A varied spell of weather on the way during the coming week.
Issued at 1500BST on Tuesday 28th Mar based on 28th Mar 00Z Model Runs
Wed 29th Mar: Into Wednesday and the broader upper pattern across the N Atlantic has high confidence given good model agreement between the ECMWF, UKMO, GEM and the GFS, for example. A large-scale and mature surface and ULL (Upper-Level Low) will be dominating within the N Atlantic. The main PFJ (Polar Front Jet) will be moving around the base of this ULL and up towards the UK, while high pressure will continue to dominate across Continental Europe. This broad-scale synoptic pattern means that the UK will be affected by a moist Tm air mass on the eastern flank of the Atlantic vortex, while frontal (baroclinic regions) zones will be pushed into many northern and western areas of the UK as the vortex to the west slowly drifts eastwards. The GFS MSLP and Jetstream chart below highlights this broader pattern well.
The frontal zones associated with the vortex to the west will move into many northern and western areas of the UK through the course of the day; there is high confidence for this. However, various regions of vorticity that will be pushed up into the UK from the SW will signal frontal wave development and this, as a result, does mean there is some uncertainty over precipitation totals and specific areas at risk of the most persistent rain. Overall though a distinct NW and SE split in rain amounts are to be expected as we move into the mid-week period. Many areas of Scotland, N England, Ireland, Wales and SW England will be affected by frontal zones through the day bringing outbreaks of rain and showers at times. Some extensive low cloud and hill fog are possible in association with the moist Tm air mass, with some poor aviation conditions likely as well. In contrast will be SE England where frontal zones will unlikely reach. This, as a result, means that SE England will maintain much drier conditions than further N and W and given the Tm air mass then in any cloud breaks and some sunshine it will become particularly warm here through the day. Our precipitation accumulation graphic highlights this general evolution well through the 24 hour period of Wednesday, with clearly very little precipitation getting into SE England than compared with further N and W.
Thu 30th Mar: As we move into Thursday and all the main GM’s (UKMO, ECM, GEM, GFS, and JMA) indicate a continued NW and SE split in conditions. The exact position and track of frontal waves that will develop from the SW and run up into N and W areas are uncertain. As a result, some errors in the forecast regarding precipitation amounts and specific regions that will see the most precipitation through Thursday are possible. However, the broader evolution through Thursday does have high confidence, and that is for further frontal rain to affect many parts of Scotland, Ireland, NW England, Wales and SW England. The 00Z UKMO FAX for 1200Z on Thu does highlight this general split, with a set of frontal waves affecting more N and W areas, but note across the SE of England still there is some anticyclonic influence from the main surface high pressure over in the near Continent. Atmospheric pressure is forecast to be over 1020mb across SE England; this is in contrast to pressure less than 1010mb across many N and W areas of the UK.
This general pattern means that further rain and showers will affect most of Scotland, Ireland, NW England, Wales and SW England through the day with pulses of rain moving north-eastwards as individual regions of vorticity track up into the UK from the SW. Again it this kind of detail that even now is somewhat uncertain, but in contrast, many south-eastern areas of England will have a much better day, mainly dry with some bright or sunny spells and when combined with the continued Tm air mass then temperatures are likely to peak on Thursday. 850mb temperatures are widely at or above +5C across England and Wales during the day but are nearing +7C or +8C across SE England. This, as a result, does mean that in any sunshine temperatures may well reach 20C, perhaps locally 21C and this is exampled well by our MR weather model for the middle of Thursday afternoon.
Additional rainfall across parts of the N and W of the UK though will signal further locally large rainfall totals, perhaps more than 30mm to 40mm in some areas, especially where orographic enhancement comes into play across parts of SW Scotland, Cumbria and perhaps especially across some southern and south-western Wales too. The 00Z GEM model for 1200Z on Thursday highlights the expected synoptic evolution well. Note the fine weather across SE England, but further N and W multiple regions of vorticity (pink shadings) move north-eastwards and maintain a risk of rain and showers across many N and W areas throughout the day.
Fri 31st Mar: Through the course of Friday the mature upper vortex to the W of the UK is expected to approach and then steadily move across the British Isles through the day. This evolution means that a more concerted ‘push’ eastwards of precipitation is to be expected during Friday, the details, however, are uncertain at this stage. Some active frontal zones are still likely to be associated with the surface low and upper vortex as it moves into the UK from the W. As a result longer spells of rain are possible, for a time at least, but the overall evolution, given the time of year, is towards a convective regime. As the upper vortex moves into the UK through the day then steepening lapse rates, when coupled with surface heating, is likely to mean a more ‘simple’ development of sunshine and showers through the day. The GFS model does a reasonable job of highlighting the eastward movement of the vortex into the UK through the day.
So, a general eastward progression of rain and showers is likely through the day with a combination of both dynamic and convective precipitation possible because some further frontal zone activity is likely, but with then more of a convective element developing from the W as the upper vortex moves into the UK. Our MR weather model highlights this potential convective component too, perhaps especially so when combined with what will still be quite high temperatures across some E and SE areas of England. MLCAPE (Mean Layer Convective Available Potential Energy) values showing a heightened risk of convective development, and hence showers or downpours, across some central and eastern areas of England and also across parts of the W UK too.
Sat 1st and Sun 2nd Apr: So, we move into the opening few days of April, and despite the change in the month we won’t see much of a change in the weather. The overall synoptic evolution from Friday will continue into Saturday. What this means that Saturday is likely to have the upper vortex directly positioned over the UK, at least for a time. As a result, Saturday is likely to produce a mixture of sunshine and April showers across the UK, seemingly perfectly timed for the opening day of the month. Using the GFS model again to highlight the broader pattern you can see that during Saturday there is an upper trough/vortex over the UK and of which will combine with surface, diurnal heating to produce the convective regime. However, upstream the pattern is becoming more amplified once again with a ridge within the jet approaching in response to trough extension just off the E of Canada.
What this means is that whilst most of Saturday will produce a mixture of sunshine and showers, in a convective regime, increasingly anticyclonic conditions will move into western areas of the UK from the west as the day progresses. This development, in association with the upper ridge/surface high will suppress convection across Ireland initially and then across the rest of the UK to end the day. This then leads on to a fine day on Sunday across most areas with some bright or sunny spells. The below forecast sounding for Manchester, on Saturday, captures the convective regime well. Quite deep instability is signalled as the upper trough/vortex moves across and combines with surface heating to produce perhaps ~400j/kg of CAPE and cloud tops up to near 24,000ft; certainly enough instability for heavy showers and thunderstorms, at least for a time.
Looking further into early April and there is quite high confidence, given some good model agreement and consistency for high pressure to be a primary feature of the weather. Details aside, it does look as though some briefly unsettled conditions around the 3rd or 4th will be followed by a build of pressure into the UK from the SW through the end of the opening week of April. The ECMWF, GFS and GEM models, along with the EC and GFS ENS, plus the EC Monthly, all indicate this build in pressure. As a result, the outlook for early April is for further fine and settled conditions to likely return, along with some pleasantly warm conditions possible too.
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