autumn Autumn 2016

[Weather-history]

7 hours ago, Frost HoIIow said:

I think his forecasts are fairly accurate, so I find them interesting. However I would be very surprised if I don't record at the very least one air frost between now and 30th November. Even during the wettest Autumn on record in year 2000 I recorded 3 air frosts in the November sandwiched in between Atlantic pressure systems. However a lot of places didn't record an air frost in that entire Autumn so maybe Ian is going for a similar disturbed one like 2000. Especially October and November with very little respite between storms.

There were only 3 days in October and November 2000 where no rainfall was measured around here. The earliest season snowfall I have seen occurred on 30th October though.

Edited September 10, 2016 by Weather-history

[iapennell]

12 hours ago, Walsall Wood Snow said:

 

Of course long range forecasts should be taken with a pinch of salt. However some are better at it than others. Also it has been said that Winter is an easier month to predict for than Summer for some reason. Also Iapennell has got a pretty good success rate with Autumn and Winter forecasts. He also seems to make his predictions with an air of confidence (best to I suppose as it makes them sound more believable) as though what he thinks is practically guaranteed to happen. Such as his assertion that this Autumn will be pretty much frost free.

 

Spring and summer are harder to predict because the Circumpolar Vortex weakens in response to the reduced temperature gradients between the Arctic and sub-tropics. This makes the jet-stream vulnerable to anomalous patches of cool or warm water, to a single depression pulling cold air south, etc. This can cause the positions of upper-level ridges and troughs to change quickly and in unpredictable ways over a matter of a few days. When that happens you can get very different wind-directions and weather-patterns over a particular area compared to that predicted earlier.

It is still possible, alas, to make broad predictions based on where the likely baroclinic zones will be, how large and persistent sea-surface temperature anomalies affect the upper Westerlies, the sea-surface temperature gradient between about 40N and the tropics gives an indication of the strength of the NE Trade Winds to be expected (and likely hurricane activity) and this will indicate the likely strength of the Westerlies needed to counteract them. Other indicators like El Nino and the QBO also give an indication of (more likely) spring/summer conditions. This will give a broad indication of conditions in spring/summer but there is a higher likelihood of the predictions proving wrong for a considerable part (or all) of the season because of the vulnerability of the Jet-stream to "flip" with relatively small perturbations.

A completely different weather-pattern to the one expected can then set in with cold north-west winds (for example), modifying sea-surface temperatures in the NE Atlantic to keep the jet-stream entrenched in the new position so that Britain has (perhaps) a cold wet summer or a cool dry summer (depending on where precisely the patch of cool water in the North Atlantic is) as opposed to an earlier prediction of a warm summer (to use the above example). I hope this makes sense with regards to spring/summer forecasting and it is why I would never even try to predict what a summer is likely to be like before early May!!

Autumn and winter are much more predictable because the increased strength of the NE Trades and (thus) of the Westerlies means they are much more stable. You can then make predictions based on the likely positions of the wind-belts based on a) where the likely baroclinic zones will be, b) where there is likely to be warmth and moisture south of these baroclinic zones to fuel deep depressions that drive the Westerlies. You can ascertain how sea-surface temperature anomalies in the autumn are likely to fix the strong Circumpolar Vortex knowing that in the winter half-year this is less likely to be flipped out of sync by a renegade depression or sudden snowfalls over Israel.

That said, even in autumn/winter things can go wrong. For example a heavy snowfall over Iran could encourage regional cooling there (due to increased surface albedo) and strengthening and southwards deviation in the subtropical jet-stream so that it smashes into the Himalayas- leading to a high-level ridge pushing right up into the Arctic. A consequent Sudden Stratospheric Warming over the Arctic would then disrupt an earlier prediction of a mild, stormy winter over Britain with bitterly cold east winds and snowfalls during January. Another example, heavy snowfall over Greenland and Iceland in late September could occur with a deep depression tracking east over the south of this Arctic region leading to a big increase in local surface albedo and accelerated cooling. This leads to a big Greenland High in October that pushes cold air south smashing through the Ferrel Westerlies over the NE Atlantic to bring hard frosts and snow to the UK when a mild moist autumn with south-westerlies had earlier been predicted. However, sudden Circumpolar Vortex-disrupting changes like this seem to be rare in the situation where you know the upper Westerlies are likely to be stable and strong because of their northwards displacement and strengthening by strong baroclinic gradients between the Arctic and North Atlantic/Pacific.

