December 9th, 2024

Hi All,

As the clear up from Storm Darragh continues, we can reflect on how the beginning of the 2024/25 Storm season (starts on September 1st) has got off to a pretty hectic start. Below I have collated all the named storm systems that have affected us and their particular tracking data as this is very pertinent to their potential for damage and in particular, flooding.

Comparing 2024/25 with 2023/24, our most damaging storms from a rainfall perspective have tended to come this side of Christmas and in my mind there is a good reason for that. With sea temperatures higher than they have been in the past, the track of a particular storm system plays a key role in determining the potential for that storm to cause excessive rainfall and more importantly, excessive rain rates that overwhelm drainage systems, break river banks and cause widespread flooding.

The warmer the sea temperature, the more energy a storm system passing over it will carry. For every 1°C increase in air temperature, the air can hold 7% more moisture, so if the storm tracks over higher than usual sea temperatures, it can pick up more energy and potentially more rainfall in a process known as cyclogenesis. The low pressure systems that brought excessive rain to Central Europe, Spain and the U.K in September / October all picked up energy from higher than usual sea temperatures in The Mediterranean, Black Sea and Bay of Biscay. It should be noted that these weren't named storm systems, some of them were another new climatic feature for us, Dana's or cut-off low pressure systems.

Using the excellent My Ocean Pro viewer on the Copernicus.eu website (find it here), we can see the difference in sea temperatures currently existing around the U.K below ;

I picked 3 points to look at the current sea temperature and you can see the results above. Comparing the sea temperature just off the south west of England and Ireland, their temperatures are relatively similar (12.08°C vs. 12.64°C respectively), but if we look at The Bay of Biscay, it is sitting at 14.58°C, some 2°C higher.

So if a low pressure system tracks up from the south of the U.K, it will have the potential for more excessive rainfall. Note, it doesn't have to be a storm system, this applies to any low pressure system. The role of increased sea temperature in energising low pressure systems is a new feature of our changing climate. Interesting I think....

General Weather Situation - w/c 09-12-24

Storm Darragh was slightly different (see above) as the storm track came in from the north west and the low pressure system created a trough in the jet stream which slowed down its movement significantly. You can see from the animated GIF above that the resulting situation from the passing of Storm Darragh is that we will enter a peak pattern in the jet stream which means high pressure will dominate our weather this week and that's good news if you need some drier weather to clear up and maybe apply a pre-Christmas treatment.

So this week we have a predominantly dry picture with a prevailing north easterly wind for a good part of the week. Now a north east wind means a couple of things in my mind. Firstly, it'll be a bit of a dull week as cloud cover pushes in off The North Sea, secondly despite a mainly dry prognosis, we could see showers blowing off The Wash into eastern and south eastern England during the week. As we get to the end of the week, the high pressure is squeezed out of the way and we switch to a north westerly wind and the chance of rain pushing across Ireland and into the U.K through Friday into Saturday together with that change in wind direction. As you can guess from the wind direction this week, it won't be a particularly warm one with 5-6°C and the risk of fog and night frosts as we get to the end of the week / weekend.

Weather Outlook - Spray Window Potential - w/c 16-12-24

So this forecast takes is into the last full week before Christmas and as above will highlight the potential for a spray window.

Next week looks to start off with a threat of rainfall courtesy of a weak low pressure system that barely raises an isobar on the GFS projection above, courtesy of tropicaltidbits.com. So potentially a wet start for the week, maybe Monday offers a drier day for England, but already rain fronts will be moving across Ireland, Wales and Scotland. For Tuesday, the risk of rain is maximised from that southerly low pressure system and then we see a brief hiatus on Wednesday morning for central and eastern areas (but not for Ireland and westerly coasts) as a deep, Atlantic low pressure system will bring strong south westerly winds and heavy rain to Ireland and the U.K during Wednesday / Thursday. This low pressure system has some pretty cold air associated it despite the wind direction, so we could see wintry showers and snow further north across Scotland. As mentioned in last week's blog, as a low pressure departs its trailing edge pulls down northerly winds, so I see a threat of wintry showers extending further south at the end of next week potentially along with night frosts.

As we approach the weekend, we see another low pressure system pushing across to continue the run of unsettled conditions and more rainfall. As this low departs on the run up to Christmas, it will pull down northerly winds and an increased risk of snowfall. No sooner has this one departed then we see another low push into the trough and this one could bring unsettled and cold conditions over Christmas itself.

Now a big caveat here, it's 16 days off (so yes plenty of time for last minute Christmas shopping 😉) so things can and most likely will change before next week.

Agronomic Notes - Spray Windows and all that...

One thing is for sure, next week looks like being a pretty rubbish / tricky one for getting on a pre-Christmas application, be that turf hardener, dew control and / or fungicide, so my consideration would be to go this week even though it is further out from Christmas.

Even though applying this week is further out from the Christmas period than one might like, if we look at the projected minimum and maximum air temperatures for the coming 12 days or so and convert those into projected Growth Potential, we can see the outlook is for minimal growth and therefore potential removal of the applied product leading up to Christmas. So if the threat of showers doesn't queer the pitch too much, I would go for this week for my treatment because the outlook for next week and potentially the week after looks pretty dire from a rainfall and wind strength perspective.

That's all for me this week, wrap up well.

All the best.

Mark Hunt