Key transition period is t+120 to t+180
Each of GFS, ECM, AIFS & Icon have their own little differences, but the broad theme is of the AH ridging pretty far eastwards over the UK and into the North Sea. It gets squeezed from the north from around t+120 as a low sinks southwards over Ireland/western Britain. Simultaneously, low pressure over the western Med moves northwards. The two areas join up right over the UK and hang around like a bad smell.
As we move into FI, GFS, ECM, AIFS all try to ridge high pressure back over the UK from [broadly] the south, but each time the low fights back to reassert dominance.
Time for changes (fingers crossed!) but I've seen this sort of set-up get established in summer - with a large and active low seemingly trapped over the UK by high pressure to W/N/E/S - many times before and it's an absolute tw*t to shift
Originally Posted by: Saint Snow