This is going to be my forecast for Autumn 2018 because the day has come now for me to release it. I'm going to explain a couple of phenomena and their impact for the coming Autumn then give a summary of what my analogues had to say as a reminder for the Autumn and last but not least, my actual thoughts for the Autumn based on everything we've looked at including analogues and models.
The current state of the North Atlantic Ocean is a strange one with a large cold blob to the northeast of the North Atlantic but very warm over towards Newfoundland. The tropics are at about average levels. This combination of sea surface temperature anomalies leads to believe a positive North Atlantic Oscillation (NAO) is likely especially given the cold blob which was last seen in 2016 before this year and therefore, this means milder than average temperatures are more likely than below average. The tropics have significantly warmed up through the Summer compared to where it was in May which will lead to a ramp up in tropical developments. This can lead to quite big changes in the Atlantic to allow patterns to change such as the NAO. The latest ensembles are perfectly split on where the NAO will go in early September indicating the uncertainty and I think the warming of the tropics is aiding this uncertainty. However, in theory, you'd think a positive NAO is the most likely this Autumn. This will be highly unusual if it verifies because the NAO has already had a record breaking long run of positivity from April to August 2018.
ENSO is currently in neutral stages and I expect it to remain so through this Autumn with no significant deviations occurring. However, models like the CFSv2 are keen on El Nino to occur beginning in the Autumn. I am extremely dubious about this as the ocean has not shown its reaction towards El Nino. Therefore, with neutral levels, ENSO is not likely to have much impact this Autumn on the weather.
The Polar Vortex, which is the ribbon of air that surrounds the Arctic Circle disallowing cold air from filtering down into the mid-latitudes, is expected to be weak this Autumn by the CFS ensembles. In theory, this should mean that northern blocking would be more likely to occur with an increased chance of northerly or easterly incursions though not necessarily a cold or settled Autumn overall. With the state the North Atlantic Ocean is in, I would think easterlies are more likely than northerlies unless a massive shift takes place in the sea surface temperatures. This weakening of the Polar Vortex is theorised to aid the chances of a Canadian Warming occurring later in the Autumn into early Winter. A Canadian Warming is a type of Sudden Stratospheric Warming (SSW) which occurs over in Canada mainly during November or December. A November Canadian Warming has occurred in 1962, 1968, 1977 and 2000; all of which were followed by cold Winters. This type of event can also lead to some cold spells or snaps in November itself as such happened in 1962 teasing what was gonna come in 1962-63.
On model runs recently, the Siberian High has appeared and it is quite early to be seeing this form on European charts especially to its extent shown. As a result, the snow cover extent through Eurasia is expected to be faster than average through this Autumn. When this occurs, it can be a sign of some colder interludes during the Autumn season but this is most importantly the case for Winter time.
In terms of long range models, well there's strong confidence from them on a milder than average Autumn which I have to agree with personally. I agree because of the state of the North Atlantic Ocean and mild Autumns occur much much more often than cold. I don't think it'll be like Autumn 2011 or 2006 type mild conditions, I think it will be an unremarkably mild Autumn. The models are far more mixed on the rainfall aspect though most are going wetter than average which I agree with again for this Autumn. Positive NAO in Autumn especially later on (November) usually means wetter than average conditions.
My analogues showed the prospects of a mixed September, a settled October whilst a very wet November for Autumn 2018. In contrast to the models which are very strong on a mild Autumn, there didn't seem to be much of a signal for the temperatures from the analogues for this Autumn. Going by the 500mb height anomalies in the analogues, September is likely to be close to average to milder than average. October is likely to be close to average though this largely depends on the exact alignment of high pressure and the wind direction. November is interesting because there have been signals in some analogues for quite a cold November and the Beijing Climate Centre model showed the possibility of this too but unsettled. The unsettled nature combined with the colder than average conditions would give away to some early wintry weather potential. However, I see this as a very small possibility and I predict November to be a stormy, mild month with a lot of rain and strong winds.
In summary,
September - A fairly mild and mixed month. Likely to be on the relatively wetter than average side but no big deviation.
October - Settled and fairly mild again. Lot of quiet weather.
November - Very wet, mild and stormy. Plenty of gales and rain.
Remember long range forecasting is not to be relied upon and is just for fun.
