It may be just me but September 2018 felt very much like we were firmly set on the road to autumn / winter and not a continuation of summer. We’ve had 3 ground frosts here last month and I can’t remember getting close to that last year. There are also some signs from nature that we are cooling down quicker this year with a lot of the Martins and Swallows already away on their migration south and I’m pretty sure I heard a Redwing yesterday, newly arrived from Scandinavia to feast on what has to be said is a mega berry crop in the hedgerows. If so that would be 2 weeks earlier than usual. Meteorologically-speaking, some of the longer-range forecasters are stating that we are in for a cooler than usual October, but we will see, long-range is notoriously fickle when it comes to our climate and the jet stream. I charted out the temperatures at my location for September 2018 and 2017 using data from my Netatmo and you can see the difference in temperatures (particularly minimum temperature)
Onto this week’s weather and the prognosis last week was for an extended stay of high pressure, so we shall see if it was right.
General Weather Situation
So we start off on Monday with an Atlantic high pressure butted up against a cold, Arctic low sat over northern Scandinavia with the result that the wind is funneling between them from the north. So we are noticeably cool to start the week, down in single figures. A dry, bright and sunny day for most areas with some high cloud across the west and just a few showers dotted over East Anglia, Wales and Co. Clare. That’s the way we should stay for most of us but Scotland will see thicker cloud through the afternoon and the arrival of heavy rain into the north-west later this afternoon. It’ll be cool enough for this to fall as sleet over elevation. Through Monday evening this band of heavy rain will move south across north-west Ireland and western Scotland, accompanied by a freshening wind blowing from the west up north but firmly from the north further south making it feel nippy even in the sun. So that wind will keep temperatures down to the low teens for Ireland and Scotland and maybe a degree or two higher further south.
Onto Tuesday and that band of rain has sunk south overnight across Ireland and down the west coast of England into Wales and that’s where it’ll reside first thing. So a wetter start for the north and west on Tuesday but as we go through the morning that rain will fizzle out over Ireland, Wales and The South West to leave a clearer and milder 2nd half of the day. Away from this rain front we see the wind change from north to north-west, but it’ll keep on blowing hard ushering in warmer air from that high pressure. So despite being windy it’ll also be warmer on Tuesday with mid-teens for Ireland and Scotland and high teens, possibly even 20°C for South Wales and the south of England. Windy and warm then but plenty of cloud around as well.
Mid-week and Wednesday sees a continuation on that theme with warm and dry conditions for central and southern England, along with Wales. Ireland stays dry as well with pleasant enough temperatures in the mid-teens. As we progress north we see more in the way of cloud cover building from the north-west and that’ll usher in some thicker cloud, rain and drizzle during the late afternoon into Scotland and later northern England. More in the way of sunshine for the east of England and again here we may see temperatures hit 20°C in the lighter winds on Wednesday. Mid-teens for Ireland and low teens for Scotland under that thicker cloud and light rain. The winds will be lighter and from the north-west / west for Wednesday.
Onto Thursday and overnight we see that thicker cloud and rain extending south across Ireland, Scotland, the north of England and North Wales so a cooler feel to the weather as we approach the end of the week. More in the way of cloud cover with a strong to moderate westerly wind as well. That rain will slowly fizzle out and dissipate but may stay stubbornly entrenched over North Wales and The Pennines. As that cloud thins we should see sunny intervals end the day across the east of Scotland, The North East and possibly further south as well. So mid to high teens for England and Wales, but cooler over Ireland and Scotland as the wind flips back to the north / north west. Those winds will be particularly strong over Scotland.
After a mild night everywhere we see a much better weather picture for Friday with a lot of dry weather around and some sunshine across the east of Ireland and England. Through the morning we see more cloud build as a prelude to rain pushing into the north of Ireland and west of Scotland. We may also see some thicker cloud into The South West and along the south coast of England through Friday morning with some of it thick enough to bring some rain. The majority of areas away from this will have a dry, warm and pleasant day but they’ll be more cloud around as we go through the evening. It’ll feel cooler again as that northerly wind provides a nip in the air. Later on Friday night we see another rain front push into Northern Ireland, western Scotland and the south-west of Ireland. Mid to high teens for England and Wales but with that thicker cloud across Ireland and Scotland, along with lighter winds, temperatures will barely scrape into double figures.
So next weekend looks a bit mixed really with that rain moving across all areas of the U.K and Ireland overnight into Saturday. There’s a risk that some of it may be heavy across The Midlands and East Anglia through Saturday morning as a low pressure pushes in from The Bay of Biscay, but the flip side is that Ireland and Scotland should miss this so here you’ll see more in the way of sunshine. As we go through the morning into the afternoon we should see a much brighter 2nd half of the day for Wales as the weather clears from the west, but it’ll still feel on the chilly side with low teens across England and Wales and low double figures across Scotland and Ireland, even in the sunshine. It’ll be pretty windy on Saturday with those isobars tightly packed pulling in northerlies so that means cool with a capital ‘C’ and a great day to do indoor jobs or tie flies 🙂 Sunday looks a much quieter day as high pressure again pushes in from the west so dry, settled with milder westerly winds and plenty of sunshine. Scotland though looks cooler and dull I’m afraid.
So does that high pressure pushing over the 2nd half of the weekend mean we are in for more settled weather or will the gathering Atlantic storms finally win the day ?
Well next week to me looks like being a transition week from high pressure to low pressure.
