Friday, 8 March 2013

Migration Forecast 2 (Part II)         (Updated Fri 8.3.13)
"The horrible texture of a fabric that should be woven of ships' cables and hawsers. A Polar wind blows through it, and birds of prey hover over it."    Hermann Melville, Letters (1851)
In Part I, we saw how seasonal changes in west Africa might be favourable for large numbers of migrants to undertake the first and most physically demanding part of their migration – crossing the Sahara. We also noted that there could be penalties for those individuals who come too far north, and too early.

Here comes that penalty now...

(Click for larger version)

… Low pressure area “A” - which is currently beating up the eastern seaboard of the USA - will consolidate and move out into the Atlantic. Deep depression “B” at 972mB (!) will also continue to track east and interact with low “C” off Greenland with fronts extending north-south, and will be off SW Ireland by midday Sunday, bringing high winds and rain with it.

The big problem is that we don't know exactly what will happen to these lows “A” and “B” during next week. There are two possibilities: they could continue on the normal path, bringing typically changeable early Spring weather to western Europe, which will soon clear. The second possibility is that a developing anticyclone “J” - which is currently off the chart, way up to the NE of Iceland – will extend south to cover most of the British Isles by mid-week.

This would bring a mass of very cold Arctic air down over the country which could, if mid-range forecasts are correct, show an influence as far south as Spain.

In this scenario, any migrants which have already made it over the desert and think that they are on the “home stretch”, might have an unpleasant surprise waiting for them in Europe. The next chart shows this forecast position but I stress that it is not very reliable yet.

(Click for larger version)

For small birds, this could stall them for several days until it changes. But ospreys are made of sterner stuff... I have gone over the weather maps for this time of year, all the way back to 2008. I compared the positions – especially wind speeds and direction – with the known arrival dates of ospreys at monitored nests. (Thanks Tiger and Chloe – your summary tables saved me a pile of time on this :)

Watching for brass monkeys
Pentland Hills, Scotland, 2004
What I found was that, although the weather conditions had some effect if it was REALLY bad, most ospreys managed to get through to their nest sites “on schedule”. True, there were exceptions to this, but remember that we don't know where most of these individuals spent the winter, so it would be unwise to draw too many intuitive conclusions.

Keep scanning the skies - even if it's snowing!

1 comment:

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