2nd October, 2023
A mini-blog this week because I am over in North Norfolk watching the biggest tides of the year roll in across the marshes urged on by the last Super Moon of the year. Tides over 5m in Norfolk are quite amazing events, not just because of the flooding but also because what comes in, must go out. So they come AND go out further revealing lots of features previously hidden.
Even here by the coast you can feel the humidity in the air and coupled with high overnight temperatures mean October has kicked off with a high pressure system in place, heavy dew and high pressure of the fungal disease type.
So what does the weather have in store for this week and next ?
General Weather Situation - 2nd October, 2023
So currently we have a ridge of high pressure pushing up from Africa and that is why we have warm humid weather both day and night. This ridge of high pressure has pushed the jet stream high above the U.K & Ireland which means unsettled low pressure systems are currently tracking across the northern Atlantic and into northern Scandinavia.
We also have a really weak low pressure slinking into The Bay of Biscay as they tend to do. This will introduce rain into the forecast today. Currently, it is tracking across Cork into Wexford and then across The Irish Sea into Wales and later northern England. Some of this rain later will push across Central England into the east later as well. North and south of this rain band will stay dry and settled.
This low pressure will cross the southern half of England on Tuesday and this will change the week's weather by pushing the ridge of high pressure southwards and introducing much cooler, windier weather by later on Tuesday, with the change taking place in the west first. This temporary blip will bring showers to western coasts mid-week and a chunk of heavy rain for western and central Scotland mid-week.
Just before high pressure builds again at the end of the week we will see more rain cross Ireland on Thursday and push into Scotland, Wales and western England on Thursday. So after a brief cooler and windier interjection this week, we will return to much warmer conditions by the end of the week with the temperature climbing into the low and maybe even mid-twenties by Sunday. Yep you read it right, mid-twenties by the weekend possibly, warm and settled.
Weather Outlook - w/c 9th October, 2023
The GFS projection above courtesy of tropicaltidbits.com for next Monday shows a very similar weather pattern dynamic to today, the 2nd of October. So that means warm, dry and settled for most of us starting next week though Scotland will be cooler and possibly more unsettled starting next week.
So will it last ?
Well if the GFS is to be believed by the end of next week we will be reminded that we are very much in October !
So the weather dynamic for next week looks like a northern-biased low pressure will bring unsettled and much cooler weather to Scotland and the far north of England and Ireland whilst further south, we start warm, dry and settled. As we progress through the 1st part of next week, the high pressure exits stage south and cooler, more unsettled air follows in behind as the low pressure sinks southwards and brings heavy rain with it to northern, central and southern England, Wales and Ireland by md-week, next week. So from mid-week, next week, we see cooler, windier and more unsettled weather push in on the back of a pronounced change in the wind direction, swinging round to the north.
This low pressure will then pull off eastwards as we enter the 2nd half of next week and pull that rain with it, though it may linger across eastern coasts through Thursday. As that low pressure exits, another pushes in behind from the northern Atlantic and we turn winder and wetter for the end of next week / weekend.
So the bottom line I think is that next weekend may be our last spell of warmth before we enter the more stereotypical October, unsettled weather and Atlantic storm systems. That's how it looks at the moment.
So just before I nip down the road to enjoy a lovely Flat White and Kanelsnegle (Cinnamon Swirl Danish to you) at The Two Magpies, I thought I'd talk about this type of weather pattern above because really from now on this autumn, it is the one to watch out for.
Identifying the type of weather pattern that spells trouble in the autumn / winter
Now it isn't rocket science, you don't have to be trained meteorologist to understand the fundamentals of why this type of weather system spells bad news for your turf management. So let's pull apart the salient points of what this GFS projection is showing and why is spells trouble for turf management.
Firstly, it is a high pressure system, so (in the northern Hemisphere) this means the winds around it rotate clockwise as I have indicated on the schematic above. So we can see that the left hand side of the system is pulling up warm humid air (because it's autumn and so unlikely to be dry air) from Africa and southern Europe into Ireland and the U.K. As an aside you can also see this isn't the case for Scotland because the temperature colouration shows it to be cooler.
