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iapennell

AUTUMN 2017 PROGNOSIS: OFTEN WET AND STORMY IN THE NORTH, FINE SPELLS IN THE SOUTH

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Congratulations with passing the Advanced Diploma in Accounting Ian and good to see you back.  Thanks for the autumn forecast, very detailed as always!

Edited by Don
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As always, a detailed and interesting forecast with a detailed discussion of the various factors at play. Thanks again Ian.

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On 9/9/2017 at 20:19, iapennell said:

 

With all this in mind, the remainder of September is likely to be a little warmer than usual with plenty of fine sunny conditions over central and southern England, Yorkshire and South Wales.  Daytime temperatures above 21˚C can be expected from time to time as high-pressure builds north and east over England.  However, at this time of year, clear night skies inevitably mean falling temperatures and minima of 5˚C or below can be expected to occur widely across England on at least a few nights during September with early-morning fog and ground-frost likely in valley-bottoms. For Scotland, North Wales and north-west England along with Northumberland and Northern Ireland the baroclinic set-up and Atlantic hurricane activity is likely to result in a more unsettled flavour to the remainder of this month.  Depressions passing to the north will bring spells of rain, strong winds from the west will be frequent and daytime temperatures will often remain the cool side of 15˚C, even in the lowlands. However, even in Scotland and the North there will be fine spells with ridges of high-pressure between depressions with maxima edging towards 20˚C but with the clear nights becoming cold enough for ground-frost at times.

 

Frontal influences will also extend further south to central/southern England and South Wales to bring some days of rain, stronger winds and maxima below 18˚C.  I do however, expect there to be more sunshine and warmth and less rain than usual for central and southern England on the whole.  For Scotland and the north-west of England, along with Northumberland, rainfall temperature and sunshine totals will be remarkably close to the long-term normal overall, but north-west Scotland will be wetter and windier than normal. Gales are likely to occur here two or three times during the second half of the month.

 

During October the cooling Arctic with the North Atlantic liable to remain warmer than usual, combined with continued hurricane activity is likely to mean a stronger atmospheric temperature gradient between the Arctic and the North Atlantic with continued injections of moist tropical warmth from the south:  This strongly suggests deeper depressions, particularly later in the month.  All things considered, with the rapid cooling of the Greenland Ice-cap and NE Canada whilst the westernmost part of Canada and the North Atlantic remain relatively warm- and with the factors discussed above likely to lead to a Circumpolar Vortex around or slightly stronger than usual for the time of year, I am confident of a so-called “three-wave” pattern with an upper ridge close to the North American Rocky Mountains. This would also place an upper ridge over Western Europe with the result being high-pressure encouraged to occur a little downwind- i.e. over central Europe whilst the upper-air pattern pushes progressively deeper depressions from coastal eastern Canada north-eastwards towards Iceland and then up into the Barents Sea.

 

Thus, during October the prevailing winds over Britain will be south-westerly, and they will increase in strength during the month.  During October these winds will often bring wet and windy conditions throughout the country.  However, the southern half of the country- all of England except Lancashire, Cumbria and Northumberland along with south and central Wales- will have a good deal of dry fine weather during the first ten days of October (there are indications that the weak upper-air Circumpolar Vortex will remain weaker than normal into October and the absence of a “Cold Patch” in the North Atlantic will encourage the jet-stream to blow further north on coming into NW Europe). The fine conditions will result from extensions of high-pressure from Europe across England and Wales:  There will be a run of warm sunny days with maxima near 20˚C with light south or south-west winds; clear nights will again result in temperatures dropping well into single figures and local ground-frost and fog can again be expected.

 

For Scotland, northernmost England and Northern Ireland and North Wales, October 2017 will be unsettled from the outset. Strong winds and rain will be frequent and it will get increasingly stormy. By mid-month the weather will turn unsettled even in the South with some wet and windy spells.  Gales and widespread heavy rain will sweep the entire country at least a couple of times during the second half of October and localised flooding is likely to occur in western Scotland, Cumbria and Lancashire. As a couple of the deepest depressions push eastwards late in October colder strong west or north-west winds will sweep the country to bring cold squally showers (with hail locally) and daytime temperatures below 10˚C as far south as Birmingham and the first snow will fall on high ground above 600 metres in Scotland, Cumbria and Northumberland- to provide a real foretaste of winter. There is likely to be at least one night with a ridge of high-pressure following such cold strong north-westerlies when skies clear and winds fall light. Air temperatures will fall close to freezing across the entire country and ground-frost will be widespread.  However, except in some rural upland areas of Scotland or sheltered frost-hollows further south, I do not expect the first air frost to occur for most of us until November.

 

The large scale upper-air pattern is likely to bring a return to some fine dry weather lasting just a few days during the second-half of October- again this fine weather- brought about by a westwards extension of the European High will be confined to central and southern England, South Wales and Yorkshire.  The fine spell is likely to be associated with cooler European air coming in from the south-east late in October and with weaker late-October sunshine daytime maxima anywhere above 16˚C are unlikely- even in the South.  Clear skies at night would encourage temperatures to drop sharply to within a few degrees of freezing point over a wide area and patchy morning fog will be widespread. However, as I have already hinted above, air-frost is unlikely to occur except in some rural low-lying valleys: Most places in England will not be getting their first air-frost of the season until November. Scotland is liable to miss out on any substantial October fine spells altogether.

 

First fortnight of October has been dry and warm in the south-east but a different story for northern England and Scotland where it has been wet (though still milder than normal overall). September has followed a similar pattern though temperatures have been close to the long-term normal in my part of the World (near nenthead in the North Pennines).  Hurricane Ophelia looks set to mark the transition to stormier conditions across Britain over the next week, but there is a prospect of drier colder conditions affecting central and southern Britain from the end of next week (according to the BBC long-range forecast).  Seem to have got the general prognosis about right, although September in the Midlands and South has proven to be a bit more unsettled than my original assertion of "Plenty of fine sunny conditions".

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I wouldn't say Northern England has been wet, it's the usual NW/SE split.
NE England right into Northumberland as had a dry month so far e.g. only 11mm here.

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38 minutes ago, 4wd said:

I wouldn't say Northern England has been wet, it's the usual NW/SE split.
NE England right into Northumberland as had a dry month so far e.g. only 11mm here.

I do believe I included Yorkshire in with Central and Southern England in my seasonal predictions for September-October as I felt it would be on the drier side of the said NW-SE split owing to prevailing south-westerly winds.  At Nenthead in the North Pennines (420 metres above sea-level) we had 148 mm (5.8 inches) of rain in September and so far this month (October) I have logged 60 mm (2.4 inches).  And we are set to get a lot more rain over the next few days, although being further east and sheltered from south-westerly winds by the Pennines and (further upwind) the Welsh Mountains you will be spared the brunt of it.

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I hate to spoil the party but is that not what you would expect from Autumn !

 

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