As we tip-toe out of February later this week we will at the same time endure one of our coldest weeks of the year. February 2018 will also go down as one of the lowest GDD months we have recorded since we started this process back in 2010. Looking at the stats from my faithful little Netatmo weather station on Weather Underground it shows that for the 26 days of February so far, we have had frosts on 20 of them and given that the last days of February are dead cert frost days, that’ll be 22 out of 28 days frosty, nearly 80% of the month, hardly a surprise then that we didn’t see much GDD or grass growth this month.
As forecasted last week, the Sudden Stratospheric Warming Event (SSW) has indeed reversed the direction of the sub-polar jet stream by pushing the main jet south of us down into The Azores and leaving us with a blocking high sat right over us. The cold easterlies that were predicted when we have an SSW have also come to pass so there we have it, we called it a week before.
It is a major meteorological event, this blocking high pressure is forcing mild, wet air right up into The Arctic causing temperatures to jump there by 35°C and no doubt leading to more concerns about the fragility of the winter ice pack, (already the thinnest it has ever been) meanwhile the politicians sit on their collective fat behinds. (Day after Tomorrow comes to mind)
My our NAO looks negative 🙁
We also have an extremely negative NAO at the moment don’t you know, NAO stands for North Atlantic Oscillation and is the relationship between what is normally high pressure in The Azores and low pressure across the Atlantic (that’s known as a positive NAO). Well our one is negative because the high is above us, the low’s are below and the oscillation is severe. Why is NAO important ? Well until it swings back positive then you can forget about much in the way of mild weather and growth. It is therefore a good yardstick to watch over the coming days for indications of a change away from this cold spell. You can see from the above that most projections are indicating that this cold weather will be with us (though not as severe perhaps) till possibly Mid-March.
I started a lot of my GDD work back in 2013 when we had a brutally cold spring and people were asking me “How far behind last year are we ?”, I couldn’t tell them definitively so I got to work on using GDD. This spring I think I am going to get that question a lot.
So as we look to the coming week and the threat is of snow, particularly on eastern coasts and The South East / South pushing across to The Midlands and the like at the end of the week. Already I can see the odd large, dry snowflake whisking by my window and this is just the start.
Last week I happened upon a smart weather chap, known as Metman James on Twitter, who is a Meteorology PhD student at Exeter University and we got chatting. He has some great GIF’s and output on his Twitter feed that highlight the weather situation and a prognosis for when it is likely to change. You can find him here.
General Weather Situation
Before I start this forecast I’d like to point it that it’s probably one of the hardest ones to call, first, when and where the snow showers will affect during the early part of the week and second the extent of the heavier snowfall event likely Thur p.m. / Friday. (and when and where it may turn to rain) So don’t whine at forecasts and the fact that it keeps changing, because it will. Just get hooked up to some proper rain (and therefore snow) radar and look not only at where that moisture is occurring but also from which direction it is coming from. That way you’re properly prepared.
Ok so we start Monday after another cracking frost (-2.9°C) and begin a cold and largely dry, bright day with some hazy cloud and sunshine. There is however a risk of snow showers spilling in off The North Sea and this risk will heighten as the day goes on. Already on the radar we can see a raft of snow showers affecting the east coast from Hull south and another band spilling in across Suffolk and Essex across London and into the south of England. Really anywhere south of The Borders is fair game today for snow showers and whilst these will affect eastern coastal areas initially, they will increasingly push inland so keep an eye out. Wales, the west coast and Ireland will have the majority of the sunshine and in it expect temperatures to leap up to a positively tropical 4-5°C. Elsewhere closer to that moderate, easterly wind, expect this to be more likely 2-3°C with a negative windchill.