High pressure systems represent air masses that are cooling and therefore descending down towards the surface of the earth. As they descend they warm up and exert more and more pressure on the earth's surface so we see the barometer / pressure increase. (hence the name).
Read about the Met Office's explanation of weather systems here
They also tend to be associated with lighter winds, which on the image above are indicated by isobars. The further apart the isobars, the lighter the winds. So we can see it becomes increasingly windier the further north you go and less windy further south and across Ireland.
As the pressure increases near the surface, water molecules are pushed closer together and combine leading to an increase in the Dewpoint temperature. The Dewpoint temperature is the point where dew begins to form when the air temperature falls to that temperature. Now, that's the general meteorological theory, but on grass it behaves slightly differently, there is a more subtle relationship between air temperature and Dewpoint temperature, especially at ground height.
Read about the Met Office's explanation of dew formation here
Now dew will only form if there is sufficient humidity associated with the high pressure system. This isn't always a given, sometimes we get dry heat in the summer with low humidity and also in mid-winter, we may get a high pressure system with frost but very little moisture / humidity associated with it. This incidentally can cause cold temperature desiccation on susceptible plant species like Poa annua.
So what we can deduce from the GFS image above that such weather patterns are associated with warming of air near the earth's surface, light winds (most times, but not always), humidity (because in this case we have a low pressure system close by feeding in moisture to the dynamic) and dew formation. Light winds incidentally are not only more conducive to dew formation but they are also associated with low E.T levels, so the dew hangs around longer as the process by which moisture is evaporated from the soil and grass is less marked.
The GIF above shows a current reading from a Davis Weather Station in the Midlands of England. I have highlighted (badly) the salient points that I made above. The air temperature is currently 16.9°C as is the Dewpoint temperature, which means water can condense from the atmosphere to form dew. Is there sufficient water in the atmosphere ? Well the humidity is 100% so I should say so !
We can also see the rate of moisture loss from the soil and grass is very low as denoted by an E.T level of 0.05mm.
So, practically if you sent the lads out at first light with Swishes, brushes, whatever, it is very likely that the surface of the grass is still wet now, some 4 hours later and will remain so until the wind picks up sufficiently to raise the E.T level and lower the humidity (because it is pushing the air around and separating the water molecules)
From a disease development perspective it is manner from heaven unfortunately.
Microdochium nivale reaches optimal growth at 15°C, we are currently higher than that and were for most of last night. Incidentally, it is also high enough for other diseases like Leaf Spot, Red Thread and of course Dollar Spot, so the stage is currently set for aggressive disease development.
Fungal diseases likes a wet plant leaf to enable them to develop quickly. They don't like high winds and a drying surface. We have a marked absence of the latter and a marked plethora of the former. So expect significant disease development until conditions abate, which by my calculations will be Tuesday onwards.
The bad news is that this will only be temporary because as I have indicated in my forecast, there is s strong likelihood of high pressure re-establishing towards the end of this week so the scenario is likely to repeat.
One last point concerning the development of disease and fungicide applications.
The fungicide balancing act..
There is a critical point where the rate of development of a disease will exceed the rate it is controlled / held back by the fungicide. The term fungicide implies it kills the fungus when maybe a truer term would be fungiostatic, which implies it inhibits the growth of the fungus.
Modern fungicides are more fungiostatic in their nature and so we have a balance between the rate of growth of the pathogen vs. the rate of growth inhibition by the fungicide. If we get our timing right and climatic conditions are not tipped in favour of the pathogen (dew formation, air temperature, etc), we achieve control or suppression. I have shown this below ;
When we have conditions like we have currently, (and have already had earlier this autumn) and look likely to get next weekend, we may see the scenario below developing....
The bottom line is your fungicide application can only do so much and that is also why I welcomed the registration of another fungicide last week because it looks like we are going to need all the help we can get this autumn until we hit that more unsettled and cooler weather.
Now it isn't just down to applying fungicides, non-pesticidal applications, dew control, grass species, etc all contribute to a robust IPM program.
Nowadays more than ever, you need them all in place.
OK, that's me done, The Two Magpies Bakery beckons...
All the best.