The impact and likelihood of a storm Aileen or Sebastian?

Looking at the latest yellow warnings issued this morning for strong winds and heavy rain later on Tuesday and Wednesday, the Chief forecaster at the Met Office thinks that the likelihood of rainfall, is higher than that of strong winds (fig 1). This is a curious decision on his part, because if anything I would have thought that it was the other way round, and that the likelihood of strong winds were just if not more likely, than that of heavy rain alone. But the Chief does think, that any impacts from the strong winds are higher than the impact from heavy rain (fig 1).

Figure 1 – Courtesy of The Met Office

I think this is a game that they play using the impact/likelihood grid that determines if a storm is to be named. As far as I can see, if they issue two alerts or more, and they’re both amber, then the storm will be named, but I’m unclear if just one of them being amber would trigger the naming process.

Figure 2 – Courtesy of the Met Office

The rain from the warm and cold fronts fairly zips across the country, but the occlusion sticks around a while across southern Scotland and hence the limited area for the heavy rain warning. According to the latest T+48 forecast chart that I can access from Met Office, it looks like they see the tightest gradients being from the Midlands and Northern England, I can’t see why they’ve taken the yellow warnings area as far north as Edinburgh, but without their model data it’s impossible to say. Obviously they’ve included gusts to 75 mph that may occur down the lee side of the Pennines.

Figure 3

The Met Office have forgotten to label the central pressure of ‘Sebastian’ (the name already given it by BIM) on their T+48 chart (fig 3), which I reckon is somewhere around 980 hPa. The UKMO solution is very similar to that of the GFS (fig 4), the only real difference is that the tightest gradients lie south of northern England, and across most of the Midlands and southern England. It will be interesting to see if this area gets adjusted southwards in the next 24 hours.

Figure 4

Author: xmetman

An ex-metman passionate about all things to do with weather, climate and clouds

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