GFS Op has mid-day temps 25C for the S and 20C for the north on Thu 3rd - but blink and you've missed it, back down to 15C/10C S/N by Mon 7th - and it's only one run out on its own!
GFS Op synoptics - LP sitting over UK Mon 24th easing away E-wards and filling with HP nosing in from the SW Sat 29th and establishing over S England 1035mb Tue 1st. That HP has a battle with LP out to the west which brings up the warm S-lies above, but in the end loses with the LP settling over N Scotland 995 mb Mon 7th and dragging in N/NE-lies behind it.
GEFS - cool at first, temps recovering to norm Fri 28th, rain and showers present until then and after that only intermittent, perhaps rather more in the N. Temp runs after 28th averaging either side of norm with the Op noted above as an extreme warm outlier in the S (and almost as much of a cold outlier at end of run)
ECM - gives more prominence to the HP from the SW Sat 29th and shows it as a large anticyclone across to Norway Mon 31st. The HP then weakens as LP pushes in from the S (1010 mb Biscay Wed 2nd) which matches yesterday's ECM but disagrees with GFS Op
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Chichester 12m asl