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  • Writer's pictureMark Hunt

December 4th, 2023

Hi All,

The first blog of December and now only a little over 2 weeks to the shortest day of the winter with the Winter Solstice occurring this year at 03:27 GMT on the 22nd December.

This marks the point when the Northern Hemisphere is furthest from the sun and is seen as the start of winter from an astronomical perspective.

Meteorologically-speaking, winter starts on December 1st and you'd have to say this year it certainly did !

On Saturday we had a lovely Hoar Frost which created some beautiful images. For Hoar frosts to form you need some pretty specific climatic conditions. During a Hoar Frost, water vapour in the atmosphere freezes onto structures which are at or below freezing point, forming ice crystals. As more water vapour freezes, these crystals grow and form elaborate patterns. Simply beautiful.

That cold air arrived with a vengeance last week with some significant frosts (we got down to -6.7°C locally) and some snowfall. All that's gone here now after milder, wetter air arrived yesterday but colder air still lies north of here because of the differing wind directions attributable to a nearby low pressure system. The image from shows how the south of the U.K is picking up milder air on a southerly / south easterly airstream, whereas further north and across the west, that same low pressure is pulling down colder air on a northerly / north easterly airstream.

This arrival of milder, wetter air on Sunday no doubt caused some of those tricky issues with the surface thawing over the frozen soil underneath. To golfers, the frost is gone, so ready to play, but actually whilst this is occurring the surface is mobile (thawed surface moving over frozen soil beneath) and extremely dangerous from a H&S perspective. Shearing of roots also occurs during this process until the point when the thawed depth imparts stability.

I wrote about this last year and produced a pdf (shown above) explaining the process which you can download if it is of use to you. You can download it here

Interestingly last year we had a similar cold snap but it occurred mid-December before milder, wetter air arrived and created the same situation to what we see now.

So with milder Atlantic air arriving to some areas, how does the rest of the week pan out ?

General Weather Situation - w/c 4th December

For the start of the week we have a low pressure system in place as the graphic from shows above. This system features rain and milder air pushing north through the course of Monday. The low will be slow-moving and no doubt responsible for some heavy localised rainfall this week unfortunately.

The leading edge of that rain will turn to snow and create some hazardous conditions for a time and it may not reach Scotland in the early part of the week. So for Ireland, a milder and dry day thankfully ahead of a much milder week than last. For England and Wales, we will see rain moving northwards, clearing from the south and west as it does so, but it'll be a slow process taking until Tuesday morning to fizzle out further north. Judging by how my local River Welland is doing a passable impression of the Zambezi, we have had significant rainfall during the thaw. So a milder, wetter start to the week for the south, colder further north with that rain turning to snow on the leading edge and some tricky conditions as it does so. So the frosts may persist further north until that milder air arrives and particularly for Scotland until mid-week. With the cloud cover arriving with the rain, it should keep temperatures above freezing for most of Ireland, Wales and England, though Wednesday morning will be close and likely frosty across eastern and north eastern areas of England. Temperature-wise 4-6°C, milder for Ireland and colder for Scotland and eastern-facing coasts.

As we get to Tuesday night, we see another tranche of rainfall push into the south west of Ireland and move across country overnight. As it does so it'll become heavier in nature so by the time we hit Wednesday morning, the south and south west of Ireland will see some very heavy rain I am afraid. This band of heavy rain will move north and west into Northern Ireland and then by Wednesday afternoon, the south west of England, West Wales and western coasts. By evening it'll be pushing eastwards and northwards across the U.K. As it does so, it'll encounter that cold air so I expect some pretty heavy snow across the north of England and Scotland as that rain front pushes across on Wednesday night. So a dry for central and eastern parts of the U.K until that rain arrives later. Ireland will clear from the south and west but that rain might not leave Leinster till Wednesday evening.

Thursday sees a new rain front crossing Ireland and pushing into the U.K, so a very wet day indeed for just about everywhere, except the far north of Scotland. The wind will pick up too as will the temperatures, so the 2nd half of the week looks to be much milder particularly across Ireland which will see temperatures nudging into the teens by Thursday. That rain on Thursday will clear the U.K from the west but may not leave the east till dusk. Much milder for Ireland, Wales and the southern half of the U.K, with temperatures in double figures. across the south, cooler for The Midlands and north. Scotland will stay colder until that rain arrives and again a threat of snow on the leading edge as it does so.

