The Isle Of Man Shadow

While the east and southeast of Ireland was being affected by heavy lake-effect snow showers this week, many people commented on a peculiar pattern visible in the shower distribution - the Isle of Man shadow, writes TMT's synoptics forecaster Fergal Tierney.

A remarkably clear shower-free slot extending southwestwards from the Isle of Man, this phenomenon had a huge effect on the 1.5 million people living in the greater Dublin area, some fifty miles to the southwest of the island. Its presence meant that while most areas were being bombarded by trains of showers, certain areas within about a twenty mile swathe enjoyed relatively clear skies - albeit just temporarily! But how did this occur?

A look at the satellite picture for December 1st shows exactly what was going on for much of the week. We can see that as the cold dry Arctic airmass flows southwestwards off the northwest coast of England, skies are clear up to a certain point. After this, the sea has modified the lowest layers of the airmass enough for cumulus to form. As they continue their track over the sea, these cumulus quickly grow in size, and start producing precipitation by around Angelsea. By the time they reach the east coast they are fully fledged cumulonimbus, complete with hail, snow, graupel, thunder and lightning! The distance required for clouds to first start to form is a delicate balance of airmass temperature and dewpoint, windspeed and sea surface temperature. The faster the wind, the further the airmass can travel before clouds can form, and vice versa.
The problem with the Isle of Man is that it's right bang near the area where the clouds start to develop, and it therefore hinders this process, resetting it back at zero again, as seen in the cloud-free zone to its southwest. The process doesn't have enough sea track left for cumulonimbus to develop, hence leading to the clear slot, or "shadow". Another factor is that the airflow diverges around the island, and when you have divergence at low levels, air must subside from above to compensate. Subsidence is the opposite of convection, and warms and dries the airmass, which tends to evaporate clouds instead of form them.

You may have noticed that there was a slight north-south toing and froing of this shadow throughout the each day, which meant that areas that were clear for a few hours then took up the slack from areas that were showery. This mostly occurred through north county Dublin, northeast Kildare and east Meath on its northern flank, to south Dublin and north Wicklow on its southern flank. This was caused by a subtle change in the steering upper winds, but I also suspect there could be some low frequency sinusoidal form to it too, linked to the diverging airflow pattern around the Isle of Man, akin to Von Kármán Vortices. This could be an interesting topic to delve further into at some stage.

In any case, this northeasterly flow has now ceased, but the next time we get one in the Irish Sea, keep an eye out for the IOMS!