Latest synoptic guidance from Matt - looking at the weather during the upcoming week. (Technical)
SYNOPTIC ANALYSIS – THE WEEK AHEAD
Issued at 1500BST on Monday 17th Oct based on 17th Oct 00Z Model Runs
Key Points
Discussion
Mon 17th: Forecast model agreement is good regarding the synoptic developments through the remainder of Monday with the main GM’s (UKMO, GEM, GFS and ECMWF) and the HRES models all indicating a low pressure with a central pressure of around 997mb to be in evidence just to the west of Scotland. This will maintain a cyclonic pattern across more northern and western areas of the UK with various ‘occlusion debris’ moving around the surface low pressure to lead to a rather convective regime through the rest of Monday. Moderate or fresh, locally strong S or SW’ly winds around the low will mean showers move through, but there is enough forcing in evidence for some of the showers to organise into persistent rain, primarily in association with the occlusions that are in evidence, for example, on the UKMO FAX chart. This synoptic regime will mean many central and eastern areas of England will experience mainly dry conditions with some bright or sunny spells, but given the strength of the SW’ly flow then some showers may well be pushed well inland to bring a few scattered showers at times. Model agreement is good regarding an upper vortex and cold pool to the north-west of the UK. This will quickly move down into the UK later today in association with a NW’ly jet streak, and this will aid to increase convective activity overnight and also aid in the enhancement of a cold front moving southeast. Some evidence of a confluent flow as well to the upper pattern tonight so the cold front moving down may well be squally with some locally heavy precipitation possible as well.
Tue 18th: As we move into Tuesday and the overall pattern is still cyclonic across the UK. The upper trough/vortex and associated cold pool will move down across the UK through the day maintain a convective regime and a resultant mixture of sunshine and showers. There is some evidence from the HRES models, ARPEGE, EURO4 for example, for a more organised region of showers, perhaps longer spells of rain, moving south-eastwards across parts of S Scotland and then down into NW England through the day too and it is across these areas of the UK that some heavy showers may well be in evidence well into the afternoon. Across more central and southern areas of England and Wales, a lot of fine weather is expected through the day with little in the way of precipitation signalled with bright or sunny spells. A moderate or fresh NW’ly wind is expected throughout the day; this locally strong at times and with a drop in 850mb temps, along with theta-e/w too then it will be quite a cold feeling day as well with many areas experiencing ‘single figure’ dew points. With freezing levels dropping to 700m to 800m then some wintry precipitation is likely across high ground of Scotland in particular, something which our HRES short-range model is indicating too.
The other important development through Tuesday of which takes places to the west and south-west of the UK is the transition of Hurricane Nicole into a North Atlantic depression. Whilst the surface low pressure in question will not directly affect the UK, what is crucial in determining the weather through the rest of the week is the development of some significant WAA on the eastern flank of the low pressure.
This development of WAA aids to markedly amplify the upper pattern across the eastern North Atlantic through Tuesday and then on into Wednesday. It is this development that allows a marked build of pressure to then approach the UK from the south-west later in the day on Tuesday. Using the GFS model to example this, you can clearly see the significant WAA and then also note the marked amplification that develops with the jet stream as the surface high pressure approaches from the south-west.
Wed 19th to Fri 21st: From the middle of the week onwards there is good model agreement between the main DET models for the high pressure to then build up and into the UK to bringing a return to a blocked regime across the UK. There are clearly some uncertainties over the details of the weather for the second half of the week, especially regarding cloud amounts and especially the risk of overnight mist, fog and frost, but seeing the high pressure builds into the UK within a relatively cool polar maritime air mass then beneath any clear skies at night temperatures will fall low enough for ground frost at least which is likely at times. The fly in the ointment within the models is, however, an upper level low and vortex to the SE of the UK over in the near continent. In what is sort of a repeat pattern to the previous week, this vortex may well come close enough to send vorticity filaments westwards under the block to enhance the risk of convective development over the relatively warm North Sea. As a result, it would seem prudent to highlight that eastern and south-eastern areas of England will generally be cloudier than elsewhere during the second half of the week with the risk of some convective development with showers possible. This highlighted well by our medium range model, but also the forecast sounding for Norwich on Thursday afternoon really is quite unstable up to and above 20,000ft. Whether this level of instability is realised nearer the time is uncertain, but it at least highlights the risk of a more convective regime across E and SE England then compared with elsewhere later in the week.
Sat 22nd and Sun 23rd Oct: Through into next weekend and the blocked regime is likely to be maintained across the UK and NW Europe. However, there are considerable uncertainties over the details at the moment. The upper vortex to the south-east of the UK may well get pulled westwards to increase the convective threat further to parts of eastern areas of England, so perhaps a heightened risk of showers here. Elsewhere mainly dry weather is, however, likely to be maintained with some bright or sunny spells, especially across the north of the UK. However, what is likely to become an important aspect of the forecast is major trough extension within the N Atlantic and eventually trough disruption with a cut-off vortex developing to the south-west of the UK as next weekend progresses. The main GM’s (UKMO, GFS, GEM and ECMWF) for example, highlight this for next Sunday. The UKMO by the 23rd has the cut-off low already present, whereas the GFS, in particular, hasn’t undergone trough disruption at the same time. However, what does seem to be the likely evolution is for a surface low pressure to be present just to the south-west of the UK with associated frontal zones in evidence too. How much inroads into the UK these get is very uncertain, but essentially more southern and western areas of the UK are likely to become more unsettled through next weekend with an increasingly fresh or strong SE’ly wind and with some rain and showers. This is likely to be in contrast to more northern areas of the UK which will maintain dry weather overall, at least for most of the weekend as things stand. At the moment though it would seem that a return to widespread unsettled conditions is unlikely, with the blocked regime likely ‘winning out’ overall as we move into week beginning Monday 24th.
850mb temperatures this week will generally be around 0C to +5C and, as a result, it will be quite a chilly week ahead. Temperatures on Tuesday across the north of the UK, in particular, will be depressed with maximum no higher than 8C or 9C and feeling particularly cold in the NW’ly wind as well. Minimum temperatures from mid-week subject to details over cloud amounts, but certainly low ‘single figures’ are likely and generally maximum temperatures through the second half of the week ranging between 9C and 15C, highest across urban areas of SE England, but many areas around 9C to 13C.