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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 example

Right-moving supercells (highly-organised mesocyclonic thunderstorms) 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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Fig8.1.9.6-19: Forecast vertical profile for Zaragoza DT 00UTC 6 July 2023 VT 12UTC 6 July 2023.  Large MUCAPE can be released after overcoming small CIN. Strong bulk wind shear.  The ENS cumulative distribution function for precipitation at Zaragoza during the 24hr period 00UTC 6 July to 00UTC 7 July 2023.  The CDFs for DT 00UTC 5 July (purple), DT 12UTC 5 July (blue), DT 00UTC 6 July (red) all show about 5% of ensemble members produce precipitation greater than the extreme of M-climate for Zaragoza.

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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:

  • 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.  


Additional Sources of Information

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