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jay007

Direction Of Thunderstorms

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Hello to all

This is my first forum post and im sorry if its in the wrong place

I have been tracking thunderstorms this year and have noticed they all seem to move from SW to NE, is this my imagination? or is this always the case? and if so why?

thank you in advance

jay

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Thunderstorms don't always travel from south-west to north-east, but is a fairly common direction for storm motion given that the flow over the UK is predominantly from a W/SW direction, due to low pressure systems that come in off the Atlantic. Thunderstorms are guided by the upper (steering) winds at around the 500mb level, and so it depends on where a trough (upper low) is positioned in relation to the UK.

Here is a 500mb and surface pressure chart from the GFS for Thursday just gone:

post-9715-025668300 1285535572_thumb.png

The colours represent heights and show the direction of the upper (500mb level) flow. So from this chart you can see that the upper flow on that day was flowing from SW-NE across England and Wales, and this is the direction the storms moved in during that afternoon. If the upper low was positioned, say, a little east of the UK then, provided the air mass was unstable, storms that develop in that scenario would travel from the east or north-east towards the west or south-west.

Hope this helps.

And welcome to the forum. :)

Hi weather 09

thank you for replying to my post

Thank you for the info its helped a little, i understand the colours and heights on the chart but i'm struggling to understand the direction of the upper flow, what part of the chart is repesented by the direction of flow?

Im sorry if this is a daft question, im still trying to get my head around weather charts lol

Thank you for your welcome :)

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Not a daft question at all. Around a trough the upper air is flowing anti-clockwise or cyclonic, and with a ridge the upper air is flowing clockwise or anticyclonic. On the chart above the trough is pretty much over the UK, perhaps more over north-west UK. So if you look at the different coloured shadings which represent heights, just follow them and that is the direction of the upper flow.

Or, instead of looking at the 500mb charts, you can take a look at this chart which shows the airflow at different pressure levels in the atmosphere (available via Netweather's Extra Subscription):

post-9715-068684100 1285540919_thumb.png

The one of most relevance in terms of storm motion will be the 500mb panel in the bottom left corner. Hope this helps.

It took me a while to digest what you was telling me but i think i have cracked it lol

Following the first chart you gave me i followed the yellow colour anticlockwise and it all make's sense thank you.

Thank you weather09 for your guidance, you have clear up a question that i have had for a while , your advice was brilliant i can't thank you enough.

jay

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Yes, the surface wind is not a very good guide to the direction that a thunderstorm will travel. As the steering winds aloft, which generally follow the 500mb contours, can quite often blow differently from the surface wind.

When winds do tend to blow different directions with height, especially backed towards the surface, then you tend to get what is known as wind shear, the more winds back towards the surface, the higher the wind shear value will be.

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I would like to thank everyone for their constructive comments in this thread- some of the specifics re. the 500mb flow, in particular (e.g. storms moving in a direction more akin to the 500mb flow than the surface flow), are things that I wasn't fully aware of.

I recall numerous instances of storms moving from SW to NE despite a southerly flow, while on 22nd June 2003, storms moved from west to east across the Tyne and Wear area despite southerly winds. This chart reveals all:

http://www.wetterzentrale.de/archive/ra/2003/Rrea00120030622.gif

...showing upper winds blowing from west to east, so that explains it.

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I would like to thank you all for your valuable input and advice it is much appreciated

thank you

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