Onto Tuesday and a heightened risk of wintry showers, this time more likely to give significant accumulations where they occur. Area-wise I would expect a similar pattern across southern England, East Anglia, the east coast all the way up to Aberdeen this time. We can also expect more significant snowfall further inland over northern England and into Wales I think as well. Similar temperatures to Monday perhaps colder across The Irish Sea as those easterlies extend their hold. Snow is even harder to forecast than rainfall so your best bet is to get onto a website that shows active radar and this will give you the heads up as to where the moisture is occurring and heading. I use Netweather for my output because it’s more national but you can get it specific for your area down to 500m2 which is pretty handy. There are lots available though, Netweather radar can be found here
Wednesday looks a re-run of Tuesday with a harsh penetrating frost and a bright, sunny, cold winters day. Again plenty of snow showers around and as the cold intensifies, the westerly reach of those snow showers extends into Wales and across the Irish Sea into Leinster. Again the majority of snow showers is along the eastern coastline of the U.K but plenty of chances of them moving inland. The wind strength will begin to ramp up on Wednesday and with a colder start to the day and stronger winds I’m not sure the temperature will get above freezing and it will be accompanied by a windchill down the wrong side of -10°C I think ! In other words, bloody freezing.
Thursday, the first day of March, will I think be the coldest day of the week with the hardest frost anticipated down to -7°C I think and possibly colder ! Overnight we have seen plenty of snow showers with accumulations right across the U.K and eastern coastline of Ireland. The suggestion is that the worst snow will be Midlands and north but I think there is plenty of time for this to change yet.
Thursday is very much the change day because we come under the influence of the southern low pressure system that is potentially due to bring heavy snow to many parts of the south of England later in the week. The first thing you’ll notice on Thursday morning is the wind strength as the isobars are packed tight together so the windchill accompanying the severe frost will be down in the -15°C region, seriously parky and not for the faint-hearted. That low pressure system south of England will begin to bring in snow showers and heavier accumulations to the south coast and up towards the M25 / M4 (Oh joy of joys) Expect more in the way of snow across Scotland and The Midlands pushing across Wales and Ireland as well, whipped along by that fierce easterly wind. High winds, bitter windchill and snow showers will continue through Thursday night.
Friday sees I think the highest threat of heavy snow however we are also seeing a temperature lift across the south of England so there’s a great deal of uncertainty of whether we will see snow or snow then rain. I’m thinking more likely the latter across the south of England, Wales and Ireland turning to snow as it moves further north. Temperatures will rise through the day with 2-3°C likely across the south and below freezing across The Midlands north but through Friday evening that milder air will push further north so snow turning to rain with a thaw is likely I think. For many Friday night may be frost-free as that thaw continues into the weekend with Sunday likely to be much milder. Bizarre, bizarre. That strong to gale force easterly wind will continue to blow hard through Friday but will moderate through the latter part of the day. At this stage I think the worst weather will be across The South West, South of England, Ireland, Wales extending into The Midlands. As I mention earlier the likelihood is that this moisture falls as snow initially giving way to rain the further west you go I think.
So the outlook for the weekend….
So we are in the grip of that English Channel low pressure system over the weekend and actually extending through Monday so that means very unsettled through Saturday with high winds, rain, sleet and snow showers principally affecting the south west corner of the U.K, along the south coast and across Ireland and Wales as well. Scotland you are the right end of the country for once but I still expect snow showers for you as well. Again the uncertainty is in what form this moisture will fall, but fall it will so a very unsettled day for Saturday, but feeling slightly milder with southerly / south easterly winds initially swinging more northwards. Across the south of England we may see temperatures up to 7-8°C, but knock that down to 2-4°C for Wales, Ireland and The Midlands north. Again uncertainty about where that moisture will fall but likely to be more rain than sleet across the south of England turning more wintry as it pushes north on Saturday evening. Sunday sees that rain band more across the north of England and perhaps southern Scotland now, falling as rain, sleet or snow depending on elevation. Milder air pushes up behind this moisture band to give a drier day across the south of England, Midlands, Wales and Ireland with temperatures on the up, touching double figures possibly across the south on Sunday. The wind will still be easterly but possibly dropping down through Saturday to a more moderate status.
