Issued at 1500BST on Thu 28th Sep based on Thu 28th Sep 00Z Model Runs
We look back on September of which has, overall, been a typically varied month. However, what has been pronounced was the chilly and wet conditions throughout the first half of the month, followed by generally warmer conditions during the second half. As we move into October and, at first sight, it does look like the start of the month will coincide with some particularly autumnal and, perhaps, rather inclement conditions as a zonal and progressive pattern develops across the N Atlantic. The upcoming weekend will be the perfect example of that, with also the interaction of two tropical features that will under-go extra-tropical transition in the coming days. The combination of a zonal jet stream, along with a pronounced baroclinic zone within the N Atlantic, means there is a heightened risk of cyclogenesis in the coming days. However, all eyes are then on a potential quick change too far more anticyclonic conditions through to mid-October.
If we temporarily focus on the short term, this will have some implications on how the long term will evolve. As we leave September behind and move into early October, it is the active jet stream across the N Atlantic that will allow for the potential development of some particularly wet and windy conditions in the coming days. The development of high pressure over NE Canada allows for CAA on its eastern flank which then combines with WAA being brought northwards, into the N Atlantic, but the remnant tropical features. These two broader synoptic features then create a pronounced baroclinic zone, with a tight thermal gradient and hence a strong jet stream.
It is that time of year now when the northern, Polar Regions cool increasingly quickly. Parts of Greenland and surrounding areas will see 850mb temperatures drop to -10C or below this coming weekend. The development of the higher pressure over NE Canada allows that colder air filter southwards into the N Atlantic, combining with tropical air brought northwards by the extra-tropical transitions of Hurricanes Maria and Lee. The thermal gradient, using 850mb temperatures as a guide, across the N Atlantic, by Saturday, is nearly 20C.
So, we move into the opening few days of October with a significant risk of some deep low-pressure systems affecting the UK. It is likely that entrained Tm air masses, along with orographic enhancement will lead to a risk of some significant rainfall totals too, across some western areas of the British Isles, perhaps leading to some localised flooding.
However, as we move through into the opening full week of October and a potential quick change in the pattern, across the N Atlantic, and into NW Europe is currently forecast. We need to take a look at the northern hemispheric view to highlight this likely evolution. What will happen is that an extending upper trough, moving southwards out of Hudson Bay and into more eastern areas of Canada. The downstream effect, as is often the case, is for the pattern to amplify, meaning that a ridge in the jet stream will then develop. The details of this evolution are certainly tricky and will potentially affect the onset and timing of any pattern change. However, using the GFS charts, it is evident how this extending upper trough, over E Canada, actually does significantly impact the pattern downstream, across the N Atlantic towards the UK by the beginning of next week and beyond.
Very quickly, the pattern across the N Atlantic is changed by this extending upper trough. From what will be a particularly zonal pattern this coming weekend and into early next week, soon becomes a far more amplified, meridional pattern across the N Atlantic. Beneath this ridge amplification is, as a result, the development of a high-pressure surface system which is then forecast to build into the UK, in association with likely WAA on its western flank. The 00Z UKMO, GFS, GEM and ECWMF all highlight this evolution, but with varying degrees of amplification. As a result, this will be crucial as to how settled the weather will become next week, or whether there may well still be further frontal incursions, especially across the north and west. The GFS is, most amplified of all models by Wed 4th Oct, UKMO not far behind. The 00Z ECMWF and particularly the GEM, are less amplified and would allow more unsettled conditions to move back into the N and W.
Within the last few days, there has been a developing trend for a more concerted build of pressure across the British Isles and NW Europe. Some of the 00Z midnight models have, however, backed away from the idea of a more pronounced build of pressure across the British Isles, keeping more southern areas drier and anticyclonic, while northern areas see a resumption of a more progressive and mobile pattern. When looking at the ENS, especially from the 00Z runs, there is a distinct split between the GFS and the EC ENS. The former does produce a more pronounced amplification to the pattern and leads to a likely settled, autumnal spell with an increased risk of frost and fog. However, the ECMWF is more zonal and less amplified, despite still having a positive pressure anomaly to the S of the British Isles. At the moment it is the more amplified pattern that is preferred, and this is exampled well by the 00Z GFS ENS between the 10 to 14 day period (7th to 12th Oct).
The feature of interest from the 00Z GFS ENS is the potential development of a large area of high pressure over Siberia and perhaps even around into the N Pacific too, which may well have implications on the developing Siberian snow cover in the coming weeks. What is also in evidence too is that the stratospheric polar vortex, while developing given the time of year, is, overall forecast to be weaker than average. While this was also the case this time last year, with little influence further down the line, it can still be important to keep the vortex ‘quiet’ as the autumn period progresses. Just like starting up an engine, once it has ‘warmed up’ then it is ready to go. Highlighting this current prediction is the forecast chart below, with the forecast continuing to run below the overall mean (solid black line) regarding zonal wind within the stratosphere.
Long-Term – Mid-October Onwards:
The crucial aspect of the forecast is whether the amplified pattern, as signalled by the GFS ENS, for example, becomes established through the second week of October. If it does then it would be particularly difficult to break down and we could well be looking at an extended period of anticyclonic conditions, and very little in the way of any stormy, autumn weather. However, if the pattern doesn’t become as amplified as is being signalled by some model output, then a more varied evolution would seem most likely, with wet and windy weather probable for more northern and western areas, while it is always drier and, at times, more settled further south.
Personally, I believe that the more amplified pattern is a sound possibility, perhaps carrying more weight than a return to a more zonal and unsettled regime. The GEFS ENS Mean v Climate Average Air Pressure out to the end of the run, 13th October, highlights a very blocked pattern indeed and of which has more of a –ve NAO look to it, and certainly not a +ve NAO pattern. Climatologically, at that time of year, low pressure would be dominating the N Atlantic, especially in and around Greenland and Iceland. However, in keeping with recent years as well, the UK has lacked stormy autumn’s and, overall, as we look further into October 2017, I believe that will be the case this year too. However, and as ever, time will tell.