• Luke Boxall

Improved way to look at the Stratosphere (SSWs) - from the current version

If you ever look at the Stratosphere in any way for help in determining the Medium Range Weather Forecast then you should read on. Even if you just look at what others put about the stratosphere.

Using the Stratospheric Vortex provides very useful information for the medium range weather forecast in Winter-time - It is quite widely understood that when the Stratospheric Vortex aloft weakens it correlates nicely to Cold weather over Mid-latitudes.

Therefore you often end up with the market being exposed to charts such as these ones from:


These charts show Zonal Wind-speed and whether it's above or below normal. The theory being - if it's below normal - cold risk and if it's above - warm risk for mid-latitudes.

However after a number of years of this style of chart being shown - it became apparent that this wasn't entirely useful - there were many times we had below normal zonal wind - but yet still warm and thus analysis became more technical and the phases "Displacement event rather than Split" became more used.

A displacement event is where zonal wind lowers because the core vortex shifts slightly from over the Polar region to be off-centered somewhere between the Pole and mid-latitudes but keeps it's strength - causing cool risk locally as Cold polar air still shifts from the Polar region to mid-latitudes but is more local as it's due to with where the displacement event is and is short-lived as the vortex snaps back inline often after these events.

The more classic scenario thought of when dealing with Stratospheric event is the Split version where such strong warming pushes into the Polar Vortex that it gets completely replacement by high pressure anomalies and the vortex itself is completely removed and weakened from the polar region and left as small left-over bits in the mid-latitude region bringing widespread mid-latitude cold risk as it breaks down the normal westerly flow over these regions.

The trouble with this approach is that it says nothing of where the displacement is - every single Stratospheric warming event is different after all and thus has different effects.

To help with this I have created a Phase index for the Stratosphere - with two axis' one being the strength of the core polar vortex - it is strong or is it weak and the second axis is where is the core vortex located over US side of the Globe or Eurasia.

By doing this we end up with a 9-Phase Diagram - each corresponding to a different state of the Stratospheric Vortex (middle Zone is Phase "9" where the vortex is near Normal)

Phase 1 sees Vortex completely split and removed from the pole - but the left-over vortex is stronger over the US side of the Globe

Phase 2 sees more of a vortex - strong displacement rather than full split but vortex pushed over to US side of the Globe

Phase 3 sees a stronger vortex shifted more towards the Pole but still US-leaning

Phase 4 sees a very strong Polar vortex - again still shifted slightly towards US

Phase 5 again sees a very strong Polar Vortex but this time shifted bit more towards Eurasia

Phase 6 sees displacement event - Vortex shifted over to Pole-Eurasia side of the Globe

Phase 7 sees a strong displacement event - vortex shifted out of the Polar region onto the Eurasia side of the Globe

Phase 8 sees the vortex completely split and removed from the pole - left-over remains stronger away from the US

Above images show 100mb Anomalies as it's the events that reach the lower stratosphere which have the most impact on the surface.

Which in itself raises another point - you need to look at the various levels in the Stratosphere to see whether an event is affecting all levels or not.

There were recent publications relating to a Warming event already over the Polar region but it needs to be pointed out that the current 10mb/50mb/100mb levels are in different Phases - 10mb is in Phase 5 with a strong vortex that's slightly Eurasia focused and lower Stratosphere has been in Phase 2/3 - weaker vortex shifted more to the US side.

(Image below shows brief mock-up of Observation for 10mb/50mb/100mb for 20-25th Oct)

This is very common this time of year - as there normally a disconnect between the Top and Lower Stratosphere until Late Nov - therefore anything you see before that point in the Upper Stratosphere normally has no bearing Lower down/Surface - often during late Nov/early Dec is a time if a Solid vortex has formed aloft - it starts working it's way into the Lower Stratosphere and that's what we see in the latest forecasts:

(Image of Forecast of 50mb from 5th November showing GLOSEAs Model up to Day 30)

Looks like by Early Dec we should have alignment in the Straosphere - one that is it Phase 5. But don't take my word for it - you can access these charts yourselve.

Send me an e-mail at and I'll give you more details.


The Weather Perspective Ltd