Thread by @Met4CastUK

1/n THREAD: Troposphere & stratosphere working together to bring cold & wintry weather.

Thought i'd do another update now models are firming up on what looks to be a fairly significant cold spell for the time of year. Where are we currently sitting & what's the likely evolution?

2/n

Atmospheric Angular Momentum has surged into +ve territory led by the 'off the charts' East Asian Mountain Torque (EAMT) event. +ve MT essentially takes momentum away from earths rotation & puts it into the atmosphere, i.e, stronger overall westerly winds within the budget.

3/n

Following this backwards in time, the above process began with the MJO transitioning through phase 4 > 5 > 6 and now into 7, this created frictional torque (air moves slower at the surface due to friction) which led to the above +MT & eventual rise in overall AAM.

4/n

This all serves to add westerly momentum into the atmosphere which results in a more disturbed jet stream. These disturbed waves then travel across the Northern Hemisphere creating peaks (high pressure) and troughs (low pressure). This can be seen on the jet stream chart.

5/n

So, on that basis alone we have forcing in the tropics creating a disrupted jet stream. This alone would increase the likelihood of blocking areas of high pressure in the higher latitudes. But.. we've also had an SSW (Sudden Stratospheric Warming).

6/n

This SSW reversed the usually westerly flow in the stratosphere to easterly, this creates -ve anomalies (easterly) that are moving down through the atmosphere & into the troposphere with the likely impacts being felt in early March.

7/n

The first warming didn't propagate downwards for reasons I'm not going to go into here, but the second & most recent warming looks like it will.

So, what does all this actually mean in terms of our weather?

8/n

Tropospheric led activity (MJO > +FT > +MT > Rising AAM) pre-condition the troposphere to blocking, the SSW then downwells into the troposphere strengthening this blocking signal. High lat blocking has been signalled for a while due to this process.

9/n

Blocking will finally come to fruition in early March and will now likely bring a cold period of weather to the UK & NW Europe. Snow is increasingly likely through next week but particularly towards the end of the week, north & east favoured initially.

10/10

Beyond next week I think the block will drift westwards into Canada opening the doors to low pressure systems from the SW. As these bump into the colder air they bring the risk of more significant snow events.