An Analysis Of Saturday's Snowfall In Ireland

As you no doubt followed on our Live Snow Blog, many areas of Ireland were affected by heavy lake-effect showers early Saturday morning as the first Arctic blast of the season took a hold on the country, writes Fergal Tierney.  

Several stations reported varying types of precipitation, including rain, hail, graupel, sleet, snow and snow pellets. Here I am going to analyse the synop reports from some of the manned stations that gave hourly reports in SYNOP format early Saturday morning, 27th November (Dublin Airport, Casement Aerodrome and Belmullet). Some of the automatic stations in the network also reported precipitation, but the type reported by these stations is not as reliable as that of a manned station, which, as we will see, can give a wealth of extra information.

Ireland at the time was under the influence of a bitterly cold Arctic airmass that originated near  Svalbard and was fed down to us in a brisk north-easterly flow via Scandinavia. 850hPa temperatures (~1400m altitude) were in the region -5°C in the southwest to -8°C in the east and north. 

The temperature profile throughout the whole depth of the troposphere followed a dry adiabatic lapse rate (see the Castor Bay sounding from 00Z Saturday below), and was therefore highly unstable, especially in flow over the still relatively warm SSTs (Sea Surface Temperatures), which were 11-12°C around Ireland, warmest in the Irish Sea. This instability led to explosive convection over the Irish sea late Friday night, generating heavy lake-effect precipitation and frequent lightning. As upper winds turned north-easterly, this convective system fed into the east and southeast coast, bringing a long period of wintry precipitation to counties Leinster and east Munster throughout Saturday morning.
IMAGE: Castor Bay sounding

It is, however, interesting to study the detail in the actual reports submitted by the observers at the three stations above, because, as it turns out, the precipitation was not the pure snowflakes that one would expect, but rather a mix of hail and snow pellets that form in such cold unstable conditions.

Below is a summary of the Dublin Airport, Casement Aerodrome and Belmullet SYNOP reports for the period 1am - 12pm Saturday. The "Weather reported" column is a direct description of the Present Weather code reported in the 7XXXX group of each report (where included), as per the Official WMO Guide

Table 4677 of this guide contains a coded list of weather types to choose from when compiling a synop report, including pure snowflakes and variants (70-79), but the observers chose more complex types to describe the intricate weather that was occurring. The Notes column includes extra information that was included in the 333 line of the reports, and it is that information that is of interest to us here.

*DUBLIN AIRPORT (68m amsl)*

IMAGE: DublinSnow.png
* Hail, Small Hail, or Snow Pellets. Click on image to enlarge
We can see that they were reporting the presence of the thunderstorms in the Irish Sea, but at the airport there was a slight frost on horizontal surfaces, in an air temperature of around -2°C. The first precipitation was reported at 5am as hail in a thunderstorm, but the definition of hail in this case is "Hail, Small Hail (graupel) or Snow Pellets". Graupel and Snow Pellets both form when supercooled liquid glazes the surface of a snowflake, forming a hail-like particle. Graupel is more like hail, with maximum diameter of 5mm. 

The description in the report gives a maximum hailstone diameter of 1cm, so this gives an idea of the updraft speeds present within the thunderstorm, a result of the vigorous heating of the airmass from the warm sea surface below. There was 1cm of new "snowfall", but the snow in this case comprised hail and snow pellets. The 6am report gave a 1cm depth of this snow lying on the ground. At 7am, a rain shower was reported as the dewpoint was on the rise through around 0°C - a result of the huge shift in wind direction from WNW around to NE, which dragged in a marine influence from the Irish Sea. There was further soft hail and snow pellets, and then a final shower of hail in the traditional sense, before the system headed on southwestwards through the southeastern corner of the country.

*IMAGE: CasementSnow.png
* Hail, Small Hail, or Snow Pellets. Click on image to enlarge
Nearby Casement Aerodrome, around 10 miles to the southwest of Dublin Airport, showed a similar situation, but with some differences in the type of precipitation reported. At 5am there was a snow/rain snow/hail* mix, with maximum hailstone diameter reported as 4mm in the 6am report. There was some snow and sleet for a couple of hours as the same windshift as Dublin occurred, followed by a traditional hail shower at 10am, which dumped another 3cms onto the 1cm layer of lying snow.

IMAGE: BelmulletSnow.png
* Hail, Small Hail, or Snow Pellets. Click on image to enlarge
Belmullet, on the extreme north-western tip of Ireland, showed much more of a marine influence, with the Arctic airmass having a much longer sea track around the northwest coast of the country. Upper temperatures were also a degree or two warmer than the east coast. Dew points were hovering at or just above zero for much of the time, falling below zero later in the period. This was reflected in the presence of a rain mix in the precipitation reports, showing how melting was taking place in the lowest layers of the atmosphere. There was not the same intensity of frozen precipitation, and the lack of extra details, such as hail diameters, lying snow, etc. shows the difference a few degrees warming can make.

The current Arctic outbreak is a very unusual feature of our weather for late November, and has brought back vivid memories of the record winter gone by. Several inches of snow were reported in parts of the southeast and west of the country Saturday, and with the cold snap set to continue, bringing easterly winds for at least another five days, these depths are sure to increase, especially in the east, where further lake-effect showers and more organised bands will blow in on a stiff easterly wind from the Baltics. 

The particular profile of the airmass has made the snow more of a pellet-form, but there have also been significant snowfalls in the traditional sense throughout the country. But whatever the technical term, it's still not the usual November rain we'd expect at this time of the year, rain that brought the severe floods to the west and south last year. Hell, it's not even official winter yet! 

Will we see more of the same in the actual winter months to follow, starting December 1st, or will he Atlantic re-establish its influence and bring us a mild one? That's the 65 million IMF Bailout question!