Autumn storm 15 Sept: potential 40mph gusts in SE, 70mph in Scotland, 90mph in Iceland! Something about the formation.

September 15, 2013 — Leave a comment


The first proper Autumn storm of 2013 is spinning up in the N Atlantic today and bringing gale force winds to Scotland and the north of England. It is a product of a strong 200mph jetstream that has been blasting between Greenland and Iceland for the last few days and something meteorologists call convergence and divergence of air masses. The jet has been strengthening at the boundary between tropical and polar air: the temperature difference is very big this time of year (think of those tropical storms bringing hot topical air to meet the cold polar air over Greenland, for example).  This temp difference causes a steeper pressure difference between the air masses and this causes the jet to speed up, converging into a narrow tube of fast winds aloft called a jetstreak (#1). Where the jet spreads out, slows down or changes direction it causes air to diverge and this causes air on the surface to rise rapidly to replace the diverging and spreading air.  Surface pressure is lowered rapidly especially on any inside leg of a northward blowing limb of the jet where there is a positive spin on air circulation encouraging cyclonic winds: this is a development region for low pressure.  These combined processes have kick started the storm between Scotland and Iceland.  The diagrams try to explain more about what’s been happening to produce this explosive depression in the North Atlantic.

Think of the jetstream as the owner of an unruly unfit DOG.  The Jet owner travels faster than the dog and pulls it along, controlling what it does, lifting it up, pulling it along, making it sit and stopping it moving by different tugs on the lead and a firm hand.  This is pretty much, in a nutshell, what the jetstream is doing to surface weather patterns.  So, come on Fido, heal boy! Diverging (spreading out) of air aloft is when air is removed from an area more quickly than it arrives; there is therefore a “deficit” of air and this will drag air off the surface to redress the imbalance which therefore causes LOWER air pressure to form at the surface, as today. Air will converge into this low pressure and rise to replace diverging air aloft. Converging air aloft causes the opposite to happen: air piles up like lorries going up a hill (air arrives quicker than it leaves) so this means air can only descend to escape… causing a high on the surface. Convergence tends to occur where the jetstream is turning SOUTH from the Poles (where cooler air has a tendency to sink).  Divergence aloft tends to favour areas where the jet it turning NORTH towards the poles and carrying comparatively warmer air which has a tendency to rise.  All these complex processes combine to create LOW and HIGH pressure on the surface.

jetstreams cause convergence and divergence

Today, gale force winds will travel down the country and reach #Reigate this afternoon. A band of heavy rain will arrive shortly after the highest winds have eased a little but they will veer to the North and temperatures will drop further: the arrival of the polar air proper. The rain is fairly short in duration overnight and is not expected to amount to more than a few mm in total.
Down in the sheltered SE there are no warnings of damaging winds but peak gusts to 40mph in exposed places are possible and this is a “near gale” Force 7 on the Beaufort Scale: impeding progress, whole trees in sway and twigs breaking off trees, that sort of thing. Also, umbrellas difficult to use (I like that one :-)!

Think of Iceland, though… winds over the ice sheets will be touching 90mph overnight tonight with temps of -10c.  Waves in the North Atlantic will likely exceed 35 feet high during this period.

The central pressure of the LOW has fallen to 964mb which is low for a mid-latitude depression. In comparison, Hurricane Ingrid today bearing down on Mexico is “only” 986mb at the moment, though peak gusts in that system are expected to top 100mph.

low forms
It will remain windy through the first part of this week, with rain, heavy at times now and then as the LOW sits and fills in the North Sea. Tmax temps in the low teens, perhaps 16c on some days but cool mornings. Temps are expected to climb through to the end of the week when, if models come right, a HIGH pressure looks to make next weekend rather nice: perhaps low 20’s next Saturday with some good weather! Hurricane Humberto looks to whizz north past the UK, perhaps brushing N Scotland on the latest charts. Things will change so check back for autumn updates.

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