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Question On Streamers - Forecasting And Formation


WhiteFox

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Posted
  • Location: Reading/New York/Chicago
  • Location: Reading/New York/Chicago

Looking at the radar this evening, I see once again that a steady line of showers has formed moving NNE from Jersey over the South of England (it looks like someone has lit a fire near Jersey and the radar echoes are the resulting smoke!):

post-1957-0-45648300-1350594356_thumb.pn

The PPN seems to emanate from a point and then spread outwards as it drifts with the flow of the isobars:

post-1957-0-62191000-1350594375_thumb.pn

The question is, how such such a feature form? What atmospheric conditions exist for such a stream to form and then prevail over such a distance? I'm familiar with such streams in the US where cold air flows over a warm body of water and creates lake-effect rain/snow, but in this case the difference in temperature can't be that great (indeed the temperature of the sea is slightly warmer than the upper air). Is it specific readings such as CAPE or Lifted Index combined with something else that is just right?

Can it be explained in fairly non-technical terms but more in depth than a cloud forms and it rains?

Edited by WhiteFox
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Posted
  • Location: Aviemore
  • Location: Aviemore

This was a convergence line, you can see on the wind map where the sw winds are meeting the southerly winds which will have effectively pushed the clouds upwards at that point to help form the line of heavy rain:

post-2-0-61204100-1350634870_thumb.png

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Posted
  • Location: Warminster, Wiltshire
  • Location: Warminster, Wiltshire

Streamers happen very often from the Bristol Channel under LOW pressure, especially if the LOW is situated so that the wind from the Bristol Channel

is blowing from the N/NW/W/SW. I have recently learnt that they can form under HIGH pressure too.

The model predictions for precipitation very rarely pick up on the streamers, and if they do they are misplaced come the actual time frame being predicted!

A recent example ; Tuesday morning had a deep LOW to the N with brisk WNW wind blowing on its's Southern flank. The previous evening's forecast for

this region was that Tuesday would be dry everywhere, however a finger of cloud was shown from the Bristol Channel coast and moving inland. I said to

myself that showers would be happening in the morning and sure enough they were blowing in from the WNW affecting quite a way inland!

I believe they form due to the temperature contrast between the sea and land.

Edited by Andy Bown
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Posted
  • Location: Reading/New York/Chicago
  • Location: Reading/New York/Chicago

Thanks gents. The wind map is great; corresponds exactly with where the showers were forming and then flowing with the direction of the wind.

What sort of height would the wind be converging?

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Posted
  • Location: Camborne
  • Location: Camborne

The Pembroke Dangler

Submitted by uk.sci.weather on Fri, 23/01/2009 - 12:32pm.

Unstable polar maritime airstreams in autumn and winter typically give showery conditions, particularly to exposed coastal areas. These showers can spread well inland where there is little shelter from the sea, such as those which travel through Cheshire towards the midlands in a NW airstream. However, in the case of the ‘Pembroke Dangler’, more is involved than simply exposure to an onshore wind.

The term was coined by Jon O’Rourke, a regular contributor to USW. It is used to describe a line of showery precipitation which forms in a NNW to NNE airflow, typically from Pembroke to Cornwall. This often results in frequent showers, or even more persistent rain, in quite a narrow band. When this occurs a narrow strip can receive quite a high rainfall total, whilst the rest of the south west is virtually dry.

It is by no means an unusual phenomenon, occurring several times a year, particularly in autumn and winter. The long fetch across ‘warm’ water from the Irish Sea to Cornwall, provides the right conditions for showers to form. The shape of the Pembrokeshire Peninsula has the effect of concentrating this unstable northerly flow, in the same way as an obstruction in a stream would concentrate the flow around it. The resultant line of often heavy showers is a convergence zone.

The long and narrow Cornish peninsula protrudes into this line of precipitation, and often has the affect of intensifying it further. Below are just 2 examples from 2008.

http://weatherfaqs.org.uk/node/216

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Posted
  • Location: Wildwood, Stafford 104m asl
  • Weather Preferences: obviously snow!
  • Location: Wildwood, Stafford 104m asl

my worst setup is the Cheshire Gap streamer, when high pressure is in charge centered over Ireland, bbc etc always give a dry day

0f course can very rarely bring snow, as on 25th Dec 04, 31 Jan 2011, 3-5 Feb 03

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Posted
  • Location: Bangor, Northern Ireland (20m asl, near coast)
  • Weather Preferences: Any weather will do.
  • Location: Bangor, Northern Ireland (20m asl, near coast)

This was a convergence line, you can see on the wind map where the sw winds are meeting the southerly winds which will have effectively pushed the clouds upwards at that point to help form the line of heavy rain:

post-2-0-61204100-1350634870_thumb.png

In this instance this will be the cause rather than sea/airmass temperature differences. Temperatures during the day were higher than the SSTs (sea surface temperatures).

Edited by The watcher
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