Use of instability products MUCAPE, MUCAPE-Shear.

CAPE: Convective available potential energy (CAPE) describes the specific potential energy of air in the lower troposphere that potentially could be released in convective storms.  It represents the buoyancy energy of an air parcel freely rising through the atmosphere and depends on atmospheric structure.

MUCAPE: is the highest (most unstable) CAPE value found within the ensemble.

CAPE-Shear:  CAPE-shear is a combination of bulk shear (vector wind shear in the lowest 6km of the atmosphere) and CAPE.  It is used to identify areas of potentially extreme convection.

MUCAPE-Shear is the value of CAPE-Shear using the most unstable CAPE (MUCAPE) found within the ensemble.

Vertical wind shear tends to promote thunderstorm organisation, although excessive wind shear can be detrimental to convective initiation by increasing entrainment of environmental air into the storm.  But if active convection is indeed established, then larger wind shear tends to be associated with higher organisation and severity of convection.

CIN: Convective inhibition (CIN) represents the energy needed to lift an air parcel upward to its level of free convection.  CIN does not indicate whether convective instability will be released, but rather provides an indication of the potential for that release.

MUCIN:  This describes the energy required to provide sufficient lift to overcome any capping inversion and to release the most unstable CAPE (MUCAPE).  It does not indicate whether convective instability will be released, but rather provides an indication of the potential for that release. 

None of the parameters give information on the cloud and precipitation.  High CAPE released within a dry atmosphere may give only little amounts of cloud.  For heavy showery rain areas or severe storms it is necessary to have moisture available at the base of convection and preferably throughout the troposphere.  Generally IFS forecasts of rain areas prove helpful in giving this information.  However, release of the instability depends on CIN being overcome or else on some dynamic uplift higher in the atmosphere. Also, additional moisture at low levels (e.g. by sea breeze) can bring development of significant showers - see section on the impact of incorrect moisture.

The section on Convective cloud processes and precipitation gives more detail.

General rule for forecasting convective storms

As a general rule, heavy showery rain or severe storms, are most likely where areas of high MUCAPE, or more especially MUCAPE-Shear, coincide with areas of forecast precipitation as long as instability is released by some mechanism.  MUCAPE and MUCAPE-Shear alone don't tell the full story.   

Example of parameter use.

Large values of CAPE lie in a zone across the Aegean Sea and parts of mid-Greece.  This is coincident with a belt of strong vertical wind shear resulting in very high values of CAPE-SHEAR (Figs 9.6.4-1 & 9.6.4-2).

In particular, high forecast values of CAPE and CAPE-SHEAR are indicated at Pilio while much lower forecast values are shown at Kavala.  At first sight this might suggest:

  • active instability in the region of Pilio with high shower activity and possibility of severe storms and heavy rainfall.
  • weaker instability in the region of Kavala with less showery activity.

Such a snap assessment would be incorrect.


  

Fig9.6.4-1: Forecast CAPE (Blue high, Red low).

Fig9.6.4-2: Bulk Wind Shear (Orange high, Yellow low).  T+24 VT00UTC 11 July 2019, DT00UTC 10 July 2019.  


 

Fig9.6.4-3: Forecast CAPE-SHEAR (Purple high, Blue low).

Fig9.6.4-4: Max CAPE-SHEAR (Red high, Blue low).  T+24 VT00UTC 11 July 2019, DT00UTC 10 July 2019.  Very high values are indicated in the vicinity of Pilio.  More modest values are indicated in the vicinity of Kavala on the CAPE-SHEAR chart but note that the maximum CAPE-SHEAR chart shows there have been much higher values during the previous 6hrs.


Forecast precipitation (Fig9.6.4-5) lies across North Greece, Albania and Bulgaria and implies sufficient moisture in the forecast atmosphere to provide precipitation.  The area intersects the northern flank of the forecast CAPE and CAPE-SHEAR.  It is this area that is more likely to see release of deep and active convection.    

