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  • Writer's pictureMark Hunt

July 10th, 2023

Hi All,


After a turbulent end to the week rainfall-wise, yet again we see the vagaries of summer rainfall forecasting with some areas getting more than 30mm over the weekend and others barely 2mm.


I did laugh when I was watching the build-up commentary for the Formula One race at Silverstone. One of the commentators said "My mother has just rang me to say where she lives, it's pouring down and she is only 40 minutes away, she lives to the west, so we could get rain here during the race.....".......


Well let's see now...... 40 minutes could be 30 miles away and yesterday's rainfall was coming from the south, not the west, so that sentence illustrated a total lack of understanding of the weather. Had they just looked at a rain radar app and tracked the direction of movement of the rain, they'd have had a pretty good idea whether it was going to rain or not during the event.


So the breakdown of June's heatwave (incidentally it was confirmed as the hottest June ever about an hour after I published last week's blog, some super computer that...) that began at the end of June continues into July. Looking at the forecast for this week and the coming weekend, that trend for cooler, unsettled weather will continue with some pretty heavy dumps of rain on the way, beginning today. Unlike the weekend's rain, this rainfall will be more widespread. Now it isn't much fun for people on summer holiday but personally I much prefer 20°C, sunshine and showers to 30°C and splitting the stones sunshine with the E.T going stratospheric.


Now that remark is sure to come back and haunt me as I sit in my camper buffeted by strong winds, lasing rain and seaweed blown off the beach decorating the awning !!


OK, let's put some detail in it..



General Weather Situation - w/c - July 10th


This lovely wind animation (from windy.com I believe) on Meteoblue's weather website (www.meteoblue.com) shows the position of the low pressure system out to the south west of Ireland and how it is pulling stronger, southerly winds into The Irish Sea. Not just stronger wind but rainfall as well. Incidentally this low pressure system has tracked west (against the normal direction of the jet stream) over the last week due to the formation of a trough pattern in the jet stream.


So Monday will begin the trend for some more widespread and significant rainfall, with rain already crossing Ireland as I type this blog. This rain will cross The Irish Sea into West Wales and The South West and will be heavy in nature. This heavy rain will then track north east into northern and north west England and then eventually Scotland. It'll also stay across Ireland most of the day. Later this evening a new rain front will push into the south west of England and track north east across central England and The Midlands. Temperature-wise, we will sit between 18-20°C with a moderate to strong south / south westerly wind, especially out west.


Tuesday sees that band of heavy rain over the southern half of the U.K still present but during the morning it'll slowly drift north and east over East Anglia. Meanwhile across Ireland and western coasts stretching up to the west coast of Scotland, we will see more showers and longer spells of rain. As we move into the afternoon, we will see the rain consolidate over the east coast of Ireland, Wales, and the west coast of the U.K. This rain will extend up to and across Scotland. During Tuesday evening, this rain will then drift south and east across northern England and The Midlands. Similar wind strength and temperatures to Tuesday.


Mid-week and a drier interlude for most of the U.K and Ireland before the wetter end to the week. Wednesday sees rain across Northern Ireland, the west and central Scotland and some showers over Central England during the day, but these look lighter in nature. During the day the heavier rain will become confined to the north of Ireland and west of Scotland leaving a mainly sunny outlook. Temperatures will range from 17 - 22°C.


Thursday sees a similar (ish) day with a more drier projected outlook at least for England and Wales. Ireland looks to stay dry until a new rain front pushes into Kerry later in the afternoon and then this will proceed north and east across the country. Scotland will see showers on western coasts from the outset and these will consolidate into more general rain across Scotland during the course of the day. Further south and east, the outlook for Wales ad England is mainly dry, still with the risk of showers in the 2nd half of the day and pleasant temperatures in the 19-21°C range. Winds will remain south westerly / southerly.


Closing out the week on Friday and that rain over Ireland has moved west overnight into West Wales, The North West and northern England and Scotland. It will remain in a diagonal pattern (/) across the U.K through most of the day stretching from South Wales up to north of The Humber. So south and east of this rain line will be pleasant temperatures with the odd shower. As we progress into Friday evening, this rain drifts south and east into The Midlands and Central England. Similar temperatures and wind direction though this will freshen across western areas.


The outlook for the weekend looks 'mixed. Saturday sees that low pressure centred over The Irish Sea and that will pull in heavy rain to Northern and north west Ireland before this moves across most of Ireland on Saturday. For the U.K, Saturday kicks off very wet across The South West and western coasts extending up to Scotland. During the day, the rain will consolidate over the U.K to bring heavier rain for most areas before this moves off east later in the day. So very wet for Ireland, Wales and Scotland and a mixed picture for England with central and eastern areas maybe seeing more of a sunshine and showers scenario but that could easily change. Sunday looks drier, still with showers across western coasts but a much drier picture for Ireland and the U.K. The exception will be Scotland and the north of Ireland where we will still see heavy rain for the south west and west through the morning and this will extend into the afternoon. Similar temperatures and wind direction again.


Weather Outlook - w/c 17th July


So this is the GFS outlook for next Monday and as you can see the low pressure has crossed the U.K over the course of the weekend and is now located across Scandinavia. You can see how this has tilted the wind direction from the south / south west to the north west, so a cooler theme to start next week. During the 1st part of the week, a ridge of high pressure will push up so by Wednesday, the wind will have changed to a milder, more south westerly orientation, so the temperatures will pick up a little. This change is temporary though because during the 2nd half of the week, that low pressure sinks south, pulls in a north westerly wind again and more unsettled conditions.


