January 11th

Hi All,

A belated Happy New Year and welcome to 2021. ‘New’ being the operative word this year with a ‘new’ rampant Covid variant, a ‘new’ National lockdown and a ‘new’ relationship with Europe. On the run up to Christmas it was an extremely chaotic situation with exports and imports. This situation is ongoing and I believe it will continue to impact us all in the coming weeks courtesy of the U.K and E.U running the agreement down to the wire and then wondering why no one knows what the rules are. It did make me shake my head in bewilderment driving down the A14 / M11 to see the gantry signs informing the population of new rules from Jan 1st for the last 4 months, when the ‘new’ rules weren’t actually decided till just before Christmas.

The news makes grim reading with the speed of growth of the new variant and the number of new cases and deaths associated with the virus. Unlike the spring, most people now know someone who has had Covid and unfortunately someone who has passed away because of it. Have to say great work by our government keeping London and The South East in tier 2 during December and then allowing all of these people to travel around the country willy-nilly when tier 4 was announced, top marks indeed to them and the superspreaders.

Life isn’t all negative though, the evenings are beginning to stretch out and on my daily permitted exercise I can walk through my town, pick up a nice cuppa and watch a Peregrine Falcon that has taken up residence in the church steeple in the middle of Market Harborough pick up its own take away (usually a Pigeon). You have to admire the adaptation of nature to human beings. We also have a resident Sparrowhawk in Market Harborough that has adapted to hunting in the High Street. It dives down on Pigeons feeding in the road and chases them right into the glazed windows of our local chemists where they lie dazed. It then picks them up and often eats them by the side of the road amongst people and cars. I have a whole host of local Blackbirds that visit the garden at first light for a feed, normally there’s 10-12 of them crammed in. As soon as the kitchen light goes on, one of them will land on the door handle and look in inquisitively. The same happens if I edge the curtains open, he’s on the window sill staring in at me and demanding food.

Golf courses and sports facilities are now closed until mid-February in England, a different date (for review) in Wales I believe and I think they remain open for two balls in Scotland. Clubs in Northern Ireland and the Republic are closed as well. After the deluges running up to Christmas and the widespread snow cover for a good chunk of the country thereafter, they’d have been closed anyway so it’s not really something that we shouldn’t expect at this time of year and mercifully it comes at a lower revenue time for most facilities and the back up of an extended furlough scheme. Since Christmas we have endured what used to be a normal start to the year, cold, frosty with snow showers, something we haven’t seen for awhile. Unfortunately for me the snow arrived just a little too late and just in the wrong place to win my Paddy Power bets for a White Christmas. It was close though and gave me something to look forward to aside from wondering how I was going to work off all of those calories and glancing nervously at the lack of intensity minutes on my Garmin. Soon fixed that though.

Onto the weather and a change from the cold, dry and frosty scenario, but maybe only for awhile…

General Weather Situation

So as you can see from the GFS output above, the cold air has retreated to the east and a mild and wet front is pushing in. Already on Monday we have had significantly wet weather across the north west of Scotland and it’s this rain front that will push down later during the day. Ireland sees a similar pattern, rain in the north west initially pushing down across most areas through the morning. The rain over Scotland will initially drift south down the west coast of Scotland / north west coast of England through the day and then into northern England. As the rain tracks east across Scotland it’ll butt up against colder air and readily turn to snow so some heavy falls are likely across Scotland this afternoon, especially across northern and eastern areas. With rain in the north, the south and east of the U.K and Wales will have a mostly dry day and noticeably milder than of late with temperatures nudging towards double figures in some areas. By late afternoon, the rain pushes down into Wales and the north Midlands and further consolidates across Scotland and Ireland as more rain piles in behind the first front. By midnight it will cover most of the U.K, except the north of Scotland which will clear after the rain pushes south allowing temperatures to tumble. Winds will strengthen through the day from the west becoming moderate to strong during this evening. Temperature-wise I’d say 7-10°C would be typical with temperatures rising further south through the night.

Onto Tuesday and we see that rain slowly moving south overnight so by dawn it’ll straddle all but the top 1/3 of Ireland and the bottom half of Wales and England. This band of rain will be slow moving so it’ll clear the east of the country first but it’ll take most of the day to clear Ireland and the south west of England. The reason for this is that we have a ridge of high pressure pushing up from the south west and by midday this will actually reverse the direction of the rain fall and instead of moving south and west, it’ll now begin to move north and east. So for Ireland and the south west of England and South Wales, you’ll see a continuation of this rain but it’ll clear the south west of Ireland as we approach dusk. For the rest of the U.K, it’ll be dry, bright but cooler with a distinct east-west split temperature-wise. Across Ireland, Wales and the south west of England expect 9-11°C , but across central and eastern areas including Scotland where it’ll be drier, brighter and cooler, expect 5-7°C. Winds will be from the north west / north and moderate to light initially swinging round to the east.

