October snow

Weather events in the Cookstown area on Tuesday, October 28th and Wednesday, October 29th were outstandingly unseasonal.  Tuesday the first occasion on which I had seen snow lying in October, was then upstaged by Wednesday with a five hour snowstorm, with snow depth averaging about 5cm.

Even in winter a combination of special circumstances are necessary for a heavy snowfall. For snow in October an almost impossible number of factors must be present, but it happened!

The previous Saturday a depression moved eastwards off northwest Scotland and gave a wet, windy day-just another wet Saturday in 2008. Rising pressure in the Atlantic cut off the advance of any further low pressure systems, this allowed the Saturday depression to slow down of southwest Norway where it actually deepened and drew down cold air from the Arctic over the British Isles.

In recent years, in a similar situation, just as cold air had set in, Atlantic high pressure surged over Britain cutting off the northerly blast, perhaps giving a few days of dry frosty weather but no snow.

On this occasion the Atlantic high pressure ridge remained in the Atlantic, blocked milder air from the west and indeed acted in partnership with the Norway low to channel Arctic air over Britain.

This set the stage, air from the arctic was driven over us. In spite of global warming and the retreat of Arctic ice, the arctic is still a very cold place. Even so, snow from an Arctic blast in October? Surely not! We must have even more exceptional circumstances.

When a cold northerly air-stream comes down, wintry showers are likely. In October these would normally be of cold rain, maybe hail, possibly sleet. Often minor low pressure troughs swing south around the parent depression and beef up the showers for a time. One such trough moved over us on the morning of Tuesday 28^th October. The timing meant that the day had no chance to warm up, the temperature was 2°C- at the worst sleet might occur.  However, as the precipitation became heavier, the heavy sleet drew down colder air from above and we ended up with over an hour of heavy snow giving a “white world” for a short time.

Events on Wednesday were similar but on a much greater scale. Sometime on Tuesday a cold air depression had developed near Iceland, it became caught up in the circulation of the deep low near Norway and headed straight for Ireland. It moved quickly, maintained a tight circulation and thus did not absorb enough milder Atlantic air to raise its temperature.

When it reached us, there was sleet for an hour or so, but just as on Tuesday as the sleet became heavier cold air was dragged down. Snow set in and continued for almost five hours.

Forecasters had expected the east of the province to get snow ( the east was expected to remain in cold air while the west with Atlantic influence would just get rain.

I am fairly sure that it was the weight of precipitation further west that turned this forecast on its head; the east never had the same heavy precipitation, only sleet and rain.

Some dates from Ian’s Diary. 

Earliest snowflakes seen to fall:                    17^th Oct 1955

Earliest lying snow:                                           28^th Oct 2008 (previously Nov 8^th 2001)

Earliest snow to 5cm depth:                           29^th Oct 2008 (previously Nov 20^th 1971)

Earliest snow to 10cm depth:                         20^th Nov 1971

Earliest snow to 20cm depth:                         26^th Nov 1965
(this was the cumulative depth on 26^th, following successive snowfalls on 25^th and 26^th, both falls from depressions not unlike that of 29^th October 2008)
Deepest snow pre-Christmas:                        8th-9th December 1967 ( about 22cm)

Deepest snow on Christmas Day:                 1968 ( about 10cm )