Thanks for that interpretation, SC. But I have a problem with trying to tease anything out of those CFS z700 anomaly charts which you posted. It's important to remember that they do display anomalies and not actual high or low pressure.
What I mean is that a positive anomaly to the north of the UK, for instance, might still indicate low pressure, but of a shallower depth than average or perhaps a low-pressure system centred over Iceland rather than Greenland.
Originally Posted by: Stormchaser
I know what you mean, and in the end some assumptions have to be made, for example that zonal conditions in winter don't tend to involve weak troughs and shallow ridges. It comes down to trying to interpret what the model has to project in order to produce those anomalies, using various typical pressure patterns taken from past experience. Unusual patterns are almost impossible to forsee even just one month in advance.
In my experience, a weak +ve anomaly usually suggests a period or two of high pressure in a given region (based on what I've seen verify) while a strong anomaly points towards a month dominated by a blocking high.
The CFS model varies a lot from run to run (November and December keep swinging around from epic blocking to very zonal), so we end up looking for what keeps cropping up more often than not. Usually that means just one or two areas of key interest, for example a lot of recent runs have shown a spell of extensive high-latitude blocking sometime between mid-November and towards the end of that month.
...then comes the probable bias that models tend to show. CFS often seems to either flatten a blocking high with lightning speed or go completely nuts with a raging jet tracking well south of normal and generating truly sensational charts. In many ways the 'average of outputs' charts are by and large a reflection of how often CFS is going one way or the other, with not much in the way of laid-back, non-energetic charts... yet they have proved to be a remarkably effective forecasting tool over the past couple of years or so.
Originally Posted by: some faraway beach