Little or no precipitation is indicated in mid-Greece (Fig9.6.4-5) which implies rather less moisture but in area of very high CAPE.  So isolated but local very heavy showers are possible and, bearing in mind the high bulk shear and CAPE-SHEAR values, local storms cannot be ruled out.


Fig9.6.4-5: Forecast precipitation (12hr). T+24 VT00UTC 11 July 2019, DT00UTC 10 July 2019.


The corresponding diagnostic charts for the probability of high rainfall (>40mm/24hr) and Extreme Forecast Index (EFI) for precipitation (Fig9.6.4-6 & Fig9.6.4-7) identify the areas at greatest risk of a major precipitation event.     

Fig9.6.4-6: Probability of total precipitation >40mm (24hr).  Green shading represents 35-65% probability.

Fig9.6.4-7: Precipitation extreme forecast index (EFI).  Red shading represents EFI>0.8, Dark red >0.9 EFI. T+24 VT00UTC 11 July 2019, DT00UTC 10 July 2019.


Forecast vertical profiles are very helpful in assessing the potential for severe events.

  • Pilo: The forecast vertical profile at Pilio (Fig9.6.4-9) shows large CAPE but with relatively dry convection, possibly released by high surface daytime temperatures.  Very little moisture is indicated and precipitation looks very unlikely.  However some moisture is available locally over mid-Greece mainly at medium levels producing possible local showery outbreaks given some form of dynamic uplift.  The relevant wind shear to consider for this is probably between medium and upper tropospheric levels rather than between lower and medium levels (the bulk shear).  Inspection of the hodograph suggests the upper tropospheric shear is not great, so shower organisation/activity would lack this element of support.  Note, however, that heavy medium level showers can penetrate downwards through underlying dry layers more than IFS forecasts tend to suggest, even reaching down to the surface.   Ensemble Control Forecast (ex-HRES) show a very humid boundary layer at Pilio, but it would require large energy input at the surface (2m temperatures above about 35°C) to overcome the large CIN and to lift the low level moisture to release moist convective cells.
  • Kavala: The forecast vertical profile at Kavala (Fig9.6.4-8) shows rather less CAPE and CAPE-SHEAR but with an almost saturated atmosphere and absolute instability at around 750hPa.  Recall also that the max CAPE-Shear over the previous 6hrs was higher.  So very active moist convection is extremely likely in the northern Greece region.  Inspection of the hodograph suggests significant shear throughout lower and medium layers allowing separation of up draughts and down draughts with persistent active precipitation cells.  

Violent storms with local hail swept across northern Greece overnight 10/11 July 2019 causing seven deaths and widespread damage.


Fig9.6.4-8: Forecast vertical profiles for Kavala, Greece. T+24 VT00UTC 11 July 2019, DT00UTC 10 July 2019.

Fig9.6.4-9: Forecast vertical profiles for Pilio, Greece. T+24 VT00UTC 11 July 2019, DT00UTC 10 July 2019.

Use of a sequence of data as early warning

A sequence of forecast EFI charts gives early indication of forthcoming severe weather potential (Fig9.6.4-10), and some idea of the confidence that may be placed on the forecast event.  In this case, northern Greece is identified as being at moderately high risk of an extreme event (EFI ~ 0.6) four days before, rising steadily to a very high risk of an extreme event (EFI ~ 0.9) two days before the occurrence of the severe weather.  Note how there is consistent indication of a very high risk of an extreme event (EFI ~ 0.9) over the Balkan states through the sequence of forecast runs.  The consistency in the areas shown at risk leads to a higher confidence in forecasts of severe weather.  Users should inspect forecast fields using ecCharts and vertical profiles as outlined above to assess forecast details, and also add in the influence of additional factors using local knowledge (e.g. regarding topographic influences) wherever possible.   


Fig9.6.4-10: Sequence of EFI precipitation charts from four EFI runs at 24hr intervals (DT 12UTC on 6, 7, 8, 9 July 2019). Increasingly high EFI precipitation values identify the areas at greatest risk.

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