Using satellite imagery to detect severe thunderstorms

Templ ref: PPT-ISO-colour.001 Doc Ref no: RTC-PRE-165.1 What to look for Cloud top temperatures (look for overshooting tops) Cloud type/structure: obviously you want Cb’s but not all CB’s look the same. Some have extensive Anvils and others not…it depends on the upper winds and wind shear environment Satellite imagery can only give you a few hints as to the size, movement and microphysical properties at the top of a storm. You cannot use it to send Nowcast warnings, as you cannot see the internal structure of the storms. Templ ref: PPT-ISO-colour.001 Doc Ref no: RTC-PRE-165.1

HIRES VIS ( November 2016: Edenvale flooding) Large, stationary CB, developed and precipitated while remaining fairly stationary Images a-e are every 15min from 1445UTC -1545UTC

HIRES VIS: 9 October 2017 Overshooting top

Overshooting Top RGB: 9 Oct 2017 Templ ref: PPT-ISO-colour.001 Doc Ref no: RTC-PRE-165.1

Convective Storms RGB: 9 Oct 2017 Small ice particles in region of overshooting top = strong updraft and possible heavy rain and hail below Templ ref: PPT-ISO-colour.001 Doc Ref no: RTC-PRE-165.1

IR10.8 colour enhanced: 9 Oct 2017 CTT below -60deg in the region of the overshooting top, and elevated small ice particles Templ ref: PPT-ISO-colour.001 Doc Ref no: RTC-PRE-165.1

Templ ref: PPT-ISO-colour.001 Doc Ref no: RTC-PRE-165.1 WV 7.3 and 6.2: 9 Oct 2017 METEOSAT paired satellite image, At left (a) 08h30UTC (10h30SAST) and at right (b)12h30UTC (14h30SAST), 09 October 2017 indicating the position of the cut-off low over South Africa in (a) and the position of the upper air dry slot, together with thunderstorms triggering within the warm conveyor belt region in image (b). © EUMETSAT 2017 Ref: Kevin Rae, chief forecaster Templ ref: PPT-ISO-colour.001 Doc Ref no: RTC-PRE-165.1

Templ ref: PPT-ISO-colour.001 Doc Ref no: RTC-PRE-165.1 METEOSAT satellite image (Meteosat 9), 02 October 2011. Historic cut-off low case. © EUMETSAT 2011   Co-incidentally this was the day of the Duduza tornado (over south-eastern Gauteng). In addition, a severe supercell type storm was concurrently observed in the Brits area, just north-west of Pretoria Ref: Kevin Rae, chief Forecaster Templ ref: PPT-ISO-colour.001 Doc Ref no: RTC-PRE-165.1

Templ ref: PPT-ISO-colour.001 Doc Ref no: RTC-PRE-165.1 METEOSAT paired satellite image (“Airmass channel” RGB composite), At left, (a) 04h15UTC (06h15SAST) and at right (b) 13h30UTC (15h30SAST), 26 July 2016 indicating the upper air cut-off low over the interior of South Africa (the system which is the subject of this report). The image in (b) corresponds closely to the time at which the Tembisa tornado occurred over central Gauteng province. “C” indicates the general location of the “cloud head” feature on the periphery of the “cold pool”. © EUMETSAT 2016 Ref: Kevin Rae, Chief Forecaster Templ ref: PPT-ISO-colour.001 Doc Ref no: RTC-PRE-165.1

Some things to remember Satellite imagery only shows the tops of storms and the internal structure is inferred from the cloud top temps, presence of ice and the overshooting top. All storms are different and CTT will be different Watch for storm motion, deviation of storms from average storm path (to the left in SA), storm speed…slow moving could cause more precip on the ground…. These images obviously only work during the day, but the Overshooting tops, WV and IR10.8 can be used at night as well. Always use the RADAR data to get the full view of what the storm dynamics look like Templ ref: PPT-ISO-colour.001 Doc Ref no: RTC-PRE-165.1

Templ ref: PPT-ISO-colour.001 Doc Ref no: RTC-PRE-165.1 More to remember Tephi’s, Satellite, NWP and other derived products should give you situational awareness leading up to the time when you expect convection to occur. Continuously monitoring the satellite imagery and RADAR products will help with the now-casting the issuing of relevant warnings. Use the examples above to help recognize favourable situations/scenario’s for severe convective development Templ ref: PPT-ISO-colour.001 Doc Ref no: RTC-PRE-165.1