The coldest/hottest you could possibly get in a calendar month.

[Methuselah]

 LetItSnow! But even I don't remember 1795! 

[Metwatch]

On 01/11/2022 at 16:37, LetItSnow! said:

Months like December 2015 and May 1833 are so interesting because they challenge our perceptions of what is possible in the record books. 

April 2011 I feel is another that tends to be less remembered about given Aprils have warmed up a fair bit. If April 2007 wasn't that warm, 2011 would have been the warmest April by 1.3C above the current third warmest which is 1865, followed by 2020. A bigger gap than between the 2 warmest Mays! The CET of April 2011 is virtually identical to the coldest June in modern times which was in 1972, and over a degree warmer than the coldest September in modern times which was in 1952.

Edited April 1 by Metwatch

[LetItSnow!]

I actually tried to answer my own question. I stitched together different daily C.E.T. means from Julys of 1976, 2006 and 2022. I took the C.E.T from the 1st to the 14th of July, 1976 and added a degree to account for global warming. I then took the C.E.T. for the 15th to the 19th of July, 2022 and adjusted a couple of the cooler means to fit in the weather pattern. I then used the daily means from the C.E.T. from the 20th to the 31st of July, 2006 and adjusted them upwards to account for the hypothetical pre-existing weather pattern.

With this it suggests a July C.E.T. of 22.4!

What would @B87and @SunSeanmake of that?

I'd also imagine in this hypothetical month that the maximum of 40.3C would be higher and probably as high as 41 or 42C and that perhaps this heat would stretch for several days. Perhaps from the 18th to around the 26th being constant highs between 35-40C.

Absolute hell. Sounds like a @CryoraptorA303theory gone mental.

Edited April 1 by LetItSnow!

[LetItSnow!]

To add to that, with some (hopefully) logical and creative liberties, here is what such a month may look like at Heathrow;

 

1st: 34C

2nd: 36C

3rd: 37C

4th: 36C

5th: 36C

6th: 36C

7th: 35C

8th: 34C

9th: 28C

10th: 28C

11th: 31C

12th: 34C

13th: 27C

14th: 27C

15th: 27C

16th: 30C

17th: 33C

18th: 38C

19th: 41C

20th: 39C

21st: 39C

22nd: 35C

23rd: 32C

24th: 34C

25th: 37C

26th: 36C

27th: 34C

28th: 33C

29th: 30C

30th: 29C

31st: 26C

Average max: 33.2C

I wonder if this will actually be within the realm of possibility in my lifetime...

Edited April 1 by LetItSnow!

[CryoraptorA303]

5 minutes ago, LetItSnow! said:

I wonder if this will actually be within the realm of possibility in my lifetime...

It probably already is, albeit a very low likelihood event.

The best way for this kind of CET to really materalise, as in, a subtropical CET, you would probably need some climate change-souped up 1976 heatwave to coincide with a calendar month, and then have the rest of the month stay very warm by day but also very humid and exceptionally warm by night. I'm thinking a week in a row of 22-24°C daily minima at coastal locations (not inversions), with the warmest nights at 26-27°C, and this with even further inland staying at 24-25°C. For this you would need some kind of ultrastable height that continues to build humidity without letting any thunderstorms materialise, similar to what Zurich had last August.

A full-on hyperheatwave that is a near-perfectly stitched 1976, 2022, 2006 line would be a very hard ask, but I doubt it's entirely impossible. Throw some just 1-2°C higher uppers into August 1995 and it will get extremely close to this and has a good chance at an idiotically long run above 28°C, possibly lasting the whole calendar month. If this occurs in July instead of August then daily lows by the end of this will be exceptionally high even without absurdly high daytime temps, and declining sunlight/increasing nighttime won't be an issue for the second major round. With this I can see a fairly guarenteed 21-22°C CET.

I was actually just talking about something like this on summer chat, how warm could September potentially get? For my estimation, I'll use the frankenstein of a June 2023-July/August 2022 summer that I created beforehand, to provide us with an extremely dry ground for max temps.

First, we'll go with the 1906 heatwave, but shifted forward by one day as August 31st was also a very hot day and we need as many hot days as we can get. We'll add two degrees to it to account for climate change and the very dry ground, and to help us, so the first four days of September all see highs in excess of 35°C and tropical nights. We then switch to September 2023, which is going to be around 2.5-3°C higher, so the 7th sees >34°C, the 9th sees high 35°C and the 10th sees 36°C at Faversham. The 11th also scratches out a 30°C in this scenario. More tropical nights are added to the roster. From there we switch to September 2003, which saw a long period of high-20s and warm nights. A few of these can probably be upgraded to 30°C. From the 22nd, we then switch to September/October 2011 shifted by a few days, which returns us to the mid-high 20s as remember, the ground is extremely dry and we're automatically reaching high temps. The peak of the heatwave sees three or four more days above 30°C and some very warm nights for the time of year, perhaps exceeding 15°C.

