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Forecasting severe convective hazards

Note: HRES and Ensemble Control Forecast (ex-HRES) are scientifically, structurally and computationally identical.  With effect from Cy49r1, Ensemble Control (ex-HRES) output is equivalent to HRES output shown in the diagrams.   At the time of the diagrams, HRES had resolution of 9km and ensemble members had a resolution of 18km.

Use of EFI with MUCAPE and MUCAPE-shear

Deep moist convection happens when deep instability (steep mid-tropospheric lapse rates), moisture and lift are all in place together.  If any of these ingredients is missing, deep moist convection is impossible. 

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Fig8.1.9.6-6: 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.

Care needed in Interpretation of ecChart Presentation.

It is tempting to simply observe on forecast charts where large MUCAPE or MUCAPE-shear EFIs coincide with high rainfall from ensemble control when assessing the release of severe convection.  Ensemble control forecast rainfall may be used in combination with convective EFIs in the short-range (up to T+48hr), but it should be remembered that ensemble control 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 ensemble control become 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.

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Fig8.1.9.6-15: 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.  Note: HRES and Ensemble Control Forecast (ex-HRES) are scientifically, structurally and computationally identical.  With effect from Cy49r1, Ensemble Control Forecast (ex-HRES) output is equivalent to HRES output shown in the diagrams.   At the time of the diagrams, HRES had resolution of 9km and ensemble members had a resolution of 18km.

Mesoscale Convective Systems (MCS)

The aim of any weather prediction scheme is to be able to forecast extreme weather events.  Some of the most significant severe convective outbreaks are associated with Mesoscale Convective Systems (MCS).

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Fig8.1.9.6-18: Comparison between HRES (9km resolution) output and observed distribution of MCS areas over Europe.  HRES data time 23 June 2021 12UTC, Verifying time 24 June 2021 00UTC. Observe MCSs are persisting during the night.

Note: HRES and Ensemble Control Forecast (ex-HRES) are scientifically, structurally and computationally identical.  With effect from Cy49r1, Ensemble Control Forecast (ex-HRES) output is equivalent to HRES output shown in the diagrams.   At the time of the diagrams, HRES had resolution of 9km and ensemble members had a resolution of 18km.

Supercell examples

Example 1:

Right-moving supercells (highly-organised 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.   A strong signal for very active convection is shown by very low values of convective inhibition (CIN) and by the moist air in the lowest layers, at the level of free convection (LFC) and at the lifted condensation level (LCL).   

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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.

Example 2:

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

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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 MUCAPE-shear EFI covers the wide area of severe convection.  Within the MUCAPE-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 MUCAPE-shear EFI signal was much stronger than that of MUCAPE 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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Note: HRES and Ensemble Control Forecast (ex-HRES) are scientifically, structurally and computationally identical.  With effect from Cy49r1, Ensemble Control Forecast (ex-HRES) output is equivalent to HRES output shown in the diagrams.   At the time of the diagrams, HRES had resolution of 9km and ensemble members had a resolution of 18km.

Considerations when forecasting Mesoscale Convective Systems (MCS) and Supercells

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

• the limited ability of Ensemble Control Forecast (ex-HRES) 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 MUCAPE and MUCAPE-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 Ensemble Control Forecast (ex-HRES).
• 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 MUCAPE and MUCAPE-shear can appear extreme but should not be ignored as unlikely. 

Additional Sources of Information

(Note: In older material there may be references to issues that have subsequently been addressed)

• Further information is available on EFI forecasting for severe convection.
• Guide to Instability Indices in ECMWF output
  
• Further information is available on derivation of CAPE and Most Unstable CAPE (MUCAPE). 

(FUG Associated with Cy49r1)