July 7th

Hi All,

Well summer rainfall as a weather forecast parameter showed over the past weekend why its accurate forecasting still evades the most-advanced of systems, A.I or no A.I.

Watching the Silverstone Formula 1, Christian Horner from Red Bull bemoaned the fact that they had set their car up for a dry race day. He said "No forecast we looked at had anything more than a 20% chance of rain and certainly nothing as heavy as what occurred later".....With the multi-million pound industry that is F1, even their rain radar wasn't accurate. I could see from Netweather.tv's rain radar that the second rainfall event was pretty fruity..."Light rain in 10 mins" came over the driver radio but it was far from light rain at some parts of the circuit.

Here in Market Harborough, we were forecast a total of 2mm over the weekend and instead on Sunday morning I woke up to 15.2mm overnight rain and then last night we had another 12.5mm at this end of the town. Entirely un-forecast but since this represented our first meaningful rain since the 3rd week of April, entirely welcome. Interestingly there was a 6mm difference between a weather station at the north end of town vs. a weather station at the southern end. (a distance of maybe 2.5 miles)

My point is weather forecasting (even with A.I) is not still an exact science, far from it and as our climate changes, we are scrabbling to catch up. Think about it, A.I works by being 'trained' on eventualities / patterns / past data and then works to recognise and predict. What if what we are seeing weather wise is new, is changing, hasn't been seen before, where's the improved efficacy now ?

GFS forecast for the next 7 days - Tropicaltidbits.com

As I look forward to this week's forecast, the talk is not of rain but of heat as another high pressure system builds into the so-called 'heat dome' shape pulling up hot air from Africa and sending forecasted temperatures into the early thirties. I type this because that's what the forecasting is saying, right ? Well not really.....both the GFS and ECMWF forecasts for this coming weekend show a dissipating heat event not a growing heat event. Now that may change but as I sit here and type this, that's how it looks with heat peaking before the weekend.

Look at the animated GIF above.

You can see how the heat builds through the week but watch across to the east, the low pressure in the north 'seeds' a cut-off low pressure system (COL) that then moves westwards (against the normal flow of weather systems) and cuts off the heat from the south.

These cut-off low pressure systems are I believe one of the reasons for forecasting inaccuracy. They are a relatively new meteorological feature and as such their behaviour is unpredictable to a certain extent. Cut-off low pressure systems (COL's I call them) were responsible for last summer's flooding in Central Europe and Spain that caused numerous fatalities. The same cut-off low pressure system that caused these events then swung west, into the Bay of Biscay and then up into the south of the U.K and gave us our wettest September on record and indeed our wettest month of the year in 2024.

So, I think this week's forecast will be determined by the behaviour of the cut-off low, yes for sure we will see heat build from mid-week, but the weekend, that hangs in the balance, we will either get a heatwave or that heat will back off dependent on the behaviour of that COL. I'd also be watching to see where it moves because currently it is tracking on the forecasts towards Denmark and that may mean more rainfall coupled with high temperatures and humidity, a calling card for Dollar Spot.

Weather Outlook

So with all the uncertainty that lies behind this week's forecast (well from the weekend onwards), you can imagine my reticence at providing one for next week !

GFS forecast for the 7-14 days - Tropicaltidbits.com

So let us see what the GFS says.....Well according to the GFS next week we see an Atlantic low pressure system try to push into our weather picture bringing unsettled weather into Ireland and the west of the U.K for the early part of next week. Then we see a ridge of high pressure rebuff that low pressure from mid-week before that low pressure pushes across again and into Denmark at the end of the week. At the same time we have a double low pressure system building west of the U.K at the end of next week / weekend. We shall see and it is certainly one to keep an eye on because any weather projection with a COL in it = unpredictable.

Smith Kerns Dollar Spot trends this summer

Agronomic Notes

So we carry on our bruising summer with another heat build up this week and possibly the introduction of humidity as well. This will push up the Smith Kerns again after it dropped back last week with the arrival of cooler weather (see graph above). There is still plenty of variability between the different geographical locations with the SK range between 17.5 (York) to 28.5 (Limerick), still above the threshold level but considerably lower than early July. With a humid and hot airstream projected to push up this week, I'd expect to see the Smith Kerns increase up to early July levels so one to keep an eye on.

Davis Leaf Wetness Sensor

One point I would raise though is that in conjunction with the formulae for Smith Kerns indicating high pressure, our findings using overnight leaf wetness data show more long duration, leaf wetness events this summer so far than last. The overnight average temperature for June 2025 was also significantly higher than the year before as well. The Davis leaf wetness reports leaf wetness severity on a score from 0-15 where 0 is dry and 15 is fully-wetted.

Here's the data from our default Thame, Oxford location ;

You can see clearly how June 2025 was different with a much warmer average overnight temperature, a longer duration of leaf wetness and a higher score. Not off the scale yet because typically we don't see humidity and leaf wetness duration increase markedly until the nights begin to draw out at the end of August and that's when Dollar Spot gets its skates on.

Image courtesy of USGA Green Section Record

USGA Green Section Record

The latest addition of the USGA Green Section Record dropped into my Inbox this week and it has some really great articles, not least an insight into Wimbledon courtesy of Neil Stubley, head of courts & horticulture at the AELTC venue. In addition, there's a great article on fungicides, mode of action and retention in the rootzone (though a lot of the A.I's we don't have access to over here of course), Autonomous mowers and it highlights that the U.S is going through its own heat dome issues. No surprise there as we share the common driver, the jet stream. You can have a read this excellent publication here

A chance to breath....

Whilst we have some cooler weather around (early this week and maybe next) and before the heat begins to rise, getting out and doing some non-disruptive, narrow tine aeration can really prove its worth. I am talking about solid tining and compact vertidraining to provide some gaseous exchange and give the grass plant's roots a chance to breath. Obviously entirely dependent on the state of your grass plant from a health / stress perspective if its appropriate or not. I have seen greens that had this type of aeration show little or no subsequent disease (Anthracnose in this case) vs. greens that didn't have it, so I am convinced of the benefits if it is possible practically.

OK, that's me for another week, let's see how that heat builds and how that cut off low pressure system behaves. I think it'll be interesting dynamic...

All the best.

Mark Hunt