Update by Eddy, Friday 11am: The #stingjet of #stormBarbara now looks like staying offshore. Hence maximum expected wind gusts can be reduced from the extreme values given below. So staying wild, but not crazy (thankfully!). See my latest posts on Twitter at: https://twitter.com/eddy_weather
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Here's how I view the development of #StormBarbara:
Barbara will be a very powerful storm, with maximum wind gusts within the #stingjet* of probably over 100mph. However, there is disagreement still between the models over the exact track of the jet. The final track of this jet will prove crucial regarding the impact of Barbara.
Currently the Global Forecast System (GFS) mean has the #stingjet remaining out to sea, to the north-west beyond St. Kilda, Butt of Lewis, Flannan Isles and Sulisker (see charts below). In this run, gusts of 70-80kts affect N and W Lewis, with up to 100kts (120mph) within the #stingjet, well offshore:
However, the all met model ensemble mean (the best estimate from all met models at the moment) indicates that GFS has slow wind bias for the region 58.0degN, -4.0degW (see next chart):
In contrast the COSMO European model (run at highest possible resolution) has a quite frightening prediction with windspeeds in excess of 115mph (180km/h) blasting into west Harris and westside Lewis at 17-18h (see next chart). If realised, this would make it almost as strong as the 'Stornoway Hurricane' of 8-9 Jan 2015.
Conclusion: With such disagreement, analyses of real-time satellite imagery will prove key as to the position and area of the sting jet / cold jet which will cause the most violent winds, as well as indicating the Langrangian (relative) speed of the system itself.
p.s. Late addition: I notice the US NAVGEM model is now pulling Barbara further NW again, in line with GFS.
Likely temporal Sequence of Events (very rough guide ± 2-3hrs):
6am: Frontal trough of storm reaches Barra, torrential rain and wind gusts reach 60-70mph.
8am: Same reaches Stornoway
9am-12 noon: Winds increase to gusts of 75mph
~Lunchtime: As frontal trough clears, a temporary 'lull' back to 60-70mph gusts
2pm-8pm: The main hooley, with the most violent winds moving northwest across the Hebrides from Barra to Lewis. Highest wind speeds are likely to last 2-3 hours in duration (depends on speed of system herself). Max gusts are as indicated above (between 80-115mph). If the #stingjet stays well offshore, this stage will be reduced in severity.
10pm-midnight: Worst hopefully over.
The Christmas Day Storm (perhaps #StormConor): It looks like it will also be a strong storm, but current model runs suggest a track further still to the NW. Let's deal with him only after ##StormBarbara!
22 Dec 2016, @eddy_weather, Stornoway
* Please note that I am using the term 'sting jet' for both sting jets and cold jets (or cold conveyor belts). This is not strictly correct meteorologically speaking - but since the public are more familiar with the term sting jet, I am using this term alone to avoid confusion.