Updated 16th April - Covering the period 23rd April - 15th May
Mean temperatures during this period are forecast to be very close to normal in the east of both Scotland and England, but up to 1C above in most western parts of the UK.
Rainfall is forecast to be near normal overall for much of the country, with some rather wet weather early in the period being offset by mostly settled weather into early May. Northern Ireland is expected to be much wetter than normal early in the period, but in contrast, it will also probably be the driest region relative to normal near the end of the period. However, particularly in central and southern Britain, there will be considerable localised variation depending on the distribution of thundery downpours near the end of the first week.
Sunshine totals are expected to be near normal for much of the country, but most likely below normal in eastern Scotland, especially counties bordering the North Sea, due to issues with haar in the first week and then frequent northerlies late in the forecast period.
This week will start off with changeable westerlies and near-average temperatures, with pressure building to the south-east of Britain but low pressure to the north and north-west resulting in rain belts heading in from the North Atlantic at frequent intervals, heaviest in western Scotland and Northern Ireland.
However, the rising pressure is then expected to build northwards to the east of the British Isles, before being shunted away eastwards at the end of the week, giving a 70% chance of a plume of warm air heading up from the south, most likely between the 25th and 27th, across the British Isles, and the possibility of one of those "two/three fine days and a thunderstorm" Spanish plume setups. With it still being relatively early in the season and the English Channel being on the cool side, there is no guarantee of particularly widespread thundery activity, but always a heightened possibility in this type of setup. Consequently, temperatures are expected to rise well above the seasonal norm during the middle part of the week with the low 20s Celsius probably quite widespread in the southern half of England. However, towards the end of the week easterly and south-easterly winds are likely to feature and this will pull some low cloud and suppressed temperatures in from the North Sea, particularly affecting eastern Scotland and north-east England, where there could end up being some marked daytime temperature contrasts between coastal areas and areas just 10-20 miles inland. By the end of the week temperatures will probably return to near normal with a generally unsettled theme and low pressure in charge.
Temperatures during this week will be above normal, probably by 2 to 3C for most parts of the UK but nearer 1C above in Northern Ireland, which will probably be too far west to catch the main plume of warm/hot air from the south. Rainfall totals are subject to somewhat more uncertainty due to the potential for thunderstorms, which tend to give localised heavy falls. It is however likely to be drier than normal for eastern Scotland, and wetter than normal for Northern Ireland. Sunshine totals are expected to be below normal in Northern Ireland, but above normal in central, eastern and north-western England and in southern Scotland.
High pressure is expected to build to the north and west of the British Isles this week, so while it will start off changeable with temperatures near or slightly above the seasonal norm, and plenty of rain associated with low pressure systems, the weather is expected to turn quieter towards the end of the week. It will also most likely turn colder, with potential for one or two northerly blasts bringing cold bright weather and scattered showers, probably not cold enough for snow on low ground, but potentially cold enough for some frost at night, which gardeners will need to beware of after a fortnight of predominantly warm weather.
Thus, mean temperatures during this week are forecast to be about 1C below the 1981-2010 long-term average over much of the country, due to a colder end to the week. Rainfall is expected to be above normal in most parts of the country due to the cyclonic start, but below normal in northern Scotland, due to the jet stream running a bit further south than normal. Sunshine totals are most likely to be below normal for most of the country, but with high pressure building to the west late in the week, this may raise sunshine totals in Northern Ireland rather above normal.
The most likely scenario for this week sees high pressure more dominant than average for the time of year, but highest pressure often centred to the west of Britain, meaning predominantly northerly and north-westerly winds for much of the time, and some Atlantic weather systems are likely to result in weak fronts toppling around the periphery of the high. Thus, it is expected to be warmer than average in the west, but cooler than average in the east (by up to 2C) due to colder air frequently heading down from the north on the eastern flank of the high. Rainfall totals are expected to be below normal over most parts of the country, but with eastern England coming out close to normal. Sunshine totals are likely to be below normal in most eastern parts of the UK, but above normal in the west.
Monthly weather forecast
The month ahead forecast, written by Ian Simpson is updated once a week - every Monday. As with any longer term forecast, it's not possible to go into much detail and accuracy can tend to drop off toward the end of the period but it can still act as a very good guide to the weather during the upcoming month.