We close out a wet week with more rain crossing Ireland overnight and into most of the U.K on Friday clearing the west as it does so. More showery in nature on Friday and still accompanied by strong south / south westerly winds. Ireland will have a drier day on Friday with some sunshine, the odd shower and temperatures till in double figures.

So a wet, windy and milder end to the week for many areas. This wet, windy and mild theme continues into Saturday with more rain crossing Ireland and the U.K, some of it heavy in nature across the south and Midlands. Potentially drier on Sunday once rain across the north of England has exited into The North Sea. So a chance of seeing some sun on Sunday in between the odd shower and temperatures likely to remain around 9-11°C, milder across Ireland and Wales, a little cooler for Scotland.

So as we edge towards Christmas, is there any sign of things dropping colder ?

Weather Outlook - w/c 11th December

So looking at next week's weather and it is an interesting mix.

As the GIF shows from above, we start next week with that low pressure still influencing the weather so a sunshine and showers type scenario I think for the start of next week. As that low pressure departs, a ridge of high pressure tries to push in and that'll swing the winds more northerly so dropping colder from Tuesday onwards. Still with showers in the weather picture although I think they will be mostly across the north, north east and east. As we approach mid-week, next week, we see a rain front push in from the north west accompanied by some very strong winds. These strong north westerly winds will be a feature of the weather from mid-week, next week onwards. They will push showers down from the north and north west and these may well turn wintry.

As we get to the end of next week we have an Omega blocking event in all of its meteorological glory with very cold air to the east of us and a blocking ridge of high pressure to the west. This will funnel northerly winds down across the U.K and Ireland with the eastern half of the country most likely turning wintry in nature I think.

The GIF above shows this clearly. Now thereafter the high pressure is predicted to push this cold air eastwards and leave us in a bit of a hiatus. in my experience this can tip either way with the cold air pushing down in the classic winter rough pattern circa winter 2010, 2013 or high pressure wins the day., Lets see how it pans out.....

Agronomic Notes

So the first issue is obviously the one I highlighted above, that of dealing with a rapid thaw where water is perched above a frozen sub-soil, as shown above.

This will have already been an issue across the weekend for Ireland, Wales and southern England up to and including The Midlands. As that milder air pushes north with the accompanying rainfall, we will see that process occur in the north of England and Scotland. It can be in my experience a bit of a fraught one for a time as the perception is the surfaces are playable from an appearance basis but they are far from it. With the thawed surface slipping over the frozen sub-surface, it means there's an injury risk to the individual and the grass plant. The problem I have seen is when there's a group of players and members all champing at the bit to go out and play and the greenkeeper is the perceived obstacle to this process. It comes down to communication and respecting his or her opinion as a professional turf manager.

Thankfully it doesn't occur too many times a year !

Being the 1st blog of the month, I take a look back now at November's stats and chat through the climatic and agronomic implications. Thank you as always to those that send their data in, from my Irish colleagues especially as I know it's time consuming to collate these.

First up are the GDD stats from our default location at The Oxfordshire, Thame.

So we can see November came in with a total GDD of 62, which is pretty run of the mill. It doesn't mark it down as a cold November or a mild one, a typical figure really. In a way it is nice to see a 'normal month' with the typical drop off from the milder early autumn temperatures, bearing in mind October's GDD total was 201.5, more than three times November's total.

That November total takes the y.t.d total to over 2,000 total GDD with a figure of 2010.9 at the end of November. That makes it the 4th highest total since 2005, with the 3 others occurring in 2006, 2017 and 2022. Dependent on December's 2023 GDD total, the year 2023 could turn out to be our 2nd highest after 2022 and 2017.

Monthly rainfall & Growth Potential Totals - U.K Locations - November 2023

Looking at the monthly Growth Potential totals for U.K locations featuring the November stats, we can see a significant drop off in growth levels, roughly 1/3rd of the previous month. This is typical across a year with the drop off to winter occurring during November. Agronomically it means a lot. Firstly, the amount of growth being generated on all areas has come down to a manageable level, which is long overdue.

