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A combination of convective EFI parameters and a precipitation forecast (probability of precipitation (PoP), available on ecCharts) highlights areas where severe convection is likely and is a powerful indicator of timing and location given some method of initial uplift to release the instability.

Fig8196.AFig8.1.9.6-1: 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.

Fig8196.B: As Fig8196.A Fig8.1.9.6-2:As Fig8.1.9.6-1 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.

Fig8196.CFig8.1.9.6-3: 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. 

Fig8196.DFig8.1.9.6-4: CAPE-shear EFI, data time 00UTC 22 June 2017, valid for 00-24UTC 22 June 2013 (as on Fig8196.CFig8.1.9.6-3).  EFI colours orange and red taken as indicating an extreme event likely.  SOT values indicate the ratio of departures of ENS forecast values from the M-climate extreme considering the greatest 10% ENS members.  The other charts show CAPE-shear values in the M-climate (derived on 19 June 2017) wherein only 1 in 10 occasions (central chart) and only 1 in 100 occasions realises more than the values shown.  The existence of significant EFI and SOT, even some days in advance, should not be overlooked, particularly if the actual forecast CAPE-shear values are much greater than the M-climate values (at say the 90th or 99th percentiles) for the area.

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Fig8196.EFig8.1.9.6-5: Maximum gusts (kph) during the period 12UTC to 18UTC 22 June 2017.  Over 100kph in Central Germany associated with the widespread active thunderstorms.

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Fig8196.FFig8.1.9.6-6: Rainfall (mm) in 6hrs during the period 12UTC to 18UTC 22 June 2017.  Over 50mm of rain fell in Central Germany associated with the widespread active thunderstorms.

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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.  Fig8196G and Fig8196.H Fig8.1.9.6-7 and Fig8.1.9.6-8 show 6-hour HRES precipitation forecasts for 00UTC on the 8 and 9 Aug 2017 as displayed by ecCharts, and it .  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) (Fig8.1.9.6-9) and CAPE-shear EFI (0.85) (Fig8.1.9.6-11) are available for 00UTC 9 Aug and show very high values.  CAPE EFI (0.6) (Fig8.1.9.6-10)  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 Figs8Fig8.1.4.5.12 and 89.6-8 and Fig8.1.4.5.13 9.6-12 (note that these charts have different but overlapping validity periods). 

Fig8196.GFig8.1.9.6-7: 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.

Fig8196.HFig8.1.9.6-8: 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

Fig8196.IFig8.1.9.6-9: 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

Fig8196.JFig8.1.9.6-10: ecChart showing 300hPa height with CAPE EFI for the 24h ending at 00UTC 09 Aug 2017 based on ENS data time 00UTC 6 August 2017

Fig8196.KFig8.1.9.6-11: 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

Fig8196.LFig8.1.9.6-12: 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.

Fig8196.MFig8.1.9.6-13: 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 Fig8.1.49.56.12.

Fig8196.NFig8.1.9.6-14:  15day meteogram with M-climate for 45N06E based on ENS data time 00UTC 6 August 2017.  An exceptional event is forecast for 8 Aug; the median lies above the 99th percentile of M-climate (green line).

Fig8196.OFig8.1.9.6-15: CDF and associated EFI for west Alps region (45N06E).  Forecasts with a data time of 00UTC 6th, as on the other plots above, are denoted by the darkest of the two dashed blue lines.  There is a consistently high EFI for rainfall (over 80%) which is sufficient for forecasting a significant and maybe an extreme rainfall event.  Some ENS members show rainfall totals close to the M-climate maxima.  The slope of the precipitation CDF shows the variation within ENS members, but all members show greater than M-climate values.  For greater confidence the slope of the CDF should be more vertical.

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• 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 (Fig8.1.4.5.1716).
• 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 (Fig8.1.4.5.1817).
• 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.

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Fig8196.P: 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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Fig8196.QFig8.1.9.6-16: Probability of Detection and False Alarm Ratio results from an initial investigation on the ability of HRES in capture of MCS.

Fig8196.RFig8.1.9.6-17:  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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Fig8.1.9.6-18: 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.

Considerations when forecasting Mesoscale Convective Systems (MCS)

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

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