Following a September of two halves with low pressure dominance replaced by high, October’s opening days shall grow increasingly unsettled, even stormy.
Thanks to a series of lows and dangling fronts separating low pressure to NW and high pressure SE, significant rain (warnings issued) is expected in Western Scotland over the next week or so coupled with periods of gale-force winds.
Here’s the ARPEGE total precip through Sep 30th through Sat 4th Oct 1200utc
ECM all the way out to 15th Oct.
2 systems push through before a notably deep, mature low (remnants of Humberto or part of) is set to cross the UK during Saturday 4th October. Exact track and depth remains uncertain at the time of writing but here’s some model solutions at the peak of the storm.
Here’s an impressive set of charts courtesy of the latest run of the GFS operational showing (in the same time stamp) an overview with rapidly deepening low approaching Ireland, sharp thermal gradient in the mid levels, driving extreme surface winds (105 mph) and jet streams on top (235 mph) forcing tremendous ascent.
GFS ensemble storm tracks as of late Tuesday 30th Sept. Note a clustering focused from central to northern Scotland…
Latest GFS and ECM show very similar tracks and depth.
Latest GFS and ECM peak wind gust projections.
The series of low of both tropical and extra-tropical shall create a very turbulent North Atlantic basin wide through the next week or so.
Beyond the stormy opening week to October, higher pressure looks to build back north over much of Western Europe for a time but there’s HIGH UNCERTAINTY whether high pressure builds north west or flattens out providing a more zonal westerly flow.
With a more active tropical Atlantic and storms transporting tropical air north, it’s very difficult to pin point a particular pattern mid to late month just as was the case through September 2nd half.
I can see higher pressure potentially building north towards Greenland perhaps sending arctic brand air down into Europe including UK/Ireland mid-month. With a likely continued uptick in tropical activity, we could even see a negative AO/NAO setup later month but it depends upon other drivers such as the MJO which through the first half of the month appears to remain over the Indian Ocean, perhaps Maritime Continent with further rising motion over the Atlantic.
CFSv2 vertical velocity for October.
The above phase 2-3, perhaps 3-4-5 phases of the MJO through October supports the below cross-model solution of neutral to positive NAO with Iberia ridge, Iceland trough but of course, lot’s of variation can be expected in between.
While looking at the polar view of the 500mb anomaly off the CFSv2 weeklies, there’s a clear trend for zonal and neutral to +NAO switching to blocky and -NAO later in October.
The longer range forecasts supports that trend negative.
Overall 30-day pressure, temp, precip forecasts show Greenland trough, Euro ridge with wet UK/Ireland.
GFS ensemble
CFSv2 monthly
The Verdict
Like with September, A lot of variation can be expected throughout October with stormy spells followed by quiet, mild & cool with a chance of a notable chill perhaps mid-month.
A near average month temperature-wise with likely wetter than average for much of UK & Ireland into Scandinavia, Warmer, drier Iberia, France, Potential flood events central/southern Europe & Balkans…
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