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Fig8.1.4.5.1: 500hPa contour heights, forecast data time 00Z 21 June 17, T+48 verifying at 00Z 23 June 17.   CAPE-shear EFI for the period T+24 to T+48 coloured Yellow >0.4, Orange > 0.7, Red > 0.8.

Fig8.1.4.5.2: As Fig8.1.4.5.1 but with HRES precipitation totals over 9hrs added: purple > 10mm.  In practice, the fact that there is precipitation indicates sufficient availability of moisture while the very high EFI indicates that unusual (i.e. climatologically high as defined by M-climate) convective available potential energy (CAPE) is available in the north Germany area.  Precipitation totals in the very active storms that are likely to form will be greater than ENS or HRES show (here HRES precipitation) and with associated significant downdraught gusts.

Fig8.1.4.5.3: CAPE-shear EFI from a sequence of forecasts data times 00UTC on 18, 19, 20, 21, 22 June 2017.  Note the increasing Extreme Forecast Index (EFI) and the Shift of Tails (SOT) above 0 and reaching above 1 at T+24 on the last forecast over North Germany and Poland. 

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Care needed in Interpretation of ecChart Presentation.

It is tempting to simply observe on forecast charts where large CAPE or CAPE-shear EFIs coincide with high rainfall from HRES from HRES when assessing the release of severe convection.  HRES  HRES forecast rainfall may be used in combination with convective EFIs in the short-range (up to T+48hr), but it should be remembered that HRES (& CTRL) is just another individual possible forecast.  In the short-range it is probably the most likely one, but in the medium-range its relative weight compared to ENS members decreases and it becomes just as likely as any other ensemble member.  Then it is best to use a probability of precipitation forecast (PoP > 1mm/24hr) rather than a simple precipitation forecast throughout the whole forecast period (both short-range and medium-range).  These concepts are discussed below using one case as an example.

All the charts below correspond to the same example. All are for data time 00UTC 6th August 2017, and we focus on the forecast for 8th August.  Fig8.1.4.5.7 and Fig8.1.4.5.8 show 6-hour HRES precipitation forecasts for 00UTC on the 8 and 9 Aug 2017 as displayed by ecCharts, and it appears an area of significant rainfall associated with an upper trough moves from southwest France to Austria.   However, precipitation data is not shown for 12UTC on 8 Aug.  Meanwhile 24-hour total precipitation EFI (0.9) and CAPE-shear EFI (0.85) are available for 00UTC 9 Aug and show very high values.  CAPE EFI (0.6) is only moderate illustrating the significant impact of bulk shear to give the high CAPE-shear EFI values.  The precipitation meteogram for the western Alps shows heavy rainfall in that area during the day and this is confirmed by data on Figs8.1.4.5.12 and 8.1.4.5.13 (note that these charts have different but overlapping validity periods). 

Fig8.1.4.5.7: ecChart showing 300hPa height with stratiform and convective rainfall (convective rainfall is plotted on top of stratiform) over the last 6 hr for T+48hr verifying at 00UTC 08 Aug 2017 based on HRES data time 00UTC 6 August 2017.

Fig8.1.4.5.8: ecChart showing 300hPa height with stratiform and convective rainfall over the last 6 hr for T+72hr verifying at 00UTC 09 Aug 2017 based on HRES data time 00UTC 6 August 2017

Fig8.1.4.5.9: ecChart showing 300hPa height with total precipitation EFI at T+72hr verifying at 00UTC 09 Aug 2017 based on ENS data time 00UTC 6 August 2017

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Fig8.1.4.5.11: ecChart showing 300hPa height with CAPE Shear EFI for the 24h ending at 00UTC 09 Aug 2017 based on ENS data time 00UTC 6 August 2017

Fig8.1.4.5.12: ECMWF chart showing stratiform and convective rainfall over the last 6 hr for T+60hr verifying at 12UTC 08 Aug 2017 based on HRES data time 00UTC 6 August 2017. Also shown are surface isobars.

Fig8.1.4.5.13: ecChart showing the probability of precipitation ≥20mm in 24hrs ending 18UTC 9 August 2017. The forecast probability of heavy rainfall is concentrated at about 6ºE, in amongst the forecast CAPE-shear EFI maxima in Fig8.1.4.5.12.

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A feed of warm and very moist air into the system is desirable for the most active MCS.

The resolution resolution  (~9km) of HRES (~9km& CTRL) allows  allows reasonable capture of the area of a large MCS –  but detail of convective areas within, nor any narrow features (e.g. squall lines), won’t be very well defined.

The current resolution of ENS (~18km) means MCSs are less likely to be represented in the forecast, and individual storms within the MCS won’t be identified or tracked.  However, the scale of an MCS is such that it can substantially alter the surrounding atmospheric environment and potentially affect larger and therefore resolved scales.  ENS can usefully identify environmental conditions promoting deep moist convection and MCSs and hence products such as the EFI of CAPE or CAPEshear can give an indication of potential for MCS development.

An investigation has shown that:

• HRES has high skill in predicting MCS in the first 24hr or so but skill falls away beyond 36hr.  Nevertheless, warnings of the potential for extreme weather are very important, even at short lead-times (Fig8.1.4.5.17).
• ENS can’t track individual storms but is good at predicting the environment that favours development of extreme convection.
• EFI for CAPE and CAPE-shear shows high skill to day 4 and there is still good correspondence between EFI and severe convective outbreaks even at day 7 (Fig8.1.4.5.18).
• HRES and ENS can discriminate well between days of intense convective activity and days of less convection – they don’t over-predict MCS.  Generally, MCSs are not predicted during periods of less active weather and convection, and this corresponds well with observations.

Fig8.1.4.5.16: Comparison between HRES output and observed distribution of MCS areas over Europe.  HRES data time 23 June 2021 12UTC, Verifying time 24 June 2021 00UTC. Note MCS are persisting during the night.

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Fig8.1.4.5.17: Probability of Detection and False Alarm Ratio results from an initial investigation on the ability of HRES in capture of MCS.

Fig8.1.4.5.18:  ROCA Diagram showing skill of CAPE and CAPE-shear Extreme Forecast Index (EFI) at recognising severe convective outbreaks (verified against observed MCS).  The area under the Relative Operating Characteristics (ROCA) curve gives an indication of skill (1.0 = High Skill; 0.5 = No Skill).  The EFI is verified against severe weather reports in the European Severe Weather Database (ESWD) averaged over the April-September periods between 2017 and 2020. .

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When using IFS output, the user should keep in mind:

• the limited ability of HRES (& CTRL), and particularly ENS, to resolve a potential MCS in detail.  Individual convective elements won’t be resolved.        
• the characteristics of the airmass, particularly the moisture content of any convergent flow.  A persistent inflow of high moisture air encourages more activity.
• changes in the forecast IR cloud output, lightning, and precipitation fields together with CAPE and CAPE-shear can point to likely areas for potential MCS formation.
• that under certain circumstances of vertical wind shear, forcing, and cloud structure an MCS can comprise one or more supercells.  The MCS can split and significantly alter the MCS’s track and development.  Alternatively some supercells can back-build and become stationary.   These effects are unlikely to be captured by HRES (& CTRL).
• most severe weather tends to occur during the initial or developing stage of an MCS.  However, heavy rain and flash floods continue in later stages of more mature systems.
• MCS tend to develop mid- to late afternoon and then persist through the evening and well into the night.  

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