Yet further low pressure dominated weather set to persist through late July and into early August
Issued at 1500BST on Tue 25th July based on Tue 25th July 00Z Model Runs
Key Points
Discussion:
Wed 26th July: Overnight and into Wednesday and the primary feature of interest will be the rapid development of a surface low pressure, across the N Atlantic, due to the left exit interaction of the jet stream. This process will be taking place through the course of Tuesday and the below graphic highlights this development well, within the short term, through the rest of Tuesday. A 160mph jet streak, within a broader diffluent trough, is allowing the surface low pressure to deepen, potentially rapidly, especially for the time of year with plenty of PVA in evidence.
The broader evolution through into Wednesday is for a zonal and indeed powerful jet stream to be present across the N Atlantic, again this all associated with this developing surface low pressure.
Through the course of Wednesday and it is the low pressure that will dominate to the NW. Active frontal zones, across many W areas first thing, will quickly spread eastwards through the day, the fronts moving through quickly in association with the zonal W’ly flow. Despite moving through quickly, some locally large rainfall totals may develop across some parts of the N and W. The more dynamic precipitation will, however, clear away to the N Sea through the course of Wednesday afternoon, following will be something of an improvement with more broken cloud allowing for some sunshine. However, as the upper vortex and surface low-pressure edge nearer to the UK, then the air mass will become deeply unstable from the W. As a result, showers will become increasingly frequent across N and W areas by the end of the day, with the atmosphere unstable to SSTs. The rapid development of low pressure will also mean quite a tight PGF, with, as a result, a fresh or perhaps near gale force W or SW’ly wind, this veering more W’ly later on in the day though.
Thu 27th July: Into Thursday and it is the upper vortex and surface low pressure that will become quasi-stationary just to the NW of the UK. There has been some model divergence over the last few days regarding the position of this low-pressure. For example, the UKMO model brings the low-pressure further into the British Isles and increases the risk of some high winds for the N and W, but it is more likely that the cyclone will remain offshore to the NW of Scotland.
However, despite this, the low pressure will be near enough to produce quite a tight PGF and, as a result, it will be a windy day for much of the British Isles. In particular across the N and W with a W or SW’ly fresh or near gale force wind expected. The broader synoptic pattern across the N Atlantic and into NW Europe wouldn’t look out of place during the early autumn period. A significant negative MSLP anomaly dominates to the NW of the British Isles, in association with the surface low pressure and with a particularly S’ly tracking jet stream. It is to the N of the main track of the jet stream that convective activity will be at its greatest as the day progresses, so across Scotland, Ireland and N England.
While many regions of the British Isles will be at risk of showery conditions through the day, it is, across more N and W areas that the showers will be driven well inland in association with the W or SW’ly wind. The showers will also develop inland as well, even with just modest surface heating given some particularly steep lapse rates. While the showers will move through quickly, they may well combine to bring longer spells of rain across parts of the N and W, and may well also bring about some local thunderstorms as well. Fewer showers are likely across the far S and SE of England, on the ‘warm side’ of the jet, with profiles here not as unstable as further N. The forecast sounding for Glasgow though, highlights the overall atmospheric profile well for the N and W on Thursday afternoon.
Fri 28th July: We end the working week with the upper vortex and surface low pressure still slow moving just to the NW of the British Isles. However, to complicate matters is a shortwave trough embedded within the W’ly flow. There is some uncertainty over how the interaction of this upper trough will aid in the development of a surface low pressure that will run up into the UK from the SW later in the day. The 00Z UKMO, GFS, ECMWF and GEM all differ slightly with regards to the depth and track of this low pressure. The other important aspect as well is that this feature will be arriving from a moisture rich environment, with a likely WBPT plume on its forward side. As a result, there could well be some heavy rain associated with this system as it moves NE across England and Wales through the latter half of Friday.
The weather across the UK will, for most of the day, be a continuation of the conditions from Thursday. The quasi-stationary vortex and surface low pressure to the NW of the UK will be slowly filling, but it will maintain an unstable cyclonic W’ly air flow across much of the British Isles. There is a good model agreement regarding this development. As a result, further showers will be widespread across many N and W areas through the day, again locally heavy with thunder possible and perhaps merging to bring some longer spells of rain as well. The showers fewer and further between across central and southern areas of England, but it is here that the persistent wet weather may well arrive later.
The 00Z UKMO FAX chart for T+84hr on 12Z Fri highlights the synoptic pattern well. The feature of interest is highlighted too, developing and approaching from the SW. Depending on the finer details of how the short wave trough and jet interact with this surface low pressure will determine the track. However, it does seem as though that persistent rain will spreading into Wales and Cornwall during the afternoon and then quickly spread NE across the remainder of England and Wales through Friday evening and into the first part of Friday night.
Sat 29th and Sun 30th July: So, we move into the weekend, and there is certainly the possibility that overnight rain may well still be in evidence across some eastern areas of England, perhaps even up into eastern parts of Scotland. Depending on the timing of this system, then, equally, the overnight rain may well have cleared. Following on behind for most of the weekend will be a cyclonic W or SW’ly air flow across many areas of the British Isles, but particularly across parts of the N and W once again.
It is likely that the vortex that becomes slow moving to the NW of the UK through the middle of this week will still be there through the course of the weekend. There are some model differences over the exact position of the centre of gravity of the low and whether more in the way of ridging potentially develop, from the SW, across more southern and south-eastern areas of England as the weekend progresses, as per the GFS. If this does occur then some drier and sunnier conditions may well develop across the SE of the British Isles, and this is catered for as part of the general forecast for the weekend.
However, for much of Scotland, Ireland, N England and Wales and it is the cyclonic W or SW’ly flow that will maintain an ongoing risk of showery conditions. The details of this are unknown for now. There may well still be some residual occlusion debris in evidence which could focus the showers in some banding features, especially across Scotland and Ireland. Overall, though, a particularly unstable atmosphere is forecast once the cyclone clear early on Saturday. A mixture of sunshine and locally thundery showers are likely to prevail through much of the weekend, especially the further N and W.
Medium and Longer Term 31st July onwards:
Looking ahead through into early August and it is, unfortunately, a continued cyclonic pattern that looks set to dominate across the British Isles and NW Europe. There has been quite a consistent signal for this within the medium and longer range Det models, along with the GEFS and EC EPS as well. Using the 06Z GEFS as a guide, you can see by the 2nd of August there is still evidence for a S’ly tracking jet and a cyclonic pattern over the UK. We’ll be taking a closer look at this within an extended outlook in a few days. However, if you’re hoping for more settled conditions into early August, then you’ll likely be disappointed.