Status:Ongoing analysis Material from: Linus, Calum
Picture
1. Impact
Between 6 and 7 August Skopje, the capital of Macedonia, was hit by flash floods and at least 21 people were killed. According to the reports, Skopje got more than 90 mm of rain in two hours. It is unclear when the rain fell, but satellite images suggest Saturday evening . Reports about the damages can be found here:
http://www.bbc.co.uk/news/world-europe-37002364
Google translate's version of the text from Miha's link below:
"This afternoon regiion Skopje was hit by the strongest storm this year, and one of the stronger over the years. Strong electric discharges were observed between 17 and 19 hours and reached number 800 (for the entire Skopje area).
The storm was accompanied by strong winds, intense rain raining continuously for about 1 hour. The nature of this storm cloud is such that cells are replenished behind one another, and therefore is the longevity of this storm."
2. Description of the event
The plots below shows the analysis for z500 and t850 (left) and short forecast for MSLP and 12-hour precipitation (right), both valid 7 August 00z. Skopje is located on the eastern edge of the upper-level trough.
The plot below show satellite image (from Dundee) for 20UTC on 6 August.
The plot below shows the 24-hour precipitation accumulations from observations received via GTS. The black dot is from Skopje and has more than 90 mm. Unfortunately we do not have any observations from Kosovo and cannot verify the northward extent of the cell.
3. Predictability
3.1 Data assimilation
3.2 HRES
The plots below show HRES forecasts for 24-hour precipitation (6 August 06z - 7 August 06z) and MSLP.
3.3 ENS
The plots below show EFI and SOT for precipitation on 6 August from different initial times (all 00z).
The plots below show the EFI for CAPE (left) and CAPE/Shear from the last forecast before the event. None of the products gave additional information about this event.
3.4 Monthly forecasts
3.5 Comparison with other centres
The plots below show 16 ensemble members from 6 August 00z for 24-hour precipitation (6 August 06z - 7 August 06z).
ECMWF
COSMO-LEPS
3.6 EFAS
Calum had a look at the EFAS forecast for the Macedonia floods, specifically at the 2016080512 forecast. Firstly the soil moistures were quite dry, in NW Macedonia the values were ~20% in all the ensemble members (this is calculated by: (forecast soil moisture - minimum climatology soil moisture)/(maximum climatology soil moisture - minimum climatology soil moisture)). Secondly in EFAS the flash flood warnings are generated from the surface runoff rather than the precipitation, so the dry soils mean that not much of the rain was translated into runoff (less than 20%). We generate flash flood warnings when there is a high probability of exceeding the mean annual maximum climatology runoff value, in the attached map the smaller dark-red pixels show the probability of exceeding this threshold, typically they were very low ~25%. The system has automatically generated 4 warning points (3 in Macedonia and 1 in Serbia) but as you can see from the graphs the probabilities of exceeding any of the return period levels is quite low. EFAS only display points on the website which have >20% probability of exceeding the 5 year return period, because this condition wasn't met at these locations they therefore weren't displayed on the website.
This case might raise an issue about how we generate runoff during an extreme rainfall event, in the flash floods we're using an empirical relationship between soil moisture and the runoff coefficient but the parameters are derived from the LISFLOOD climatology run so the same problem might emerge from the original model results. LISFLOOD uses the Xinanjiang model (aka VIC/ARNO) to generate surface runoff, maybe we should test how suitable it is for extreme flash flood events.
4. Experience from general performance/other cases
5. Good and bad aspects of the forecasts for the event
- The extreme rainfall was missed over Skopje, but present further to the north in the forecasts
- Negative influence of very dry soil in the EFAS forecast?