Met Eireann rainfall weather forecast chart for Storm Dennis
Met Éireann says the first impact of Storm Dennis will be felt from Saturday morning and be felt right through the weekend according to the forecaster which believes it could be a record-breaking storm.
The forecaster has already issued weather warnings from the storm and also outlined what to expect and what is causing the stormy weather in a weather commentary on Storm Dennis by Paul Downes, Meteorologist, Forecast Division.
He says Storm Dennis is undergoing rapid cyclogenesis in the Western Atlantic. While staying to the northwest of Ireland, Met Éireann expects Storm Dennis to produce some wet and windy weather over Ireland this weekend.
It warns that flooding issues around rivers are on the cards due to elevated levels, particularly in the midlands, west and south. It says an any heavy rainfall would cause issues.
He outlined how the storm could impact as follows:
Saturday: The first round of wet and windy weather will reach the Atlantic coast on Saturday morning spreading countrywide during the morning easing later and Status Yellow wind and rain warnings are in operation.
Sunday: The core of storm Dennis (actually a hybrid of Fridays Low and Storm Dennis) will gradually fill and weaken as it tracks south-eastward towards Ireland bringing squally showers with a risk of thunder. A widespread Status Yellow wind warning is likely to be issued and possibly Orange level winds locally (especially in the vicinity of squally showers along Atlantic coasts) can be expected.
Monday: Winds will gradually ease through Monday, but not dissimilar to this past Monday showery conditions can be expected with wintry conditions possible.
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Explaining what is causing the storm Mr Downes said the strength of the Jet stream (an atmospheric river) is determined by the temperature gradient closer to the surface, called the zone where warm and cold air meet the baroclinic zone (a strong temperature gradient that separates warmer air from colder air and often the battle field that spans storms).
"The ongoing battle between warm air over the south central Atlantic and a cold pool over much of Eastern Canada has led to a strong level baroclinic zone fuelling a powerful jet aloft with several waves tracking across the Atlantic. The lead wave of the two has been the driving force for this first deep low ~933hpa currently tracking northwestly and occluding south of Iceland.
"Warm air being pushed up along the US east coast meeting very cold air over Eastern Canada is the driving force for this powerful jet, with winds up to 225 knots (~416 km/h) at its core, currently aiding the rapid development of Storm Dennis. Dennis will develop rapidly today (Friday).
"To put this into perspective, rapid cyclogenesis is defined as a drop in pressure of greater than 24 hPa in 24 hours, this storm is forecast to drop by around 70 hPa in 24 hours as it rapidly tracks north towards Iceland and with a forecast central pressure in the low 9-teens it will rival the lowest ever pressure recorded in the north Atlantic of 914 hPa off the coast of Scotland on Monday 11th January 1993.
"The core of Storm Dennis is expected to stay well to our northwest as it rapidly strengthens before pirouetting (the Fujiwhara effect) around Fridays low and comes back to visit us on Sunday as it slowly weakens," said Mr Downes.
Read also: STORM DENNIS WEATHER WARNINGS