If this year's stratosphere watch will be half as interesting as 2017-18's was, then we'd be doing very well.
To start off, let's have a look at what's going on currently in the stratosphere. This is the time of year when the Polar Vortex really begins to reorganise itself and the stratosphere cools down along with it. Recently, the stratosphere has gone even cooler than average, quite substantially so as shown by the JMA stratosphere analysis below of both 10hPa and 30hPa in the stratosphere.
These cooler than average stratospheric temperatures usually equate to an intensified jet stream. Why we have not seen this and even had a cold snap caused by a northerly wind is because the stratosphere and troposphere have been off with each other, they've been disconnected. A recent example of this was in late Autumn 2016 when we had a fairly cold but settled November. However, by the time meteorological Winter opened, the troposphere began to react to the stratosphere and we had a mild December. January 2017 almost had a SSW but it just never materialised. Just a bit of fun recent history there for you.
The first signs of the stratosphere and troposphere reconnecting is the first week of November of which looks pretty unsettled though models have downgraded the extent of the wet and windy weather as we've gotten closer to the time (they could go back the other way, you never know in weather world).
The GFS ensembles forecast the zonal winds at 10hPa to weaken through the first half of November (see the green lines). The black line is the 1979-2016 averages of the zonal winds at 10hPa. This is a significant drop from where we are now but not far from average. The other lines (apart from the red dash line which is 2017-18; notice the big plunges in February 2018 associated with the SSW event) are of the CFS ensembles. They continue to forecast weak to very weak zonal winds through the second half of November into December. Sunday's run even showed 3/4 going for some kind of stratospheric warming but today's run has gone back to weak/very weak zonal winds rather than reversed. Just gives away their poor reliability but it's interesting how often they've picked up on these weak zonal winds or a stratospheric warming to occur in the final two months of 2018.
The previous two ECM runs have been showing an Aleutian Low and a big ridge of high pressure to the northeast of Europe at +240 hrs. These are two things that can be precursors to stratospheric warming events. The ECM clusters are in full agreement on the block of high pressure to build to the northeast of Europe and dominate here during the first to second week of November 2018.
According to Eric Webb, we are in the most favourable part of the QBO cycle for a major SSW to occur. Other SSW/Canadian Warming (CW) events that have occurred at this same time in the QBO cycle include January 2013, January 1985 and December 1978.
https://twitter.com/webberweather/status/1056939035963375617
Comments