If one knows the Upper Westerlies are likely to be weaker and travelling further south (and thus a bit more unpredictable), more likely developments such as SSW events over the Arctic and huge blocking highs over Scandinavia can be factored into the seasonal forecasts. In Britain and during the winter months all quadrants of wind-direction other than from between south and west tend to bring markedly colder-than- normal weather. So if one has to conclude the Upper Westerlies will be weaker and travelling further south (i.e. an expanded Circumpolar Vortex) it is almost a foregone conclusion that the winter will be icy to varying degrees because in this situation winds reaching Britain in winter could come from all manner of directions but southerly, south-westerly or westerly!     

[lassie23]

On 08/09/2016 at 12:05, stainesbloke said:

Seeing as we were robbed of a summer month in June, am quite happy for a warm summery September.

If I remember rightly, we were robbed of an entire winter, so roll on snow in October.

[I remember Atlantic 252]

17 minutes ago, lassie23 said:

If I remember rightly, we were robbed of an entire winter, so roll on snow in October.

agree with your quoted post and your post Lass!

[Nick L]

26 minutes ago, lassie23 said:

If I remember rightly, we were robbed of an entire winter, so roll on snow in October.

I was tempted to mention this too

[stainesbloke]

45 minutes ago, lassie23 said:

If I remember rightly, we were robbed of an entire winter, so roll on snow in October.

We have been robbed of more than one winter in recent years, the same as quite a few recent summers. Perma-Autumn is the phrase I've seen mentioned a lot and it's quite apt.

[Nick L]

The number of above average months we've had in recent years, a good run of below average ones would be quite a nice change.

[stainesbloke]

43 minutes ago, Nick L said:

The number of above average months we've had in recent years, a good run of below average ones would be quite a nice change.

I agree, if they are not in summer. Below average Autumns and Winters would certainly be a big change with potentially much more interesting weather.

Edited September 10, 2016 by stainesbloke

[Walsall Wood Snow]

5 hours ago, iapennell said:

Spring and summer are harder to predict because the Circumpolar Vortex weakens in response to the reduced temperature gradients between the Arctic and sub-tropics. This makes the jet-stream vulnerable to anomalous patches of cool or warm water, to a single depression pulling cold air south, etc. This can cause the positions of upper-level ridges and troughs to change quickly and in unpredictable ways over a matter of a few days. When that happens you can get very different wind-directions and weather-patterns over a particular area compared to that predicted earlier.

It is still possible, alas, to make broad predictions based on where the likely baroclinic zones will be, how large and persistent sea-surface temperature anomalies affect the upper Westerlies, the sea-surface temperature gradient between about 40N and the tropics gives an indication of the strength of the NE Trade Winds to be expected (and likely hurricane activity) and this will indicate the likely strength of the Westerlies needed to counteract them. Other indicators like El Nino and the QBO also give an indication of (more likely) spring/summer conditions. This will give a broad indication of conditions in spring/summer but there is a higher likelihood of the predictions proving wrong for a considerable part (or all) of the season because of the vulnerability of the Jet-stream to "flip" with relatively small perturbations.

A completely different weather-pattern to the one expected can then set in with cold north-west winds (for example), modifying sea-surface temperatures in the NE Atlantic to keep the jet-stream entrenched in the new position so that Britain has (perhaps) a cold wet summer or a cool dry summer (depending on where precisely the patch of cool water in the North Atlantic is) as opposed to an earlier prediction of a warm summer (to use the above example). I hope this makes sense with regards to spring/summer forecasting and it is why I would never even try to predict what a summer is likely to be like before early May!!

