With just 7 days to go until the beginning of the meteorological spring, the last thing you want to see in long range weather model output is the possibility of a Polar Vortex (PV) split or Sudden Stratospheric Warming (SSW).
Friday’s GFS (US weather model output) certainly suggests that at least a displacement of the colder air in the polar stratosphere could become displaced by the second week of March.
The PV is a band of strong westerly winds that forms in the stratosphere between about 10km and 50km miles above the North Pole every winter. When the vortex weakens, shifts, or splits, the polar jet stream often becomes extremely wavy, allowing warm air to flood into the Arctic and polar air to flood south.
A SSW refers to a swift jump in polar stratospheric temperatures. The warming is preceded by a slowing then reversal of the westerly winds in PV. Such disruptions of the PV can lead to cold air outbreaks in the mid-latitudes, in place like Eurasia, Europe and the United States.
It should be noted that despite there being an estimated 25 major SSW events during the past 40 years, a relatively small number have led to a severe cold outcome in Ireland.
For example, a major SSW around February 10th, 2018, produced the infamous Beast From The East of late February and early March that year. A SSW in January 2019 had little impact on Ireland's weather, which, in fact, remained relatively mild for the rest of the meteorological winter. Another SSW in January 2021 produced cold conditions in Ireland and Britain, but it was much less severe that the 2018 outcome.
Should the GFS model output from Friday verify, it would take another 2.5 to 3 weeks for the impact of a PV split and any accompanying SSW to be felt in the mid-latitudes. By then, we are looking at a risk of cold weather at the end of March into early April, an outcome nobody wishes to see materialise.
For now, it’s one to keep an eye on.