Wednesday sees that ridge of high pressure consolidate the rain over Ireland and the south west of England / Wales and by dawn it’ll be on the move again clearing the south and west of Ireland to track north and eastwards across Ireland. The same is true across The Irish Sea with rain moving up from The South West into Wales and the West Midlands. Now we still have cold air across the eastern half of the U.K so as this rain moves north and east, it’s likely to turn wintry on the leading edge and so we may see some sleet and snow around at lower elevations before it turns to rain. This band of rain and wintry showers will again be a slow-moving one and the eastern side of the country may not even see it during the daylight hours as its progress gets halted across the middle of the U.K. So for Ireland starting wet but clearing across the south and south west to leave rain firmly camped out across the top 1/2 of the country. For the U.K, rain across the south west and west initially moving north and east and turning move towards a mix of wintry showers and rain across central areas. Scotland will see that west-east divide with the west coast receiving the Lion’s share of the moisture and central areas / east staying colder but drier. Again that east-west split with Ireland, Wales and the west sitting at 9-11°C vs. the eastern side of the U.K sitting closer to 4-6°C. Winds will be lighter on Wednesday whilst maintaining that easterly bias.

Onto Thursday and dawn finds most of the U.K still sitting under that band of rain, sleet and snow whilst Ireland for once starts nice and dry, but cooler. As we progress through Thursday morning, the band of rain and wintry showers pushes eastwards but we will see more rain come in off The Irish Sea into Wales so any drying out is only temporary. As we progress through the day, that band of rain and wintry showers moves across to affect the eastern side of the country clearing the west as it does. Ireland looks to stay dry, bright and cool. Temperature-wise we will still see double figures across the west but only 5-7°C for central, northern and eastern parts. The wind will strengthen on Thursday across central parts and push in from the south east / east.

Closing out the week on Friday we see a much drier end to the week with a cold night, a widespread frost and a clear and sunny start to the day. That’s the way it looks to to stay across most of the U.K and Ireland. During the afternoon, a band of showers will push into Connacht and the north west of Ireland and these may become wintry in places. Winds will be lighter but still from the east and all in all, it’ll be a nice winters day.

The outlook for the weekend looks mixed. Saturday sees a band of wintry showers pushing across Ireland and across The North East. These showers will push into the western half of the U.K overnight falling as rain, sleet and snow. By Sunday they will be affecting Wales and the north west of England / western Scotland with some moving inland. Later more in the way of rain will arrive for the western coastline of the U.K. Ireland should see a better Sunday after those showers clear eastwards overnight on Saturday to leave a colder but drier Sunday. For the U.K, mixed across the west but drier, colder and bright with overnight frosts across central and eastern areas.

Weather Outlook

So above is the projected GFS output for next Monday and you can see why we end up with a largely cold and dry weekend courtesy of high pressure pushing in from The Atlantic. As we move through Monday, a cold low pressure begins to push down from Iceland and this will sandwich the U.K between high and low pressure. When this happens we know we get strong winds forming so Monday will start calm and settled but the winds will begin to ramp up into Tuesday and beyond with rain and wintry showers pushing into Scotland. This cold, low pressure will sink south and drag colder air in behind it pushing the mild air away further south. So as we go through into Wednesday we see a much colder, northerly airflow with I think snow showers for many places particularly in the latter part of next week and into the first part of the weekend. This cold theme continues into the week after next I think with a pronounced northerly airflow and it’ll possibly mark January 2021 as one of coolest January’s for some time.

Agronomic Notes

GDD update – December 2020 – U.K Location – Thame

So December 2020 finished the year off in pretty typical fashion recording a total GDD of 36.5 for the month and comparing similarly with the last 5 years although the rainfall probably didn’t !

So we can see that the total GDD for 2020 came in > 2000, making it 3 out of the last 4 years that this has occurred. This got me thinking and I wish it hadn’t because I then number crunched monthly stats from 2005 – 2010 to calculate both total GDD, G.P and total rainfall for the year so I could look at the data. Sometimes I wish my mind would just leave things be, but no, not to be denied, here are the stats ;


It makes interesting reading to me.

Firstly, in terms of GDD and rainfall, there is no statistically valid trend to either. That is to say that the relationship between year and total GDD is not a strong one and the same goes for rainfall. This doesn’t come as a big surprise to me because you only have to look at the inherent variability in our weather due to us being an island to know that we don’t have stable weather trends. The devil will be in the detail of course when it comes to maximum rainfall falling over 24hrs and of course maximum and minimum temperature but I’d have to devote most of the rest of my life to dig into this much data. And of course this data is for just one location and a dry one at that. If I did the same exercise for a location in South Wales or Devon / Cornwall, the south and west of Ireland it would of course be different. Looking at the rainfall in 2020 vs. 2019, it represents a 21% increase which is considerable and it will interesting to see if this trend is consistent at different locations when I get round to doing this.