Here is what the daily maxes would look like for the month:

September 20HH:

1st: 35.4°C Pershore
2nd: 37.0°C Kew Gardens
3rd: 37.6°C Cambridge
4th: 36.6°C Faversham
5th: 33.3°C Otterbourne
6th: 34.5°C Kew Gardens
7th: 35.6°C Wisley
8th: 32.4°C Cavendish
9th: 36.2°C Kew Gardens
10th: 36.5°C Faversham
11th: 30.3°C Faversham
12th: 23.7°C Wisley
13th: 26.6°C Chivenor
14th: 29.7°C Kew Gardens
15th: 30.2°C Kew Gardens
16th: 30.9°C Kew Gardens
17th: 30.7°C Kew Gardens
18th: 27.0°C Stradbroke
19th: 26.5°C Kew Gardens, Faversham
20th: 30.4°C Kew Gardens
21st: 31.1°C Faversham
22nd: 25.0°C Cambridge
23rd: 24.9°C Shoeburyness
24th: 26.2°C Cawood
25th: 30.0°C Colwyn Bay
26th: 31.8°C Kew Gardens
27th: 32.2°C Cambridge
28th: 32.4°C Burlands Farm
29th: 32.3°C Coningsby, Santon Downham, Cambridge
30th: 31.3°C Coningsby

Surely this would be one of the ten hottest months ever recorded. The first heatwave would also be one of the longest stints above 30°C on record and would be comparable to July 2006. The very high humidity by the end of the first heatwave could probably also lead to a daily minimum challenging the all-time record outright, let alone September's.

May 1833 is a weird one, while it can be assumed to have been very warm, as it occured before modern meterological study it is impossible to say for sure how warm it actually was and it's likely that there are inaccuracies. If the numbers are close to accuracy then a modern repeat would be positively mindblowing.

[LetItSnow!]

@WYorksWeatherAn open place to discuss any months maximum/minimum potential if desired, going off our quick chat in the model thread.

My reason for my belief is on one of the GFS runs the 19C line was getting into France not especially far away off the continent though that's been cut back on the latest run. It does make me feel it is a possibility in the future though. I mean, if we could see the 10C line bathe the entire country on February 5th, 1869, goes to show anything's possible!

And just for fun, but what would it take to get the 30C line over our shores? An extremely bad scenario no doubt and one we may never see (hpefully). What would that even equate to in terms of a maximum temperature in the UK?

[WYorksWeather]

Probably worth continuing discussions from the model output thread in here - @LetItSnow!, @raz.org.rain.

As a reminder to those who didn't see the earlier discussion, prompted by this chart posted by sebastiaan1973 in the model thread:

In short, we could theoretically see the 17-18C isotherm into the south east corner, and some areas on the continent just to our south possibly touching 20C.

My judgement was that a full 24 hour average of 20C in April, as opposed to it possibly just clipping us, is so unlikely as to be discounted. But I suggested that with a 24 hour average of 17-18C, you could achieve 29-30C even in early April, up to 33-34C by late April.

Other thoughts welcome!

[LetItSnow!]

 WYorksWeather In case my point may have been misunderstood, my 20C line musings are not about this upcoming spell even, just the fact that if we see that that is almost a possibility now, it seriously makes me question the state of our climate and the extremes that are possible soon. I actually find it very concerning to be honest. 

[WYorksWeather]

 LetItSnow! I think again we have to distinguish what we mean by 'over the UK'. If it theoretically just clips the south coast, well then obviously it depends for how long. We know though that we got a 24-hour average of around 24-25C in July 2022.

Theoretically therefore, I see no reason to suspect that if in some ridiculous world we managed to get a 24-hour average of the 30C isotherm, you could have a night that stayed above 30C for at least one station, followed by a daytime temperature of 45C.

Again though, I would say that if we're going to get the 30C isotherm in, it'd probably just clip us, so more realistically if we averaged 27-28C over a 24-hour period, applying the same logic as for April, take a couple of degrees off, maybe a night-time low of about 28-29C, and a high of 43C or so.

 

[LetItSnow!]

