Originally Posted by Danno
What do you see are the issues? And how are they going to play out?
Replying to this here Danno as it's the Winter thread after all. First, let's look at the current state of the north Atlantic. This is the latest global sea surface temperature anomaly from NOAA for 16 August 2018. I have divided the North Atlantic into three because this is how we see if the profile is favourable to negative NAO which is the key to cold and snow in our country - though there can be cold and snowy months with positive NAO such as January 1984 but those kind of months are few and far between. The North Atlantic is in a positive NAO state with bands of cold-warm-average. In May, this was cold generally though warm over to the eastern seaboard of America and the cold anomalies were in the shape of a horseshoe which led to the hot Summer as it promoted blocking over Scandinavia and from the Azores to us, 1976 featured a very similar profile. This is a clear sign of cold AMO. However, since May, the tropical Atlantic has warmed up significantly but still only average by this stage of August and the peak of the Atlantic hurricane season is just around the corner - usually mid September - so expect some hurricanes to form if the Atlantic continues to warm up and vertical wind shear dies down. If an El Nino does take off, do not expect this to happen as vertical wind shear continues to increase with El Nino which in turn leads to a decrease in hurricanes.
An active Atlantic hurricane season is one of the things we look at for Winter as it dumps a lot of warm water in the North Atlantic Ocean which can then be advected into the North Pole and promote northern blocking. 2010 was an active hurricane season. 1995 was an active hurricane season. 1887 was an active hurricane season. These are three examples of years that had active hurricane seasons and were followed by cold winters. However, it's not impossible to receive a cold Winter with an inactive hurricane season, 2009 was such an example and was followed of course by 2009-10. 2009-10 was an a moderate El Nino Modoki and as a result had increased vertical wind shear which led to the inactive hurricane season but as it was El Nino Modoki, we had a cold Winter. El Nino Modoki is very good for cold Winters in the UK and Ireland! The El Nino which is forecast by the models and agencies this year is a modoki and I expect it to be a modoki but there is uncertainty on if it will reach El Nino threshold which is 0.5c or more above average. 1990-91 was an El Nino Modoki but did not reach the threshold though it was a cold Winter with a notable cold and snowy spell in early February 1991. Anyway, I'm going off on a bit of a tangent, thought you'd might like that extra info with this post.
Another thing I'd like to point out is the Norwegian Sea - notice the significantly warmer than average SSTs there. Very similar to last year! Now look at the 500mb height anomaly reanalysis for 2017-18, there was plenty of northern blocking there which we theorised last year there to be as a result of the state of the Norwegian Sea, similar to come this year?
The SST profile in the Atlantic we want to see is one like this from 14 December 2009, bands of warm-cold-warm. What is the least favourable for a negative NAO in our current SST profile is the cold anomalies to the south of Greenland, we would want to see them disappear by Winter time and not to mention, the warm anomalies off the coast of America. They are two problems with the current state of the North Atlantic which are not favourable for negative NAO especially with a more powerful Polar Vortex as Winter gives. Things can change like they have since May as the Atlantic is more favourable now than it was in May but it's getting kind of late especially as it takes quite a bit of time for the temperatures in the North Atlantic to change unlike say the Irish Sea or the North Sea. Nevertheless, in spite of 1976 being similar to 2018 in terms of the Atlantic SST profile, 1976-77 was a cold Winter with a cold December and January but mild February.
The NAO forecast from May was for positive NAO in Winter 2018-19 using the UK Met Office's methodology but don't think this is the be all because the Atlantic was in a positive NAO state in May 2009 which led to the UKMO forecasting a mild Winter for 2009-10 - see SST chart below for May 21st 2009.
The Atlantic SST profile was in a big mess in May 2009. It didn't know what it wanted to do.