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SPRING/ EARLY SUMMER 2018 WEATHER PREDICTION Apologies that this is a bit late, I have been busy during the worst blizzards to hit northern England in the last 39 years: However, I now have my seasonal prediction for Spring, covering late March, April and May 2018. Given this is so late I will add a prediction for June! At the time of writing the Circumpolar Vortex and Stratospheric Westerlies around the Arctic are much weaker than normal and are predicted to remain so for the next three weeks. Almost all (20 of 21 modellers) predict the Stratospheric Westerlies to remain weaker than usual for the next three weeks (https://www.weatheriscool.com). The predictions on some days for the Stratospheric Westerlies at 60N, both at the 10 mb and 30 mb level, show a negative speed, in other words mean easterly winds. This has implications for the weather for up to a month beyond the end of the forecast period for weather-patterns in the lower atmosphere. In other words, we are looking at a situation whereby there is likely to be frequent high-pressure over northern Scandinavia and over Greenland through the remainder of March and for much of April. The pattern of sea-surface temperatures is also interesting. After a cold February and early march sea-surface temperatures around the UK are about 1C below normal for the time of year. There are colder than usual waters in the southern Norwegian Sea and in the North Atlantic around 50 to 60N and to the west and NW of the UK. Meanwhile warmer-than-usual waters (with anomalies over 3˚C above normal) are found off the east coast of the USA. Significantly warmer than normal waters (anomaly up to 2˚C), are also found in the Bering Strait and Barents Sea. The Mediterranean Sea overall was about 1˚C warmer than normal for mid-March. Arctic sea-ice has been at record lows for early March in the Bering Strait around Alaska and in the Barents Sea and around Spitzbergen, but sea-ice extent has been close to the mid-March norm off eastern Canada, around Greenland and just north of most of northern Russia. There was more sea-ice than usual for mid-March in the Baltic Sea between Sweden, Finland and the Baltic States. The Quasi Biennial Oscillation is strongly easterly at the 30 mb level over the Equator and it has also turned easterly at the 50 mb level. The Sun is now very quiet, with few solar flares and sun-spots predicted over the next thirty days. Both these factors signify weaker Westerlies in higher latitudes than normal. In the eastern equatorial Pacific sea-surface temperatures are locally more than 1˚C below the long-term norm so we have La Niña conditions and sea-surface temperatures over all equatorial ocean surfaces are (on average) fractionally below the seasonal normal. The impact of the Madden Julian Oscillation, a convective large-scale weather-pattern that encircles the globe is diminished by La Niña. The Madden Julian Oscillation leads to large-scale convective waves that penetrate the Stratosphere and which can often disrupt the Stratospheric Circumpolar Vortex in high latitudes, La Niña often (but not always) prevents this happening. However, by May the Stratospheric Westerlies at high latitudes tend to break down and reverse due to the 24 hour solar- heating effect on the Polar Stratosphere so the workings of the Madden Julian Oscillation and the effects of La Nina become much less important. For earlier in the Spring, the starting point is very weak Westerlies both aloft in the Stratosphere and in the troposphere at higher northern latitudes. In late Spring the Circumpolar Vortex weakens anyway, though factors such as a quiet Sun and easterly QBO high over the Equator would, if anything, point to even weaker Westerlies aloft but the impact is not as great as during winter and early Spring. The patterns of sea-surface temperature, sea-ice cover and less thick ice than normal over the Central Arctic Ocean (https://www.nsidc.com/arctic-sea-ice) would suggest weaker baroclinicity in the troposphere around the Arctic but with the Vortex shifted a hundred miles or so in the direction of Europe. This will be important later in the season when other global influences, such as La Niña or the easterly QBO have less of an importance. Most of the equatorial waters being slightly cooler than normal, would hint at a weaker global circulation (though the impact on the winter hemisphere would be greater), with a predisposition towards blocking in high latitudes. We can now use the above information to make a forecast: Starting with the second half of March the above analysis indicates further periods with strong blocking patterns over Scandinavia leading to further very cold easterly or north-easterly winds with the air originating over north-west Russia. These will bring snow-showers to the North East, Yorkshire, East Anglia and the South East of England whilst frontal influences bring some heavier rain or snow to the South-West. Eastern Scotland will also get snow-showers, however amounts will be nothing like we saw at the start of March. Western Scotland, NorthWest England and the English Midlands along with much of Wales will