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  • Writer's pictureMark Hunt

February 6th, 2023

Hi All,

Well after some double figure temperatures and very little rain (for most places except Scotland and Northern Ireland), you could almost forgive yourself for thinking Spring had started early. Walking around the Hambleton Peninsular at Rutland Water yesterday, my favourite patch of Snowdrops have appeared and there's some lovely Celandines at the start of the walk as well. There was even some nice warmth in the sun.

Our weather patterns at the moment concern me. Normally from January to March, we have a strong Transatlantic pattern in the jet stream with a fast jet stream wind speed pushing low pressure systems from west to east. Sometime towards the end of March, this pattern typically changes and we run into blocking patterns where the jet stream loses speed and forms undulations. These can sometimes give rise to dry, cold or warm, dry spells of weather. So I am looking at a weather pattern more typical of April in early February.

What I think we have currently is a slow-moving jet stream which has formed into a Diffluent block where it is split in two by a dominant high pressure system that is vectoring any moisture north and south of us. Why I am concerned is that this type of weather pattern tends to occur later in the spring and is rare in February I'd say. Diffluent blocks tend to be longer-lasting as well, so currently I can't see a clear weather signal that puts an end to this. Sometimes it is there, but it isn't consistent. You can read more about Diffluent Blocking Patterns on The Met Office's website here

Looking at historical UK weather, you have to go back to 1932 to find the driest February when we had 8.8mm of rain for the month. Now the problem is we didn't even know that the jet stream existed in 1932, so we can't pinpoint the reason for this, though I bet it was the same feature. Interestingly in that year, a very dry February was followed by a reasonably wet March, a wet April and May before we hit a dry this space. It's a things that make you go hmmm moment I think...

General Weather Situation - w/c 06-02-23

So with high pressure in charge we shouldn't expect too much going on this week although the first half of the week will be characterised by frost in the morning. As is very often the case, the frost may not appear until daybreak as this is often the coldest part of the day and crucially before the warming effect of the sun has increased temperatures.

The graph below shows the air temperature this morning just south of Birmingham and you can see how it kept falling till just before sunrise. Sometimes it can keep falling after sunrise, it just depends on the dynamic and topography of the location.

One of the other considerations is of course soil moisture, in that some areas of the country had 40-50mm in the first two weeks of January and precious little since. So the ground is drying out, we have less moisture and more air present and this means temperature changes (either way) are more volatile. Quicker to drop, quicker to rise. These snap ground frosts are for sure a feature of a drier than usual situation at this time of year in some parts.

So Monday to Wednesday looks like being frosty for the southern half of the U.K, but for Ireland, the west and north including Scotland, more cloud cover will likely keep temperatures above freezing. There's also a risk of fog for Tuesday and Wednesday over the southern half of the U.K because of light winds. They'll be a little rain around for north west Scotland on Monday but this will fizzle out as high pressure holds the reins.

So the southern half of the U.K, including Wales will be colder than the north, Scotland and Ireland where I think you'll see 8-9°C daytime and 4-6°C night time temperatures. Further south, we have already dropped to -4.5°C this morning and I think that's likely to be the coldest day of the week because we will have more cloud cover on Tuesday and Wednesday. So maybe -1°C here , rising to 5-6°C.

The second half of the week sees stronger winds from the south west kick in which should mean slightly warmer day and night temperatures as we move through the week and of course remaining dry with more in the way of sunshine. The same goes for the north, Scotland and Ireland, with temperatures nudging into double figures later in the week and staying around 5-6°C overnight. That's also the way the weekend looks, mild (ish) with a south westerly wind and sunshine and clouds.

Weather Outlook - w/c 13-02-23

Above is the animated GIF produced by for this Monday and next Monday and it is a bit of a spot the difference really isn't it ?

A Diffluent block reflected in a wildly undulating and split jet stream means very little change going from this week into next. 10-day projections are also extremely volatile, alternating between continued high pressure and a Bay of Biscay low breakdown at the end of next weekend. So at present next week looks dry, settled with plenty of cloud cover meaning dull and a bit dreary with I think plenty of fog around and maybe just skirting ground frost sort of overnight temperatures and 7-9°C through the day dependent on where you are.

Agronomic Notes

Well really a continuation of last week's notes, not surprising when you consider we are in a similar weather pattern. Dry weather definitely means time to catch up on winter projects that were delayed during the very wet weather of December and the first part of January. Punching holes to allow surfaces to vent and encourage root growth at a time when shoot growth will be limited is a big tick in my books and just tickling surfaces along with light foliars as and when. After all, we are only 3 weeks from the start of March so no need to do anything drastic to surfaces, just let them breath and if timely, remove a bit of organic matter :)

Day length is beginning to stretch out towards spring and summer and you can see from this image taken on the Sun Seeker 3D app that the sun is higher in the sky now compared to the Winter Solstice. The green line is the arc of the sun on the date of the Spring Solstice (20-03-23), the yellow line is the arc of the sun yesterday (05-02-23) and the light blue line is the arc of the sun on the Winter Solstice (20-12-22)

Longer days mean more light or the potential for more light anyway.

Looking at our light levels for the year-to-date as measured on an Apogee SQ-212 in a Central England location, you can see we are edging up slowly towards the bottom sufficiency range for Lolium perenne, (11 mols per m2), but as yet nowhere near Agrostis currently. That said, I think a day when we get > 10 mols per m2 does tend to benefit bentgrass, even if it isn't sufficient for strong photosynthetic conversion of light energy into sugars and growth. Cloud cover and therefore light levels are I think one of the trickiest weather parameters to predict because of the associated variability and the effect of local topography.

This theme of local topography and its effect on localised weather is something I have mentioned before but it hit home again as I sat in the audience awaiting my turn to talk at the excellent U.K Lawncare Association annual conference down at Tewksbury. Big thumbs up to Kate, John and Richard and all the UKLA group for putting on an excellent conference.

As I sat there I watched some rain clouds push in from the west bringing very light rain to my location. A few miles away they butted up against the range of hills to the east of Tewksbury and fell as heavy rain. What was happening here was the rain clouds were having to rise to clear the hills and as they did so, they cooled and the water vapour they contained condensed into moisture / rain. So we know the windward side of a hill / mountain will always picks up the majority of the rainfall and the leeward side, the minority. I wonder if that feature of the local topography is taken into account when it comes to a weather forecast ?

OK, short blog today because I have a packed things-to-do-list but next week I'll look back at January in detail and hopefully we may see an end to this diffluent block ?

All the best.

Mark Hunt

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