Autumn and winter are much more predictable because the increased strength of the NE Trades and (thus) of the Westerlies means they are much more stable. You can then make predictions based on the likely positions of the wind-belts based on a) where the likely baroclinic zones will be, b) where there is likely to be warmth and moisture south of these baroclinic zones to fuel deep depressions that drive the Westerlies. You can ascertain how sea-surface temperature anomalies in the autumn are likely to fix the strong Circumpolar Vortex knowing that in the winter half-year this is less likely to be flipped out of sync by a renegade depression or sudden snowfalls over Israel.

That said, even in autumn/winter things can go wrong. For example a heavy snowfall over Iran could encourage regional cooling there (due to increased surface albedo) and strengthening and southwards deviation in the subtropical jet-stream so that it smashes into the Himalayas- leading to a high-level ridge pushing right up into the Arctic. A consequent Sudden Stratospheric Warming over the Arctic would then disrupt an earlier prediction of a mild, stormy winter over Britain with bitterly cold east winds and snowfalls during January. Another example, heavy snowfall over Greenland and Iceland in late September could occur with a deep depression tracking east over the south of this Arctic region leading to a big increase in local surface albedo and accelerated cooling. This leads to a big Greenland High in October that pushes cold air south smashing through the Ferrel Westerlies over the NE Atlantic to bring hard frosts and snow to the UK when a mild moist autumn with south-westerlies had earlier been predicted. However, sudden Circumpolar Vortex-disrupting changes like this seem to be rare in the situation where you know the upper Westerlies are likely to be stable and strong because of their northwards displacement and strengthening by strong baroclinic gradients between the Arctic and North Atlantic/Pacific.

If one knows the Upper Westerlies are likely to be weaker and travelling further south (and thus a bit more unpredictable), more likely developments such as SSW events over the Arctic and huge blocking highs over Scandinavia can be factored into the seasonal forecasts. In Britain and during the winter months all quadrants of wind-direction other than from between south and west tend to bring markedly colder-than- normal weather. So if one has to conclude the Upper Westerlies will be weaker and travelling further south (i.e. an expanded Circumpolar Vortex) it is almost a foregone conclusion that the winter will be icy to varying degrees because in this situation winds reaching Britain in winter could come from all manner of directions but southerly, south-westerly or westerly!     

So you're saying there's still at least some hope for this coming Winter then despite how things stand at present? If so I'll take that as even if we do end up with a mild Winter I don't want to believe it's a full gone conclusion just yet as Autumn is the season when I start looking forward to and dreaming of Winter again and been as it's only 10 days old I don't want to think we've got nothing to play for this early.

[Guest]

If we have another mild, windy and wet Winter and Autumn im off to Montenegro, and it'll probably be both colder and 'snowier' there, even if it is in the Med.

[I remember Atlantic 252]

4 hours ago, Nick L said:

The number of above average months we've had in recent years, a good run of below average ones would be quite a nice change.

maybe happen April to Sept 2017

[cheese]

7 minutes ago, BREIFMAN11 said:

If we have another mild, windy and wet Winter and Autumn im off to Montenegro, and it'll probably be both colder and 'snowier' there, even if it is in the Med.

Well, Montenegro isn't that much milder than the UK in winter.

[Nick L]

4 minutes ago, cheese said:

Well, Montenegro isn't that much milder than the UK in winter.

In the Montenegro capital, "it rarely snows for more than a few days per year" - yep, sounds just like here!

[iand61]

12 hours ago, Weather-history said:

There were only 3 days in October and November 2000 where no rainfall was measured around here. The earliest season snowfall I have seen occurred on 30th October though.

remember that early snowfall.

heavy overnight rain led to some flooding up here but then in the space of just a few minutes it changed into large flakes which covered in little time at all.

by the time it stopped we had about 3 - 4 inches of snow and a school closure gave my daughter an unexpected day off.

[Guest]

50 minutes ago, Nick L said:

In the Montenegro capital, "it rarely snows for more than a few days per year" - yep, sounds just like here!

Forgot to mention, Montenegro Kotor. Up in the mountains, you can see the leftovers of Winter even in the roads.. the drivers are absolutely bonkers there.

[Frost HoIIow]

12 hours ago, Weather-history said:

There were only 3 days in October and November 2000 where no rainfall was measured around here. The earliest season snowfall I have seen occurred on 30th October though.

