Synoptic guidance blog - 28th February.
Issued at 1500GMT on Tuesday 28th Feb based on 28th Feb 00Z Model Runs
Wed 1st Mar: Into Wednesday and model agreement is good regarding the broader synoptic evolution across the UK and the N Atlantic in particular. It is prudent to highlight that a cut-off high, both at the surface and within the upper atmosphere, is set to develop over parts of Greenland and Iceland. It is this development that aids in generally pushing the jet stream on a more S’ly track and stops low pressure systems from developing and tracking on their more usual climatological track towards the west and then north-west of the UK. This developing pattern is exampled well by the GFS model, note the cut-off in the jet stream developing to the NW of the UK, with the overall jet stream pattern highlighted and emphasised.
All the main GM’s indicating a slow moving area of low pressure to be in evidence to the NE of Scotland during the day and this means a showery start to Wednesday across the north with some of the showers wintry too. Otherwise for most of the UK it is a dry start, albeit chilly with some frost and icy patches, but with some early spring sunshine too. As Wednesday progresses though and an occlusion is set to try and move into the UK from the west and south-west, but is expected to make limited progresses northwards across the UK, potentially becoming rather slow moving across parts of Ireland, Wales, Midlands and into parts of N England during the afternoon and evening. Given a continued Pm air mass on the northern flank of the baroclinic zone, then some sleet and snow is possible across higher ground of Wales, perhaps the Midlands and N England as the day progresses, but nothing too widespread is to be expected. Quite a strong W or NW’ly wind will also develop across southern areas of the UK as well. This general evolution is highlighted well by the 00Z GEM model, noting more in the way of frontal precipitation across parts of England, Wales and Ireland, but with a more convective regime in the north. Some reasonable amounts of vorticity and forcing are associated with the fronts so some moderate or heavy rain is possible and this may also play a role in allowing rain to turn to sleet and then snow through latent heat/evaporation cooling processes across central areas as the day progresses.
Thu 2nd Mar: Into Thursday and some residual precipitation is likely to be evidence across some central areas of the UK, the left-overs of Wednesday’s frontal zone. However, low pressure to the W and SW of Ireland will send another set of frontal zones into the UK from the W and SW through the day, to again affect parts of England, Wales and Ireland. The slow moving low pressure to the N or NE of the UK is still expected to be present during the day, and this will maintain a more convective and showery regime across Scotland in particular, again the showers potentially wintry on higher ground as a rather cold Pm air mass remains in place across the UK. The track of the frontal zones eastwards across central areas through the day is quite complex, and there is the possibility that the extreme south and south-east of England may escape with a dry day with some bright or sunny intervals. So, a very varied period of weather over a short distance is expected during the day on Thursday, and this is highlighted well by the T+60hr FAX chart from the UKMO, which is similar to the unmodified GM at the same time frame. With higher pressure just to the south, an active frontal zone moving eastwards through central areas and a more convective, showery regime in the north. The general track of the jet stream has been overlaid as well to again highlight quite a S’ly tracking feature and note to the cut-off high pressure to the NW of the UK.
Fri 3rd Mar: As we move into Friday and model agreement remains good regarding the broader synoptic developments, and that is for an upper trough to extend down towards Iberia through the course of the day. It is this that allows for cyclogenesis to take place as a complex region of baroclinic features come to together to aid in the development of a surface low pressure that will then move up into the UK from the South. This type of scenario is notoriously difficult for models to get a grasp of, especially the details, and tracing some of the developments back, these can be directly related to happenings, upstream, over parts of the E USA in the next few days. The 00Z GFS highlights the extending upper trough, as represented by the lower 500mb temps and GPH. Note the cut-off high pressure over parts of E Greenland and just to the north of Iceland, this maintaining a more blocked regime at higher latitudes which continue to help to distort the N Atlantic pattern somewhat, meaning the rather ‘typical’ cyclonic tracks and developments aren’t to be expected. The general track of the jet stream has been overlaid as well.
