Prolonged heavy rain episode in Lithuania on 5-8 July 2007 Izolda Marcinonienė Lithuanian Hydrometeorological Service

The parameters of very heavy rain in Lithuania DANGEROUS PHENOMENON mm/≤12 h 2-3 months amount of precipitation /≤5 days CATASTROPHIC PHENOMENON ≥80 mm/≤12 h OBSERVED on 07/07/2007 At 6 meteorological stations (MS) There was 2,5 months amount of rain (202 mm)/4 days at 1 MS 86 mm/12 h at 1 MS 85 mm/10 h at 1 MS

Diagnosis The period between 5 and 8 July, 2007 is of special interest, when an upper low with sharp trough approaching from Southern Europe triggered the deepening of a lower tropospheric cyclone within unstable warm and wet airmass boundaries over Southern Ukraine. Especially intense cyclone development and powerful release of energy of instability occurred in the occluded front slowly moving from Belarus to South-Eastern Lithuania (on 6th day) and to the North approaching Latvia and moulding into expressive occlusion spiral (on 7-8th day). The centre of cyclone in the upper level coincided with the centre of a low-level cyclone – it determined the prolonged nature of the process. Confluence at the surface associated with upward vertical motion in upper surface layer provided heat and moisture for powerful Cb clouds and heavy rains forming over Lithuania and neighbouring countries.

HIRLAM forecast for night 6/7 of July (mm/3h)

HIRLAM forecast for 7 of July, (mm/3h)

Total precipitation during the period 5-8 of July (monthly rate 79 mm)

Cyclone track, July, 2007

MSG IR and surface pressure at 00 and 12 UTC,

MSG WV 6,2 µm and ECMWF geopotential height at 300 hPa at 00, 12 UTC

Wind speed (m/s) in Vilnius, at 00,12 UTC, , HIRLAM

Wind direction (°) in Vilnius, at 00,12UTC, , HIRLAM

Equivalent potential temperature (°K) in Vilnius, at 00, 12 UTC, , HIRLAM

Temperature advection at 00 UTC, (Vilnius 55°/25°), ECMWF

Temperature advection at 12 UTC, (Vilnius 55°/25°), ECMWF

Potential vorticity ( 0,2=2 PVU) at 00 UTC, , ECMWF

Vorticity advection at 00 UTC, , ECMWF

MSG RGB Night Microphysical (00 UTC), HRV (12 UTC),

MSG RGB Airmass, at 00, 12 UTC,

Conclusion The conditions within the airmass boundaries – high equivalent potential temperature, wind shear in the lower troposphere and a potentially unstable stratification of the air in a rather deep layer – were favourable for a rapid cyclogenesis to occur. The most intensive convection appeared over the E and W Lithuania at the point of coincidence of the wet bulb maximum potential temperature and the strongest convergence. Different MSG images together with analysis of meteorological parameters helped to both predict and explain the processes behind this unusual event in the Baltic States. ECMWF, MetOffice and DWD numerical forecasts and HIRLAM as well provided very good guidance on the cyclogenesis 2-3 days in advance of the event. It allowed forecasters to predict weather conditions quite well and issue their warnings in time. It was the second time when such heavy and long rain period occurred in Lithuania over the last 30 years) and a significant damage had been done – mainly by rivers floods. Moreover, it spoiled the Lithuanian open-air festival of songs and dances.

Lithuanian festival of songs and dances starts...

...and ends