Blog looking at the potential for very wet and windy conditions across southern England Wednesday night, though for now, strongest winds with perhaps 70-80 mph gusts, look to only affect Channel Islands and northern France.
After a spell of dry weather late last week and into the past weekend for many, wet and windy conditions return this week and look to last for the foreseeable. A meander in the jet stream allowed high pressure to build over the near continent keeping many dry and very mild in southerly winds over the last few days, away from the far northwest. However, the high retreats east starting this week, with a strong and zonal jet streak ploughing across the North Atlantic, stoked by a plunge south of cold arctic air over central and eastern North America creating a tightening thermal boundary. This strong jet stream will bring increasingly unsettled and windy conditions to the UK this week.
The main parent low will spin around to the northwest of Britain towards Iceland, but we're keeping a watch on a fast-moving secondary low, that was over New Foundland at midnight last night, and will head towards the UK over the next 24 hours riding the jet stream.
The low will ride east on the colder polar side of the strong westerly jet stream, the associated frontal system continuing to occlude mid-Atlantic. The low will initially be more of a wave than a closed low as it makes the trans-Atlantic crossing, but some deepening, for a time, is modelled on Wednesday, as the low moves under the developmental Left Exit region of the jet lying just to the south of the UK.
Weather Models today have trended towards running a low further south and a bit shallower across southern England Wednesday night, compared to previous few days runs, which deepened the low more rapidly and deeper, sending the low northeast across Wales and northern England and bringing 70-80mph gusts in the southwest. But today, models have increasingly tracked the low further and further south, on the 12z runs moving northeast across southern England and on across the Midlands and exiting east coast of England.
The triple point of the frontal system, where lift will be maximised, moves northeast across southern England and East Anglia Wednesday evening, bringing pulses of very heavy rain, perhaps convective in nature, so bringing a risk of surface water flooding. Flood alerts are beginning to rise again across southern England, given the saturated ground from the wet start to the month, with the additional heavy rain perhaps triggering some flood warnings too over the next few days.
Also, the tightening pressure gradient on the southern side of the low moving northeast brings the risk of gales or severe gales through the English Channel and along the south coasts. 00z ECMWF showed 70-80mph gusts through the Channel, including the Channel Islands, perhaps 50-60mph gusts along the mainland south coast and a little inland, 30-40mph inland across southern most counties.
The 12z model runs out so far, GFS, ICON and UKV appear to have shifted the swathe of severe gales further south still, with Channel Islands and the coast of northern France seeing the highest gusts
The Met Office have issued yellow warning for rain along southern counties of England east of Dorset and in force between 5pm Wednesday to 6am on Thursday:
Following heavy showers on Wednesday, a band of heavy rain will move northeastwards on Wednesday evening, followed by further heavy showers overnight. 15 to 25 mm rain is likely to fall widely, with 30 to 40 mm in a few places over a 12 hour period. With the ground already saturated, this may lead to some flooding. Strong winds are also likely, with gusts 40 to 50 mph possible along coasts.
So, for now, it looks like mainland southern England will be spared severe gales, though Channel Islands could see 70-80mph gusts early Thursday, along with the northwest coasts of France. But as the situation is still very fluid and fast moving, with regards to how the low may still move more quickly under the left exit region of the jet stream, there is potential it may deepen quicker and track a little further north than modelled, such small errors could mean southern and southeast England could be back in the frame to experience gales. There were a notable number of ensembles that showed deeper solutions this morning bringing the strongest winds over 60mph across the far SE of England.
So worth bearing in mind if up early on Thursday morning to watch out for flooded roads. Coastal showers following on Thursday and Friday, with no other secondary lows or frontal systems forecast, with the parent low to the northwest driving the weather. A heavy rain band looks to spread across all parts later on Saturday, as the next frontal system moves through.