 WYorksWeather By over the UK, I mean that I can forsee an opportunity in the coming years to get a southerly with low pressure further west and an exception surge of warmth that gets the 20C up to C England so perhaps the term "over" is quite loose. I mean I don't see a 20C line clearing Lerwick come April or even May, but in an exceptional scenario I could see it climbing to the Midlands sort of area, maybe a bit further north. If the pressure was high it could perhaps last 24 hours. I'd imagine it would occur in the second half of the month.

[WYorksWeather]

 LetItSnow! Oh yeah I didn't think you meant 20C over most of the UK, when I say over I really mean that at least a reasonable area (say county-sized) would record that average for 24 hours, as opposed to the isotherm clipping the far SE coast by 5 miles for 3 hours, say.

 

[CryoraptorA303]

 WYorksWeather What's this, there's a plume coming to the UK next weekend? How likely is this to materialise (not the insane 29-30°C scenario of course, just the general likelihood to see some warm weather)?

[WYorksWeather]

 LetItSnow! In relation to this point (realised I didn't really answer it), I think I might also have been misunderstood. What I'm saying is that the 17-18C isotherm just clipping us is probably equivalent to an average of 15 850hPa for 24 hours, but then if you upgraded it a bit, made it later in the month etc., you could get it up to 17-18C. I just think it's very, very difficult to imagine how we would get an average of 20C.

Of course, this then runs in to the art of what is possible. I don't think in 100 current Aprils you would expect to see an event like that. In 1000 Aprils, maybe. In 10,000 Aprils, almost definitely.

And that is a different question of course to what will be possibly in the future, for that reason I'd say that although the chance is so low as to be discounted in my view at the moment, I wouldn't say the same if you asked me in 2050.

 CryoraptorA303 Actual warm weather? Very unlikely, the pattern doesn't support it at the surface. We're talking about an exceptional 850hPa plume for early April. Tonight's ECM 12z operational has the 17-18C isotherm clipping into the SE.

[CryoraptorA303]

 WYorksWeather Ah, I only just saw the isobars...

This would probably lead to severe thunderstorms over the south. We've had enough rain lately.

[LetItSnow!]

Upon thought, perhaps I am jumping the gun a little, but still - if low pressure wasn't so strong at the moment there is the possibility we could have tapped into it further and got some very warm temperatures out of it, mid/upper 20s perhaps, but as you stated, very unexceptional temperatures at the surface and a one day wonder. Indeed I imagine a lot of daytime temperatures for the first week of April will be unexceptional, just very mild by night.

I have to wonder if the rising Hadley cell is playing a part in this weather pattern at the moment. Global drivers that have been similar to 1998 may be playing out but with high pressure to our south, instead of easily barrelling southward and introducing colder northerlies, the lows are further north and basically slamming against a European high. Not to say the former pattern isn't possible (May 2023 had weak pressure to our south a lot at times), but perhaps it's an example of an old weather pattern spun with a new AGW twist.

Or it could be that April 2024 would have always had this exact weather pattern but that it's amplified to be extra warm aloft and that without AGW it would be 3-10C uppers but instead its 10-18C uppers now.

[CryoraptorA303]

 LetItSnow! Well whatever happens, hopefully the much-anticipated ENSO flip coming in the next couple of months flushes this wet regime out. Anymore excessive wetness will write off this year's harvest for a lot of the country I imagine, the fields are already looking more like swamps round here and it's only just starting to recover with the first ~3 weeks without much in the way of heavy rain since January. Good luck cultivating anything of worth in that if April picks up right where the first half of March left off.

[WYorksWeather]

 CryoraptorA303 I still think with temperatures at the 850hPa level that high, somewhere will record low or even mid 20s if it comes off, and probably some extremely high minima overnight Friday into Saturday before it. The ECM OP is showing 22C, so maybe 23-24C in practice if this verified?

For minima, it's showing around 15C, quite oddly, around 9pm Friday evening, with a warmup overnight.

We still don't have the 12z ensembles yet, so I can't really say how likely it is to verify, or even if it is right at the top of end of possibilities. But for it to be shown by an OP run at day 5, I'd have to say probably no less than a 10% chance?

I'll say more once I've been able to look at the ensemble, which should be out in another half an hour or so.

[WYorksWeather]

 LetItSnow! Just had a quick look actually, and the highest CET mean for early April is no more than 16C. The pattern mentioned above with widespread 20C by day and minima no lower than 13C would probably come very close, depending on the exact timing of the airmass, and whether it overlapped the 9am-9am period well enough to qualify.

If the minima did overlap the CET period, that is how it could be an exceptionally noteworthy event. I just checked, and there is no CET record minimum above 13C for the whole of April. If we did exceed that, and in early April as well, not even the end of the month, it would be truly exceptional.

