February 20th, 2023

Hi All,

Well the media is already forecasting a return to winter and 15-20cm of snow on the way, this week blardy, blah. As discussed last week, the Sudden Stratospheric Warming (SSW) has taken place and we now await to see if it will impact on our weather down in the Troposphere. Certainly from the 10-day GFS output, we can already see a shift in the direction of flow in the jet stream with a reversal from the more usual west→east direction to east→west occurring at the end of February.

This means any 'weather' related to the SSW event won't actually happen until we move into the start of March. So if any period is going to be affected, it is the first half of March and since that is beyond the scope of forecasting accuracy, you can see why what you're reading is just typical media crap, nothing else.

Does a SSW event increase the chance of colder weather in the first half of March ? Yes for sure....Does it definitely mean the first half of March will be cold We can't say that yet....

Netweather.tv have an excellent narrative on the SSW, how it is unfolding and where it may or may not be leading. It is worth reading....here . It does highlight the increased probability of colder weather for the first half of March.

To my mind it really depends on the position of cold air to the east / north east of us around this time as to what weather we are likely to get but almost certainly we will experience a run of colder easterlies and that we know seldom makes growing grass a pleasurable experience.

Here's how the GFS forecast looks like as we run towards the end of February ;

So we have a Diffluent block pattern forming with high pressure to the north and low pressure to the south. This will funnel cold air in-between in an easterly / south-easterly direction. The jet stream is effectively split with the northern portion still flowing west→east, but the southern portion will flow east→west and the blue block arrows show how the low pressure will move westwards instead of eastwards.

So after a lovely dry February when lots of you got plenty of early work done, we look to be heading for a cooler spell of weather and a slower start to spring proper potentially.

No great shakes there, it happens quite often nowadays that March and April are slow growth months, we just have to prepare correctly and manage the inevitable expectations now that the lawn has started growing at home (albeit temporarily) :)

General Weather Situation - 20-02-23

So for this week we have a gradual loss of that lovely mild air that has made my walking in the Leicestershire and Rutland countryside so pleasant as of late.

The week is split into two parts really, Monday and Tuesday will be mild and largely dry with a strong to blustery westerly / south-westerly wind. They'll be rain across the west and north west of Scotland through Monday and into Tuesday, but elsewhere we remain dry with temperatures in the mid-teens for some.

Later on Tuesday we see a band of rain move into the west of Ireland and this will move across Ireland overnight, pushing into Scotland, the north west of England and North Wales by Wednesday morning. Wednesday is the change day as the wind swings round to the north west and that will take the edge of the temperature, with temperatures expected to be 8-9°C. This change in wind direction will push that rain across southern areas in the form of blustery showers through Wednesday, It'll remain cooler and unsettled across Ireland and Scotland with plenty of showers as well.

Night time temperatures will drop as well and hover in the low single figures area, enough for ground frost in places where skies clear going into Thursday. More rain arrives on Thursday for Northern Ireland and Scotland and drives south across Ireland and into northern England, petering out slowly as it moves south. Some places will miss these showers and remain bright and cool all day with that north westerly wind remaining in situ. Again temperatures sitting in the 8-9°C sort of range with that cooler wind.

Closing out the week on Friday, the wind swings round to the north and that pegs temperatures back further. A mainly dry picture on Friday, except for showers extending down eastern coasts during the day. If anything that wind will strengthen during the day on Friday to make it a chilly affair even where you see the sun.

The outlook for the weekend is really more of the same with the wind now fixed in a NNE direction and pushing the odd chilly shower into East Anglia from The Wash. Mainly dry though for most of the U.K & Ireland with plenty of sunshine. Cooler night temperatures may just give a ground frost in sheltered areas dependent on wind and cloud cover obviously. So cool, dry and bright with that wind backing off in intensity as we go through the weekend.

Weather Outlook - 27-02-23

So this one is the tricky one because of the issues I have highlighted above in terms of the events that are yet to unfold following the SSW event, but here goes anyway.

Next week looks like starting calm and settled most likely with ground frost. It will most likely be a week of easterlies due to the Diffluent Blocking Pattern shown above which will funnel winds from this direction. As such I think it'll be a dull and cool week with some showers from Tuesday onwards pushing across from The Wash and The Channel. Towards the end of the week is when life gets potentially interesting as the winds strengthen and swing round to the north east.

Interesting because as the GIF above shows from www.tropicaltidbits.com, it is to the north east where the cold air sits and that sets the stage for a cold air incursion towards the end of the first week in March which may well be wintry. Now this is more than two weeks away, so as always it is heavily caveat-laden, but I think if we are to see a wintry consequence from the SSW event, it'll be around this time.

Agronomic Notes

This week, I wanted to show the two types of days we typically get in the spring and how they result in a very different growth proposition for the grass plant.

Note - the charts are using data generated every 15 minutes from a Davis Vantage Pro2 Groweather station and the Growth Potential is calculated for that period. (and so is slightly different from if you just entered the min and max temperature into the usual Growth Potential equation)

So the first type of day I am looking at is when we have a hard ground frost but a very mild afternoon.

This type of day is typical of March and April and probably causes the most issues from a turf management perspective. Clear skies overnight have meant there's been a sharp frost with temperatures dropping down to -2.4°C.

There's likely to be a frost delay if you have that policy in place on your course, with the air temperature not rising above freezing till 10:00 AM. Thereafter it rises rapidly to hit a peak of 13.5°C before the air temperature rapidly falls as the sun sets.

The red horizontal line is drawn at G.P=0.4 and symbolises when there is enough temperature to initiate good spring growth.

So on this type of day, that period is condensed between 12:00 - 4:30 PM and the total Growth Potential for the whole 24-hour period day is 0.02, in other words very little.

The second type of day is when cloud cover maintains mild overnight temperatures so there's no pronounced slump at either end of the day. You can see that the result of the milder night temperature is a much longer duration of Growth Potential, beginning at 4:00 AM and extending all the way through to 11:30 PM. So even though the maximum day time temperature (12.9°C) is lower than the example above, the fact that the night time temperature never dropped below 9.7°C have made the difference.

So on this type of day, the period for actual growth is extended to between 04:00 AM - 11:30 PM and the total Growth Potential for the day is 0.48, in other words , a good spring days growth.

The bottom line is we need to focus on night temperatures to determine if we are likely to get enough hours of actual Growth Potential for you to see a nice clip rate, recovery from aeration and / or disease scars.

Night temperatures are the key to spring growth.

If I chart out the Growth Potential from the start of the year, you can clearly see the two different types of day, the first type where we have a pronounced short peak of G.P and the second where we have extended G.P over the whole day.

Interestingly you can see how we got some nice growth in the first half of January (wet and mild = cloud cover = higher night time temperatures). This then dropped away as we moved into a period of heavy frosts and cold days before a brief uplift at the beginning of February. More of the same cold nights and mild days then followed before some milder nights put us back into positive G.P last week. In the west, this would have happened earlier in the week because of more cloud cover.

So this time next week I'll have more of a handle on the effects of that SSW and whether that early signal for a wintry second week of March is growing in probability or declining. Until then, stay safe, take care and dig out those buffs and thermals because they're going to come in handy again !

All the best.

Mark Hunt