We had a light covering that morning, but according to the Met Office Boltshope Park near Durham reported 15 cm of snow in less than 3 hours. Not bad for October.

[northwestsnow]

I really enjoy autumn,esp when we are lucky enough to some continental conditions with some sort of block in or around scandy, something magical about a misty cold afternoon in late october/november, just takes me back, pea soup,potato hash, all the different colours of the falling leaves,splendid!

[Walsall Wood Snow]

25 minutes ago, northwestsnow said:

I really enjoy autumn,esp when we are lucky enough to some continental conditions with some sort of block in or around scandy, something magical about a misty cold afternoon in late october/november, just takes me back, pea soup,potato hash, all the different colours of the falling leaves,splendid!

Yeah thats what Autumn weather should be about, especially around Halloween and Bonfire night. Not gales, rain and above average temperatures, even though that weather type is associated with Autumn it can be like that any time of year and often is.

[Summer Sun]

100F in London on Tuesday on the feels-like temperature

[DAVID SNOW]

1 minute ago, Summer Sun said:

100F in London on Tuesday on the feels-like temperature

How many times are you going to post this?

Yes, it looks hot on Tuesday!        Not for me though.

[Summer Sun]

4 minutes ago, DAVID SNOW said:

How many times are you going to post this?

Yes, it looks hot on Tuesday!        Not for me though.

 

Pretty good consistency it must be said...

[DAVID SNOW]

6 minutes ago, Summer Sun said:

Pretty good consistency it must be said...

From you, that's for sure

[Summer Sun]

9 minutes ago, DAVID SNOW said:

From you, that's for sure

I try my best

[iapennell]

4 hours ago, Walsall Wood Snow said:

So you're saying there's still at least some hope for this coming Winter then despite how things stand at present? If so I'll take that as even if we do end up with a mild Winter I don't want to believe it's a full gone conclusion just yet as Autumn is the season when I start looking forward to and dreaming of Winter again and been as it's only 10 days old I don't want to think we've got nothing to play for this early.

@Walsall Wood Snow, Absolutely. However, it is still early September and there is several weeks to go before the Circumpolar Vortex and higher-latitude storm tracks settle into their winter patterns. Furthermore the sea-surface temperature anomalies, changes in hurricane activity, the El-Nino Southern Oscillation (ENSO) and the Quasi Biennial Oscillation (QBO) of wind-patterns high up over the Equator ALL have profound implications for the weather we can expect this coming winter and they seem to be in a state of flux.

I made a prediction for dry and sunny conditions in September-October over England and Wales back at the end of July based on how some of these important large-scale patterns were playing out: They have all changed in quite significant ways since, for example there was a large patch of cooler-than-normal water in the North Atlantic to the west of Britain at the end of July which is now significantly warmer than normal for the season and the QBO has turned Westerly due to some quite unexpected disruption. So the factors which I based my original forecast upon have changed; what I do when I discover this is that I update my prediction in the light of these changes.For instance, in the light of these changes, I am now expecting October to be warmer than normal and increasingly wet and windy over the north of Britain and with blustery conditions reaching all parts by the end of the month.

And indeed, even when one makes a firm prediction on the basis of variables one is confident will hold through the season (because of the stability of storm-tracks and the positions of the subtropical high) there always remains a small probability of things that can happen that knock the global winds off-balance. The global atmosphere, despite having some seemingly stable features is fluid and perturbations can arise (for example with wind-currents) to cause a cascade of increasing knock-on effects to bring about surprising shifts in expected weather-patterns. There are so many possible perturbations like this that not even the most powerful computer models can predict accurately which will occur and which won't. This is the Lorenz "Butterfly" Effect which affirms (like rolling a dice) that there is an element of random chance of wind-currents and sea-surface temperature over different locations that affects the fluid dynamics of our global atmosphere! There is also the possibility of happenings that meteorologists may not know about such as sudden Solar Flares or a major volcanic eruption a few months down the line that could invalidate the parameters under-pinning one's forecast.

So you can never ever be 100% certain of seasonal weather-predictions!

[Walsall Wood Snow]

2 hours ago, iapennell said:

@Walsall Wood Snow, 

Thanks iapennell for your kind reply