Much has been made in recent days of a forecast Sudden Stratospheric Warming (SSW) in the opening week of January and how this meteorological phenomenon could lead to a repeat of the 'Beast from the East' conditions of December 2010 and February 2018.
A Sudden Stratospheric Warming (SSW) refers to a swift jump in temperatures in the stratosphere over polar regions that is sometimes linked to the onset of colder weather in Ireland and Britain. History tells us that less than half of all SSWs lead to an outbreak of severe cold weather here and therefore, no conclusions can be drawn at this early stage.
Ireland is already in a blocked setup whereby high pressure to our west is preventing the more typical winter regime of Atlantic weather systems approaching Ireland at bay. Next weekend, most guidance currently shows the current spell of cold weather relaxing somewhat, with the result that temperatures will edge upward to near normal for the time of year.
IWO Senior Forecaster Peter O'Donnell says, "Some guidance suggests that cold air is not going to be pushed very far east by this weak Atlantic resurgence, and there could be a "battleground" scenario developing by middle of next week (around the 13th-14th) where cold air clashes with the milder Atlantic air along a frontal boundary through some parts of Ireland (most likely west Ulster to north Leinster on current indications). This may lead to some snow or sleet developing in Ulster, but longer term the weak milder regime is shown holding on for a while."
"Many following developments in the stratosphere seem to think that a second cold spell is quite likely later in the month, so this may not be the end of cold weather if it does turn milder next week," Peter added.
"The stratospheric polar vortex (SPV) is a rotating vortex of cold air high up in the stratosphere over polar regions," says Met Éireann Meteorologist Paul Moore.
Mr. Moore continues, "It forms every winter in the Northern Hemisphere and the strength of the Northern Hemisphere SPV has a significant influence on the Northern Hemisphere mid-latitude weather patterns. When the SPV is in a strong symmetrical state, it helps to maintain westerly momentum in the troposphere over polar regions and in the mid-latitudes, leading to our typical winter pattern of a strong west to east North Atlantic jetstream, bringing storms and weather fronts over Ireland from the west or southwest and keeping it mild and wet. When the SPV is in a weakened or non symmetrical state, it can lead to the weakened jetstream behaving more like a meandering river with blocking high pressure forming in our region, bringing the possibility of colder, drier weather."
"When a weakened SPV goes through further disruption, it can lead to what is know as a Sudden Stratospheric Warming (SSW) event, which refers to the reversal of the zonal winds in the Stratosphere from westerly to easterly, along with a rapid jump in temperatures in the winter polar stratosphere that leads to a complete breakdown of the SPV. Every SSW is different and less than half of them lead to colder conditions in Ireland. For example, the SSW in February 2018 led to the ‘beast from the east’ and storm Emma, whereas the SSW in January 2019 had no significant impact here, due to the easterly winds not propagating down into the troposphere from the stratosphere," added Mr. Moore.
Met Éireann reporting on the current state and SPV forecast
The SPV has been below average strength through most of December 2020 (pink line—left panel below) and is forecast to continue weakening (orange line –left panel below) to below zero m/s, which means changing from westerly to easterly. This signifies a technical SSW. The mean zonal winds at the bottom of the stratosphere have also been at very low levels (pink line— right panel below), and are forecast to decrease to record low levels over the next week (orange line– right panel below). This has been, and continues to be a contributing factor in the blocking high pressure systems we have seen in our region lately. The lack of westerly momentum at the bottom of the stratosphere helps to slow down the North Atlantic jetstream in the troposphere and leads to it meandering like a river, north to south, resulting in a more blocked pattern in the mid latitudes. More.
The potential for much colder weather is as high today as it has been in early January for many years but this in itself does not mean we are guaranteed such an outcome. An interesting few days of weather model watching lie ahead as the impact and extent of the SSW on our weather becomes clearer. It will be 10 to 14 days before we witness the ultimate outcome. Until then, treat the newspaper headlines containing 'snowmageddon' and 'Beast from the East' with a huge pinch of salt (and grit). The 'Pest from the West' is never too far away.
The IWO long range weather forecast for Ireland is updated by Peter O'Donnell each day on this website. In the meantime, this explainer video from the Met Office outlines the story behind SSW events.