Thunderstorms cause dramatic flooding in Tuscany

By Kristina Petraityte, HansPeter Roesli, Jochen Kerkmann (EUMETSAT), Alessandro Fuccello, Davide Melfi (Italian Air Force Meteorological Service) and Sheldon J. Kusselson (NOAA NESDIS)

During 10 and 12 November 2012, the Italian regions of Tuscany and Umbria have been affected by a rapid succession by a series of thunderstorms generating intense precipitation. The areas around Massa Carrara, Grosseto and Orvieto were the most affected; here heavy rainfall caused dramatic flooding of streams and rivers (about 200mm over Tuscany, with maxima of 300–350mm over Grosseto area). A short description of this event with a rainfall analysis is given in the report from Alessandro Fuccello and Davide Melfi. A precipitation analysis from NASA (based on satellite data) was published on NASA Earth Observatory.

The main process started on 11 November when a deep trough with a cut-off low formed over Spain/Morocco, see animation Airmass RGB, 11 November 00:00 UTC–12 September 21:00 UTC,. This cut-off low helped to push moist air towards the southern and central parts of Italy, while the northern part remained in drier air. On 12 November numerous thunderstorms formed along this (moisture) convergence zone, which had formed over the Tyrrhenian Sea (just south of Elba island), and moved towards the Tuscany mainland. The storms persisted for many hours, re-forming in the same area along the surface convergence line.

The moisture transport toward Italy can be monitored with the help of NOAA's Blended Total Precipitable Water (TPW)) product (Kusselson, et al., 2009). This product is produced hourly and combines the latest polar orbiting microwave TPW product images from the NOAA-15, 16, 17, 18, 19 and Metop-A satellites with those of the Defense Meteorological Satellite Program's (DMSP) Special Sensor Microwave Imager and Sounder (SSMI/S) F-16, 17 and 18. Since most of the moisture in the atmosphere is below 700hPa (Kusselson, 1993), the Blended TPW product for this case (see Figure 4 and related presentation) shows the precise location of the highest concentration of low level moisture (moisture plume or atmospheric river), which showed up well from the subtropics of northern Africa to the mid-latitudes of northern Italy.

Overlaid on the Blended TPW image is a set of arrows showing the 700hPa wind flow through the highest moisture — indicating the greatest moisture transport and insuring a continuous supply of low level moisture to the potential flood threat area. Low level wind flow parallel the highest moisture was allowing for the continuous transport of deep moisture over multiple days to the area of central Italy, increasing the moisture or at the very least replacing existing moisture that had been released as rainfall, thus, increasing or prolonging the heavy rain and flooding threat for Italy on 12 November (see presentation, figure 4 lower left image).

And how do we know there was a high amount of moisture? In addition to the precipitable water values, the Blended TPW Percent of Normal product (lower left images of figures 6, 8 and 10), that takes the Blended TPW and compares it with a 1988–1999 satellite climatology from the NASA Water Vapor Project, shows how anomalous the moisture from North Africa to Italy was for the time of year. Above normal moisture transport, as seen in the lower left images of figures 6, 8 and 10 into an already high moisture/heavy rainfall area acted to replace moisture that had already been released from the clouds, thus prolonging the heavy rain and flooding threat as long as lift and forcing by upper level features such as jet, PV anomalies, diffluence were continuing to be present (see upper left images of Meteosat-9 Airmass RGB in the same figures for these forcing mechanisms).

Additional content

IR10.8 image at 13:00 UTC
Ice Particle Size RGB image at 13:00 UTC
Animation IR10.8 images (08:00-13:00 UTC, animated )
Animation Airmass RGB (11 September 00:00 UTC - 12 September 21:00 UTC)

References

Kusselson, et. al, 2009: An Update on the operational implementation of blended total precipitable water (TPW) products, AMS 89th Annual Meeting, 23rd Conference on Hydrology, January 11-15, 2009, Phoenix, AZ.
Kusselson, S.J., 1993: The operational use of passive microwave data to enhance precipitation forecasts. 13th Conference on Weather Analysis and Forecasting, Vienna, Virginia, AMS 434-438.