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More extreme extra-tropical cyclones can give rise to swathes of damaging winds (Fig9.3-6). Verification using many case studies has shown that in most cases both the mean winds and the gusts are reasonably well predicted, on average.  That is to say if the cyclone shape, central pressure and pressure gradients are well-forecast, then in general so are the winds.  There is also no clear evidence to suggest that extreme cyclones are under- or over-deepened on average, though some case studies show that the onset of the phase of most rapid deepening can sometimes come a little too late (meaning, for a standard eastward-moving low, a little too far east).  In turn this tends to mean that the onset of very strong winds may actually be further west than predicted, but conversely those very strong winds may actually not extend as far east as in model output.  A particularly difficult strong wind swathe to predict is the one associated with the rare phenomena known as the "sting jet".  Evidence from a few cases suggests that IFS gusts are not biased but are representative of the real gusts experienced during a sting jet event.  However because this phenomena is intrinsically difficult to predict, and because the damage swathe associated with it is narrow, and typically short-lived, one has to expect large errors in forecasts for specific locations in potential sting jet scenarios.   Because of these factors and the often finely balanced nature of extreme cyclogenesis events (that relate to sting jets) one must expect some jumpiness in wind forecasts in these situations, even at relatively short ranges.  Nevertheless, the ensemble can help a great deal with highlighting the intrinsic uncertainty in gust forecasts in very cyclogenetic situations.  Users are also encouraged to make use of the cyclone database products also extratropical extra-tropical cyclone diagrams when dealing with strong winds related to cyclogenesis events.  These products were designed, in part, with that particular use in mind.

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The guidelines in Section 1.1.6 hold for most cyclones, but very small cyclones pose extra difficulties. They may be beyond the capabilities of the IFS horizontal resolution.  And even very small cyclones can have a sting jet associated.  Accordingly forecasters need to treat ensemble output with particular care when there is potential for very small cyclones to develop - say with lateral dimensions of order 200km or less.

1.1.7 Local winds  due to orography

Detail of winds to the lee of and around isolated mountains inland or mountainous islands are not likely to be captured.  This is because:

• horizontal resolution (~9km) may not capture the geographic features sufficiently.
• smoothing of the orography is likely to underestimate the mountain heights (possibly by factor of 4 in isolated mountainous island). 

Additional Sources of Information

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• See the impact of Cycle 47R3 on representation of wind gusts.
• Read more on convective gusts.
• Read about new EFI parameters for forecasting severe convection (which contains informative case studies, where there are references to gusts associated with active convective systems).
• Watch a comprehensive lecture on Model Physics: Concepts, Practice and Products (36sec delay in start, winds and gusts discussed from 31min15sec to 36min40sec).
• Read a 2015 paper on Extratropical extra-tropical Cyclones and the strong wind phenomena that are associated

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