be drier and brighter during the cold easterly/north-east spells but still cold. Daytime maxima will range from near 7˚C in the south of England to 2 to 3˚C in North East England and Scotland though upland areas in Scotland will remain below freezing-point by day during these cold spells. Frost will be widespread at night during these cold north-easterly spells as skies clear inland, minima will be locally -5˚C or colder from the Midlands northward. A brief milder spell with south-westerly winds will occur around the 25th March. This will bring rain (and mountain snow) to Scotland and NorthWest England along with parts of upland Yorkshire and Northumberland and North Wales. Coastal and upland gales are possible in all these regions. Northern Ireland can expect similar weather. Daytime maxima will be near 10C in the lowlands in these more northerly areas. For the Midlands, the North East lowlands, lowland Yorkshire along with eastern and southern England and South Wales brighter warmer weather is likely with less rain, temperatures of 14 to 15˚C can be expected quite widely and nights will be frost-free for a time. The very end of March will see a return to icy north-easterly winds with snowfalls in the North East, Yorkshire and eastern Scotland, drier brighter conditions elsewhere and the return of air-frosts at night. March looks set to be the coldest for five years with mean daily temperatures around 3.5 to 4˚C over much of England, averaging over 2˚C colder than average. The departures from normal look set to be about 2˚C below in eastern Scotland; but nearer 1˚C below in western Scotland. Rainfall looks set to be above normal in East and North East England, South West England and in Eastern Scotland but a little below normal in the West Midlands, North West England and western Scotland. As we head into April we can expect the alternation between icy north-easterlies and milder showery westerlies to continue. About half of early and mid-April will be dominated by westerlies with depressions taking a track just to the north of Scotland and moving east into the North Sea. A good deal of cold wet showery weather will affect the northern half of the British Isles, with showers in Scotland and northern England likely to be accompanied by sleet or even hail at low levels but with snow in the mountains above about 600 metres. Strong westerly winds will affect coastal areas of the NorthWest, North Wales and Scotland with gales possible in some places. Daytime temperatures will be near 10˚C in the northern and Scottish lowlands, possibly a bit more where the Sun comes out. There will still be the possibility of frost at night as the generally cool showery Maritime Polar airstream will lend itself to frequent clear skies inland with winds falling light. The Midlands, South Wales and the South and East of England won’t escape showers during the showery cool Westerlies of early-mid April, but they will be lighter than further north and there will be more in the way of sunshine. Daytime temperatures of near 14˚C can be expected to occur quite widely, so it will feel like Spring at such times. Clear skies at night will lead to temperatures falling close to freezing point and sharp ground-frost will occur. During the first three weeks of April, quite possibly as an extension of the cold north-easterlies expected to set in at the end of March, there will be a spell of five or more days when strong high-pressure over northern Scandinavia and/or near Iceland will lead to much colder drier east or north-easterly winds affecting the country. This will bring what is widely known as a “Blackthorn Winter” as it coincides with the time when blackthorn trees normally blossom, at least in the lowlands of the Midlands and South of England! The North Sea will be near its coldest by this point and with the air bring slightly less frigid coming across from northern Russia convective snowfalls near the East Coasts of Scotland, North East England, Yorkshire and East Anglia will be less (and less intense). Alas, there will still be snow-showers in most of these locations but except on higher ground in the North and Scotland will be unlikely to lead to significant snow-cover. Coastal Kent and around London is likely to see any showers fall as rain or sleet as the north-easterly winds will just not quite be cold enough to bring snow. On the western side of Scotland, to the west of the Pennines in North West England and across the Midlands, Wales and southern England the cold north-easterly winds are likely to lead to dry, bright conditions although the north-easterly wind will still feel cold. Again, the South West of England is likely to be affected by frontal systems moving into the Bay of Biscay whilst cold north-easterlies affect the rest of Britain which will lead to some rain and sleet locally, though snow will still fall on Dartmoor and Exmoor. It will be cold nationwide with maximum temperatures below 8˚C even in the South during the April spell with north-easterly winds. Daytime maxima in the lowlands of North East England and in Scotland will be near 5˚C and will remain below 0˚C in the Scottish mountains, where snow is liable to