New statistical approach for hail storm nowcasting: case study of severe thunderstorm developed on 28 June 2007 over northeast Bulgaria Boryana Markova 1,2, Viara Rafailova 2, Rumjana Mitzeva 1 and Guergana Guerova 1 1 Faculty of Physics, University of Sofia, Sofia, Bulgaria 2 National Institute of Meteorology and Hydrology – Varna, BulgariaINTRODUCTION A slow-moving shallow low-pressure system passed over east Bulgaria and west part of the Black Sea on 28 June The 0000 UTC 500 hPa analyses showed a deep low over Scandinavian Peninsula and the Baltic Sea with strong south-westerly flow over Balkan Peninsula. Based on the 0000 UTC real aerological sounding at Bucharest, a hail producing thunderstorm was suspected to develop in the afternoon. However, the severe thunderstorm, with cloud top reaching 14 km and maximum reflectivity above 65 dBz, developed in the morning hours instead. Several days before the thunderstorm an early heat wave spread over the Balkan Peninsula (Balafoutis, 2007) with maximum temperatures in Northeast Bulgaria reaching 37-42◦C. During the heat wave period (20-28 June) convective activities with isolated lightning and thunders were recorded. The most severe thunderstorm developed on 28 June 2007, the last day of heat wave. Egg-size hail particles and severe wind gusts up to 28 m/s were observed around 0830 UTC and caused substantial crop losses in villages of Northeast Bulgaria. In this region severe hail storms are rare compared to the hail storm frequency in Northwest and south Bulgaria. Classification function Correct classification % all thunder ordinary F (th,r) = CAPE Li K Bucharest0000 UTC0300 UTC0600 UTC CAPE, J/kg Li, degrees K, degrees 41-- F F(th,r) ≥ 0 Thunderstorms F(th,r) < 0 Ordinary clouds Dobritch0000 UTC0300 UTC0600 UTC CAPE, J/kg Li, degrees K, degrees F  77.9% of thunderstorms were correctly classified. METHODOLOGY Based on discriminant analyses (StatSoft Inc., 2001) classification function F, combination of CAPE, Li and K indices was derived using 112 aerological sounding in summer 2006 with precipitating clouds developed over east of Bulgaria. Bucharest 0000 UTC Surface analysis, 0600 UTC 28 June 2007 The classification function F was calculated using the proximity soundings at 0000, 0300 and 0600 UTC from Dobrich, a town situated nearby to the region where the severe hail storm occured. Using CAPE, Li and K indices a new classification function F was computed and presented in the table 2. Aerological sounding from Bucharest at 0000 UTC was characterized by relatively low CAPE and low absolute value of Li, indicating weak instability. Only the calculated K value indicated the probability of high convective potential. Comparison between the indices from the real aerological sounding at 0000 UTC (table 1) and proximity sounding (table 2) shows:  Higher indices in the proximity sounding than from the real sounding;  The indices values increased gradually with time;  The highest value being at 0600 UTC just before the thunderstorm. The calculation of cumulative percentage for thunderstorms cases at 1200 UTC for summer 2006 revealed that only in 5 % of the cases F value was greater than the calculated maximum F = 2.21 on 28 June Histogram of probability distribution and cumulative percentage (solid line) of F for thunderstorms in summer The red column is F value for 28 June Diurnal variation of F for two stations in northeast Bulgaria Razgrad (Rz) and Dobrich (Db) on June The classification function F at station Dobrich:  has a maximum at 0006 UTC, just before the thunderstorm;  decreases sharply after 0006 UTC. CONCLUSION This study shows that use of proximity soundings close in place and time is beneficial for nowcasting of severe storms. The preliminary results indicate that the derived classification function F has a potential as an additional nowcasting tool. REFERENCES Balafoutis, Ch. J., 2007: The recent heat waves over Balkans as an indicator of climate change and a signal for new planning decisions. Developments in Tourism Climatology - A. Matzarakis, C.R. de Freitas, D. Scott, StatSoft, Inc. (2001). STATISTICA (data analysis software system), version Aerologigal sounding Buchurest 0000 UTC Radar reflectivity at 0731 UTC showed mature stage cloud. Hail on ground was reported at 0831 UTC It was reported in a local newspaper: “Egg size hail and severe wind gusts caused damages to the crops in several villages in the region of Dobrich” Photograph by M.Petrov AIM The aim of this work is to test new nowcasting tool that will improve the operational forecast skills for severe hail storm events. Table 1 Table 2 ANALISIS