The season of mists and mellow fruitfulness feels like it’s getting into gear, at least here in south Devon, with the Culm valley full of fog this morning, after a clear and rather cool night for August, with temperatures at Exeter down to just 6.9°C. Notice the high overnight minimum temperatures from across the southern North Sea (fig 1).
That speculative blog that I wrote last week about how ex-Hurricane Gert might change the zonal flow to meridional, may not have been too far of the mark, as the outlook from the GFS model for the next week looks distinctly anticyclonic across southern areas (fig 2). I wonder if the remains of Harvey will reinforce that even more by this time next week as the GFS suggests it will.
The panel of eight charts are all from the GFS model runs of the last eight days (fig 1). They display the same forecast chart for midnight tonight (Saturday the 20th August 2017), but all from different model runs and forecast ranges, from the oldest of T+192 (last Friday midnight’s run), to the latest T+24 (this Friday midnight run). I’ve done this to see just how much the forecasts differs as we get closer to T+0, the forecast day we are interested in. The chart at the top right is from the furthest out (T+192) and the chart at the bottom right is closest to the forecast (T+24), I did this to compare just how the GFS was handling the approaching remnant s of ex-hurricane Gert on Saturday night.
As you can see, the forecast for Saturday night has changed quite radically in the last week. The rain from the approaching warm front was at first [T+168] expected quite quickly over Western Scotland and Northern Ireland, but from Thursdays run the model slowed it down and brought the area further south. The ridge of high pressure that was at 15° west at T+192 also changed position and alignment quite a lot in each run, and by the end is aligned NW-SE rather than N-S as it was at T+144.
What I’ve learned from this little comparison of the GFS, is that there is not a great deal of consistency before T+72, and it’s only from then that things start to firm up, but even when have, this by no means that the model has locked onto the correct solution.
Earlier this week I was spouting on about how ex-hurricane Gert would usher in a change of type and that by the end of next week summer would have made a home back. That was on Thursday, but the latest guidance from the NWP models suggest that once the low (that was the remains of ex-hurricane Gert) has finally struggled to cross the country, which looks like it will take most of the week, things will have changed very little, and mobility will be resumed after just a brief incursion of warm and humid air from the continent for a couple of days at the start of next week. Having said that, I did hear on the BBC weather that the models are having a torrid time on what effect Gert will have on the forecast for later this weekend, but looking at the latest run from the GFS and the UKMO models, their solutions for Monday do look remarkably similar (fig 1 & 2).
The remains of Hurricane Gert in the coming few days will break the spell that the weather over the UK has been under since the 20th of July. It looks like, for whatever reason, it will halt the zonal westerly flow of the jet stream that’s plagued the country for the last four weeks (fig 2), and bend the upper air pattern to become more meridional (fig 3), and allow the formation of a large blocking anticyclone to form to the west of the British isles. Of course, this all depends on the latest NWP all holding true for the five days or so.
There’s still some more unseasonably windy weather to come between now and then, particularly later on Monday, as an extratropical low, spawned by the remains of the tropical air from Gert, deepens and tracks southeastward across Northern Scotland and into the southern North Sea (fig 1). This marks the end of the zonality of the last four weeks, as pressure build strongly behind the low. The last anticyclonic spell across the British Isles was between the 16th and 20th of June. It won’t be a spell of hot weather, because the flow will be northerly at first, it should be dry, and sunny so day time temperatures should be very pleasant, but the nights may be quite cool if not cold.
Looking further ahead, and according to the GFS, the theme seems set to be high pressure across the eastern Atlantic, with a north or northwesterly flow down across the British Isles and much of northwest Europe. Hopefully, this will finally put an end to the mobility of the last month, we shall see!
The mobility in the weather patterns that started around the 20th of July, continues unrelentingly into the third week of August according to the latest forecast charts of the GFS model. The only difference that I can see this week is that the discrete areas of low pressure are generally a little to Iceland than to Scotland, and high pressure gets closer to southern areas by next weekend.
In fact the whole week can be summed up as changeable, windy at times, with spells of wet weather, cool at first but gradually becoming warmer by next weekend.
Tuesday doesn’t look half bad, bright and breezy with showers more frequent in the north and west. Next Saturday looks a fine day across the whole country, if this T+156 forecast can be trusted, as does next Sunday away from Scotland, when it looks almost summer like in the south, but this is not the first time that the GFS has promised this scenario in the last few weeks.
I wrote an article about KISS this week, and the forecast chart below (fig 2) is another perfect example of “not caring one hoot “about that principle looks like, from the forecasts in Barad-dûr. I often wonder what forecasters in the other Met agencies across Europe make of these convoluted forecasts and analyses from the Met Office.