A blocked pattern dominating once again.
Issued at 1500GMT on Tuesday 17th Jan based on 17th Jan 00Z Model Runs
Wed 18th: Into Wednesday and the dominant synoptic pattern across the UK will be that of a large surface anticyclone and a large amplification to the upper pattern across NW Europe. The main centre of gravity of the high pressure is likely to be over the near Continent, and it is on the southern flank of the high pressure that a very cold air mass will dominate across France and into Spain. For the UK though, this means relatively mild conditions on the northern flank of the blocking high.
The UKMO, ECMWF, GEM and GFS all indicate a surface high pressure of around 1040mb to be situated across parts of Germany through the day. With the main upper ridge generally being across the UK, there is some disconnect between the surface high pressure and that of the upper ridge, but all-in-all the weather across the UK remains distinctly anticyclonic. The main uncertainty, as will be the case throughout the forecast period, is cloud amounts and distribution.
The high pressure is situated in a location that is allowing a relatively mild S or SW’ly air mass to come up into the UK around the western periphery of the high pressure and this is bringing a lot of cloud. The colder and drier air mass across into the near Continent though is likely to influence cloud amounts across the far south of the UK through the day, and it is across more southern areas of England that the best of any cloud breaks will be with some sunshine after a chilly start with local frost. For most of the UK it is relatively mild, mostly cloudy or overcast and with some patchy light rain and drizzle possible. The on-going risk of low cloud and hill fog is likely too across higher ground of the north and west of the UK with some very poor visibility at times. The GFS model, as an example, highlights the overall situation well for Wednesday, noting the large amplification to the pattern over NW Europe in association with the high pressure and blocking regime.
Thu 19th and Friday 20th: As we progress towards the end of the week and the overall synoptic pattern isn’t expected to change. All the main forecast models are in good agreement for high pressure to remain the dominant feature of the weather across the UK and this maintaining the benign and stagnant conditions across many areas of the UK. The GEM, ECM and UKMO from the 00Z runs all indicate atmospheric pressure to be above 1030mb across most of the UK and with perhaps highest pressure developing across parts of England and Wales with a central pressure near 1036mb.
The weather through the rest of the working week is then, clearly, expected to be very similar to the previous few
days. With the anticyclone dominating the weather than many areas of the UK will have drier than average conditions with little in the way of any active weather. The main uncertainty will again be cloud amounts and distribution. Forecast soundings do indicate further extensive cloud across many areas of the UK with little signal for any widespread cloud breaks. However, this doesn’t preclude the potential for more local and regional cloud breaks to develop at times during the end of the week and where these appear then some overnight mist, fog and frost are possible.
Equally, the cloud, most probably Sc or CuSc type may well be thick enough still to produce some patchy light rain and drizzle in places too, exactly where is unknown but some western hills of England, Wales and Scotland could well see some patchy light precipitation at times. Winds for most of the UK will be light and variable beneath the area of high pressure so where cloud is present it is unlikely to move very much and remain in place over the same areas for some considerable time. The below forecast sounding for Manchester during Thursday highlights the general atmospheric profile that is likely to dominate across most areas of the UK to end the week. Despite a subsidence inversion, in association with the anticyclone, there is a region of significant moisture within the boundary layer and it is this region that is likely to lead to some extensive Sc CuSc type clouds leading to further overcast and often grey weather for most areas, despite some cloud breaks in places.
Sat 21st and Sun 22nd: As we progress into the coming weekend and the UKMO, GEM, ECM and GFS continue to highlight the dominance of high pressure across the UK but with the high pressure gradually easing away as the weekend progresses. This process is then what is likely to lead to a general progression towards more unsettled conditions beyond the weekend and through into the final full week of January, but more on that within the Medium and Longer term section.
Into the weekend, and atmospheric pressure is still forecast to be above 1030mb across most areas with the 00Z ECM, GFS, GEM and UKMO all indicating an on-going blocking pattern. The jet stream will track to the north of the UK with weather systems being held at bay to the west and north-west of the UK, as shown on the below charts from the midnight runs for 00Z Sat 21st.
The general forecast for the coming weekend is then similar to that of the previous days with many parts of the UK maintaining fine and settled conditions across all areas with some bright or sunny intervals developing, but still with often large amounts of cloud. One aspect of the forecast that does seem to be in evidence though is that as the main centre of gravity of the high pressure declines to the east as the weekend progresses there is then enough of a feed off the near Continent to allow for a colder air mass to filter into the UK. To what extent is uncertain for now, but across England and Wales at least temperatures are likely to decline towards the end of the week and into the weekend. Whilst 850mb temperatures don’t show much of a significant drop; theta-e does highlight this progression towards a generally colder air mass quite well, noting the green shadings (relatively mild weather) on Friday giving way to blue shadings (colder air mass) through the course of Saturday and into Sunday.
So, whilst the uncertainties over cloud amounts are likely to continue for a few more days yet, the weekend is likely to generally become colder across England and Wales and with a heightened risk of overnight frost. Whether the air mass is cold and drier enough to help break up the moisture within the boundary layer is yet to be seen, but there is likely to be more in the way of cloud breaks developing across England and Wales as the weekend progresses. Clearly given the time of year and this advection of somewhat colder air off the near Continent then any cloud breaks will lead to a risk of frost, and this is being picked up well by our MR model for Saturday night and through into Sunday morning for example. So, the weekend as a whole is likely to be quite chilly, especially in England and Wales, but with most areas maintaining predominantly fine and settled conditions, with some brighter weather at times, but equally with further large areas of cloud very slowly moving around beneath the area of high pressure.
There is a developing signal for lower pressure to become more influential as we progress towards the end of January and probably beyond too into early February. The blocked regime over the next 5 to 7 days is forecast to ease away. As is often the case, the period of transition is uncertain, with some models indicating more southern and eastern areas of England hanging on to fine weather into next week, whilst it generally becomes more unsettled, milder and windier further north and west. After a sustained period of inactivity so far this winter there is also some evidence too that the MJO will become more pronounced (and also be more of a useful forecasting tool) and move into phase 2, as shown by the UKMO forecast below, this, for January does equate to a more unsettled regime.
So, despite some uncertainties over the forecast as we move into early next week during that transitional period away from a blocked regime to something more unsettled, confidence is quite high to suggest that the end of January and into early February will likely become more cyclonic across many areas of the UK. Details aside at that range, this will mean a generally more mobile period of weather with spells of wet and windy weather and also with generally mild conditions too. Highlighting this overall trend towards more cyclonic conditions are the GFS ENS, noting the large drop in pressure from all ENS members compared with the short term and with a clear increase in precipitation signal too, the below being a N England example.