In the past few years, demands on flash flood forecasting have grown. The Flash Flood Indicator (FFI) is a system used at the Czech Hydrometeorological Institute for the evaluation of the risk of possible occurrence of flash floods over the whole Czech Republic. The FFI calculation is based on the current soil saturation, the physical-geographical characteristics of every considered area, and radar-based quantitative precipitation estimates (QPEs) and forecasts (QPFs). For higher reliability of the flash flood risk assessment, calculations of QPEs and QPFs are crucial, particularly when very high intensities of rainfall are reached or expected. QPEs and QPFs entering the FFI computations are the products of the Czech Weather Radar Network. The QPF is based on the COTREC extrapolation method. The radar-rain gauge-combining method MERGE2 is used to improve radar-only QPEs and QPFs. It generates a combined radar-rain gauge QPE based on the kriging with an external drift algorithm, and, also, an adjustment coefficient applicable to radar-only QPEs and QPFs. The adjustment coefficient is applied in situations when corresponding rain gauge measurements are not yet available. A new adjustment coefficient scheme was developed and tested to improve the performance of adjusted radar QPEs and QPFs in the FFI.

Radar measurements are one of the key data sources for detection and nowcasting of convective storms and accompanying severe phenomena. Radar algorithm estimating probability of hail occurrence was first implemented in the Czech Hydrometeorological Institute (CHMI) in 2004. Development of new radar products estimating probability of severe hail occurrence and their visualization has taken place in the CHMI in recent years. This article describes evaluation of radar products which, combined with temperature sounding measurements, determine probability of hail occurrence and provide estimates of maximum hail size. These products are operationally produced in CHMI and visualized in a new JSMeteoView2 application which serves as the main tool for visualization of radar data and other remote sensing data in combination with detailed geographical layers. The article discusses performance of radar hail products tested on a large set of data and their operational usability when estimating hail presence.

An updated version of JSMeteoView2 web-based application has been developed in the Czech Hydrometeorological Institute (CHMI). It has been used as a primary application for display of weather radar data in CHMI forecasting offices. It is used primarily for displaying of radar data which are available with horizontal and vertical resolution of 0.5 km. It also enables display and combination of other types of data such as data from synoptic stations, data from meteorological satellites, lightning data and requests for intervention of integrated rescue system. Users can use various geographical layers to precisely localize individual meteorological events. New JSMeteoView2 layer displaying results of object based CELLTRACK algorithm is used specifically in convective situations. CELLTRACK identifies cores of radar reflectivity 44 dBZ and higher and tracks these identified reflectivity cores. It also displays various direct and derived characteristics of the identified cores based primarily on radar data but also on lightning data. Among other reflectivity core characteristics various hail products were implemented. They are based on Waldvogel algorithm, severe hail algorithm described by Witt and also Vaisala HydroClass algorithm based on dual polarization weather radar data. Long-term evaluation of these hail products was carried out using records from European Severe Weather Database and also on continuous observations from CHMI professional stations, which enabled assessment of probability of detection but also assessment of false alarms occurrence. The contribution will introduce the updated JSMeteoView2 application with main focus on description of CELLTRACK object layer and overview of implemented hail products. The contribution will also describe methods used for evaluation of hail products and results of this evaluation.

On 24th June 2021 a devastating F4 tornado occurred in southern Moravia (SE of Czechia) that directly caused 6 fatalities and about 200 injuries. Approximately 200 houses and buildings out of about 1200 damaged ones had to be torn down. It has been the most severe tornado in modern history of Czechia.Besides other data sources, the event was captured by C-band polarimetric Doppler radars of the Czech Weather Radar Network (CZRAD) as well by the LINET lightning detection network. Contribution will present analysis of these data and discuss their utilization in operational severe storm nowcasting in the Czech Hydrometeorological Institute. Operational nowcasting algorithm CELLTRACK for convective storm nowcasting and web-based application JSMeteoView2 for radar and lightning data analysis and display will be introduced.The pronounced Doppler radar signatures of supercell storms like the hook echo, bounded weak echo region and the mesocyclone signature will be shown together with signatures derived from the polarimetric radar moments such as RHOHV ring, ZDR arc/ring or KDP column.Contribution will also present comparison of various radar and lightning characteristics of tornadic storm with characteristics of all other storms identified over Czech Weather Radar Network domain by the CELLTRACK nowcasting algorithm during the investigated period of 24th June 2021.