July 19th


Hi All,

Below is the GFS schematic from last weekend that gave us and Eastern Europe a mini-heatwave as a peak in the jet stream allowed hot air to push up as predicted.

Now, there’s always two sides of the weather coin as we know and for Western Europe, they were caught in a jet stream trough, under a near static low pressure which was trapped between the two peaks as you can see above. So The Netherlands, Belgium and Germany received record rainfall with 6 inches (150 cm) falling over 24 hours. In some areas the rain rate was higher with 207 mm (8.28 inches) recorded in just 9 hours. So far 120 people have lost their lives and the scale of the damage to the affected countries infrastructure will run into billions of euro. Truly a weather catastrophe and I wish my European followers of this blog all the best if you and / or your families have been affected.

The propensity of the jet stream to form into peaks and troughs first caught my attention back in the winter of 2010 when we went from an extremely muggy November to a freezing December in the blink of an eye. It has fascinated me ever since. One of my ambitions is to study and understand the weather more in depth than my current level of knowledge but rather frustratingly when we talk the jet stream we know we have limited data as it was only discovered less than a 100 years ago.

A German meteorologist, Heinrich Seilkopf (1895-1968) is credited with coining the term ‘jet stream’ when he used a special German word, strahlstromung, literally meaning “jet streaming,” in a 1939 research paper that described high-speed winds in the atmosphere. Perhaps the original observer of the behaviour of a jet stream (not ours of course) was Wasaburo Ooishi, a Japanese meteorologist, who discovered high speed funnels of air in the 1920’s. His findings remained unshared because of World War II, but when the Americans first launched high altitude raids on Japan, they discovered ferocious winds caused them to arrive at their targets earlier than expected or to use more fuel as they flew into strong headwinds. I knew about this but what I didn’t know is that the Japanese also used the same jet stream to launch balloons loaded with bombs towards America, some 400 or so reached their target, much to the incredulity of the Yanks. All because of their knowledge of the jet stream.

Back down to earth so to speak,  we can be sure that the U.K & Ireland will experience more weather extremes because we lie on the path of the sub-polar jet stream and it is these ‘extremes’ that will continue to challenge our industry with an ever present requirement to invest in (at first sight) a contradictory requirement for both irrigation and drainage !

Scientists assert that these extremes and by association, the changed nature of the jet stream, are a feature of climate change as the poles warm and less temperature differential exists between the poles and the equator. Less temperature = less energy and if you’ve ever stood on a bridge and watched the current in a river, where the current slows (because of less energy), you’ll notice it forms meanders and it is these meanders that mimic the behaviour of the jet stream. Peak or trough, take your pick, both can be equally devastating.

For now we have to live with a warm / hot summer peak….but what comes after that ?

More of the same or will we be picking up a trough ?

General Weather Situation

So a pretty straight-forward week coming up with that high temperature, high pressure dominating for the first 3 days of the week when we will see our hottest temperatures despite the ever-present north wind. So Monday through to Wednesday we will see long periods of unbroken sunshine, temperatures in the mid-twenties for Ireland and high twenties / low thirties for Scotland, Wales and England. The change comes later in the week……On Thursday we will start to see a change as a ‘BOB’ low pressure comes into play (as forecast a week ago and shown above). The first indication that things will be on the move will be when the wind turns eastwards on Thursday and we drop 3-4°C of temperature, followed by a similar drop on Friday and accompanied by some more cloud cover. Cool beans because I’m off on Friday chasing Mullet on the south coast :). As we head into Friday evening we see the first rain from this ‘BOB’ low pressure push into The South West and then slowly move north and east.

Now as we have seen before with this type of weather system, it tends to be slow-moving, affect the southern half of the U.K and be somewhat unpredictable. It will also introduce moisture into a high temperature cell which means the potential for thunderstorms will increase as we go through the weekend. Currently we are looking at a slow-moving band of rain arriving late on Friday night and moving slowly up country on Saturday before fizzling out. Ireland and Scotland look to miss this pretty much entirely although there is a chance of a shower on Saturday for the east of Ireland. Overnight into Sunday we see this low pressure pull in more rain from the east and some of that rain will be heavy. That band of rain will affect the southern half of the U.K on Sunday tracking slowly westwards towards Wales. Some of that rainfall could be very heavy and localised. Temperature-wise, a much nicer low twenties beckons once we pick up that easterly wind.

Weather Outlook

So as you can see from the GFS output above, we start next week in a trough with a jet stream that’s been pushed south somewhat. Does the trough remain or does a new heat plume displace it ?

Well the indications are that next week we will settle into a spell of pleasant weather after we see the BOB-low exit stage right on Monday. So this may mean a wet start to the week for the east of the country before drying up on Monday p.m. and through Tuesday. I expect temperatures in the low twenties, a pleasant cooling wind and nice conditions really. As we go through next week, a new Atlantic low pressure comes into play which will bring showers and cooler weather to Ireland on Tuesday night / Wednesday and then move eastwards into the west of the U.K during Wednesday. I’d expect the rest of next week to traverse into a ‘sunshine and showers’ type affair with a north westerly wind, cooler high teen temperatures and rainfall for all parts of the U.K, but particularly the north and north west. Now I know we have some sun worshipers out there but when I walked yesterday in the rising heat, everywhere was deserted. Fine if you like just sitting there doing nothing but if you’re an active type (as I like to think I am) then this heat is a P.I.T.A 🙂

Agronomic Notes

So with a hot temperature turf status currently affecting us, there’s plenty to discuss….