I also decided to look at total Growth Potential year on year because this more accurately represents grass growth. Currently we use GDD for a lot of growth models and particularly for product-based recommendations. I believe in future we will change to Growth Potential or rather should change to Growth Potential. It’s not a right or wrong decision, it’s just development and improvement in my mind.

The problem is that in the middle of the summer heat, GDD will suggest that the plant is still growing and therefore adding on GDD when in reality we know that once we reach say 27°C, Poa annua checks out in terms of growth and shuts down. GDD doesn’t reflect the negative effect of high temperature on grass growth whereas Growth Potential does. It does this because it is based on a formula that is orientated around an optimum temperature for cool season and warm season grass growth. Typically I use 18°C to reflect an optimum average temperature for Poa annua.

It’s probably best to illustrate the difference between GDD and G.P by way of an example.


  1. Growth Degree Days – Model calculated by ;

GDD = ((Max Air Temperature + Min Air Temperature) / 2 )  –  (Base Temperature °C )


Example Day 1                                                                         Example Day 2

Maximum Air Temperature =  20°C                                 Maximum Air Temperature = 30°C

Minimum  Air Temperature =  18°C                                 Minimum  Air Temperature =  18°C

Base Temperature = 6°C                                                      Base Temperature = 6°C

GDD = (20 + 18) / 2 = 19                                                     GDD = (30 + 18) / 2 = 24

19 – 6  (Base Temperature = 6°C)                                    24 – 6  (Base Temperature = 6°C)

= 13 D°C (13 Growth Degree Days)                         = 18 D°C (18 Growth Degree Days)

So according to the GDD model, we would see a likely 38% increase in the potential of the plant to grow according to temperature on Day2 vs. Day1 (18GDD vs. 13GDD)

Now if I saw 13GDD as a course manager / superintendent I would think the grass would be in flush mode and it would be all hands to the mowers, PGR at the ready….But if I saw 18GDD I’d be thinking that it would be too high a daily GDD to generate excessive grass growth. Now you can’t tell this from the GDD formula but you can if you use the Growth Potential formula.

So now let’s plug in the same two days into the Growth Potential calculation which is a lot more complex as a mathematical formula…..


  1. Growth Potential model calculated by ;

The formula above returns a value between 0 and 1.0, where 0 = No Growth Potential and 1.0 = Maximum Growth Potential


Example Day 1                                                                         Example Day 2

Maximum Air Temperature = 20°C                                       Maximum Air Temperature = 30°C

Minimum  Air Temperature = 18°C                                       Minimum  Air Temperature = 18°C

If you do the maths or have a trick calculator, then the G.P comes out as ;

GP = 0.98    Day 1                                                                     GP = 0.77 Day 2

So according to the Growth Potential formula for the same two example days that we used in the GDD example, the difference in growth potential is 21% less on day 2 (0.77) vs. day 1. (0.98)

Day 1 would represent near optimum growth if you consider that the maximum number that G.P can return is 1.0. But day 2 would represent less growth than day 1 because of the high temperatures.

So GDD calculates an increase between day 2 and day 1 whereas Growth Potential calculates a decrease. In reality a day with 30°C maximum temperature and 18°C minimum temperature would constitute a high stress day for a plant species like Poa annua because the maximum air temperature exceeds its optimum range and it would shut down its stomatal pores and reduce its growth rate. The GDD calculation doesn’t reflect this reduced growth due to high temperature whereas the Growth Potential calculation does. So G.P is a good indicator with respect to plant stress levels in the height of summer. In summer 2020 at the end of July we saw the daily G.P drop to 0.3 at our trial sites reflecting the very high day and night temperatures and considerable plant stress. It was this magnitude of stress that kicked off Anthracnose through August and September.

Going back to our graphs what we are saying is GDD is an accurate yardstick of outright temperature because it measures the average between maximum and minimum daily temperature but it isn’t an accurate yardstick for potential grass growth.

Here are the two graphs of total yearly GDD and G.P using data from The Oxfordshire from 2005 to 2020 (And a big shout out to Sean for recording and sending that data for the last 15 years, cheers matey, couldn’t do these stats without it !)

You can clearly see the difference between the two models and in particular how the G.P was lower in both 2018 and 2020 because of the high temperatures experienced during the summer in those two years. So yes we are measuring higher temperatures year on year if you look at the GDD chart above but it isn’t resulting in more grass growth because during certain periods of the year the temperature exceeds the optimum for grass species like Poa annua but also Festuca now I come to think about it. I think this is a trend that will continue long past the time I’ve hung up my PC or more accurately with the way my laptop has been working during the number crunching segment of today, thrown it out of the window :). Now if I could just chart out yearly E.T since 2005 that would be really interesting (stop it!)

OK, quite a heavy blog today and certainly my mind feels more than a little frazzled. Next week I’ll look back at December 2020 using the data you kindly provided and discuss the plus / minus (copyright Bernie Schenk) of aeration during lockdown with high Leatherjacket populations. Food for thought maybe ?

All the best.

Mark Hunt