Why is prolonged mild and dry so rare in winter?

[Summer8906]

This links in with the "mild and dry months" in the historical forum.

It does seem to be quite rare for a mild, dry period to set up in early or midwinter (Dec and Jan) for a prolonged period of many weeks, though more common in Feb.

Thinking of the kind of synoptics which are zonal but with the jetstream further north, and an anticyclone over the channel or northern France.

Such synoptics did of course persist for two and a half months in winter 1988/89 and there have been a few shorter periods, but more generally, mild/dry setups tend to become something else (usually mild and wet) in a couple of weeks.

Surprising in a way, I'd have expected a strong high in the Channel or Northern France and a strong but northerly zonal jetstream with lows over Iceland to be a self-perpetuating pattern in the same way that wetter, more cyclonic zonality is.

So I'm wondering what people's thoughts were as to why prolonged mild and dry is so rare in the early/mid winter?

Edited December 9, 2023 by Summer8906

[WYorksWeather]

There are plenty of people on this forum better placed to answer this than myself, but I'll give it a shot. 

First issue - anticyclonic weather doesn't naturally produce heat in the way that it does at other times of year. The strength of the sun is very weak, so a UK-based high typically cools in situ rather than warming as it can do e.g. in late February. So any sustained settled spell with high pressure over the UK will tend to be at best average to slightly above. Therefore, you usually need the high pressure positioned somewhere to our south, to draw up a mild south-westerly airflow around it, but still with the high ridging into the UK so we keep the weather settled.

The second problem is that in terms of weather regimes, the jet stream is much more powerful in winter compared to at other times of year, due to much larger temperature differentials between the poles and the tropics. So there's generally a greater tendency to bring in flat westerlies which are not going to be especially mild, and are likely to be wet - you need that south-westerly airflow, and you're less likely to get it as the weather regimes don't favour it.

Finally, and this should go without saying, you also need a reasonably warm airmass! Obviously it depends on what you mean by mild, but generally if you're looking for something to deliver mid-teens, you need something approaching a 10C 850hPa airmass, which is not a common scenario in December or January.

So, the only way to get prolonged settled weather that is also dry would be typically to have a strong anticyclone positioned to our south, and ridging into the UK, pulling in south-westerlies around it. This would need to stay almost exactly in place - a move to the north would create a UK high which would then rapidly cool in situ, and a move anywhere else would probably open up the opportunity for the jet stream to use the UK as target practice, driving in wind and rain from off the Atlantic.

Essentially, so many pieces need to come into place, and then if you want long duration, they also have to stay in place. It's not enough to just get a high pressure setup. The orientation has to be absolutely right.

As examples, here are some of the charts that have historically delivered very mild conditions in early and mid-winter.

18.7C recorded on 28th Dec 2019 (warmest December day on record):

16.3C recorded on 1st January 2021 (warmest NYD on record):

18.3C recorded on 26th January 2003 (joint-warmest January day on record):

You can see the general trend - warm 850hPa airmasses, perfectly positioned highs to draw in south-westerlies. Not the sort of situation that is ever likely to persist for long.

 

[LetItSnow!]

13 hours ago, Summer8906 said:

So I'm wondering what people's thoughts were as to why prolonged mild and dry is so rare in the early/mid winter?

Probably been explained already in depth by the above poster but the short version is that the spells required to raise the temperature to exceptional levels in winter is usually unstable and forced by a lot of energy from the Atlantic. Months like December 2015 and even February 1998 always have a lot of energy in the atmosphere that is pumping up all that moisture laden air (February 1998 may have been dry across England and Wales but remarkably wet with flooding in parts of Scotland) and it just can't stick around. The patterns that bring hot spells in summer usually just give faux cold scenarios apart from in rare, special atmospheric circumstances, February 2019 a remarkable example.

[Summer8906]

Thanks for the replies - I'm aware that most highs will not "draw in" warm air, but just querying why a zonal type with the jetstream further north and a strong high to the south of us isn't (usually) self-perpetuating. In other words it's rare for the jetstream to "stick" further north even if it's zonal - what usually happens is the jetstream goes south and it changes from dry to wet.

A good example of what tends to happen is seen on the very last frame of today's GFS 00z (26 Dec 00z). There's still a high over Northern France but the jetstream seems to be sinking south further west in the Atlantic. This almost always happens at some point, with 1988/89 being the notable exception. Wouldn't mind betting that if the model was to continue beyond that, the jetstream would descend south to produce a wet and windy end to the year - not to say this will actually happen of course!

Edited December 10, 2023 by Summer8906

[Weather-history]

December 1988-January 1989 was particularly dry, south of the Border and obviously it was remarkably mild. That’s one of the longest very mild dryish periods I can think of.

