Well I've been in "t'North" 4-and-a-half years now. It's only because it was a great summer when we
bought the farm that I know there can
be summers up here...
We visited the farm 9 times over the 4 months it took to buy it. Brilliant summer sun every time. We moved in on Oct 16th 2006 and it rained from that lunchtime until February.
Fabulous spring the first spring, lambing was a breeze, warm weather no rain plenty sun plenty grass. On June 12th a quarter of a million tonnes of water fell on our farm and that rain didn't stop until end August - and then only briefly. Next two springs and summers were wet. The backends and winters were wet too.
2009 & 2010 the winters brought snow such as hadn't been seen in these parts for decades. But snow always used to 'blow' in previous times, thankfully the recent snows lay quietly.
2010 was finally a dryish spring and summer. But cold. The spring grass never grew, ewes had no milk, lambs cold and hungry, coccidiosis hit. Never had a t-shirt day all that year, only one day stooking hay when the work warmed you enough to take the long sleeves off.
Winter 2010 came a month early. Everything carried on being a month early. More grass end March than we'd had by end May the previous year. Now we're back in winter having had some summer in April.

I think you are right, hughesy. They 'forecast' on the basis of probability. Since the patterns have been random for the last 5 years, I forecast a fifth year when all the weather forecasts are wrong and we have to use our own judgement and local weather-sense to decide when to cut the grass for hay.