It also means the longevity of applications, be that nutrition, dew control and probably most importantly, turf hardeners and pesticides becomes more in line with what we expect. If you work on a combined longevity of 10 from a Growth Potential perspective for an effective, systemic fungicide application (so that's a short list right?), we can expect to get more than 4 weeks if the application was made in November, whereas if we go back to October, that longevity may have been as short as 14 days, if it was made early on in the month when it was warmer.

Comparing the two months below using data from our default location, the drop off in growth is clear to see ;

No great shakes in terms of differences / variability between locations away from what we would expect from a Growth Potential perspective.

Rainfall is another matter though....

Rainfall is dependent on so many climatic factors but also localised topography impacting which areas get high rainfall and which don't.

It isn't just a case of saying areas in the north and west are the wettest, that argument doesn't cut it anymore if you look at the data.

I remember 15 years or so ago doing a talk to a BIGGA section and I was discussing the climate projections for came from the Met Office and they broadly went along the lines of increased rainfall due to a warming climate (tick that box) and areas to the west and north experiencing increased annual rainfall vs. lower in the south and east. No great shakes there, tick in the box ?

Don't think so.....

Sure if you look at the data above, the south west locations are the wettest in England, but we also see locations in the south and south east (Brands Hatch, Sevenoaks, Guildford) are picking up more rain than The Midlands and even further north.

That definitely wasn't in the climate projections I looked at 15 years ago....

It is why I have very little faith in climate projections looking forward when it comes to the U.K and Ireland for that matter. Not because I don't think our climate is changing and will continue to change but because I don't think we have a handle on the biggest driving force behind our climate. I am talking about the jet stream of course. If we don't clearly understand how that is likely to change in the future, how can we reliably predict rainfall and temperature variations ?

The reason why southerly locations are picking up more rainfall is because we are seeing more BOB's - Bay of Biscay low pressure systems. These tend to impact most across the south west, south and south east coast of England. BOB's are becoming more frequent because of a shift in the position of the jet stream. Nobody (IMHO) predicted that change and its impact on rainfall patterns.

Makes you think doesn't it ?

Monthly rainfall & Growth Potential Totals - Irish Locations - November 2023

Pretty similar Growth Potential stats to the U.K ones range-wise aside from the inevitable high marker that is Valentia.

Approximately 50% of the Growth Potential in November vs. October so the same comments apply about extended longevity from applications, which is positive.

For the sake of completeness, below is a comparison of the October and November daily Growth Potential using data from Cork, you can see the drop off clearly.

You can see how October's daily G.P were significantly higher than November's as we would expect and also how the U.K stats for October show much higher daily G.P, but as we move onto November, they are much more similar. Both sets of G.P figures show the marked drop off at the end of the month when the cold weather arrived. See how the pattern is more gradual in the Irish stats vs. the U.K's ? That is because the cold air pushed in from the east so it slowly made its presence felt slowly in Ireland vs. a much quicker transition for the U.K.

I'd argue that the same expected and unexpected variability is present in the Irish rainfall stats. So we see the usual west vs. east difference but the south and south east locations of Cork and Wexford are showing very high rainfall totals vs. just up the coast in Dublin. I realise there is usually a difference but this year there is just over a 70% y.t.d rainfall difference between Cork and Dublin !

Thankfully November's totals for Wexford and Cork were more 'normal' compared to October's excesses !

Are Cork and Wexford's higher totals due to the effect of those southerly-orientated low pressure systems I wonder ? Possibly so....

Next week's forecast will give us a glimpse of that all-important weather run in towards Christmas and the likelihood of a spray window to get on that tonic, hardener, fungicide, dew control, delete where applicable....

Now my little old head is spinning, lots of stats in this week's blog, hopefully they are useful. Tempus fugit though and I have scarcely burnt a calorie so far today. So it's grab a quick, light tea and an hour of training (torture) beckons on the Wattbike with my fav trainer Emma 😮

All the bet.

Mark Hunt

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