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SSWs and unsettled weather

Published 6th January, 15:12

The new year has brought a continuation of the unsettled and mild conditions which characterised the last days of 2022. Can we expect a change during the next couple of weeks? What about the longer term?

Wet, windy and milder than average

Computer models have been quite consistent in recent days so there is a reasonable degree of confidence in the medium term outlook. The London GEFS plot below is consistent with recent updates. It shows forecast temperatures at approximately 1500m above sea level (850hPa level) on the top half and precipitation on the lower. The plot may appear as a jumble of lines, but the key points to take from it are quite straightforward:

1) Temperatures at the 850hPa level are close to or above the average for most of the time. The thick black line shows the 30 year norm and not many runs dip below it and those which do don't stay there for long. 

2) An ongoing risk of rain. Each spikes on the lower half of the plot shows a forecast of precipitation at the given time by one of the runs in the model.

3) Could the precipitation be snow? Very probably not! The snow row at the very bottom shows the number of runs in the model which are forecasting snow to fall on the given day. The maximum possible value is 33, but on this update it peaks at 3 on January 17th. In other words the highest chance on any of the coming 16 days is approximately 9%.

 

GEFS 06z London 850hPa temperatures and precipitation

London GEFS air temperature and precipitation forecast

Perhaps surprisingly there isn't much differences showing on plots for the northern half of the UK. Temperatures are generally a little lower as is typically the case and there could be snow at times over the hills. However, at low levels rain is much more likely, although I wouldn't discount transient snow. Will things change during the last third of the month?  

High up in the stratosphere

In recent days there has been discussion about the possibility of a Sudden Stratospheric Warming (SSW). What is an SSW?

Firstly it's worth quickly explaining what the stratosphere is. The atmosphere is made up of a number of layers and the bottom one is called the troposphere. It goes up to about 10km and is where the weather occurs. 

The next layer is the stratosphere which is between approximately 10km and 50km above the Earth's surface.

The important thing to understand is that developments in the stratosphere can have an impact on what happens in the troposphere and therefore on our weather.

Sudden Stratospheric Warming (SSW)

An SSW occurs when the winds in the Stratospheric Polar Vortex (poleward of 50°)  weaken or reverse. Cold air descends and temperatures in the stratosphere rise. An SSW is a warming of up to 60°C (the amount temperatures increase by, not the absolute value) in a few days. They occur about 6 times per decade. 

In the weeks which follow an SSW the shape of the jet stream in the troposphere is changed. The impact is that the chance of cold arctic air being pulled down to mid latitude locations such as the UK increases approximately 2 weeks after an SSW due to the jet stream pattern becoming wavier or more amplified

Is an SSW about to happen?

According to the computer model data the answer is no! However, there are indications that the chance will increase in late January. 

Where can I see if an SSW is forecast?

You can now check the Stratospheric forecasts on The Weather Outlook. The charts update every 6 hours and explanations of what they show are provided.   

GFS stratosphere chart
GFS 10hPa wind forecast

 

TL;DR

In the short to medium term the focus is on unsettled weather with close to or above average temperatures. The risk of colder snaps is greater in the north, but even there nothing out of the ordinary is showing. Snow will be mostly restricted to high ground in the northern half of the UK and conditions may be favourable for the Scottish ski resorts.

In the longer term the chance of colder spells perhaps starts to increase. Computer models have been suggesting high pressure having somewhat more influence and if that happens nighttime frosts could return more widely. Also, developments in the stratosphere possibly come into play more as we head into February.

Latest charts


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