Single Status Update
Hello Ian. I was wondering if was possible to relate how a Summer like 1976, 95 might have shaped up in regards to long range forecasting in the May before these incredible summers happened. I found your long range forecast for this year great reading regarding Angular momentum and the many other variables involved and it made me think 'what would cause perfect conditions as in 76, 95? Thanks.
Mr Wilson, It is interesting to note that in both 1976 and 1995 the hot summer conditions were preceeded by cold weather in the Spring (particularly March). Cold spring conditions also preceeded warm dry summers in 1975, 1983, 1984, 1996, 2006 and (particularly) 2013. But in recent years hot summers have followed warm springs such as in 1990, 2003 and 2014.
The forecasting of a summer based on conditions the previous season are not, therefore, likely to be met with success. It is also harder to predict summer conditions overall from two or three months out because the Circumpolar Vortex is relatively weak in the summer (owing to weaker subtropical to Arctic temperature gradients and weaker NE Trade Winds that reduce the need for stronger Westerlies at higher latitudes to offset them to satisfy Conservation of Angular Momentum laws).
The weakness of the Circumpolar Vortex in late-spring and summer means that it is prone to rapid changes to the numbers and positions of upper ridges and troughs caused by relatively small interruptions like a change in sea-surface temperatures locally or a depression over the Alps, for example.
Nonetheless there are certain key indicators such as patches of cool North Atlantic water up-wind, which if localised near the UK would impart anticyclonic curvature to the upper Westerlies close to Britain and cause a fine warm summer. Warm water right across the North Atlantic with Arctic ice well-retreated and with corresponding baroclinic gradients pushed into the Arctic would encourage depressions to shift to well north of Britain leaving high-pressure to move in from the south.
I hope this helps.