Status:Ongoing analysis Material from: Ivan
Discussed in the following Daily reports:
http://intra.ecmwf.int/daily/d/dreport/2014/06/23/sc/
http://intra.ecmwf.int/daily/d/dreport/2014/06/24/sc/
1. Impact
An unstable air mass triggered a lot of convection accompanied by heavy thunderstorms, hail and torrential rain across Bulgaria from 15 to 19 June 2014. Climatologically June is one of the wettest months over most parts of the country. Nevertheless rainfall accumulations exceeded monthly means in some regions, especially in eastern Bulgaria. In NE of Bulgaria more than 100 mm fell just in 24 hours on 19 June causing a dozen of fatalities, most of them during a deluge in the residential area of Asparuhovo in the coastal city of Varna.
120-hour rainfall from 06UTC on 15th June to 06UTC on 20th June 2014 (left) and 24-hour rainfall from 06UTC on 19th to 06UTC on 20th June 2014 (right) including observation (numbers) and gridded analysis product with LAM ALADIN and rain-gauge observations. High-density observations and gridded analysis courtesy of the National Institute of Meteorology and Hydrology, Bulgaria.
2. Description of the event
HRES analyses show that there is a strong synoptic forcing that led to severe weather. There was initially a moderate easterly flow of humid air near the surface. The soil itself was very wet as well following few days of heavy convective showers. At the time of the heaviest rain over NE Bulgaria in the evening on 19 and in the morning on 20 June a rapid cyclogenesis occurred over the Black Sea just offshore the Bulgarian coast. The deep cyclone that formed moved to the north and then to the east turning the wind from E to NW over Bulgaria.
HRES analyses of MSLP and 10-metre wind (left) and soil moisture analysis before the flash floods on 19th June 2014 at 00UTC (right).
Satellite RGB Air mass animation showing strong convective activity over the Balkans and cyclonic circulation over the Black Sea (EUMETSAT).
The animation above from the Romanian meteorological radar network shows a long-lasting quasi-stationary convective cell over Varna, just in the region where the deluge occurred. You can see that cell on the right side of the animation just over Varna on the Black Sea coast. It is striking that the cell had been staying over the same place for more than 2 hours producing heavy rain. As a consequence a large amount of water washed away cars, houses and caused the death of about 10 people in Varna. (Animation is created with data from the National Meteorological Administration of Romania available free on the web).
3. Predictability
3.1 Data assimilation
3.2 HRES
The plot above shows comparison of three consecutive short-range HRES forecasts versus the observed 24-hour rainfall (top left panel). Observations are displayed with numbers while the shading represents a combination of LAM ALADIN and observations to create precipitation analysis on a regular grid. The analysis data are kindly provided by the Forecast Department of the National Institute of Meteorology and Hydrology in Bulgaria. It shows huge amounts of rain over NE Bulgaria. Rainfall totals of 155 mm were recorded in two stations and few other with rain above 100 mm. Although this is a 24-h accumulation it actually fell in just in few hours. The convective nature of the rainfall makes forecasting of this event very difficult. Indeed, the short-range HRES forecast gives considerable amounts of rain but not surprisingly the largest totals are misplaced. What is a little bit surprising is the fact that the latest forecast seems worse than previous two over NE Bulgaria where the largest amounts were recorded.
Longer-range HRES forecasts (see the figure above) vary much more but they had kept forecasting heavy rain quite far in the medium range.
Synoptic-scale forecast (see the figure above) about the cyclogenesis seems to be good in the HRES.
3.3 ENS
The flash floods occurred on 19 June in the evening and in early morning on the 20 June when just in few hours more than 100 mm of rainfall were recorded in NE part of the country. Flash floods occurred there. The analyses show that 99th model climate percentile for 24h total rainfall was exceeded in many places in the east of Bulgaria. Some local heavy showers were observed in some far western regions of the country as well. If we compare observations with the 95th model climate (M-climate) percentile, we'll notice areas of local torrential downpours in many parts of Northern Bulgaria as well. The shortest range EFI forecast matches quite well the areas affected by heavy rain but the EFI has never reached extremely high values close to 1 over NE Bulgaria. This could be due to the convective nature of the rain and the large uncertainty in the forecast that we might expect in such cases. Note that there is 6-hour shift between the validity of the EFI forecast and the accumulations represented by the observations: EFI is valid from 00 to 00UTC whilst accumulations are for periods of different length starting from 06UTC.
Observed 24-hour total precipitation compared with the model climate. Red triangles denote accumulations that exceed certain model climate percentiles (95th or 99th percentile). Green triangles denote observations that exceed 99th model climate percentile. The shortest range EFI/SOT forecast is also displayed. The CDFs from the most affected regions show that the forecast was quite uncertain and precipitation amounts were underestimated although they appear extreme compared to the M-climate.
The EFI forecast provided good guidance throughout the forecast from longer to shorter lead times. At the beginning, 7 days in advance, positive SOT over Bulgaria showed a risk of abnormal rainfall whilst the EFI was positive but still values were low. Closer to the event EFI became higher.
EFI forecasts for 24-hour total precipitation with observations on top. Green triangles denote observations that exceeded 99th model climate percentile.
The same signal we get by looking at longer accumulation periods, 3 and 5 days. The most extreme rainfall is again in NE parts of Bulgaria, with rainfall totals exceeding the 99th percentile of the M-climate in many places in the east of Bulgaria and also more locally in some western parts of the country. EFI provides good guidance throughout the forecast.
Observed 72- and 120-hour precipitation compared with the model climate. Red triangles denote accumulations that exceed certain model climate percentiles (95th or 99th percentile). Green triangles denote observations that exceed 99th model climate percentile. The shortest range EFI/SOT forecast is also displayed.
Longer lead time EFI/SOT forecasts for 72- and 120-hour total precipitation. Green triangles denote observations that exceed 99th model climate percentile.
3.4 Monthly forecasts
3.5 Comparison with other centres
4. Experience from general performance/other cases