The weather scene is that we have a deep northerly, low pressure slowly squeezing away an entrenched southerly high pressure residing over The Bay of Biscay through the course of next week. So it looks like it’ll be settled and largely dry for the south of England, Wales and the south of Ireland for the first part of the week. Further north across Ireland and Scotland you will feel the influence of that low pressure with stronger winds and rain through the 2nd half of Monday, Tuesday and Wednesday. At this stage it doesn’t look like that rain will extend its grip to the south but all areas will have a predominantly south-westerly airflow. Thursday looks like being a bit of a hiatus for all areas before I think a succession of low pressures finally push the high out of the way and bring rain and strong winds for the end of next week / weekend. My guess is with a south-westerly airflow we will see mild temperatures day and night but with an ever-present wind hopefully not the still, muggy balmy nights we just don’t need at this time of year.
First off we will start with a look-see at our usual GDD data from The Oxfordshire location for September 2018.
Looking at the stats from a GDD perspective September 2018 looks pretty similar to September 2017 from a growth potential perspective and pretty average really which is surprising to me because I would have bet on it being a lower GDD month. (Just goes to show what I know)
Cumulatively that puts us in a very similar, if near identical position to 2017, which again highlights the way we have caught up so much from this perspective. That said I know you’ll say well we might have had very high GDD through June, July and August, but it didn’t actually reflect in growth because the grass plant was moisture-limited and for most species the temperature was above-optimum for growth. That is of course one of the drawbacks of GDD in that it has no ‘top out’ whereas Growth Potential does, so when I get the time I’ll plot the same stats from a G.P perspective or I might just throw that job Wendy’s way (cue running for office door sounds 🙂 )
Soil Moisture Status
It was only a week ago odd that I was driving along the M1 in Yorkshire aquaplaning between Costa Coffee stops so I know for some of you this next bit will seem a bit academic. We know it’s very much been a north-south split rainfall-wise this autumn so far and whereas the north has had some pretty horrendous rainfall totals, the same cannot be said from The Midlands south.
Here we are still extremely dry, the rivers and reservoirs are still at below summer level so I have continued by exercise from earlier in the summer for soil moisture surplus / deficit using data sent in by Sean at The Oxfordshire, a location that tends to be on the dry side I’d say and quite representative for that reason.
So here’s where we were at the end of September last year at this location ;
A net moisture deficit of 149.4mm from June 1st to September 30th, 2017, which reflects a change of -3.7mm from the end of August 2017. So in other words during September last year the difference between accumulated rainfall and E.T was -3.7mm, i.e we lost 3.7mm from the soil over the month when you add up the accumulated rainfall and subtract the accumulated loss of moisture by E.T.
Let’s see how we look for 2018 at the same location ;
We sit at -311.8mm, so more than double the moisture deficit of 2017 for the same period and what’s more if we look at September 2018 in isolation, we continued to lose more moisture by E.T loss than we received by rainfall. Whereas in September 2017 the moisture deficit was -3.7mm, the figure for September 2018 was – 37.1mm, so we have continued drying down through September. So we are dry and strange as it seems, some areas will need irrigation to avoid going under moisture stress, particularly outfield areas where you may have overseeded.
That’s why we saw this information above from The National Drought Group (yes there is such a thing) declaring that the water supply situation in the southern half of the U.K is still fragile and good autumn / winter rainfall will be required to avoid water restrictions next year. I am grateful to Glenn Kirby of Syngenta for the link in one of his tweets…Read about it here
What of disease pressure through September ?
Well I think it has been a mixed picture with less disease pressure in general than last September and that’s down to humidity differences between September 2018 and September 2017, there is a caveat though. (more on that later)
If you look at the chart above which plots humidity across September 2018 and 2017, you’ll note in general a lower level for this year than last with shorter periods when it exceeded 90%, the critical value for disease pressure. Last September we had 6 peaks during the month when the humidity > 90% for extended periods of time vs. only 3 this September and the periods have been shorter. I have also highlighted the period at the end of September 2017 when we had a peak of humidity which carried into October, this generated high Microdochium pressure so we were already on the back foot from that perspective.
Now The Oxfordshire is a pretty open-aspect site, so in general humidity levels are lower than other sites which may be tree-lined. Data from my Netatmo comparing this year and last shows a different picture and that’s because my back garden sits in shade for a good % of the day and is less-affected by the wind.
So you can see in general much higher humidity levels because of the closed-aspect of the garden and it also shows the period at the end of September 2017 when we had extended periods of high humidity. In both locations, the end of September 2018 has shown lower humidity levels than the previous year so I think we are running into October in a better situation this year on the whole. Now for the north and west where you have received more rainfall this may not be the case but your ‘advantage’ is that you run cooler temperatures, particularly overnight, so the disease pressure tends to be earlier in the autumn rather than later.
Before I leave the chart above you’ll note that during this month there were some high humidity periods, notably around the 6th/7th, 20th, 21st and 23rd and these would have generated disease pressure on a shaded site.
That I think is the story of the autumn so far, lower disease pressure overall, but on shaded sites, it has still been significant.
Disease pressure looking forward
Looking at the stats for the coming week they illustrate my point above about the north-south divide with respect to temperatures and humidity. With low pressure influencing conditions in the north of Ireland, Scotland and northern England, it’ll be cooler and windier, so less risk of high overnight temperatures and high humidity and therefore disease.
It’s a different picture in the south as the winds are set to be lighter, particularly overnight and once we pick up the warmer air on Tuesday that’ll represent high disease pressure on Tuesday, Wednesday and Thursday nights. Much will depend on the intensity of wind on your site because if it continues to blow overnight then the humidity will be low, but if the winds are lighter and / or you have a sheltered site, I would expect higher localised humidity and consequently high disease pressure I am afraid.
Ok that’s it for this week, I’m off on my travels to Switzerland and Germany, celebrating a 12th Birthday with Meteoblue in-between 🙂
There will be no blog next Monday because I’m off for the day for an event which I may or may not chat to you about in future blogs depending on how it goes of course.
All the best.