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Forecasting Severe Convective Hazards

Use of EFI with CAPE and CAPE-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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Fig8.1.4.5.6: 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 CAPE or CAPE-shear EFIs coincide with high rainfall from HRES when assessing the release of severe convection.  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.

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Fig8.1.4.5.15: 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.

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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• 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.  

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). 

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