friday into Saturday before the Bank Holiday turns more unsettled but milder.

Last weeks highlights

Much of last week was dry and rather warm with plentiful sunshine, a splendid spell of Summer-like weather across Wales, however it is still only April and clear skies and calm conditions by night meant fairly widespread frosts on many night up to Thursday. Sea breezes kept some coastal areas much cooler in spite of the sunshine, on Monday Aberporth only reached a chilly 12c cf. 15 to 17c inland.  Tuesday had another frosty start in many places, as low as -1.5c, at Llanwnnen, but then  was another sunny warm day, most places getting above 17c, with Porthmadog, North Wales, the warmest spot in the UK max 21.3c. Llanwnnen reached 21.1c (70f) on Tuesday making a massive diurnal (daily) temperature range of 22.6c and one the the greatest I have ever recorded. Sea breezes affected the coasts though and none more so than at Aberporth where the max was only 13.4c so almost 7c cooler than Llanwnnen just 18 miles away but crucially well inland.

Similar on Wednesday although the Midlands saw more cloud afternoon. The gradient (along the pressure isobars) ENE wind overrode the sea breeze effect so that even coasts experienced the warmth of up to 20c across WW as at Porthmadog, Llanwnnen and Pembrey, with even the oft-chilly Aberporth managing 19c! For much of Wales sunshine was virtually unbroken Monday to Thursday with largely clear blue skies giving an estimated 50 hours in these 4 days at Llanwnnen and being typical, so a very sunny spell.

The weather began to change Friday with much more cloud and a little rain over Wales, although still quite warm and mostly dry for the Midlands. An active cold front edged down Saturday pm turning it decidedly cold and wet over North Wales but still quite mild to the south of it, at 1600 BST it had dropped to just 4c on Anglesey while at Milford Haven, Pembs it was 15c. Although we have entered a more unsettled weather pattern this weekend, for much of Wales and the Midlands away from more northern parts there has been little rain, for most no more than a mm or two. So no matter what happens in the coming week a dry April is assured.

The week ahead

Fine and sunny this morning across WW and the NW Midlands, but cloudy for the rest of the Midlands. Sunday afternoon largely dry and most places will see sunny periods although the cloud and spits of rain may stick into this evening for the SE Midlands below Northampton to Stratford--upon-Avon. Cooler and fresher than of late, highs 11 to 13c with moderate north winds. Just the chance of a shower for North Wales late in the day, with a few showers skirting coastal WW overnight too otherwise dry.  Clear periods and light winds means a widespread air frost tonight, down as low as -2c in places, and one of the colder nights of April 2015.

With pressure low to our north through Monday and Tuesday we are in a cool, essentially showery NW flow of air which originated over the Arctic. Both days will see dry, sunny spells, but there will be some sharp showers about, these with hail in places and even some sleet or snow for the higher ground above about 400m (most likely Monday evening). Most of the Midlands dry and bright until well into Monday afternoon but a more defined line of showers looks set to track SE late Monday afternoon into the early evening, so most places eventually catch a shower or two. Cool highs on Monday of between 9 and 11c with a freshening WNW breeze afternoon. Breezy with showers dying out away from windward coasts on Monday night, a widespread ground frost inland, locally getting just below 0c. Tuesday and again a sunny spells and showers mixture, with perhaps a longer spell of rain during the afternoon which could end up quite cloudy. Chilly highs between 8 and 11c and a breezy day.

Clearing skies on Tuesday night means another widespread ground frost at least and locally even a touch of air frost as winds should decrease towards dawn.  Wednesday at best another sunny intervals and showers day, at worst it could turn out wet and windy as there is the potential for a new low to cross the UK at some stage during the day, although this scenario not set in stone as yet. Another cool day in any event no higher than 12 or 13c. Yet again Wednesday night could see a widespread ground frost developing once any rain or showers clear. Back to sunny spells and scattered showers for Thursday, lighter winds so not feeling too bad in the sun, highs between 11 and 13c though still below average for the end of April. Some places will no doubt miss Thursday's showers. Thursday night looks cold and frosty, -3c possible in places.

A frosty dawn to Friday but some decent sunny spells during the day as a ridge of high pressure edges into the west, this should have the effect of dampening down any showers that may develop, and although there is likely to be a few knocking about for many places quite a nice, dry and bright day. Still quite cool on Friday though maxima 12 to 15c but with only light breezes. Another widespread frost likely again Friday night with largely clear skies expected. A chance that a tenuous ridge is over the UK during Saturday, so just about another mostly dry and bright day, however indications are that low pressure and rain fronts to our SW will be edging in later in the day.

Unfortunately, pressure looks set to fall as we progress through the Bank Holiday weekend with low pressure centred to our west, at least this does draw up milder (if sometimes moist) south to SW winds so frost concerns should have alleviated by then. Details impossible with no clear signal, but Sunday and Monday likely to see some spells of wet weather, but no real washout is anticipated, and at times it could well be dry, bright and mild. The latest GFS output in fact keeps the Midlands largely dry on Sunday and Monday but I feel this unreliable so prepare for some rain at some stage over the holiday period.

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