In terms of rainfall prospects, the 1st part of next week will be a sunshine and showers type affair, with temperatures more like October I reckon than July (high teens) but then that low pressure pushes more significant rain through on Wednesday / Thursday to all areas along with strong winds and cooler temperatures before a hiatus at the end of the week.


I say hiatus because our present weather dynamic for July is for a low-lying jet stream position which is keeping extreme heat at bay and allowing a more changeable, unsettled weather aspect to dominate from The Atlantic.


If I mystic meg it, this pattern could continue to dominate till the jet stream nudges up again. There is a signal that this might occur towards the end of July, during the last week, setting the scene for a warm / hot end of July / start of August.


Agronomic Notes


Speaking to a good number of superintendents lately, it is clear that June's weather brought many challenges.


One of these related to greens speed and this was particularly the case when the prolonged dry spell was broken by rainfall. Now although we were irrigating in June, as I have commented upon before, coverage was really poor.


The consequence of poor coverage is that some areas of a surface receive excessive rainfall and others inadequate amounts. Now we know the wind direction was pretty fixed during the 5-6 week dry period that began in early May, so there wasn't a great deal of change in the areas that received above optimum and sub-optimum water by irrigation. When the rain arrived, all areas got moisture and so the areas that were moisture-limited suddenly started growing and we experienced a growth flush.


As discussed last week, the high temperature also shortened the longevity of PGR applications so we had a perfect storm of moisture-enabled growth, reduced efficacy of PGR's and optimum GDD / Growth Potential from temperature-perspective.


In this week's blog, I will take a look around the U.K and Ireland at June's rainfall and Growth Potential figures to analyse the geographical variability in these two parameters. What you'll see that in terms of Growth Potential there was very little geographical variability across all areas and so it was a common issue on sports facilities and especially golf with that ever-present vocal minority who want 'faster' greens.


Before I do this analysis, let's look at the maths.


Optimum Growth Potential in a single day is set at 1.0, that is to say that a cool season grass plant (there is a separate figure for warm season grasses) can grow at an optimum rate from a temperature perspective when the average temperature is 20°C, or in my G.P calculations, 18°C, because I feel this more accurately reflects the impact of stress on a mixed Poa / Agrostis sward.


So in other words, if we have a month of optimum growing conditions in June, it would be 30 days multiplied by 1.0 = 30.


Now look at the total G.P for the different locations across the U.K & Ireland and see how close they were to this optimum figure.


Growth Potential and Rainfall Totals - June 2023 - U.K Locations




So you can see the range of monthly total G.P figures varied from 23.70 - 27.04, with most locations between 25.5-26.5 = 85 - 88.3% of optimum. This illustrates my point that from a Growth Potential perspective, June 2023 was a high month from a total perspective.


Aside from the Scarborough figure, it was super-consistent regardless of geographical location. For example, Dumbarton in Scotland was 26.45 and Okehampton in Devon, 26.42 !


This suggests that the weather pattern that caused the prolonged high temperatures in June 2023 was generalised across the whole of the U.K.


As you'll see shortly, the same was true for Ireland.


In terms of rainfall and of course, summer rainfall, the pattern wasn't quite as consistent....

Typically June's rainfall amounted to between 25-50mm across the U.K, but the majority of that rain was in the last few days of the month.


Growth Potential and Rainfall Totals - June 2023 - U.K Locations



For Ireland, we can see a very similar picture in terms of Monthly Growth Potential with some locations like Shannon and Valentia even closer to the theoretical optimum figure of 30. So the range for Ireland varied from 25.1 - 29.0 = 83.7 - 96.7%, which is even higher than for the U.K locations. So a similar issue in terms of flushes of growth potentially and very consistent stats across the whole country of Ireland.


Rainfall-wise, June continued the trend for a dry month with rainfall arriving in the 2nd half of the month. When this rainfall arrived, the potential for a growth flush was high, as some of you who are measuring clip yield will no doubt testify.

More variability in rainfall of course across the Irish locations with the south west and west as usual receiving the majority vs. the south east and east of the country. Dry for sure for Ireland and that comes after a dry May, but as mentioned earlier, that dry spell broke in the 2nd half of June for Ireland and since then it has been cooler and unsettled.


Those Growth Flushes in June..


So I started off this blog discussing the growth flushes in June that made superintendents lives tricky for a short while due to the combination of optimum Growth Potential and rainfall, the latter arriving after a sustained dry spell.


We then saw a sustained flush of growth that meant greens cut at 05-06:00 am (for example) had already grown significantly by the afternoon and evening. So players arriving after work for example would have encountered significantly more growth and therefore slower greens as a result. No doubt leading to that oft-inaccurate and resented remark "Have you cut the greens today "....Ignorance is bliss eh ?


To highlight this point, I have charted out the dynamic Growth Potential, so this isn't just taking the daily maximum and minimum temperature for that calculation but the maximum and minimum temperature per time interval as measured by the Davis Vantage Pro Weather Station. In this case it was every 15 minutes on a station located in Milton Keynes.


I have highlighted two specific periods when you can see that the Dynamic Growth Potential doesn't dip below 0.7 and hits 0.95 - 1.0 during the day. (optimum in other words)


Both of these periods coincide with rainfall.


With the consistently high Growth Potential recorded across the vast majority of locations for the U.K & Ireland, the above scenario was repeated on many sites during June 2023. With the arrival of a cooler, Atlantic airstream in early July, the Dynamic Growth Potential dropped back as night time temperatures in particular took a dive.


OK, that's me for this week, next week the blog is likely to be later in the week.


All the best....


Mark Hunt


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