The block to the east is looking increasingly resilient. This is helped by the fact that there is now agreement on the low to the west of the UK during the middle part of next week locating almost as far south as ECM had it on yesterday's 12z - which means the jet doesn't look like being so flat as we head towards the weekend.
Despite this, GFS still insists that the jet will attempt to ride over the blocking high rather than undercut it.
Comparing GFS with UKMO for day 6 (+144 hours, GFS left, UKMO right), we see the following:
Notice how the GFS chart has a ridge aligned SW-NE through the Azores, which reflects the angle of the jet in that area. It's firing right at the UK. To the SW of Spain, an area of lower heights represents a shallow trough in the jet which is being cut-off (becoming a closed loop separate from the main jet stream) as a result of the strong, flat jet progressing towards the UK. During the following days this supports a ridge to the north of it, which builds through the UK as a strong feature, keeping away the strong Atlantic storms.
The UKMO chart features a stronger trough west of Spain, which has not yet become cut-off. The question is whether it could remain attached to the main jet days 7-8, causing it to angle more towards Europe, as happened on the ECM 00z op run. That run had a stronger trough than the 12z UKMO run shows. As it stands, the UKMO setup certainly seems to give the blocking high a boost - it looks like the Atlantic jet would have a lot more trouble riding over the top of that compared with what GFS goes on to show.
The UKMO evolution could also save us from another long-duration stalled front event, which is what GFS projects for Thursday through Friday as the boundary between Atlantic and Continental air masses generates frontal system along a shallow disturbance travels.
The GFS version of events is arguably a lot better for the UK longer-term though, as it provides a lengthy run of largely dry conditions.
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