January 1898, 1916

It’s a tricky set-up as the high pressure needs to have enough influence to keep it relatively dry but not so much so that you end up with inversions and therefore colder surfaces temperatures eg 1963-64, that was a very dry winter. It was colder further south than north because of this. 

[summer blizzard]

23 hours ago, Summer8906 said:

This links in with the "mild and dry months" in the historical forum.

It does seem to be quite rare for a mild, dry period to set up in early or midwinter (Dec and Jan) for a prolonged period of many weeks, though more common in Feb.

Thinking of the kind of synoptics which are zonal but with the jetstream further north, and an anticyclone over the channel or northern France.

Such synoptics did of course persist for two and a half months in winter 1988/89 and there have been a few shorter periods, but more generally, mild/dry setups tend to become something else (usually mild and wet) in a couple of weeks.

Surprising in a way, I'd have expected a strong high in the Channel or Northern France and a strong but northerly zonal jetstream with lows over Iceland to be a self-perpetuating pattern in the same way that wetter, more cyclonic zonality is.

So I'm wondering what people's thoughts were as to why prolonged mild and dry is so rare in the early/mid winter?

Others have described it well however the two simplest reasons are.. 

1) High pressure in winter essentially traps cooler surface air when it builds behind a cold front, this makes it difficult for a high to sustainably produce mild weather unless it remains in place several days

2) In winter the continent is relatively cool and thus a persistant SE flow will tend to draw relatively cool air while the SW flows capable of producing warmth are by their nature less likely to persist because the westerlies will tend to try push the high east. 

[baddie]

If its going to be mid AND dry in winter, there is like a 90% chance it will happen in February. It will just make it feel like an addition to spring, but could tigger wet Marches. This year was an obvious example, as was 2019 but to a lesser extent

Likewise, mild and wet Februaries lead to warm and sunny Marches, with examples being 2011, 2014, 2020 and 2022

Edited December 11, 2023 by baddie

[MP-R]

44 minutes ago, baddie said:

If its going to be mid AND dry in winter, there is like a 90% chance it will happen in February. It will just make it feel like an addition to spring, but could tigger wet Marches. This year was an obvious example, as was 2019 but to a lesser extent

Likewise, mild and wet Februaries lead to warm and sunny Marches, with examples being 2011, 2014, 2020 and 2022

2020 was a wet March though. The jet so turbo charged still that it took until lockdown to blow itself out.

[LetItSnow!]

11 hours ago, MP-R said:

2020 was a wet March though. The jet so turbo charged still that it took until lockdown to blow itself out.

March 2020 was actually drier than average in all overall regions but it was wet in the extreme northwest and southeast though. 

[MP-R]

5 minutes ago, LetItSnow! said:

March 2020 was actually drier than average in all overall regions but it was wet in the extreme northwest and southeast though. 

There’s as much wet (if not more) as dry on there. Being in the white zone, I had roughly 120% rain which is indeed wet.

This is what a dry March looks like:

[SunSean]

11 minutes ago, MP-R said:

There’s as much wet (if not more) as dry on there. Being in the white zone, I had roughly 120% rain which is indeed wet.

This is what a dry March looks like:

Yeah those rainfall anomalies are so misleading. How on earth is 75mm considered a dry brown colour it's well above average!

[MP-R]

10 minutes ago, SunSean said:

Yeah those rainfall anomalies are so misleading. How on earth is 75mm considered a dry brown colour it's well above average!

The anomaly maps are indeed a bit dubious at the best of times. I guess the actual rainfall amounts have to be gauged somehow. It’s a tricky one as I wouldn’t class the 50-100mm range as particularly dry or wet (from a West Country perspective at least) but hey ho. No different to 30°C being shown as world-endingly red on weather maps nowadays…

Anyway, won’t take the thread more off topic

[LetItSnow!]

9 hours ago, MP-R said:

The anomaly maps are indeed a bit dubious at the best of times. I guess the actual rainfall amounts have to be gauged somehow. It’s a tricky one as I wouldn’t class the 50-100mm range as particularly dry or wet (from a West Country perspective at least) but hey ho. No different to 30°C being shown as world-endingly red on weather maps nowadays…

Anyway, won’t take the thread more off topic

When the percentages for England, Wales and Scotland are 77, 81 and 88, you can confidently say it was overall dry even though isolated pockets of wet areas may exist. We have statistics for a reason. 

