Archives for September 2012

All eyes on York after worst September storm since 1981

Paul Hudson | 14:02 UK time, Wednesday, 26 September 2012

The deepest area of low pressure in September since 1981 has caused many of Yorkshire's rivers to burst their banks - although the rain was not quite intense enough to cause serious flooding of property like in the autumn of 2000.

Cumulatively since the rain started falling on Sunday evening, to when it stopped during the early hours of this morning, Ravensthorpe in Richmondshire has had the highest rainfall in the country with 131mm (over 5 inches).

At Leeming, 100mm (4 inches) of rain has been recorded in the same time period. This is almost double what would normally be expected in the whole of September (Average 52mm).

Furthermore, the station recorded its wettest September day, with 76mm falling in the 24hours to 10am on Tuesday morning. Weather data has been collected at the RAF station since 1945.

All eyes are now on the River Ouse catchment, whose tributaries are the Rivers Derwent, Aire, Don, Wharfe, Rother, Nidd, Swale, Ure and Foss, all of which have been high in the last 24 hours.

The Environment agency currently has 2112 properties on flood warning along the length of the Ouse catchment.

Current forecasts suggest the river will peak at midnight in York at no higher than 4.7 metres above normal. It is currently flowing at 4.5 metres above normal.

This would be higher than the peak in January 2008 of 4.5 metres above normal, but quite a bit lower than the record which was set in November 2000 of 5.4 metres above normal.

It will give comfort to the residents of York that during that record breaking river level, the main flood defences, including the Foss barrier, held.

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Yorkshire at risk from renewed flooding

Paul Hudson | 15:19 UK time, Monday, 24 September 2012

Heavy rainfall in the next 30 hours could lead to yet more flooding across parts of Yorkshire. Current indications suggest some Pennine catchments of North & West Yorkshire could be most at risk.

The very active weather system which I wrote about here on Friday and is currently affecting much of our region could produce 50-75mm in total in some parts of Yorkshire - with around 100mm possible in one or two exposed Pennine spots, see diagram below.

There are several reasons why such high rainfall totals are expected.

Firstly, some of the air is of tropical origin, having been drawn from ex-hurricane Nadine as it formed late last week; secondly, the weather front bringing the rain will become slow moving across the North of England, leading to a long period of rainfall; thirdly, air is being pushed up over the Pennines by a strong Northeasterly wind at first, enhancing rainfall, hence some of the largest totals will be across the eastern upslopes of the hills.

With the water table and soil moisture levels higher than average especially in western areas following the wet spring and summer, which you can read more about here, flooding is a distinct possibility as we head through tonight and tomorrow, as the region's rivers take the strain once again.

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Autumn blast on its way

Paul Hudson | 15:12 UK time, Friday, 21 September 2012

A deep area of low pressure looks set to dominate our weather late in the weekend and into next week as the official start of autumn (which is the 22nd September this year, defined as the day the sun crosses the equator into the southern hemisphere) coincides nicely with the first meteorological blast of the season.

The track of this low pressure is still open to question, and has been causing real problems for forecasters in the last few days.

It's down in part to ex-hurricane Nadine which is in the mid-Atlantic near the Azores.

Computer models invariably struggle to handle these storms because such systems are relatively small but intense, and as such can be difficult to model with any consistency.

What Nadine does in the next few hours will help determine where an associated area of low pressure will develop and track, drawing in some of Nadine's warm sub-tropical air, hence the uncertainty regarding the timing and location of potentially heavy rain and strong winds for the UK.

The chart below is the best estimate of its position at midday on Monday, based on the UK Met Office Global model.

The current best estimate for our area is for the heavy rain to hold off until late Sunday, with much of next week then very unsettled, at times windy, with showers or longer spells of rain.

On a more positive note, if timings remain the same, it does mean that much of this weekend across Yorkshire and Lincolnshire will be fine and sunny, albeit cold at night with a risk of rural frost.

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Wet end to September raises autumn flooding concerns

Paul Hudson | 15:13 UK time, Friday, 14 September 2012

The second half of September look set to be dominated by low pressure, with little chance of a prolonged dry settled spell of weather.

All computer weather models are indicating that temperatures are likely to be below normal, with rain expected at times across the UK - although there will be some dry, bright days especially in the south and east at first.

Heavy rain on wednesday led the River Aire to burst its banks into the flood plain near Skipton, shown in the picture below.

This highlights the fact that the water table remains high, following the wettest April on record, and the wettest summer in 100 years.

Since March, there hasn't been a prolonged spell of dry weather which would naturally allow the land to dry out and the water table to fall, and should the traditionally wet months of October and November materialise, then a renewed risk of flooding is a distinct possibility.

The floods of autumn 2000 were unprecedented in their scale and severity, and such was the volume of rainfall that fell, that it became the wettest autumn in the Central England data set which goes back to 1766.

But the flooding was made far worse because there were no spells of settled, dry weather in any of the previous months.

April 2000 was the wettest on record and June 2000 was also very wet, with the River Ouse rising to its highest ever June level, as parts of the region experienced flooding.

But crucially July and August 2000 - the warmest two months of the year, when evaporation rates are at their highest - whilst not classed as a washout, saw no period of prolonged dry weather over a period of weeks which would have allowed the land to dry out.

So in 2000, far less rainfall in October of that year would have caused some flooding, because the land was already so wet.

We can only hope that whilst the rest of September is likely to see more rainfall, the rest of autumn shows some improvement, or further flooding may become a reality.

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Global temperature update

Paul Hudson | 15:27 UK time, Friday, 7 September 2012

Global temperatures in August were 0.34C above the 30 year running average, according to the UAH satellite measure, showing a small increase on the previous month, shown below.

It's the 3rd warmest August in this particular satellite data set which began in 1979.

Using the more standard 1961-1990 average used by the World Meteorological Organisation, global temperatures were approximately 0.593C above average.

The recovery in global temperatures since the turn of the year has in part been due to a rise in sea surface temperatures in the equatorial Pacific, as 'La Nina' conditions faded, shown below.

Most computer predictions expect positive sea surface temperature anomalies in this part of the world to persist into next year, with weak El Nino conditions becoming established.

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