UK meteorology

JohnnyO said:
Thankies Chris, we've still got the driving rain and high winds, though thankfully no tornadoes.( as yet ).

JohnnyO. o/
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That's a pity, Johnny - it's been lovely here this past couple of days. I think you may see something of an Indian Summer this year to balance-out the bad rub of the green so far this summer.
 
chrisbell said:
That's a pity, Johnny - it's been lovely here this past couple of days. I think you may see something of an Indian Summer this year to balance-out the bad rub of the green so far this summer.
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Promises, promises. We had thunder and lightning with flash floods two nights ago . Just dry would do me, even without the summer bit. Our Frenchie is shrinking to a pug now !

JohnnyO. o/
 
There have been some analyses by weather enthusiasts of the long-range seasonal models. I'm going to make the pre-output explanation this weekend's 'lesson' as I've got to explain what this means before I can discuss what the models are showing for the autumn.

Unlike the short-range models I normally mention (GFS, Met Office and ECMWF), the seasonal models are not deterministic; not only they do not seek to predict actual temperatures, rainfall levels etc., but they are based upon algorithms that modify the output in the same way for the same initial variables that are entered. What this means is that they use meteorological theory and past data such that the model always performs the same way for a given set of initial conditions, to quote Wikipedia. By contrast, these long-range seasonal models are stochastic (statistical), which means that they include an element of chaos or randomness, which, beyone around 10 days, renders deterministic models useless due to the huge downstream effects of tiny changes in the initial conditions, as Johnny mentioned earlier with regards to his joke about hurricanes in Wakefield.:D As I understand it, statistical models run the same sequence several times with the same initial conditions, allowing chaos theory to modify the output each time, then average across the runs. Their output, therefore, shows whether atmospheric pressure, temperature and precipitation in a given area is expected to be below average, around average or above average in map form.

What these models are suggesting is that, at least during September and October, the UK as a whole is likey to be warmer than average, possibly significantly so. November may continue warmer than average, but, here, model consensus disappears as some outputs suggest a marked cooling. In terms of rainfall, the season as a whole seems likely to be wetter than average, though a couple of models, including the respected CFSv.2 seasonal model suggest that it may be more high pressure-dominated, particularly further south and west, while northern and eastern districts might be closer to average. By contrast, the monthly output of the same models shows the season overall to have average rainfall masking some quite strong month-by-month swings, with September being wet, October very dry, and the aforementioned south west/north east divide in November. Interestingly, as we are nearly in September, the shorter-range deterministic models for the first week or so of the month are out, and these show warm and dry weather predominating away from the far North, suggesting that, if the monthly CFSv.2 output is correct, the weather is likely to go downhil rapidly mid-month!
 
Netweather's chief forecaster has posted a convective forecast for later today and the overnight period:

http://www.netweather.tv/index.cgi?action=convective;sess=

Synopsis

An upper trough axis lies SW to NE across the far NE Atlantic and a cut-off upper low sits over Iberia to the west of an upper ridge over central, southern and eastern Europe. At the surface, a weak cold front will slide slowly SE across England and Wales, with a warm and huimid airmass across SE UK and an unstable showery flow across NW UK. A shallow area of low pressure covers the near continent which will deepen slightly and bring an area of heavy and perhaps thundery rain across SE UK tonight.

... SE UK ...

The NW edge of a plume of very warm and humid airmass residing on the near continent will sit across SE England today and overnight. Isolated elevated heavy showers, perhaps with sporadic lightning, may occur where Ac cas becomes deep enough with subtle forcing along NW edge of plume. Otherwise, overnight there is an increasing risk of embedded elevated thunderstorms within pulses of heavy rain developing and moving NE from N France / East English Channel across SE England ... as baroclinic zone and surface convergence developing over northern France in response to falling heights/temps from the NW create increased ascent of plume over near continent. There is a risk of minor localised flooding from these storms, mainly Sussex and Kent overnight.

... N. IRELAND and W SCOTLAND ...

With steepening lapse rates from the NW, as upper trough / cold pool slides SE, convection may become deep enough amongst the showers expected here to produce sporadic lightning and small hail.
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