What’s up with the Weather this Winter?
An unusually high frequency of southerly or south-westerly winds persisted throughout December.
Warm and moist tropical air masses brought very mild, wet and stormy weather over Ireland.
December 2015 was one of the mildest on record in most areas and the wettest on record in parts of
the West, South and Midlands. The widespread flooding in some parts of the country was exacerbated
by already saturated ground following above normal rainfall in November. The combined rainfall
totals for November and December were the highest on record in some parts of the country, in
particular at Shannon, Cork and Knock airports.
For further information on extremes and other details, please see the provisional weather summary
for December 2015 from the Climatology & Observations Division in Met Éireann.
Jets streams play a key role in determining the weather. Jet streams generally don’t follow a straight
path, normally have peaks and troughs. This December there was a pattern of continuous troughs and
ridges across the North Atlantic and over Ireland giving us a mild predominantly south to southwesterly airflow, see Figure 1. This resulted in a prolonged spell of very mild and very wet weather
during which record temperatures and rainfall amounts occurred.
Further Information: What is the Jet Stream? NOAA Online Jet Stream School
(a) Desmond (5-6 Dec 2015)
(c) Frank (29-30 Dec 2015)
(b) Eva (24 Dec 2015)
Figure 1 (a)-(c) Images from California Regional Weather Service, model winds at 300hPa.
For comparison, picking one of the driest, calmest, coldest
day in Ireland during December as Sunday 13 December
there is a marked difference in the Jet Stream structure,
disjointed and to the north of the island, see figure below.
Temperatures dropped to -3.1°C at Markree station (in Co
Sligo) and 0.8mm of rainfall reported in Mullingar (Co
Westmeath) with the daily 10-min mean wind speed measured
a light breeze (4.7 knots, 9 km/h, 5 mph).
Figure 1(d)
Follow tweets from @METclimate for more information on the storms and @METeireann for the
latest forecasts.
Heavy rainfall from Storm Desmond followed by Eva and Frank contributed to widespread damaging
floods and were associated with strong south to south-westerly flows across the North Atlantic.
An ‘atmospheric river’ formed on a stronger than normal jet stream which resulted in large quantities
of rainfall falling over parts of Ireland during Storm Desmond, Figure 2.
Figure 2. IMERG image - Storm Desmond
It is very difficult to determine the exact causes of the recent extreme weather. Some studies suggest
that the El Nino event in the Pacific is likely to increase the risk of strong jet streams, heavy rainfall
and stormy conditions during winter. Climate Change models suggest that with rising temperatures
leading to an increase in the water carrying capacity of the atmosphere we can expect an increase in
the frequency and intensity of severe weather events. However climate needs to be assessed over a
number of decades before we can confidently identify genuine trends, as opposed to natural
variability. At present we can point to the unusual persistent strong west-south-westerly air flow over
the country as the predominant causes of the extreme weather this December, without attributing a
definitive cause.
Further Information: Ireland’s Climate: The Road Ahead, Met Éireann