I think my main issue here is that basic meteorology told you that in slack unstable conditions, showers were most likely to develop inland, and then drift (in this case general to south) in the gradient wind. In west Cornwall this favoured showers along the spine of the Lizard, which is exactly what happened. However, the forecast chance of rain, and temperatures, were the same on a coast where sea breezes were forecast, or the wind was just plain onshore, as elsewhere. An onshore sea breeze was correctly forecast for Penzance, so where was the rain coming from?
In the event the coasts of Cornwall were the UKs warm spots. Officially Bude (18C) unofficially Lands End (18.4C), Penzance 17.8C. The 17.8C was achieved around 11:00, when the forecast temperature was 14C. It was much cooler inland under the cloud.
I keep hearing of the smaller & smaller grids used for forecasts, yet a perfectly predictable evolution of yesterday's weather over Cornwall was totally unforecast. Shame really, as I find the model output a week ahead quite remarkable most of the time, yet converting it into a weather forecast continues to be a real issue.
From THE SENNEN COVE DIARY - MAY 29TH - SUNDAY
What? Another rip gribbler? Surely not. This morning there was a bit of cloud out to the east and the north, nothing to worry us at that moment but a change nevertheless. The breeze had moderated and gone around to the east - a potential benefit to the surfers if the swell picked up later - but enough cooling to make believe the skin is not frying with each passing minute. More aftersun goes on the shelf.
The forecast would have put many who believe their smartphones off going to the beach, where it was lovely. Another photo from yesterday.