So next week looks like being another complicated one to forecast but at present I expect us to remain unsettled with that low pressure lying in a trough created by a southerly positioned jet stream. That means the low will stay south and then gradually move north and east through the early part of next week so I expect heavy rain, sleet or even snow for the first part of next week down south, drier for Ireland, the west and north though. As we go through the week those easterlies will become northerlies so no quick return to mild weather but perhaps not as cold as this week with less in the way of frost. So a drier end to the week possibly as that low pressure system moves away but staying cold.
As you can see from these wilting Hellebores in my frozen flower bed we have frost desiccation at work this week in some parts of the country. Out walking yesterday it was amazing to walk across 2 ploughed fields that last week left most of the field on my boot such was the stickiness of the soil, to this week when it was powder dry on the surface. This is the result of frost desiccation where water is frozen in the soil and therefore unavailable to the plant, it may also be due to water within the plant itself freezing. As temperatures increase the plant re-establishes its internal osmotic balance and the wilt corrects itself.
Cold temperature desiccation can lead to discolouration of shallow rooted grasses for the same reason as we see in the hellebore above. I took this picture many moons ago and in it you can see bleached patches of I think Poa annua var. annua which were subject to cold temperature desiccation. I expect to see the same occur this week, such is the severity of the frost and in some parts of the U.K, dryness of the soil as well.
For all of you guys that have aerated and topdressed recently then I expect you’ll also see some frost heave where the sand is pushed back up to the surface by the result of ice expansion in the soil down the core holes.
On the subject of soil dryness, I was asked last week to do a rainfall comparison that highlighted what a wet winter it has been across the north-west of England. Frost desiccation won’t be an issue here unfortunately. As you can see from the rainfall comparison above of 3 areas of England, The North West has experienced nearly 150% more rainfall than Oxford or Birmingham. This recent run of cold and frost is just the thing those guys need.
When are we likely to see growth again ?
With many areas of the U.K only experiencing 2 days of growth in February but still experiencing winter wear, it is likely that the pressure for growth will ramp up as we get into March as tees, walkways and winter season pitch areas begin to show the signs of a long winter with very little recovery growth. As discussed at the beginning of this blog, the critical point to watch for is when the NAO switches back away from its current negative value. What this weather jargon means to you and me is that in order for the weather to right itself, the high pressure needs to move away and be replaced by a milder, westerly airstream and that effectively would translate to a positive NAO.
So what are the weather models saying ?
If we look at the projection for the NAO, it isn’t expected to hit neutral and swing to positive until the middle of March, the models predicting the jet stream position also agree. So I don’t expect us to see much in the way of mild weather until w/c 12th March, that’ll be 3 weeks by then of cold weather with easterly and northerly winds dominating. Now of course all this is a long way off and things can change one way as quickly as they change another but we have had an SSW and the associated easterlies, so a cold 1st part of March is looking increasingly likely.
Thaw on frozen ground risk…
For the areas in line for the main snowfall (south of England, Midlands, Ireland, The South West, etc) on Thursday night / Friday there is a risk of flooding on Friday / Saturday I think. This is because the hard frosts of earlier in the week will have frozen the ground solid to a significant depth and so snow thaw / and / or sleet / rain (depending on what we get) on top of this will have nowhere to drain causing water to perch on the surface. Another potential issue is instability in the surface as it begins to thaw over a frozen rootzone, this can be extremely tricky on sloped areas and certainly presents a Health & Safety hazard until the thaw is deeper and the surface regains stability. One to watch in areas that receive the penetrating frost and then heavy snow / sleet rain and temperature rise shortly afterwards.
Ok that’s me this week, bit of a short agronomic blog but the focus point is more weather than agronomic at the moment. Wrap up well this week, especially on Wednesday and Thursday and keep an eye peeled on the weather radar.
All the best.