JERSEY Met is to consider issuing ‘rainfall event warnings’ in future to enable Islanders to prepare for severe weather.
The agency was this week caught by surprise by two storms which rolled in on consecutive days, bringing extreme conditions which were not predicted by initial forecasting models.
The first – an intense low-pressure system which barrelled in from the Atlantic on Sunday night – was expected to send the worst of its weather over the eastern half of the Channel.
But the storm, eventually named Storm Gerard by Météo France, changed track late Sunday evening – hitting the Island with severe-gale-force-nine winds and gusts of 70mph which felled dozens of trees.
The following day Jersey Met forecast that a second storm would bring between 15mm and 20mm of rain, before clearing southwards.
But the weather system effectively stalled over the Island, dropping almost 60mm of rain and causing severe flooding in Grands Vaux.
If the heavy rain had been predicted, Jersey Met says it would have notified other government departments.
It would not, however, have issued a public flood warning, as this does not currently form part of its portfolio of weather alerts.
The agency is now working with other government departments, during its analysis of the week’s weather events, to determine whether formal rainfall warnings should form part of future forecasting.
Explaining the lead-up to the arrival of the first storm, Paul Aked, head of meteorology at Jersey Met, said: ‘During the weekend we were monitoring a rapidly developing low-pressure system, moving towards the islands from the west.
‘Given our size and position in the Atlantic, it is extremely difficult to predict weather systems like this and there were large uncertainties around this particular system’s track and depth.
‘Details of how strong the winds would be and where would be most affected did not become clear until late Sunday evening, when we began to see the track of the low pressure moving the most severe winds from Normandy and the eastern half of the Channel to come closer to the islands than expected.
‘This brought severe gales from the north/north-west, a wind direction in which trees here are more vulnerable. As a result, a large number of trees were felled, exacerbated by the saturated ground from recent heavy rainfall, and we also saw localised structural damage.’
Mr Aked said that an orange wind warning – the second-highest level of alert – was issued before the worst of the winds hit, but that 24 hours earlier it was simply not expected to ‘develop into such a vigorous system’.
On Monday, forecasters were monitoring the development of the second low-pressure system, which was expected to deposit between 15mm and 20mm of rain overnight and into the following day.
‘In reality, the fronts became slow-moving across Jersey, resulting in a longer period of heavier rain than forecast, with 58.6mm at Howard Davis Farm, equivalent to two weeks of rain falling during this event,’ said Mr Aked.
He added that the department used a combination of computer models and local knowledge and experience to forecast weather, and that ‘this leads to accurate forecasting most of the time’.
But he admitted that it was ‘sometimes difficult’ to forecast finer details during ‘certain weather situations’, such as that which led to the flooding on Tuesday.
Mr Aked added: ‘Jersey Met do not currently issue inland flood warnings, but as part of our post-event analysis, alongside other departments across government and non-government agencies, we will be working together to review this week’s weather events to ensure we provide the best services we can moving forward. That may see more formal rainfall event warnings coming into use in the future.
‘Irrespective of the present or future criteria that might trigger individual warnings, had we foreseen the actual amount of rainfall on Tuesday, we would have shared this with our emergency and planning colleagues.
‘This is part of our normal responsibilities and was not done, solely because the modelling and forecasting did not predict such a high rainfall total.’
