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Possible Stormy Spell 13th to 19th Nov

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Author Topic: Possible Stormy Spell 13th to 19th Nov  (Read 3752 times)
Martin Mc Kenna
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Maghera, N. Ireland

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« on: November 11, 2009, 09:12:25 pm »

Model Guidance continues to oscillate between different scenarios with regards the low coming NE Friday Evening through Saturday.  The 12Z GFS has a strong interaction with the shortwave coming east across the Atlantic, effectively locking in the PVA the trough and combining it with the diffleunce of the Left Exit of the 160knt Jet coming towards the SW.  There is a fairly consistent signal for a somewhat elongated low to form in the SW approaches into Friday, as the low latitute baroclinic low combines with the colder air trough to its NW, effectively forming a elongated low pressure system SW of IReland.  What is not clear is as to which of two possible low pressure systems will develop

The 12Z GFS really develops the southern low with an elongated low extending NE, this brings Storm force 10s to the Channel and potential 600m gusts to 70knts through the Channel which post cold front could see surface gusts to 65knts along the Channel.  There is also a swathe of strong winds ahead of the squally cold front which goes through Friday Night with gusts to 60-65mph in places

The 12Z UKMO GM however prefers to develop the northern centre with a low over Kinross vs N Wales in the GFS - The Pressure gradient is therefore not as intense across England and S Wales, the southern centre relaxes as it moves NE whereas in the GFS it is maintained as the main low centre. This is frequently a problem for numerical models with elongated low centres they have difficulty choosing which centre to develop and which not to...  At this stage its difficult to see which will be correct, and in terms of wind rain the two stories are similar - except the GFS has a stronger 2nd bout of wind on Sat. The 12Z GFS Ensemble gives a general indication for WEt and Windy conditions but does not have the required resolution to determine which centre will develop.  As the timeframe nears the NAE and Higher Resolution REgional Models should be able to provide a better steer on the potential development.

Beyond that there are more significant differences as early as T+60 in the vicinity of the Azores, the UKMEt has a slower more developed low which effectively has time to get picked up by another trough and this moves NE as a deepening low early on monday.  The GFS has a much flatter faster feature running NE, the source region is the complex low SW of Bermuda which contain the remnants of Ida, and therefore will be subject to further model differences

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