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Extratropical Cyclone Diagrams
Overview
A comprehensive set of post-processed ensemble products use a feature-based approach to represent objectively the location and behaviour of near-surface, synoptic-scale features typically associated with adverse weather (eg fronts, frontal waves, cyclonic features). Co-location masking, a feature-type hierarchy and a minimum separation threshold, are all used together to help keep all cyclonic features 300km or more apart. Mean sea level pressure, estimated from 1000hPa geopotential height and temperature, is shown as a reference point on many charts. A tracking algorithm is used to follow the cyclonic features as they evolve in each ensemble member. As a severe weather event approaches, the products can:
- indicate an increasing risk of a major storm in the area of interest,
- highlight the track that the storm is likely to take,
- suggest the degree of confidence that can be placed in that track (see Fig8.1.9.8).
Fig8.1.9.1: To view extratropical cyclone forecasts:
- On charts page, click on extratropical cyclones.
- On extratropical cyclones page select the extratropical cyclones diagram.
- Select nominal data time of the forecast of interest.
- Select feature of interest - in this case a barotropic low (black).
- Display of product (in this case ensemble member tracks, 1km wind speeds, 300hPa wind speeds, central pressures and vorticity).
- Step to next or previous nominal data time of interest.
- Select alternative presentation of forecast features (e.g.fronts, dalmation charts etc) from drop-down menu.
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View directly the Cyclone Database Products.
Example Charts
The extratropical cyclone diagrams provide a comprehensive display of the variation between the forecasts of each member of the ensemble regarding positions of fronts, depth of depressions, and strength of winds at 1km altitude. In the examples the features near Brittany relate to an extreme windstorm which in terms of European losses was the major windstorm of thr 2016-17 winter. For interpretation of the charts see a guide to using cyclone database products.
Fig8.1.9.3: An example of a chart showing positions of fronts diagnosed from ensemble members (see legend below chart for details) illustrating the variation in positions. DT 00UTC 03 March 2017, T+84 VT 12UTC 06 March 2017.
Fig8.1.9.4:An example of a "Dalmation Plot" showing the centres of cyclonic features coloured to show an analysis of the cyclone class as derived from ensemble members (see legend below chart for details) showing the variation in forecast positions. DT 00UTC 03 March 2017, T+84 VT 12UTC 06 March 2017. Note that not all the spots denote genuine low pressure centres; it is only the barotropic lows (black spots) that are guaranteed to be.
Fig8.1.9.5:An example of a "Dalmation Plot" showing the centres of cyclonic features, coloured to show an analysis of the forecast maximum wind strength, at 1km altitude, within 300km of each centre derived from ensemble members (see legend below chart for details). Chart highlights show the variation in positions and intensity. DT 00UTC 03 March 2017, T+84 VT 12UTC 06 March 2017. Note several members suggest a maximum wind of 65-85kn in the vicinity of northwest France.
Fig8.1.9.6: An example of a chart showing the percentage of ensemble members predicting a cyclonic feature point will track within 300km in a 24-hour period T+72 to T+96 (i.e. 00UTC 06 March to 00UTC 07 March 2017). DT 00UTC 03 March 2017, T+84 VT 12UTC 06 March 2017. Only cyclonic features with a maximum wind speed exceeding 60kn at 1km altitude within 300km of the centre at some point in the 24h period are included. A probability greater than 60% (darker orange) is shown over the western English Channel and NW France. For a cyclonic feature moving west-to-east in this part of the world the strongest winds will ordinarily be found to the south of the low track. This needs to be taken into account - indeed it is important for the user to not misinterpret the shading on these strike probability charts as being like a simple wind gust probability chart.
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Fig8.1.9.8A(top): Forecast tracks of frontal wave (arrowed in Fig8.1.9.7) from ensemble members.
Fig8.1.9.8B(left): Forecast central pressure (hPa) of each depression developing from the frontal wave as identified by the ensemble members.
Fig8.1.9.8C(right): Forecast wind strengths (kn) at 1km altitude within 300km of each depression developing from the frontal wave as identified by the ensemble members.
The ensemble CNTL is shown by thin green lines. Most ensemble members forecast the track of the selected cyclonic feature to curve towards Britain before moving SE into northwest France. Almost all ensemble members deepen the low, some to below 995hPa with winds at 1km altitude reaching more than 60kn and a few greater than 70kn. The threat of severe weather is clearly shown but it is necessary to inspect the ensemble members, meteograms, EFI charts etc. to identify the associated risk. DT 12UTC 04 March 2017.
