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Synoptic Guidance - Unsettled and Milder


Blog by Matt Hugo
Published on 14th February 2017 14:56

Issued at 1500GMT on Tuesday 14th Feb based on 14th Feb 00Z Model Runs

Key Points

  • A particularly changeable period of weather across the UK as both high pressure and low pressure dominate
  • Most unsettled conditions in the days ahead will be across parts of the north and west
  • Many central and southern areas of the UK will have further dry conditions overall
  • Lack of any real significant jet stream activity means little in the way of any significant cyclogenesis
  • High pressure set to be a dominant feature of the weather in the days ahead just to the south of the UK
  • Temperatures on the rise and generally much higher than of late, so mild or very mild overall
  • Little in the way of any wintry weather in the coming days with relatively mild nights and with little frost

WV Analysis

Discussion:

Wed 15th Feb: Into Wednesday and model agreement and consistency is good regarding an area of low pressure that will track just to the NW of the UK through the course of the day. There is some slight variation in the predicted central pressure with the GFS having the low at 0000Z Wed at 1004mb, but the ARPEGE, for example, nearer 1000mb, but overall the synoptic pattern is well modelled by the various forecast models. A large amplified pattern exists to the east of the UK, this the residual pattern of the cold and blocked weather of the last week. However, the low pressure tracking to the NW of the UK during the day will be the dominant feature as Wednesday progresses. Model agreement is good too for the main forcing associated with this low pressure to generally be rather insignificant and essentially not particularly well aligned with the main surface fronts. As a result, a rather showery and sporadic nature of the precipitation pattern is expected on Wednesday with showers and some longer spells of rain tracking SW to NE across the UK. It will be across more southern and south-eastern areas of England that conditions will be driest, but even here some light showers are possible. The general lack of any significant forcing and vorticity is highlighted well by the 00Z GEM model for 1200Z on Wed. Note the overall weak and fragmented vorticity structure, and this matches the fragmented precipitation signal too.

Model comparison

However, despite this one aspect of the forecast which is in evidence is that given milder conditions and generally higher temperatures move into the UK in association with an mT air mass, then there is likely to be some element of instability across more western areas of the UK. Some small amounts of CAPE are in evidence in some forecast models and, as a result, some of the showers across parts of Ireland, Wales and SW England could be heavy and perhaps even with a rumble of thunder too, something which our SR HRES model highlights.

CAPE

Thu 16th Feb: As we progress into Thursday and model agreement is good for Wednesday’s low pressure to be tracking just to the north of Scotland early in the day before then clearing away to the NE as the day progresses. As a result, it will be across the northern half of the UK that conditions during Thursday will be most unsettled with a continued risk of some showers and perhaps longer spells of rain. There is some evidence of forcing interacting with the surface low pressure during the day, so some locally heavy precipitation cannot be ruled out, but this likely restricted to parts of Scotland. For most of England and Wales though it will be a relatively dry and benign day generally given more of an anticyclonic flow and some bright or sunny spells are possible too. As the low pressure clears away to the north-east then WAA from the SW of the UK, in association with an upper trough over the western N Atlantic, will allow for a general build in GPH across most areas of the UK to end the week. But this build in pressure will be contaminated by weak frontal zones attempting to run into it from the SW and also given what will be quite a mild and moist boundary layer given the mT air mass. The general evolution at the start of Thu is highlighted well by the GM’s, note to the large low pressure over the W N Atlantic, it is this feature that has brought the extreme snowfall and blizzard conditions to parts of Canada in recent days.

It is on the downstream side of this low pressure that the WAA will allow for the rise in pressure to end the week, especially across England and Wales and this is annotated below.

Model comparison

Fri 17th Feb: As we progress towards the end of the week and, again, model agreement regarding the broader synoptic evolution is good and hence high confidence in the overall forecast. There are uncertainties over the details mind, especially regarding the potential development of some dense fog across more central and southern areas of England on Thursday night and into Friday. Given quite a mild and moist air mass in place over the UK, then any cloud breaks on Thursday night into Friday morning could well allow fog to develop given light winds beneath the building ridge of high pressure, certainly something to be aware of. Overall though Friday itself will be dominated by a ridge of high pressure across most areas of the UK, which will mean predominantly fine and settled conditions for many with little in the way of any active or descriptive conditions. There are some uncertainties over cloud amounts, but again given that the air mass beneath the ridge will be of an mT origin then forecast soundings do show quite a moist boundary layer conditions, so whilst some cloud breaks are possible, overall quite a lot of cloud leading to grey and overcast conditions are likely. As Friday progresses though and then particularly towards the end of the day, a shortwave upper trough will be approach from the west, with an associated surface frontal zone and this is then likely to move into the UK bringing some frontal precipitation to parts of Ireland, Wales and western areas of England into the overnight period. The GFS model, using 500hPa and MSLP highlights the upper ridge over the UK towards 1800Z on Fri, but then note the approaching upper trough from the west in what essentially leads to a short-wave, trough-ridge-trough-ridge pattern over NW Europe to end Friday, as annotated.

