Penellype's Weather Channel

[Penellype]

The local weather station rainfall measurements show that we had 21mm rain (over 3/4 inch) in 10 minutes between 5.04 and 5.14pm today. The rainfall rate peaked at 122.4mm/hour. My allotment is under water

So far this year it has been too cold, too hot, too dry, too wet and frighteningly thundery. Can I send it back and order a different one please?

[bazzaboy]

The local weather station rainfall measurements show that we had 21mm rain (over 3/4 inch) in 10 minutes between 5.04 and 5.14pm today. The rainfall rate peaked at 122.4mm/hour. My allotment is under water

Sorry to hear your allotment under water, Penellype, these storms seem very localised and difficult to predict: we're only about 50 miles south of you and eventually got less than half of the promised 11mm in two sessions at 1.30pm and 4.00pm., no thunder, mainly a warm sunny day....

[Penellype]

Thanks bazzaboy. Yes the showers were very localised indeed. The next village to us, Strensall, which is about 3 miles away, had just 2.5mm rain all day according to the weather station there.

[bazzaboy]

The rainfall rate peaked at 122.4mm/hour.

I think that's officially "cloud burst" territory, isn't it, I don't know if the Met Office still use that term? I had a look at your News flashes, Penellype, desperate when that torrential, I do sympathise. I can recall what we called "cloud bursts" when I was a child (in Kent in those days), we thought they were great fun, doubt our parents agreed but leaky roofs weren't our concern and flooded roads and gardens a bit of an adventure. I've just read that they're caused by warmer-than-normal ground (as now) tending to cause upward air currents which prevents it raining until the droplets have achieved a larger mass than usual, so it doesn't rain until the droplets are huge enough to overcome the upward airflow. Years later I can see the dangers of this. The fact that this can occur should also make us very wary of building on floodplains: the oldies spent much of each Winter "hedging and ditching" for a reason!

[Penellype]

I think that's officially "cloud burst" territory, isn't it, I don't know if the Met Office still use that term? I had a look at your News flashes, Penellype, desperate when that torrential, I do sympathise. I can recall what we called "cloud bursts" when I was a child (in Kent in those days), we thought they were great fun, doubt our parents agreed but leaky roofs weren't our concern and flooded roads and gardens a bit of an adventure. I've just read that they're caused by warmer-than-normal ground (as now) tending to cause upward air currents which prevents it raining until the droplets have achieved a larger mass than usual, so it doesn't rain until the droplets are huge enough to overcome the upward airflow. Years later I can see the dangers of this. The fact that this can occur should also make us very wary of building on floodplains: the oldies spent much of each Winter "hedging and ditching" for a reason!

It was definitely cloudburst type weather - I think the current description is torrential. It reminded me of the videos you see in news reports of hurricanes (without the extreme wind) when there is lashing rain that you can barely see through.

You are quite correct that this sort of convective weather, where a heavy shower or thunderstorm appears out of "nowhere" in a very short time is caused by heating. Hot air rises (convection) and if the upper air is unstable (it was very unstable on Monday due to an upper "trough") this leads to the formation of huge clouds. The same process leads to tornadoes in America (and occasionally here, although we don't often have enough heat and instability for the sort of tornado found in America). The updraughts in a towering storm cloud are enough to throw an aircraft around like confetti, so pilots try to avoid them, small water droplets haven't a hope of escaping, so you get these huge drops of rain, often hailstones which can be really big in extreme cases.

On Monday there had been thunderstorms about all afternoon, and we had a fairly long one around 4pm. That cleared away and the sun came out. Hot August sunshine, wet ground, concrete (roads etc in the village and York not far away), unstable air... perfect conditions for rapid convective cloud formation. I am always wary in this situation as I know how fast things can change, and you can't always see it coming. If you look at the radar in this sort of weather you will tend to see the thunderstorms at their strongest over cities and major roads - this is no coincidence as concrete, brick and tarmac radiate much more heat than vegetation.

[Penellype]

Massive uncertainties in the forecasts for next week and the bank holiday, with weather models completely disagreeing (high pressure and hot vs low pressure and cool). The problem is another tropical storm, Ernesto, which is currently in the middle of the atlantic and is likely to get caught up in another low pressure system and head vaguely in our direction. You can see it here on the chart for mid day Tuesday:



By Friday the GFS has this over Scotland as quite a deep low which would bring rain and gales to the northern half of the country:



The ECMWF does something completely different:



The remains of Ernesto are circled in blue.

This highlights again the difficulties the models have with these small intense areas of low pressure (hurricanes and tropical storms). I can't even find it on the GEM (Canadian) model chart for Tuesday, but by Friday it appears to be somewhere off the coast of northern Norway!