accumulate where it falls. Night skies will be clear during the spell of icy north-easterlies, except along the East Coast and in the far south-west so nighttime temperatures on the coldest nights will drop well below freezing point, particularly as the wind will fall light inland. Again minima below -5˚C will occur locally from the Midlands northwards, so gardeners beware! The cold snap will not last beyond a week and will be superseded by a return to milder showery west or south-west winds. During the last ten days of April there is confidence in high-pressure developing over the UK for a time, aided by the still cold seas around the country and the weakening of the Circumpolar Vortex as this retreats northwards. This high-pressure is likely to be centred over and to the west of the country extending as a ridge from the subtropical-high over the Azores. The vast bulk of England and Wales will enjoy fine sunny conditions for a few days; with temperatures reaching a very warm 20˚C or above inland, although coastal areas will be considerably cooler. Clear skies at night with light winds will still allow temperatures to plummet and frost is likely inland, even in the South. Scotland and the far North East of England is liable to miss out on this fine spell to some extent, chilly north-westerly winds will bring more cloud and a touch of rain to coastal areas. Temperatures in lowland Scotland will remain below 14˚C during the fine spell further south. Clear skies inland at night will still see temperatures fall below freezing-point locally. The end of April or the beginning of May will see a return to showery west or north-westerly winds across the whole country as a four-wave Circumpolar Vortex gets properly established. These winds will be lighter as the depressions responsible for them are likely to be weak and slow-moving. The air will be cooler than normal for the time of year, thanks in part to below normal sea-surface temperatures upwind so daytime maxima will be no more than 15˚C, even in the Midlands and South whilst maxima of 12˚C will be normal for Scotland and the North where hail and sleet is still likely to accompany showers. Again, the Polar Maritime airstream responsible will mean clear skies inland on most nights; this means widespread ground-frost and localised air-frost from the Midlands northwards. Average temperatures during April 2018 will be near-normal in the South of England but colder than normal elsewhere, with the departure from the seasonal normal over 1˚C below the April normal over a wide area. Mean daily temperatures will range from 9˚C along the South Coast to 7C in the lowlands of the North West and a chilly 5˚C in the north of Scotland. Rainfall totals will range from a little below normal for April in the Midlands and the South of England and in South Wales, to around the seasonal norm in South West and North West England and North Wales but a little wetter than normal for much of Scotland, North East England and Yorkshire. (CONTINUED BELOW)
There is a real possibility (according to my Spring 2016 forecast) of an Arctic blast at the end of April or beginning of May that will bring a short spell of unseasonably cold weather following which, with night skies clearing, air frosts will be widespread and temperatures could go down to -3C in lowland Scotland and the North of England. Given the right conditions, with dry ground, a sustained flow of air from the central Arctic for several days and with a dry clear night and winds falling calm in a ridge of high-pressure following such Arctic airstreams some surprisingly low temperatures can occur just about anywhere in the country (even as late as May). In mid-May 1935 an exceptionally cold Arctic airstream affected Britain which brought snow-showers as far south as London and Wiltshire. Clear skies occurred at night with the northerly airstream falling light and the air temperature fell to a garden-devastating -8C at Rickmansworth in Hertfordshire (this is on the outskirts of London). This could happen again, possibly this spring: If a strong high-pressure forms over Greenland following the passage of a depression into Scandinavia the northerly winds could be strong and come from a long way north bringing an airmass which is close to freezing point even at sea-level across the English Midlands and South in early May (the Central Arctic remains a source of some surprisingly cold air until early May when its mean temperature is still around -10C). If skies clear and winds fall light as a ridge moves in from the west (a likely scenario following a spell of northerlies) air temperatures near the surface would fall a further several degrees (and more than 10C colder in some favoured frost hollows). This means that severe air frosts with minima of -5C or colder remain a small possibility across most of the country until surprisingly late in the spring. At my home at just over 400 metres' elevation in the North Pennines I have recorded a minimum temperature of -9C in April (in 1990) and -5C in May (1984). Moreover, as recently as last year I recorded -5C overnight 26th/27th April and one local gardener lost the entire contents of his Polly-tunnel!