GFS model

Sat 18th and Sun 19th: As we progress into the coming weekend and, at the moment at least, all four main GM’s (UKMO, ECMWF, GFS and GEM) indicate this upper trough moving across the UK, so it is a potentially grey and wet start to the weekend. However, the main surface fronts will be moving into a region of subsidence, in association with the upper ridge and by the time we get to the end of Saturday another rise in GPH is likely to be following from the SW, especially across more central and southern areas of the UK in particular. As a result, Saturday at the moment will start quite grey and damp across many central, western and northern areas of the UK with just a slow improvement through the day as frontal precipitation fizzles out. SE England is likely to maintain further dry conditions overall, perhaps with some brighter weather here, and parts of Scotland and Ireland may also see some during the afternoon for a time. Overall though Saturday is yet another non-descript and rather a benign day of weather as both low pressure and higher pressure essentially cancel each other out. However, what should be highlighted is that there is a chance though that the upper trough moving into the UK from the W could well be sharper and with a greater amount of forcing associated with it, this isn’t the preferred solution for the time being but both the ECMWF and GEM highlight this towards the end of Saturday. Note how compared with the GFS and the UKMO both have a sharper trough.

Model comparison

If this does become the dominant synoptic evolution by the time we get to the weekend then the frontal system moving into the UK late Friday and through Saturday will be a far more active feature with generally more in the way of precipitation associated with it. This is something to bear in mind as whilst this isn’t the most likely evolution at the moment, it is these kinds of details that could change between now and the end of the week which would lead to a wetter day on Saturday for most. Using the 00Z GEM as an example of this more developed scenario, note clearly the more pronounced vorticity associated with, the sharper upper trough and a far more pronounced signal for precipitation as Saturday progresses.

Model comparison

Once this feature is out of the way then Sunday is likely to see a build of GPH once again from the SW in association with WAA, and it is likely that more southern areas of the UK on Sunday will have yet another relatively quiet and benign day of weather with some sunshine, but with also variable amounts of cloud. High pressure extending NE from the Azores is currently the preferred pattern as we move through Sunday and over the top of the high pressure, some limited mobility becomes established. As a result for parts of Scotland, N Ireland and perhaps N England then Sunday could well become cloudier, breezier and with some rain and drizzle at times, this perhaps primarily WAA driven, so most of the precipitation on Sunday developing on western up-slopes of the north and west of the UK. This overall pattern is exampled quite well by the MR model, noting higher pressure extending into the UK from the SW, but with a more unsettled, WAA driven precipitation-type regime across the north and parts of the west in conjunction with another spell mT air mass conditions.

WAA

After what has been quite a cold period of weather over the last week or so the outlook is certainly for far more milder conditions across the UK in association with more in the way of mT air masses affecting most areas. As a result, temperatures will steadily rise in the coming days and, overall, end up slightly above or above average with some particularly mild or very mild days at times. As a result, there is little risk of any wintry weather looking forward and with little in the way of frost by night too. Our own GEFS ENS mean vs Climate Average temperature between the 18th and the 20th, highlights this general milder-than-average outlook in the coming days well.

Milder than average temperatures
Medium and Longer Term 19th onwards

As we look ahead to the final week or so of February and if anything compared with last week there has been a shift towards a greater likelihood of milder and more unsettled conditions dominating across the UK as February, and winter, draws to a close. The MJO has indeed progressed into Phase 8 which, as discussed in last week’s guidance, does support, statistically at least, a progression towards a –NAO regime within approximately 10 to 14 days’ time. The main GM’s highlight this progression into a relatively high amplitude Phase 8 now.

MJO

However, despite this development it doesn’t always mean that a more blocked regime will develop and compared with last week I do believe that the chances of a more blocked pattern, leading to a –ve NAO regime is lower now for the end of February and into early March. Certainly not impossible, but when looking at the broader picture, using the usual medium and longer GEFS and EC ENS, the overall evolution is for an unsettled regime, especially across the north and west of the UK, with higher pressure to the south. The 00Z GFS and ECM 8-10 day mean highlight this overall broader +ve NAO pattern if anything well, noting the strong signal for higher pressure just to the south of the UK, but with lower pressure to the north and a broad W’ly regime overall. As a result there now only seems to be a low risk of any wintry conditions returning to the UK before the end of February now.

Model comparison

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