So, the general forecast for Friday is for another chilly start across the north of the UK with frost and possible icy patches too, especially across some central and southern areas of Scotland and across N England where the flow is generally weakest. The low pressure to the N and NE of Scotland is still expected to be there and is modelled, by the ECMWF, UKMO, GFS and GEM to still bring some convective activity to parts of Scotland, but with less activity than previous days. It will be a reasonable day across the north of the UK with some bright or sunny intervals and with relatively light winds. However, the extending upper trough, as discussed, will allow for surface cyclogenesis to take place and with associated frontal rain then moving northwards into England and Wales as Friday progresses, so a wet end to the working week here. The rain may turn to sleet and snow on the leading edge as it moves northwards, but this is a highly uncertain development for now. The developments of this low pressure are particularly uncertain, especially the track and how far north the frontal zones will get, but a general north and south divide in conditions is to be expected as Friday progresses. By the end of Friday, essentially 0000Z Sat 4th, the UKMO, GFS, GEM and ECMWF all have a surface low pressure either over or just to the S or SW of the UK, with an unsettled regime in evidence across England and Wales at times.
Sat 4th and Sun 5th: So we move into the coming weekend with quite a lot of uncertainty over the details in particular. However, the broader synoptic pattern across the UK and the N Atlantic is still one that is primarily being dominated by a cyclonic evolution. The low pressure that will try and move northwards during Friday is likely to become slow moving and potentially ‘trapped’ across parts of the UK during Saturday. As a result, a particularly wet day is expected across parts of England and Wales on Saturday, perhaps with some sleet and snow on the higher ground of Wales, N England and perhaps across into Ireland too, but this kind of detail is simply unknown at this stage. The best conditions of the weekend are likely to be reserved for Scotland which, at the moment at least will be furthest away from the main low pressure and associated frontal zones, but even here some precipitation, perhaps wintry, could move in during Sunday. The UKMO and the ECMWF hold the low pressure over parts of E England, with a potentially cold E or NE’ly under-cut on the northern and western flank which could well increase the risk of wintry precipitation. Conversely, the GFS and GEM are more keen to track the low northwards, especially the GEM, which has it passing NE Scotland by 0000Z 5th. Again it is this uncertainty over the track of the surface low pressure that leads to some significant differences in the details for the weekend’s weather. Overall, though, the emphasis is on a lot of wet weather across England and Wales, with a cold under-cut possible across N England in particular, whilst Scotland and perhaps parts of Ireland experience the best of the conditions as the weekend progresses. The low pressure in question has been highlighted at T+120hr.
Temperatures through the rest of the week and into the weekend will, overall, be near or slightly below average for the end of February and early March, but especially across the northern half of the UK. Milder conditions will develop, at times, across central and southern areas of England in particular as S or SW’ly winds develop. But many northern and western areas of the UK are expected to remain within a generally cold Pm air mass, and again this will have implications on precipitation at times in the coming days, especially on the boundary where frontal zones become located. Some overnight frosts and icy conditions are possible at times too, especially across Scotland, Ireland and N England. Highlighting this general north and west, south and east divide in temperatures is the GEFS ENS mean v climate average between the 1st and the 3rd of March. Clearly note that the further north and west you are then, overall, it will be colder than across more southern and eastern areas of England.
Looking further ahead and after what was a particularly high amplitude phase 8 MJO within the last 7 to 10 days or so, no impacts from that have been felt really across the N Atlantic. This just goes to highlight that despite, at times, global drivers (MJO et al.) being quite pronounced they don’t always lead to what statistically can be expected. The MJO is now in phase 2 and potentially move into a phase 3 state and of which generally relates to some sort of +ve NAO regime, but phase 2 and phase 3 don’t often have a specific/strong impact on the N Atlantic pattern.
Equally, March is often a month of variations and extremes due to what is clearly a time of changing seasons. In recent years March has almost been an extension of winter with little in the way of any early spring warmth and the given the likes of the extended EC EPS and the EC Monthly information then, at the moment at least, there is little evidence that point towards an early start to spring. The general evolution and thinking at the moment are that after the more cyclonic start to March blocking patterns may well then become more influential as the month progresses. But, as ever, the location of the blocks will be crucial as to the weather across the UK, and it is near impossible, at the moment, to say where any blocking may well develop. However, just using the 00Z GEFS ENS mean for the 9th of March, can you see that there is some evidence of blocking over parts of Greenland which wouldn’t generally correlate to a +ve NAO regime. All-in-all, March can often be a very difficult month to predict with some short and sharp changes in conditions expected, but I currently favour a more blocked month as it progresses and, if the block(s) are in certain positions then some late cold weather is certainly not out of the question.
