Mesoscale Convective System over central Italy

By Alexander Toniazzo, Nicoletta Roberto, Marco Petracca, Alessandra Mascitelli, Claudio Giorgi and Silvia Puca (H SAF)

These strong thunderstorms, consisting of an ensemble of organised convective elements, reaching up to 35000km², persisted over this area from 01:00-12:00 UTC. This system produced strong winds, hail, lightning and intense precipitation, plus a tornado that hit Civitavecchia on the Lazio coast at about 07:30 UTC.

In general, numerical weather prediction (NWP) models cannot accurately predict this type of weather event, due to the speed of development and its large extent. However, for this case near-real-time H SAF products were used to observe the whole event from the start, including during its development. This was done by estimating, every 15 minutes, the instantaneous precipitation from P-IN-SEVIRI-PMW (H60B) product, taking advantage of the availability of passive microwave (PMW) products such as P-IN-SSMIS (H01 new release) and P-IN-MHS (H02B). Italian ground-based networks consisting of weather radars, rain gauges, and lightning sensors were also used to monitor the event during its full growth.

On Thursday, 8 September, as announced in the bulletins and in the alerts issued by the Italian Civil Protection Department the day before (on 7 September) and on the same day, intense thunderstorms were recorded throughout Toscana, Umbria and Lazio Regions.

The first intense thunderstorm area hit the coast of Toscana in the early hours, with very heavy rainfall. The second storms developed southwards in the coastal area between Lazio and Toscana, with strong gusts, hail and a tornado in Lazio. The two storms moved to the east and decayed slowly, eventually merging over Umbria and Marche. The storms developed in the early hours of 8 September over the very warm sea waters of the Tyrrhenian Sea, very close to the coast.

Figure 1 is the ECMWF 500hPa synoptic analysis at 00:00 UTC. A fast trough, associated to a deep low pressure over the British Island, can be seen moving over France towards northern Italy. Strong thunderstorms, with heavy rainfall, hit some parts of northern Italy during the night.

In the Mediterranean area the sea temperatures were still very warm and warm air masses were advected with southerly winds.

Figure 2 is the mean sea level pressure and temperature in 850hPa (ECMWF model). The pressure was high in the Mediterranean, and very warm air masses, coming from northern  Africa, were advected towards the central areas and towards Italy.

It is in this air, that the very large thunderstorm areas developed. The lack of cold air in high layers usually impedes convection, but on this occasion very warm areas over the sea, associated with local wind convergence, led to the formation of very strong thunderstorms (MCS).

Figure 3 shows the high resolution winds in low layers (10m)  (MOLOC of ISAC-CNR). It is noticeable that the winds converged in an area east of the Toscana coast. The upward motion of the very warm and moist air determined by this convergence in low layers is the leading factor for the development of the storms over the sea.

Weather models do often underestimate, or misplace, the strong precipitation associated with the development of storms over the Mediterranean during the autumn months. So, for Civil Protection, good monitoring and nowcasting is needed to observe, and then track, the development and evolution of the thunderstorms and associated precipitation. Therefore, H SAF products, with their ability to track and estimate precipitation over large areas, including over the sea, are very useful.

The full development of the thunderstorm from inception at 01:00 UTC  off the coast of Tuscany, can be seen in the animation (Figure 4). It moved slightly from west to east and decreased in latitude, following the coastline between Tuscany and Lazio. Looking at the estimates of precipitation intensity, it appeared to reach maximum precipitation values in the morning (at 08:30 UTC), in the hinterland of the Lazio region.

Additional H SAF products are available for storm observation — those using passive microwave sensors aboard low Earth orbit (LEO) satellites. Among these, it is interesting to observe the P-IN-SSMIS product at 05:46 UTC (Figure 5), which observed an intensity of precipitation over 100mm per hour near the coasts of central Italy.

A further interesting satellite acquisition is that of the MHS sensor on board the NOAA-19 satellite. The resulting P-IN-MHS (H02B) product (Figure 6) identified the large MCS storm hitting the regions of central Italy at 07:38 UTC.

Organised convective cells with hail were observed over Lazio coast at 08:35 UTC by ground-based radars which measured reflectivity up to 55dBZ (Figure 7), and a probability of hail (POH) up to 100%.

Intense electrical activity is also associated to convective cells — in 15 minutes, ground-based sensors registered 2521 strikes over central Italy (Figure 8) .

Related content

South Africa floods (April 2022) and Mediterranean Cyclone (October 2021) event reports (H SAF)