GFS certainly seems to be on its own tonight with regards to how the Atlantic jet stream evolves from day 4 onward and the associated behaviour of the mid-Atlantic ridge and troughs located south of it.
With ECM's extended ridge over a cut-off low to the south now well supported by UKMO and JMA, and GEM giving it some thought, that looks to be the form horse.
The mid-Atlantic/Greenland ridge and Scandi trough combination was actually first depicted by the ECM model, on last night's 12z op run, so it's curious that while this turned out to be just a wobble from that model, GFS has decided to run away with it today. Perhaps the answer lies with the MJO projections, which show a low amplitude MJO capable of at least a weak influence:
http://www.cpc.ncep.noaa.gov/products/precip/CWlink/MJO/CLIVAR/ALL_emean_phase_full.gif
The GEFS resolutely steer the MJO from phase 3 to phase 1 via phase 2, but the Canadian and Japanese model ensembles (JMAN and CANM) head the other way, while ECM has it hanging around in phase 3 without much sign of moving along.
See what phase 2 can bring in August:
http://www.americanwx.com/raleighwx/MJO/MJO/AugustPhase2all500mb.gif
This looks a lot like the GFS 12z op run. The September phase 2 mean looks nothing like any of the output, so the response seems to be characteristic of August despite the change of month.
Here's the phase 3 mean for August:
http://www.americanwx.com/raleighwx/MJO/MJO/AugustPhase3all500mb.gif
The ECM and JMA 12z op runs fit well with that.
Meanwhile Phase 4 in September is a good match for the GEM output:
http://www.americanwx.com/raleighwx/MJO/MJO/SeptemberPhase4all500mb.gif
...so perhaps that model is seeing a response more in tune with the long term average for September.
I don't usually give the MJO much weighting for our weather, but the similarity of those means to the current model output really caught my eye.
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