Martin Rowley wrote:

snip

Surface charts don't always show the whole story, particularly with
blocked anticyclonic features.


Or even, as in a notable event in the mid-seventies, in an apparently
typical boring warm sector. Steady SW wind, 200ft cloud base, 1500M
visibility, steady barometer, then 5 minutes later the barometer had fallen
7mb, the cloud was on the deck, visibility 500M, and the wind had increased
about 10kts. After another five minutes the pressure was about a millibar
shy of where it had been before the fall and everything else was back to
normal.

Graham