James Peacock

UK rainfall anomalies in the opening week of August 2025 are big on gradient.

From over three times the usual in the far north, to less than one twentieth of it in the central-far south.

www.roostweather.com/ukobs/rain_m...

Fruity

While on a short stroll just now, an acorn fell from a tree right onto my foot, which inadvertently booted it into the distance.

I then glanced up to briefly meet eyes with a rather dejected looking squirrel 😂

From time to time the GFS forecast model produces seriously wild scenarios, such as today's 12z which would take the August Hadley mean CET to-date from 115th warmest as of yesterday to 3rd warmest as of 23rd.

I'm not saying such a hot scenario isn't possible, but...!

As for which scenario will be correct, I'm starting to wonder whether this will be one of the rare occasions where GFS is onto something.

Reason being, while GFS has been near-steady, both the ECMWF & UKMO det. runs moved ex-Dexter more slowly than their prior respective runs.

...with the UK & Ireland hot in the south, warm in the north, & gradually more prone to scattered showers & thunderstorms as the week progresses.

Of note is that both versions of events aren't as dry-looking as we were seeing in the previous few days' updates.

Sit rep: The standoff continues regarding the behaviour of ex-Dexter early next week.

ECMWF & UKMO's det. runs have it approaching the UK Mon-Tue, leading to hot then briefly wet & in places thundery weather.

GFS' has it drifting around east of the Azores instead...

The CFSv2 model hasn't been having a great time with these events. For early August, the earliest 8 runs in this plot favoured neutral to cool conditions in the Nino 1+2 region, which is where it's currently most anomalously warm.

A theme of 2025 has been intermittent 'El Niño Costero' events, in which the waters just west of northern South America become anomalously warm, while those further west are near or below average.

Currently, the further-west waters are in a mixed state with pronounced eddies.

For a change, the western Mediterranean Sea's surface temperatures are currently close to the 1991-2020 average.

However, a heatwave is about to spread across the region, so those SSTs will be on the rise again.

Meanwhile, SSTs are extraordinarily high to the NW & N of Europe.

GFS is being stubborn with its slower-moving version of ex-Dexter. Its latest run still has that system hanging around just east of the Azores as of next Tuesday, while hot weather gradually spreads northward across the UK & Ireland.

If UKMO & ECMWF don't budge this will be quite a standoff.

The Hadley Central England Temperature mean for 1st-5th August is a 1°C below the 1991-2020 average.

Yet it's 0.4°C above the 1961-1990 average. So what's cool by recent standards was 'typical' back then.

Forecasts suggest the CET begins climbing on 8th, perhaps considerably.

If 2025 does have a dry August, it will be very interesting what happens in the autumn.

The drought of 1976 broke in spectacular fashion right at the end of August, in fact 5 of the following 6 months saw over 100 mm in my local area. That autumn stands 2nd wettest on record.

Mar-Aug 1976: 143.3 mm
Mar-Aug 2025 so far: 92.9 mm

So for a driest Mar-Aug, 2025 will need to see less than 50.4 mm, which is just below the 1991-2020 average.

Feb-Aug 1976: 171.7 mm
Feb-Aug 2025 so far: 149.1 mm

2025 needs a pretty dry August for that one, under 22.6 mm.

Local to me, 2025 continues to be the driest on record since the start of spring (16% less than 1976!), the 2nd driest since the start of February (8% more than 1976), but outside the driest 5 since the start of January.

The outlook? Little rain likely for at least 10 days.

The 00z ECMWF det. was a very dry run for much of southern England, while the 00z GFS det. was so more widely, in England & parts of Wales.

This is in keeping with recent ensemble guidance. If you're in one of those areas & saw much rain in the past fortnight, that might prove very valuable.

As for that wetter interlude in the UKMO & ECMWF runs, it barely affects south(east)ern England.

They keep the air too stable there for showers & thunderstorms to get going - but whether that's accurate, we can't know for sure. There's a lot to be sorted out regarding TS Dexter.

So UKMO & ECMWF have more of a heat build next Sat-Mon than GFS, but then a brief wetter interlude that GFS doesn't have.

Despite this discrepancy, all three runs have high pressure across the UK & Ireland by next Thursday (UKMO & GFS by Wednesday), with warm or hot weather.

Ex-tropical cyclones often lead to forecast model shenanigans & ex-Dexter is no exception.

Of the latest det. runs, UKMO's & ECMWF's bring it toward the UK early next week as a decaying feature but disagree on how quickly, while GFS' traps it near the Azores instead.

Now how's this for a plot twist: When looking at purely the summer through end of July, it's the mean daily min that's well above the prior record highest, while the mean daily max is 3rd warmest - hanging out with the legendary summers of 2006, 1976, & 2018.

Another view of it which also shows that the mean daily minimum has been relatively unremarkable, but still among the warmest dozen years.

Note that the apparent positive correlation is actually a result of the long-term warming trend in both daily max & daily min.

In a dataset spanning over 80 years, breaking a prior record for a 5-month period by a margin of 1.14°C is absolutely astonishing.

You can see how distanced 2025 is from the rest.

I wonder how many appreciate just how far outside historical precedent the daytime warmth has been, on average, since the start of spring in southern England.

E.g. local to me it's been 2.83°C above the 1991-2020 average when the prior highest was 'only' 1.69°C in 2018.

...as well as the first one without a single sub-21°C max.

July 2025's daytime warmth here was extraordinary to a degree that the mean daily maximum can't fully convey, although it was a whopping 2.9°C above the 1991-2020 average & the 6th highest on record.

July 2025 really stood out here for having not one day with a max temp lower than 21.7°C.

Closest historical Julys are 1976 (3 days in 20-21°C range, lowest max 19.5°C) & 2022 (5 days in 20-21°C range, lowest max 19.6°C).

So 2025's is the 1st July without a single sub-20°C max.

The GFS ensemble strongly persists this dry signal all the way out to 16 days time.

It feels a bit strange talking of continued (instead of returning) rainfall shortages in Wales & SW England when the UK & Ireland has just been through about half a month of changeable weather.

Meanwhile, north-western Europe will see a return or continuation of anomalously dry weather, with only occasional, mostly small amounts even as far north as southern Scotland.

For most of Wales & SW England this follows a drier than usual July; drought persisting or returning.

August started out hot in the southwest of Europe but relatively cool in the northwest, but change is now underway.

Hot weather will spread north-eastward during the next week or so, as a substantial heatwave unfolds.

It's unclear whether it will extend into southern UK.

Delivering to Verwood about 15 seconds of 8-12 mm per hour rain rates, this squall would have been very forgettable were it not for a brilliantly clean clearance.

With the designation of tropical storm Dexter by the NHC at 03:00 UTC today, 2025's Atlantic hurricane season reached 4 named storms just 6 hours later than 2024's did.

Last year then took 9 more days to reach 5, but 2025 might do so within 6.