So, take any forecasts beyond the weekend with a pinch of salt for now until we see where this storm is actually heading. In the meantime there will be some rain for most places tonight into tomorrow as 2 cold fronts move south east. These may get stuck over the southern half of the country giving quite a wet day tomorrow. Behind the fronts it will be cooler, fresher and probably showery, but not the thundery heavy type of showers that we had earlier in the week. The showers should die out as a weak ridge of high pressure builds for a couple of days, although it could still be showery in the north and west. After that it appears to be anyone's guess!

[Penellype]

I think I have the name of the storm wrong in the above post - Ernesto is moving in much more quickly than this and will arrive here as a low pressure on Sunday. The uncertainties surround the development of another storm in the atlantic next week (not yet named), but otherwise the above was correct as of last night. The sea surface temperatures in the atlantic off the north east coast of America are unusually warm, and this will be fuelling these storms, which are forming much further north than the usual hurricanes (which start off the west coast of Africa). This will be adding to the model uncertainty as it is not "normal".

Sorry for any confusion.

[Penellype]

Another couple of warm and humid days (less so in Scotland) with a lot of cloud and patchy drizzle or showers. Tomorrow into Thursday a cold front moves south across the country bringing some rain and a much cooler air mass as the jet stream moves south. By Saturday we are in a northerly wind as a ridge of high pressure builds to the west:



This will lead to some chilly nights over the weekend, particularly for Scotland, Ireland and the northern half of England. If you are in a place that is vulnerable to frost keep an eye on the forecast temperatures as there are some 4s and 5s appearing in some of the model runs.

With impeccable timing, the bank holiday is looking cool and showery at best, although there is disagreement between the models over the track of a small low pressure system. Another front could move across the country from the west on Monday. After that it looks like the high pressure could build into the following week, bringing a possible dry and warm start to September (but not a return to a heatwave). This is a long way off and things can easily change, but early September often has high pressure so it would not be a surprise if this were to happen.

The main message is to keep an eye on those night time temperatures over the weekend, and be prepared to close greenhouses etc if necessary.

[Penellype]

After a cold, wet and miserable day today, things should improve tomorrow as a ridge of high pressure arrives. It will remain fairly cool though, with a few showers.

It should be warmer and drier on Tuesday, although a cold front moving down from the north west may keep north western Scotland and Northern Ireland rather wet. This front will move south on Wednesday, breaking up as it does so. There probably won't be much rain on it (a few days ago models were predicting a wet day on Wednesday from a low moving up from the south, but this is now expected to barely graze the south east). However the front will introduce cooler air, and as high pressure builds behind it we are once again looking at some rather cold nights. Ground frost is not out of the question as far south as the midlands in vulnerable areas on Wednesday and Thursday nights. The ensemble chart for York shows the roller coaster temperatures well:



Uncertainties become greater as we get to September. Most of the models agree that high pressure is likely to be around, but its position is by no means certain, and this is crucial for determining temperatures at this time of year. It is therefore likely to be fairly dry, particularly so the further south you are, but as far as temperatures are concerned we need to wait until nearer the time. Some models are producing temperatures in the high 20s (probably followed by thunderstorms), others are not so warm.

[sandspider]

Thanks Pen.

Any thoughts on whether the winter will be a cold / long one? I saw somewhere that most signs pointed to it being a comparatively mild one...

[Penellype]

Thanks Pen.

Any thoughts on whether the winter will be a cold / long one? I saw somewhere that most signs pointed to it being a comparatively mild one...

It is far, far too early to say what will happen in 3-6 months time. The models are having enough trouble agreeing on what happens next week!

There are various factors that need to be considered when forecasting for the UK winter. Some of these are already known, some are not. A few of them are:

Solar cycle - we are currently almost at solar minimum of the 11 year solar cycle, meaning that the sun's activity (seen as sunspots) is very low. This is thought to increase the chances of high pressure over the north pole in winter, which can lead to a cold winter for the UK. The cold winters of 2009-10, 2010-11 and 1962-3 were all within a couple of years of solar minimum, but other cold winters were not, and some of the past solar minimum winters were not cold.

QBO - this is a regular weather pattern over the tropics which has an influence on the direction of the winds high in the atmosphere, and switches direction roughly every 18 months (ish). A westerly QBO enhances the jet stream and leads to an increased likelihood of a mild, wet and windy winter, an easterly QBO damps down the jet stream making it more easily blocked off by high pressure, increasing the likelihood of a cold winter. The QBO has been in its easterly phase for about a year, and is expected to start to transition to westerly any time now. There is a time lag between the transition and its effects, so we will start the winter under the influence of an easterly QBO. However there have been very cold winters with westerly QBO (eg 2009-10) and not all easterly QBO winters are cold.

ENSO (El Nino) - This is a warming (El Nino) or cooling (La Nina) of the tropical pacific sea surface between Indonesia and Peru, typically peaking around Christmas. The heat from an El Nino warms the planet while La Nina cools it down, and this has a big effect on the rainfall patterns in some parts of the world. How much depends on the strength (ie how big the deviation in temperature is from normal). The effect on the UK is small, but El Nino appears to lead to milder winters and La Nina possibly to cooler ones. Last winter we were in a weak La Nina, and currently the temperature is around neutral. The forecast models want to develop an El Nino by winter, but there isn't much sign of this actually happening and they are starting to back off the idea, so this has to go down as one of the unknowns at the moment.

