Mark Hunt weather corner; 111
How to spot a wrong 'un
In my series of articles this year, I have talked about how trough patterns in the jet stream are behind the increasing frequency of extreme weather as it is labelled by the tabloids.
I have also highlighted how rain rate measured in mm per hour is the real villain of the piece when it comes to flooding as drainage systems are overwhelmed by the sheer intensity of modern rainfall dynamics.
In this edition, with Storm Babet fresh in the collective memory, I thought it would be helpful to talk through the process of identifying one of these excessive rainfall events in advance. What characterises them, where you can find the advance information and how do you interpret it?
So, what characterises a bad storm system from a rainfall perspective? There are 4 to look out for:
- The formation of a trough pattern of cooler air in the dynamics of the jet stream. Remembering that it forms at the boundary of warm and cool air when you look at a GFS projection (Global forecasting system), depicted by a dark green line, more on this later.
- The presence of a slow-moving, low pressure system when you scroll through the projections. Ordinarily a strong jet stream speed will push a low pressure system across the U.K & Ireland in 12-18 hours, a low pressure system hanging around for longer will normally result in high rainfall totals.
- In general, (but not always) the lower the pressure at the centre of a low pressure system (measured in millibars), the more intense the weather system. Storm Babet, as an example, measured 956.4mb here in The Midlands, whereas typically an Atlantic low pressure would be in the region of 990-1000mb.
- The last is an obvious one - forecasted rainfall totals. If you're seeing high rainfall totals on consecutive days and these stay stable as you get closer to the event, you know the characteristic of the rainfall. What you won't see (yet) is forecasted rain rate figures.
Where do you find the forecast projections?
There are various websites you can use.
I use one called www.tropicaltidbits.com. It is a free-to-use site but asks for a donation as a Patreon (if you intend to use information for this site regularly), which I think is fair enough. This site has huge amounts of weather information but, for the purposes of this article, there are two areas that provide the most useful information for turf managers.
www.tropicaltidbits.com/analysis/models/?model=gfs®ion=eu&pkg=z500_mslp
This will take you to a GFS forecast of weather system movements over a set time period.
You can scroll through 6 hours at a time and watch how the weather systems move. What you are looking for is a low pressure weather system (characterised by a large capital 'L' in red). It will have the atmospheric pressure in millibars written next to it - remember, the lower the pressure, the more intense the low pressure system.
If you are interested in wind strength and direction, look at the closeness of the isobar lines on the charts and remember that low pressure systems rotate anticlockwise and high pressure systems clockwise (in the northern hemisphere that is).
Below is the schematic for Storm Babet from 20-10-23 06:00am in 6-hour intervals
Notice how the position of the centre of the low pressure system (marked by a red 'L') does not move in the more familiar west-east trajectory, rather it tracks in from the south and moves northwards, up the eastern side of the U.K. Another point of interest, rainfall events that follow this pattern (originating in the Bay of Biscay) tend to give more reliable rain to the south and east of the U.K compared to Atlantic low pressure systems which typically bring rain to the west, north west and north typically.
You can also access a rainfall pattern projection on the same
website;
www.tropicaltidbits.com/analysis/models/?model=gfs®ion=eu&pkg=mslp_pcpn
Visit: www.weatherstations.co.uk
Send your questions for Mark to: editor@pitchcare.com