Page History

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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.

Fig8.1.9.6-3: 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. 

Fig8.1.9.6-4: CAPE-shear EFI, data time 00UTC 22 June 2017, valid for 00-24UTC 22 June 2013 (as on Fig8.1.9.6-3).  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.

Fig8.1.9.6-5: 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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Fig8.1.9.6-12: 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.

Fig8.1.9.6-13: 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 Fig8.1.9.6.12.

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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.

Supercell

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examples

Example 1:

Right-moving supercells (highly-organised mesocyclonic thunderstorms with cyclonic flow at the mesoscale) developed over NE Spain producing giant hail and floods in Zaragoza.  Large deep-layer shear (over 30 m s^-1 0-6 km shear) coexisted  with a quite large MUCAPE.  ENS mostly about 1500J kg^-1 with extreme above 2000 J kg^-1.   Very low values of convective inhibition (CIN) in the moist air in the lowest layers and the level of free convection (LFC) and lifted condensation level (LCL) were strong signals for the very active convection.   

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Fig8.1.9.6-20: Channel-9 and Channel-12 imagery VT 15UTC 6 July 2023 showing supercells over NE Spain.

Considerations when forecasting Mesoscale Convective Systems (MCS) and Supercells

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

Example 2:

Example 2:

During the winter months, one can easily downplay the signal from the convective EFIs.  This is because the model climate for CAPE and CAPE-shear don't have particularly extreme values at that time of year.  Thus almost any signal of CAPE or CAPE-shear is often portrayed as extreme.  However, high CAPE and CAPE-shear values should not be underestimated.

In the case illustrated (Figs8.1.9.6-21 & 22) the structure of the convective EFIs patterns differs.  These are:

• extreme CAPE (for the season) across southern Ireland to northern England. 
• extreme CAPE-shear (for the season) across Celtic Sea towards Wales.  In this example CAPE-shear is dominated by the extreme wind shear.

Severe thunderstorms, even supercell thunderstorms, can develop where the CAPE and CAPE-shear areas overlap and high storm helicity as all the necessary ingredients for severe convection are in place . 

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Fig8.1.9.6-21: Forecast Extreme Forecast Index for CAPE with Shift of Tails for 24hrs to VT 00UTC 27 Dec 2023, DT 00UTC 26 Dec 2023.

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Fig8.1.9.6-22: Forecast Extreme Forecast Index for CAPE-shear with Shift of Tails for 24hrs to VT 00UTC 27 Dec 2023, DT 00UTC 26 Dec 2023.

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 Fig8.1.9.6-23:Forecast wind hodograph relative to storm movement.  Curved forecast hodographs in the lowest 3 km give high helicity relative to the  storm - even in the lowest 500 m.  This is one of the predictors for tornadoes.

Example 3:

A major outbreak of severe convection occurred on 26 May and CAPE-shear EFI covers the wide area of severe convection.  Within the CAPE shear areas 43 tornadoes and hail very large hail (over 50mm diameter) occurred mainly in the west of the area and severe convective gusts were reported mainly in the east.  Tornadoes and very large hail usually occur in the presence of supercells.  The CAPE-shear EFI signal was much stronger than that of CAPE EFI. This gives information about the environment – large instability but also presence of strong deep-layer wind shear.  In such kind of environment well-organised convection tends to develop if enough lift is provided.

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Fig8.1.9.6-24: Observed severe weather over USA 26 May 2024 (left diagrams). EFI forecast of CAPE and CAPE-shear  DT 00UTC  24 May 2024, VT 00UTC 26 May to 00UTC 27 May 2024 (right diagrams).  The CAPE-shear EFI signal is much stronger than that of CAPE EFI which illustrates the importance of strong instability together with significant shear when forecasting severe convective weather.  

Considerations when forecasting Mesoscale Convective Systems (MCS) and Supercells

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

• the 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.
• that observed surface temperatures and dew points may differ from forecast values.  Users can then assess possible modifications to the lowest levels of the forecast vertical profiles and amend the convective inhibition accordingly.
• Winter M-climate EFI values are low and moderate values of CAPE and CAPE-shear can appear extreme but should not be ignored as unlikely. 

Additional Sources of Information

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