[Don]

3 hours ago, Methuselah said:

But even I don't remember 1795! 

What, not even in a previous life?! 

[WYorksWeather]

 CryoraptorA303 Final comment on this - a solution at least as extreme as the ECM OP is actually a 15-20% chance going by the count of ensembles. Very strong agreement for the warmest areas to at least exceed 19C raw temperature, about three-quarters of the ECM ensemble. Warmest ensemble chart I could find by trying to match the warmest areas on the GFS map to named towns was for East Dereham. The warmest temperatures look to be roughly on a line from Grantham to Peterborough, and then running east from there to Norwich.

Details may vary, of course - still more time for upgrades, downgrades, and timing shifts.

Overall I think both the minimum and mean CET date records are under threat, plus possibly the minimum CET monthly record even, as it really doesn't rise much by late April.

[CryoraptorA303]

15 minutes ago, WYorksWeather said:

I still think with temperatures at the 850hPa level that high, somewhere will record low or even mid 20s if it comes off, and probably some extremely high minima overnight Friday into Saturday before it. The ECM OP is showing 22C, so maybe 23-24C in practice if this verified?

If I had to guess from the chart I'd say somewhere in the region of 23°C at Cambridge, Santon Downham or Coningsby (seriously, GO AWAY Coningsby or I'll investigate you for doping!). If there is higher sunshine than expected then that could become 24°C but I wouldn't hold my breath. A slight realignment seeing the hottest air over London instead could see the Heathrow-Kew Gardens-St James's Park chain at 24°C with one of them just barely scratching out a 25°C in exceptionally high sunshine. As the record for April 6th is 23.9°C at Santon Downham from 2011, the London realignment scenario is likely daily record territory. The current East Anglia alignment could pull off a 24°C in high sunshine but it'd be a very close record.

However, the minimum for Friday may be more worth talking about.

24 minutes ago, WYorksWeather said:

For minima, it's showing around 15C, quite oddly, around 9pm Friday evening, with a warmup overnight.

15°C is getting to record break territory. The current map is showing around 15°C at Langdon Bay, so assuming a slightly more ideal scenario, perhaps the lowest temps arriving a couple hours earlier when it's not completely dark yet or more cloud cover, the 15.9°C record set during an inversion in the April 2018 hot spell could be under threat. Unfortunately an inversion won't happen this time as the wind will be quite strong (hence the SE coast seeing the highest overnight temps), so inland areas can prepare for a very uncomfortable night, especially if the actual minimum of the day is at around 9pm.

Overall quite strange to see that the April daily minimum record could be under threat by a scenario that you'd more expect to be setting mild records in the colder six months of the year. Then again the May record was also set in this manner, actually very close to this scenario with Folkestone having the record, although I'm fairly certain much higher pressure was involved in that May/June 1947 heatwave. That heatwave interestingly also peaked in daytime temps in northern East Anglia. Could this year be some kind of parallel to 1947? While its remembered for the summer heatwaves, 1947 also saw a very cold and wet February, and if I recall, either March or April (or both) were very wet as well. So was the August 1990 record actually, that saw very high overnight uppers and 23.7°C at Brighton. 2022 went the inversion route, but the uppers were so high that it didn't matter at that point. I don't think August 2003 had any inversions either, but the uppers were either slightly lower overnight or the wind wasn't as strong as that didn't quite reach August 1990's record.

[LetItSnow!]

Chilling things down a bit, I did the inverse of my hottest ever month theory and decided to try and create the coldest possible month ever. Same logic; I took three different Januaries with extreme cold spells (1841, 1881 and 1895) and modified the numbers ever so slightly in order to make sense for the surface conditions (same as I did for residual heat in July XXXX).

The result was a January with a C.E.T. of exactly -5.0!

Edited April 1 by LetItSnow!

[CryoraptorA303]

 LetItSnow! Can someone get onto the big man upstairs about a supereruption so this can happen?

[Geordiesnow]

 WYorksWeather I think the amazing and alarming thing for me is the plume we are seeing is the second one quickly after the first one that has hit Germany eastwards smashing temperature records and is going to absolutely rapidly melt any snow cover well to out east which of course in turn will lead to more warming.

The amazing thing for me usually if you see a pool of warm/hot air moving northwards, it dilutes to the south but this is not the case because Africa/tropics have been at record breaking in terms of warmth.

It's going to be an interesting summer in the northern hemisphere I feel, the affects of El Nino and climate change will surely make their presence known and it's all ready starting and we barely started April nevermind summer! 

 

[baddie]

 CryoraptorA303 On the 6th/7th??? Warm-ish, Dry-ish, Bright-ish, but extremely windy