Fig8.1.9.9A(top): Ensemble Extreme Forecast Index (EFI) and Shift of Tails (SOT) charts for mean daily 10m wind speed (left) and M-climate for this (right) at 99th quantile (typically 1 in 100 occasions in the ensemble realises more than the values shown). DT 00UTC 03 March 2017, T+48 to 72.
Fig8.1.9.9B(bottom): Ensemble Extreme Forecast Index (EFI) and Shift of Tails (SOT) charts for maximum 10m wind gusts (left) and M-climate for this (right) at 99th quantile (typically 1 in 100 occasions in the ensemble realises more than the values shown). DT 00UTC 03 March 2017, T+48 to 72.
EFI exceeding 0.7 in much of France suggests unusual winds, and 0.8 in some places suggesting very unusual winds for those locations. Meanwhile areas of non-zero SOT suggest a genuinely extreme event is possible.
Considerations when dealing with small cyclones
The spatial resolution utilised in generating the extra-tropical cyclone charts (~50km) is rather larger than the resolution intrinsic to the ensemble, and is primarily applicable to monitoring mid-latitude depressions and their associated features. This is partly by design, in that we are trying to capture "synoptic scale features", and not every minor nuance in the model fields. Mid latitude depressions typically have a length scale of order 1000km and the program can extrapolate realistic central pressures from the surface pressure pattern. As the feature is followed through the forecast period, feature specific plumes of central pressure, upper and lower altitude winds, and vorticity are plotted. These are presented in the subsidiary diagrams. However, resolution of the input data means a reduced capacity to correctly represent certain aspects (such as depth or maximum 1km winds) of small, deep vigorous cyclones, where the length scale is < ~200 km, say. Nevertheless, the forecast positions of small vigorous centres are normally well captured.
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Fig8.1.9.11: Extra-tropical cyclone chart considering a small depression in the Mediterranean Sea. DT 00UTC 28 Sep 2018. The initial position of the low centre is identified by ensemble members and the subsequent forecast tracks and plumes are fairly similar. The higher resolution of the medium range ensemble allows a fairly good representation of small vigorous features. Confidence may be placed in extra-tropical cyclone charts associated with larger mid-latitude depressions and the trend of vorticity and other parameters will give a fair indication of development or decay..
Considerations regarding Identification of Fronts
Level at which fronts are identified.
Fronts are identified using a vertically-interpolated level that is everywhere 1km above the model orography. This terrain-following approach, and the choice of 1km, have many advantages:
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Choosing the level 1km above orography offers the best of both worlds - it is close enough to the surface to be mostly co-located with surface frontal signatures (e.g. a frontal pressure trough) but far enough away from the surface to be representative of the lower troposphere while not being over-influenced by discontinuities in the orography. In general the 1km level is lower than the 850hPa level and actually represents the real model airmass over mountains and not a less meaningful underground extrapolation (see Fig8.1.9.12).
Identification of type of front
A thermal variable (wet bulb potential temperature, θw) is used in order to incorporate a moisture component. It is quite common practice to use a similar variable (equivalent potential temperature, θe) around Europe. In tests it was found that using a pure thermal variable like temperature regularly generated spurious dry fronts downwind of topographic barriers (from the Foehn effect).
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- complex topography
- complexities and even uncertainties in land/sea/lake/ice boundaries
- atypical humidity structures (for example the warm airmass may be dry and the cold airmass moist)
Correspondence with surface isobaric troughs
Where frontal activity is weak there will not necessarily be an associated surface isobaric trough. Around anticyclones, relatively inert cold fronts can give a change in weather type (e.g. from cloudy and mild to clear and cold) whilst exhibiting a very weak isobaric trough or indeed no pressure trough at all. However, active fronts normally are associated with marked surface pressure troughs on charts.
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Fig8.1.9.12: Selection of altitude used in the identification of fronts.
Additional Sources of Information
(Note: In older material there may be references to issues that have subsequently been addressed)
Read the guide to using cyclone database products for interpretation of the charts.
Watch a comprehensive lecture on extratropical cyclone diagrams (10sec delay before start).
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