I think people realize by now that I create these threads when there is potential of highly abnormal even record breaking weather but that it doesn't necessarily mean that records being broken is likely. 

Anyway looking at the charts I am highly intrigued by the upcoming cold spell. My snow obsession has lead me to pour over the charts looking for unusually cold summer charts, and in comparison with the likes of July 1888 (the best, although probably still untrue contender for July/August snowfall) it does compare favourably. To emphasise I do not expect snowfall at lower levels, though we should certainty expect hill snow above 700m in the Grampion region (possibly even lying summit snow) which is an achievement in itself at this time of year. 

Wednesday and Wednesday night are the best contenders for the records of Lowest daytime maximun and lowest night-time minimun for July respectively.

We need 7.5C to beat the lowest daytime minimun (GFS predicts basically 7.5C in sheltered Scotland)

We need -2.5C to beat the nightime minimun (GFS goes for about 0C in sheltered spots, but I suspect the minimum could be a little lower than that) 

Also what is the record low 850hpa temp? It could feasibly get down to -4C which I'm sure has never happened before (1888 was nowhere near) 

Good thread, Q.

Perhaps we should have a reality TV show on the Forum - Weatherswap. Richard gets to live in Jiries' shed for a week, and George heads to the East coast of Scotland. 

Are mountain sites allowed for minima? 

Cairngorm forecast!

Very interesting. The maximum temperature record certainly looks under threat. The high res temperature charts for both GFS and NMM show temperatures in parts of the Highlands not doing any better than 7C on Wednesday.

Mountain sites such as Cairngorm are not allowed. Most sites over about 200m are excluded. Aviemore at 220m is allowed.

I thought mountain sites were only excluded for lowest maxima? In normal circumstances a mountain would not usually record the lowest temperatures. 

Where are you on 850s GW?

850s of -3C, maybe even -4C are something I have never come across in July/August before.

In fact August snow actually seems more likely than July snow as 850s in the arctic are much lower (-10C in August at the NP is possible).

So to see -6C in Iceland in the 2nd week of July is remarkable. 

My understanding is that the mountain sites are excluded from all temperature records in the UK but I might be wrong. The mountain sites are actually defined by the Met Office as those above 500m asl. They are highlighted at this link - there are only 6 of them. http://www.metoffice.gov.uk/public/weather/climate/

I could just image.. Richard would be complaining there wasn't enough sun getting through to the shed, and Jires comparing it to a winter in Chicago 😄

RE mountain top:

The spine of England, parts of Wales, and large parts of Scotland have officially recognised Sub Arctic climates due to the elevation. It's no wonder elevated sites are excluded.

Not sure on the 850's. Just looking at the days with record low maxima and minima in July the coldest 850 I can see in the archive is 9 July 1986. But I'm sure there must be others below 0C in the more distant past.

http://www.wetterzentrale.de/pics/archive/ra/1986/Rrea00219860709.gif

Here are a few more cold 850 charts from July

21 July 1884 around -2C in Shetland http://www.wetterzentrale.de/pics/archive/ra/1884/Rrea00218840721.gif

25 July 1884 the -5C line makes it as far south as the Faroes http://www.wetterzentrale.de/pics/archive/ra/1884/Rrea00218840725.gif

26 July 1884 the 0C line plunges well down into England http://www.wetterzentrale.de/pics/archive/ra/1884/Rrea00218840726.gif

But surely this is a pointless search because of the nature of 850's?

-5C upper in July over Scotland in:

1) Sunshine would easily see temps lifted to low teens.

2) Overcast conditions with rain would see them struggle.

3) Overnight could possibly see a frost, however days are extra long still in July in Scotland.

Agreed. But I think Q is just trying to understand what the record low 850 temperature is in July. A low 850 temperature does not necessarily mean low surface temperatures.

Interestingly as I was pulling up the data for 1884 I see that London got close to recording a ground frost. It was reported that the temperature fell to 32F on the grass on the morning of 24 July 1884. If it got that low in London there must have been a widespread grass frost elsewhere.

Well of course the surface temperature would vary considerabely although note the 10C (moist)/15C (dry) rule which I mentioned for the record heat last week also works in this context:

-5C upper would correspond to 5C in moist air and 10C in dry air. Realistically it will be a few degrees higher than that at the 850hpa level so its pretty accurate. 0C with constant rain would correspond to a surface temp of about 7C and be enough to break the record.

Also the snow benchmark shouldn't change that much. -6C will still be a good benchmark for snow at sea level even in July, so if we get -3 or even -4C that's telling us there is abnormally cold air over us for the time of year.

That 1884 stuff is pretty amazing. Are there any snow reports?

Subzero maybe. But -3C is much much more impressive than -1C.

No snow reports. There was a lot of thundery activity  - some of it quite severe during that month so I expect there was hail.

The cold weather arose after a low pressure system developed off the east coast of Scotland and moved north into Norway dragging in a cold northerly flow behind it. Clear skies overnight caused the low temperatures.

Btw I'm looking at the metoffice report for 1888 (the month where I said snow allegedly fell) and I quote:

"The most remarkable fact in connection with precipitation was that early on the 11th snow showers occured in many parts of the country, even as far south as the isle of Wight"

I can see why many people say that this was probably a miss report for soft hail.

I see no snow

(thats for the Cairngorms, btw)

I remember reading about the 1888 event in Stirling's old "Weather of Britain" book, where there were reports that northern fell country was lightly dusted down to about 1000 feet or so. Temperatures in the south varied between about 6 and 12 degrees so perhaps soft hail was more likely there.

IIRC Hubert Lamb's updated Climate History and the Modern World noted this summer as one with unusually widespread sea ice and areas of low SST in the Iceland/Faeroes area (said to be comparable with the late 1600's) which would have lessened any modification of the air mass and made snowfall more likely.