Hurricane season (Atlantic) - hurricanes suck up large amounts of heat from the tropics and dump it in the arctic. There is a theory that an active hurricane season increases the chances of high pressure over the pole in winter. Currently the hurricane season is very quiet, which has been due to cool sea surface temperatures in the tropical atlantic. This is starting to warm up now, and we are only part way through the hurricane season. It is anyone's guess whether the warmer seas will pep up the hurricane season, and if they do, whether this will affect the winter.

North Atlantic sea surface temperatures - there is a theory that a "tripole" distribution of sea surface temperatures in May leads to a similar thing in autumn, and that this favours a cold winter. A tripole consists of a warm area around southern Greenland, cold in the mid latitudes and and warm near the tropics. This May we had almost the exact reverse of a tripole and there is still a notably cold pool of water south of Greenland, which some people think contributed to the pattern that gave us the hot summer. If this is correct, the same pattern in winter would give us a very cold winter.

Siberian/Scandinavian snow cover - early extensive snow cover over Siberia and Scandinavia in October is thought to increase the chances of a cold winter. Obviously we have to wait until October for this.

I have to say that the 2 real "knowns" - low solar activity and the easterly QBO both tend to favour a colder winter.

In complete contrast, David King (who did get the summer quite wrong) says it will not be particularly cold in his autumn forecast http://www.weatherwithouttechnology....0November.pdf:
"For the weather prophets that predict an even harder winter than last year, I wish them
well; however, nature makes the rules and nature has made no provision whatsoever for
anything like a long hard winter; Although this is just the autumn prediction I can tell you now that the winter is equally as benign, with precious little snow for most of us, maybe some frosts, especially in February [the simple reason for this being that the hottest days in June give the coldest days in the following February – June 2018 has been exceptionally hot] – therefore February will have some very cold nights – but also dry sunny days."
Interestingly he also appears to predict a prolonged drought and another hot summer next year.

I don't know about that - I tend to prefer science over old weather lore, but science has precious few answers for long range forecasting at the moment (only one of the long range weather models predicted the hot summer).

We shall see.

[Nicos]

According to the hawthorn bushes in the hedgerows around here, it's going to be an extremely cold winter. Never seen so many berries in all the time I've been here!

[Penellype]

We seem to have a lot of some kinds of berries - the elderberry trees at the allotment are laden with them. Whether this means a cold winter or not I don't know - it could equally be related to a hot, dry summer and the trees reacting to stressful weather in the past. I think this is more likely than them having some magical ability to predict the future.

However, weather is all connected, and it could be that hot and dry summers tend to lead to cold winters. I have no real evidence of this. The most similar hot summers I am aware of are 1868, 1870, 1921, 1976, 1983, 1999 and 2006 (information from https://www.gavsweathervids.com/autu...r-forecast.php).

From the CET (Central England Temperature) records...
1868-9 remains the warmest winter on record (since 1659), with all 3 months well above average.
1870-1 had a very cold December and January and a warm February
1921-2 had an average December and warm January and February
1976-7 had cold December and January and around average February
1983-4 was around average for December and January and had a cold February
1999-2000 was around average in December and January with a warm February
2006-7 was very warm throughout.

Statistically the sample is far too small for any sensible conclusions, but with 2 of the warmest winters on record (1868-9 and 2006-7) in the sample of 7, it is hard to make a case for the old saying "hot summer, cold winter"! Only 2 of these winters could really be described as very cold.

The hot summer of 1995, which is not in the above list as August was hotter than July, was followed by a cold winter. Like 1976 and 2006, 1995 was a year which was very near solar minimum, as were 1868 and 1921.

[sandspider]

Thanks Pen. Time will tell!

[Penellype]

Lots of uncertainty in the models at the moment.

Currently we have high pressure around, which has given the south and east a fine and dry weekend. It has been cloudier with light showers in the north and west. Over the next couple of days a cold front will move south across the country, bringing some rain and cooler temperatures behind. Once again we will be in a situation where vulnerable areas in the north may see ground frosts although the days will be pleasant enough.

Uncertainty increases towards the end of the week, with the models developing a low pressure over the country by the weekend, with the high pressure moving north. This would be cooler and more unsettled than the current conditions.

Uncertainties are increased by the development of tropical storm Florence in the tropical atlantic. The track of this storm is proving difficult for the models to deal with and this is causing widely different outcomes from the various model runs. However the general picture looks likely to be cooler and more unsettled than forecasts have been implying recently.

The latest ensemble forecast for York shows significant uncertainties as early as Tuesday:



It is however quite clear that the temperature is going to drop mid week.