More on topic of the the thread, I wonder if the winter of 1999/2000 for the south would be a close match. December 1999 was wet but sunny (and did see snow on the 18th) but January and February were both dry and sunny. February 2000 seems to have been wet in the northwest but more sheltered further southeast.  Trevor Harley writes that it was the first February (or winter month?  He’s quite vague about it) ever that saw every day of the month see at least 8C or higher. It was also a ridiculously sunny winter with 155% of the average sunshine in London for the winter as a whole. 

[Catbrainz]

A little late but Ill make a guess. Mild and dry in winter would suggest a Bartlett pumping up southerlies and blocking Atlantic rain. It can change if the high moves, if it moves north and becomes a UK high things tend to get chillier with less of a southerly feed, move south and it ether ends up in a more northerly pattern or turns into a westerly wet zonal pattern. Basically a Bartlett moving anywhere will ether make for a cooler or wetter or both pattern. The jet stream is more active in winter so its hard for a sustained high to keep itself up. 

[Weather Enthusiast91]

I wouldn't say that prolonged mild and dry is rare. But I would say that mild AND sunny is, until we get to around mid Feb.

Mild and dry during the winter is almost always dominated by cloud, just like this current spell for example.

 

Edited December 17, 2023 by Weather Enthusiast91

[Scorcher]

Wasn't the winter of 2011/12 pretty dry and mild up to February?

As others have said, it's hard to get a stable high pressure in the right position that sticks around at this time of year.

[Summer8906]

On 19/12/2023 at 09:39, Scorcher said:

Wasn't the winter of 2011/12 pretty dry and mild up to February?

As others have said, it's hard to get a stable high pressure in the right position that sticks around at this time of year.

December 2011 was wet, it changed to much drier around Jan 6th. So Jan 2012 was mostly a rare mild and dry month, probably the best example of such since 1989.

Edited December 20, 2023 by Summer8906

[MattStoke]

Our mildest winter weather comes from the south west, which is also a very wet direction due to long sea track and cyclogenesis. Weak solar energy and long nights mean heat doesn't build when high pressure is centred over the UK, unlike in summer.

I detest mild winter weather because it's also grey, miserable, wet, windy and boring.

Edited December 20, 2023 by MattStoke

[Freeze]

December 2015 was very mild (almost warm) and fairly dry down here by December's standards. I realise though that it was extremely wet further north and west.

[Summer8906]

I think what I'm trying to get at is: if there's a west-east zonal pattern with a high centred over northern France, say, and the jetstream is running west to east further north, e.g. across northern Scotland, it often seems to be the case that the jetstream then moves south displacing the high and bringing wet and windy to all areas.

One might think that the high over northern France would be replaced with a new high over the Azores building in (as often happens from around Feb to Sep) and the jetstream would remain further north due to the persistent belt of high pressure keeping it there. Instead "something" forces the jetstream south. See for example what happened in January 2020 when there was a brief high mid-month. However it's generally not a block forcing the jetstream south, as what results is usually wet zonality, rather than any kind of blocked pattern. (see Feb 2020)

1988/89 and Jan 2012 are amongst the rare examples of the kind of persistent pattern I've been thinking of in the early/mid winter, though the pattern becomes much more commonplace in February.

Edited December 20, 2023 by Summer8906

[Summer8906]

16 minutes ago, Freeze said:

December 2015 was very mild (almost warm) and fairly dry down here by December's standards. I realise though that it was extremely wet further north and west.

I suppose Dec 2015 was the halfway house between a true mild and dry month (e.g. Jan 1989) and a mild and wet month. Consequently down here it was very dull and frequently drizzly, but there was little heavy rain.

Jan 1990 would be another of this kind of in-between setup, as that was very mild and rather dry for much of the month (even the Burns storm didn't produce much rain here, it was a bizarre "sunny storm" where I was in south Hampshire) before turning first drizzly around the 19th or so and then more generally wet later, though ironically as I said the wetter systems were those other than the Burns system. ISTR this month was actually drier than average in the far south.

I remember the mid-month weekend of Jan 1990 (which was dry) was unearthly mild, particularly after dark. With a run of snowy winters still in recent memory, it was unlike anything I'd experienced at the time, even 1989 wasn't like that. Despite the general character of winters since, I don't think I've experienced anything quite that mild in Jan again. Not sure if 1990 remains the mildest Jan since 1916 or whether something else has overtaken it.

Edited December 20, 2023 by Summer8906

[Scorcher]

3 hours ago, MattStoke said:

I detest mild winter weather because it's also grey, miserable, wet, windy and boring.

Our sunniest day this month (5.7 hours) had a max of 12.6C. So  mild air doesn't necessarily mean dull all the time, and colder air certainly doesn't guarantee brightness either. That cold spell last December was rather dull here mostly- we recorded no sunshine at all for 4 consecutive days between the 10th and the 13th.