June 13th - A bit of a heat plume going on...

Our current weather continues to defy accurate forecasting beyond 5 days. Looking at the GFS and ECMWF outputs since last week we have gone from a cool and wet trough event to a heat plume and then something in the middle. Pretty much out of nowhere this week we will though pick up a transitory heat plume that'll push temperatures into the high twenties and most likely low thirties the further south you go in the U.K.

Some of that heat will make it to the south and south east coast of Ireland as well. As you can see from the animated GIF above, it won't affect all of the U.K & Ireland and looks currently to reach as far north as The Humber / North Wales. Further north, we will see temperatures 10-12°C lower, with a cooler and more unsettled theme, courtesy of a northerly Atlantic low pressure. Just off Portugal you can see a BOB (Bay of Biscay) low pressure and it's this weather feature that seems to be the hardest to predict in terms of behaviour / tracking / timing. I still expect it to impact our weather early next week in some shape or form.

What goes around comes around....

Last week I mentioned that I worked in agriculture as a seed contractor / crop inspector back in the late 80's. At the time, one of the new trends / fads was to grow a mix of varieties in the same field in order to reduce the risk of disease.

Last week I attended Cereals '22, an outdoor agricultural show, with Prodata. Walking the NIAB plots, I smiled to see one of the latest 'innovations'. The image above shows a stand of winter wheat consisting of 4 different cultivars. The strategy is to grow a mixture of varieties to decrease disease susceptibility, now that sounds familiar !

It struck me how linked amenity and agriculture are in some cases because our job is to introduce new cultivars into our existing grass sward in order to create diversity and introduce resilience in terms of disease management. The image above is the same.

In either scenario, monoswards are not the ideal and doomed to failure. You don't see naturally-occurring monoswards across crops and grassland. To create and maintain one is to act against the natural forces of selection and requires more inputs, be that pesticides, nutrition and the like.

I saw this with my own eyes back in the late 80's when there was a Winter Wheat variety called Slejpner. It was high yielding and a farmers favourite. One year it occupied a huge acreage across the U.K and overnight became susceptible to a specific Yellow Rust variant. The rust took just 3 weeks to move from the east of England to Scotland because so many fields were of the same variety, it was easy for it to move from one field to the next. The variety crashed in popularity. A lesson learnt.

Onto the weather...

General Weather Situation

So I'm going to split this week into two parts forecast-wise, Monday to Wednesday and Thursday onwards.

The first part of the week is pretty similar day to today with warm temperatures in the high teens on Monday and building towards the mid-twenties by the time we reach mid-week. The only rain on the weather map is for the north and north west of Scotland which will see showers pushing in each day, with Wednesday perhaps featuring the most significant rainfall. We begin to lose that strong wind in the first part of this week but it'll swing round from south to north and back again. So warm and settled for the 1st part of the week with pleasant temperatures for the bulk of the U.K & Ireland, though always highest across England and Wales.

As we approach Thursday, that heat really starts to build and push temperatures upwards from the mid-twenties to the high twenties and beyond for the south of England. I think currently the hottest day will be Friday with temperatures dropping away thereafter as the heat plume subsides. It wouldn't surprise me if we see a thundery breakdown overnight Friday into Saturday as that southerly BOB feeds moisture into the weather pattern across the south of England. This will be accompanied by some torrential downpours for sure.

Part of the reason for the temperature drop on Saturday is a change in wind direction from southerly on Friday to northerly / north easterly on Saturday so expect to see 10°C lower temperatures for the weekend. Ireland looks to pick up some of this pleasant weather but probably peaking at 22-23°C on Friday before a showery and thundery breakdown. Saturday could see significant rainfall later in the day for northern England, The Midlands and then central and southern England later on Saturday night. By Sunday this rain passes through and we are left with a much cooler feel to the weather courtesy of a biting north easterly wind. Scotland looks to stay sort of high teens weather with some rainfall as mentioned earlier for the north and west and this will push into central areas later in the week with Friday looking potentially wet for the west and south west. Sunday will feel chilly.

So you can call this week's weather event a mini-heatwave because it won't last !

Weather Outlook

Now the jury is still out in my view for next week because of the projected behaviour of two low pressure systems. Those of you travelling to Glastonbury may want to pack some wellies and wet weather / mud gear just in case. That said, the current projection is for the low pressure to subside and dissipate as we go through the early part of next week and affect France rather than the U.K. Now I've seen this before and all it takes is one flip northwards and the south of the U.K ends up cool and wet so the jury's out for me.