Anthracnose and general plant stress

As you may recall from my blog on June 14th, I predicted that it would act as an Anthracnose trigger, with the heat of early June being followed by rainfall from mid-June onwards. Well if the Anthracnose fungal spores did indeed germinate and enter the plant, this week’s weather may move them from being in a biostatic state to a necrotrophic (turf damaging) one.

Failing that and continuing the good news, this week’s high temperatures followed by rainfall this coming weekend and next week will provide a 2nd Anthracnose trigger for 2021.

Now I know we look at air temperature as the trigger for Anthracnose, but to me it’s quite amazing how the soil temperature (particularly in the surface) behaves during weather like this.

Here’s a trace of air temperature vs. soil temperature at 25 mm depth from a high sand content rootzone and a clay-based one at our Great Dunmow turfgrass research site.

Top graph – High sand content rootzone / Bottom graph – High soil content rootzone

So the two graphs show the gradual decline in moisture status over the last 7 days, despite irrigating the high sand content rootzone at 60% of E.T, taken from the Davis Vantage Pro weather station.

The top one from a high sand content rootzone, the lower one from a high clay content rootzone, look in particular at the temperature fluctuations on a daily basis ;

High sand content rootzone – 17.5 °C @ 04:25 am – 28.5°C @ 13:18 pm

High clay content rootzone – 19.5 °C @ 04:42 am – 25.0°C @ 15:00 pm

So the high sand content rootzone cools down more at night and heats up more in the day whereas the high clay content rootzone retains more heat at night but takes longer to heat up during the day and reaches a lower maximum temperature than the high sand content rootzone.

This is of course due to the water-holding capacity of the respective rootzones, with the high sand content dropping to nearly 10% vmc, vs. the high clay content that is currently sitting around 40 % vmc.

The higher the water content, the lower the air content and as we know gas molecules move faster than liquid molecules so the effect of temperature is more marked (either way) on a rootzone that has a higher air content vs. a lower air content.

Now you’re probably saying, geez that guy is running his moisture down to 10% in the top 25 mm, that’s a lot lower than I am currently…

Well firstly soil moisture meters measure moisture at a deeper depth so we aren’t exactly comparing ‘Apples with Apples’, but also this is a new construction, it has a brilliant root system and of course I have no soil-compacting people trampling all over it (which is actually a blessing and a curse because it means it will build up organic matter quicker). So far, so good though and currently I’m replacing E.T at around 60% because I’m not brave enough to go lower.  What the above does tell us though is that a high sand content green will easily exceed the trigger point for Anthracnose in the top 25 mm which is where the majority of organic matter sits and coincidentally that’s where the spores are as well. A heavier soil, push up green will be less likely to hit that temperature and by virtue of the buffering capacity of the higher soil content rootzone, it should be subject to less high temperature stress. I say ‘should’ because that’s entirely dependent on the level of O.M present, which is likely to be higher on a high soil content rootzone than a high sand content one. (Remembering that organic matter heats up faster than rootzone)

So we definitely have a trigger going on from Anthracnose this week from both a high temperature perspective in terms of spore germination and a stress signal for the previous one in early June. You can see how high temperature stress is decreasing the daily Growth Potential with the most marked decrease for Central England (and by virtue of association, Wales this week), with lower decreases for Scotland and Ireland.

Here’s the Meteoturf data for 3 locations….

So be on your guard, mitigate stress by alternating cutting and rolling if practically feasible and rather than inputting just nutrients, supplement either a foliar or surfactant application with a biostimulant. Once we reach the weekend, the turf can breath a sigh of relief as can all you irrigation technicians and things will settle down a little.

Another point I’d make before wrapping this blog up in time for the 1 p.m. publishing deadline is the number of GDD we are piling on at the moment. If you look at the highest total (which is for Central England although the other two locations aren’t far behind) at 115 total GDD for the last 7 days (at 6°C base), that works out at every 8 -9 days for a Trinexapac-ethyl (TE) application (every 130 GDD I tend to work on using a 6°C base). So there’s a high potential for ‘rebound’ growth this week if like most you’re applying on a fortnightly basis. Remember frequency of application is much more important than rate when it comes to TE. It’ll also be interesting from a technical perspective to see how the Prohexadione calcium-based PGR’s work in this weather as both cool temperature and high temperature performance is reckoned to be a distinguishing factor between these and TE. You can read Bill Kreuser’s excellent paper on this very subject here

Right, that’s about all I can type this week in my attic office without expiring from my own form of heat stress, time to drop down the house and hydrate.

That’s another thing to remember for this week, plenty of fluids and sun block, a tan for the vain is another persons melanoma and there’s nothing clever about I can tell you….

All the best…

Mark Hunt