I wrote this for an article so the wording might sound weird.
Recently, the GFS model has been consistently picking up on a northerly to occur around the last weekend of October 2018. It’s been showing this for the past few days but today, it has gone quite extreme with the cold levels by October standards. The charts below are of the 500mb height synoptics and very cold 850hPa temperatures which the midnight and six o’clock runs of the GFS generated today. That is a proper northerly blast with cold air coming straight from the Arctic and a mid-Atlantic ridge setting up ascending to Greenland. Out of the two, the GFS 0z was clearly the more extreme one of the two as it brought the -9 isotherm into Scotland and set up a block of high pressure in Greenland with a centre of around 1065mb.
These kinds of synoptics are among the coldest you could receive for this time of year in the UK and Ireland. October 2008, exactly ten years ago, brought the most widespread October snowfalls since 1974 and for some, their first October snowfall since 1934 showing just how rare such a scenario is. Even London had snowfall in October 2008 which is probably unheard of to some people. Could we have the same pattern occur exactly ten years later? It would be an amazing coincidence but extremely unlikely.
The Arctic Oscillation is an index that shows you whether there is above or below average heights over or around the Arctic Circle. The more positive the AO is, the deeper the below average heights are i.e. low pressure thus disallowing cold from descending into the mid-latitudes where the UK and Ireland are. The more negative the AO is, the stronger the above average heights are i.e. high pressure thus sending cold down into the mid-latitudes. The AO has been in an unusually long positive state since the Beast from the East and the Son of the Beast events earlier in the year with only a few temporary blips into negative AO during the period such as late July into early August or late June (see the top of the chart below). Positive numbers (1, 2 etc) reflect positive AO whilst negative numbers (-1, -2 etc) reflect negative AO.
The GFS ensembles (the red lines at the end) are forecasting a drop in the AO into quite negative territory by the third week of October showing there is the suggestion of blocking to occur over the Arctic which will in turn send cold air down into the mid-latitudes. The question though would be, where would it do so? The latest GFS runs highlighted above show the cold air coming to the UK and Ireland via northerly winds. This would be a huge change on the weather since April.
The North Atlantic Oscillation is an index that shows you the difference in mean sea level pressure between the Icelandic Low and the Azores High; two elements which make up the zonal pattern that give the UK and Ireland mild Winters and cool Summers. The more positive the NAO is, the deeper the Icelandic Low is and the stronger the Azores High is. The more negative the NAO is, the weaker the Icelandic Low and Azores High are – sometimes even can be swapped around which many cold Winters have. Positive numbers (1, 2 etc) reflect positive NAO whilst negative numbers (-1, -2 etc) reflect negative NAO. Like the AO, the NAO has been in an unusually persistent positive NAO state since April, in fact, unprecedentedly so. This has been as a result of the very strange North Atlantic sea surface temperature (SST) profile that 2018 has produced.
However, the GFS ensembles are not only picking up on a change in the AO to negative levels, they’re doing the same with the NAO for the third week of October onwards. This would be completely different to anything we’ve seen since April with high pressure forming somewhere near Iceland. This would support the northerly scenarios the above GFS runs show today but no guarantee of course.
Meanwhile on the other side over towards Alaska, the GFS is picking up on a trough to be placed here around the same time it wants to deliver a northerly airflow to the UK. This would be yet another major change on the weather recently as blocking has achieved record heights in this region. This is another thing that would support the northerly verifying for the end of October as the jet stream is not aligned from the north of Alaska going southeastwards into the eastern seaboard of the US. If it were, cold conditions would take place in the eastern US and then blow up the jet stream across the North Atlantic delivering stormy conditions to the UK and Ireland. This would not be the case if the trough were to verify.
There is no doubt that the media will jump on the hype train and release scaremongering headlines such as EARLY WINTER ARRIVING WITH BLIZZARDS COMING TO LONDON or something like that. Ignore them! Do not be heeding any attention to them as they are a complete waste of your time.
This northerly could verify and bring the first widespread snowfalls of the season very unusually early but there is a higher chance of it not verifying especially given how we’re talking just around two weeks away and other models such as the more accurate in terms of verification ECM have yet to generate out as far as when the GFS shows the northerly to occur. Will be keeping a close eye on it as always with everything in weather world. It’s all just for fun right now though the bigger picture itself is very intriguing.