Thread by @Aigle_e

🔧Record weak #SPV, weak MJO propagation, deeply negative AAM & emergent La Niña will be the main subseasonal drivers of the upcoming month, prompting wet conditions across Europe & near-normal temperatures throughout September. 🧵

📉🌧️It is now well-acknowledged that the coming month will likely be thermally near-normal to below-normal across Europe & wetter than average, especially in the first half of the month, thanks to an active rail of low pressure transiting throughout the continent (+NAO/SCAND) ⤵️

Without any sign of strong heat ahead, September could be a relatively "normal" month, climatologically speaking, without this preventing the occurrence of milder epiphenomena. Behind this dynamic, a deeply negative AAM & weak MJO activity works to maintain disturbed conditions⤵️

🌊A transition into a very weak MJO phase 6-7 and a rapid return to phase 1 under Niña-like AAM is very likely to contribute to the upcoming unstable pattern, maintaining Greenland-Scandinavian High Lat. blocks, as currently shown in extended-range forecasts ⤵️

📉 Deeply La Niña-like negative AAM seems to be mainly fueled by a strong negative IOD event underway, sticking convection over the Maritime Continent, strengthening La Niña and in turn contributing to making the AAM negative through a prolonged action-reaction cycle. ⤵️

❓ Such conditions is very likely to continue throughout September, making a generally wet & standard or cool September very likely in Europe. JMA composite for -IOD in September match very well with what is currently forecasted in NWP. ⤵️

⚒️ These conditions are also close to the trend observed in the past three decades, compared to the period 1940-1989, which was more zonal & likely enhanced by a combination of both record weak #SPV & cooler Atlantic SST pattern prompting such a synoptic. ⤵️

🍂 Overall, I would expect a continuation of disturbed & cool conditions in 🇪🇺 over the next few weeks, w/o excluding the possibility of a possibly significant mild spell, given the persistent strong background warming. A near-normal September would be a blow to the C3S forecast.

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