Once again the GFS and ECMWF projections are at odds for next week with both calling a settled start to the week but the latter projects a north Atlantic low pressure to sink south through the week and bring cooler and unsettled weather to all areas by mid-week. The GFS is more for the typical north-south divide with cooler and unsettled weather for Scotland and the north of England, but staying drier and warmer further south. You can see below how markedly different the output is for Wed 22nd June between the two models ;

I am on my hols next week so I kind of hope that the GFS output turns out to be correct ! :)

Either way as approach the end of next week, both models have that northerly low pressure pulling down cooler and unsettled weather across the U.K & Ireland, the difference is in the timescale.

Agronomic Notes

The arrival of some serious heat this week, albeit in a short-lived form, marks our card that summer is knocking on the door although I always maintain that June is a 'change month' rather than the real thing. The heat this week is hot enough to provide plant stress and comes on the back of some windy and warm (ish) days of late.

High temperature and high wind is a lethal combination from a grass plant's perspective and one that is maximised if the relative humidity is low. When the humidity is high (>90%), there's plenty of moisture in the atmosphere so moisture loss from the soil and grass leaf surface is low as it is difficult for water to evaporate into an atmosphere that's close to being saturated. Think of being in a hot, humid climate, your clothes quickly become saturated with sweat because it can't evaporate. Of course the converse is true and when relative humidity is low (<60%), moisture is readily lost from the soil and grass plant.

The graph below shows data over a 24-hour period on the 12th of June, 2022 from a Davis Vantage Pro Groweather station at The Oxfordshire ;

You can see how the E.T begins to build as relative humidity begins to fall, rising to a peak at 14:00 pm when 0.58 mm was lost over an hour period as the humidity hovered around 60% . That might not sound a lot but it is the equivalent of 5,800 litres across a hectare, in an hour.

Think about how useful this data is. For instance if you were hand watering, the most useful time to do this would be from 13:00 pm in order to counter-act the period of highest E.T.

The last 4-5 days around here have been characterised by low relative humidity during the day, high winds and reasonably high temperatures and so E.T levels have peaked at > 5.0 mm per day on some sites, that's quite high and easily high enough to put the grass plant under stress.

Here's some cumulative data showing E.T loss this month at different sites across the U.K ;

Location E.T loss in June to date E.T loss in last 5 days

Dumbarton 34.82 mm 8.25 mm

Sevenoaks 35.71 mm 19.31 mm

Guildford 37.69 mm 18.43 mm

Thame 42.39 mm 20.53 mm

Milton Keynes 40.41 mm 20.30 mm

It is interesting to me that two sites at different ends of the U.K (Dumbarton and Sevenoaks), separated by some 400 miles as the crow flies have near identical moisture loss by E.T for the month but vary significantly over the last 5 days. That's because Scotland had a lovely start to the month with high E.T and then this has dropped back. Further south it has been the other way round.

Agronomic Strategy

Dealing with plant stress is more effective as a preventative strategy than it is as a reactive one.

Preventatively you need to have your grass plant regulated before the stress arrives so the growth rate and therefore water requirement by the plant is reduced. Limiting free radicle accumulation in the plant tissue by using proven biostimulants is also a great strategy. Having a free-draining but non-hydrophobic rootzone is also key because moisture retention is the last thing you want as temperatures rise. Water holds a fraction of the oxygen that air does and warmer water even less, so you don't want your grass plant roots sitting saturated in warm water because you risk hypoxia (oxygen deficiency). That's why sometimes aeration prior to stress is so effective at combatting it because the plant can access good levels of oxygen in the rootzone surrounding the root hairs. I remember once seeing two greens that had been vertidrained prior to stress stay Anthracnose free compared to other greens that had received no aeration prior to the stress.

This stress peak is pretty short-lived, maybe 2-3 days down south but remember when you look at the stats above, the grass plant will have been up against it from a moisture-loss perspective for a week now before this temperature peak arrives and that is a different ball game. (pardon the pun)

Enjoy the heat, I'll be having a week off from the blog but will be back fresh as a daisy in a couple of weeks time 😎👍

All the best.

Mark Hunt