A case study of heavy storms in Basque Country: 30-31 August 2015 event J . E g a ñ a 1 , 2 , S. Gaztelumendi 1 , 2 , O. Príncipe 1 , 2 , V. Palacio 1 , 2 1 - Basque Meteorology Agency (EUSKALMET). Parque tec nológico de Álava. Avda. Einstein 44 Ed. 6 Of. 303, 01510 Miñano, Álava, Spain 2 - TECNALIA, Meteorology Area. Parque tecnológico d e Álava. Avda. Albert Einstein 28, 01510 Miñano, Ál ava, Spain. Abstract Introduction During the 30 and 31 August 2015, in the In warm seasons (from May to September), storms generated by deep convection Basque Country heavy storms occur, leaving are frequent in the Basque Country. They do not only produce heavy rainfall in a short in some places strong wind gusts and heavy period of time, but they can also originate hail and strong wind gusts. showers with hail. On day 30, wind is the most Generally, surface pressure patterns are not very defined in these situations and a relevant aspect associated with stormy activity barometric swamp or relative thermal lows appear. In high levels, southwest wind generated due to a instability line that crosses usually predominates and there is often a trough configuration with its axe over the west the territory from west to east. On day 31, of the Iberian Peninsula. This configuration generates positive vorticity advection, storms characteristics favors very heavy therefore air rises. Besides, in this period, warmth accumulated during the day favours showers exceeding 30 mm in one hour at atmospheric thermal instability. When instability degree is high, storms might be very some points due to the formation of active and vertically quite developed and the top of the clouds could reach the convective cells moving from southwest to tropopause. northeast. In many cases, storms are created in the south or southwest outside the Basque This episode is studied considering Country, although they can also be generated within. This depends on the convergence synoptic and mesoscale features. In order to areas in low levels and instability degree above the Basque Country. In this region, characterize the synoptic environment during storms usually move from southwest to northeast. This is the most frequent movement this episode, different synoptic maps are on southwest flow in high levels. In some circumstances, deviation in the storms analyzed in the main levels; 300 hPa, 500 hPa movement might be relevant in relation to the main flow, as other factors are Figure 2. Geopotential height and 850 hPa topographies, sea level considered. In these situations, it should be taken into account instability index and and Wind at 300 hPa the 30 Figure 1. SLP, Geop-isoterm 850, Geop-isoterm 500 images from 12 UTC 30/08/2015 to 06 UTC 31/08/2015. pressure, and some instability indexes, other parameters, such as wind shear and storm relative helicity, in order to estimate and 31/08/2015 12 UTC. dynamic instability and other relevant activity degree of the storms. parameters are taking into account. In this In the Basque Country, Total Totals Index (TTI) values of severe storms are usually analysis we use registered data from the over 53 and Lift Index (LI) under -4 (Egaña et al, 2011). In these cases, rainfalls are automatic weather stations network and usually very intense and more than 10 mm can be accumulated in 10 minutes. remote sensing data available in the area. Moreover, hail might have over one centimetre diameter and strong wind gusts might be Environment produced. On 30 and 31 August, surface day. As a result of all of it, thermal and blows and this favours that storms move pressure pattern is not very defined (see dynamic instability grows. In 850 hPa level, from southwest toward northeast (see Fig.1). During the 30th, a small secondary the 31st, tongue of warm air extends from Fig.1 and Fig.2). low moves from south up to the Bay of south of the Iberian Peninsula and reaches On the other hand, instability index are Biscay, across the west of the Iberian 22-23 ºC. The next day, it moves off to the relevant. TTI reaches 54 and LI -4. Total Peninsula, and very active instability lines south, therefore temperature is not so high Column Water is also noteworthy, with Figure 4. Evolution of the squall line on 30/08/2015. PPI and MAX with are associated. Meanwhile, in 500 hPa (see Fig.1). values between 30 and 35 kg/m2 (see Figure 3. TTI, LI index and Precipitable Water the 31/08/2015 00 and 12 UTC. lightning activity products of Kapildui radar. level, a trough is situated over the Atlantic On 30 August, south wind Fig.3). Ocean, in the west of the Iberian predominates in surface. However, from Peninsula. Its axis is located parallel to the the morning on the day after, northwest coast of Portugal and it has cold air inside wind starts to predominate, at the (-12 to -14ºC) (see Fig.1). This air mass beginning in the west of the Basque contrasts with the sea surface temperature Country and afterwards in the east. Thus, that is around 21ºC, taking also into temperature rises more in this area. In high account the heat accumulated during the layers of the troposphere, southwest wind Figure 6. Kapildui radar images: MAX with lightning activity from Figure 5. Radial velocity from the Kapildui radar. PCAPPI 2 km. 30/08/2015. 12:42 to 19:52 UTC and Episode description ZHAIL product. 31/08/2015. Day 30: Day 31:

Maximum day temperatures reach very high values due to south wind and warm air in low Maximum temperatures are not as high as the previous day, although they reach 30- layers (22-23°C at 850 hPa). It reaches around 38ºC in some areas of the Cantabrian watershed, 32°C in the northeast of the Basque Country. At midday, first convective cells start forming in where south wind effect is more considerable. In addition to heat accumulated during the day, the southern part of Basque Country. Convective cells move from southwest to northeast and region is situated on the right of the trough axis and close to the depression that moves over the they activate as they approach the Basque Country. northwest of the Iberian Peninsula. All this favors a sufficient dynamic increase to transform First rainfalls appear in the southwest between 12 and 13 UTC. Cells activate and move accumulated energy during the day into storms in the afternoon. especially in the east of the Basque Country, leaving moderate to heavy rain, locally very This instability situation causes a squall line longer than 300 km, from the Cantabrian sea to the heavy and accompanied by hail (see Fig.6 and Fig.9). In storm areas, wind intensifies and south (see Fig.4). At the end of the afternoon, this line begins to form and approaches the Basque very strong gusts are recorded, but much less than the previous day. Later around 18:30 Country about 20 UTC. Thundershowers are preceded by a strong wind intensification (see Fig.5). UTC, new storms come from the southwest of the Basque Country and leave again heavy Southwest wind gusts of hurricane force are registered in some mountain areas and wind gusts are and very heavy rain, but this time they are more scattered throughout the region (see Fig.9 very strong even in non-exposed areas (see Fig.7 and Table 1), resulting a sharp increase of and Table.2). temperatures. After these phenomena, rainfalls are produced and most of them are moderate, likely It is remarkable the amount of lightning recorded during the event, they are distributed accompanied by some hailstorms during the first phase. First rainfalls occur in the southwest of all over the territory, but especially in the south (see Fig. 8). 1536 lightning strokes are Alava and quickly spread to the rest of the region, from west-southwest to east-northeast. They are registered in total. The most intense and positive cloud-ground lightning is recorded at 20:24 locally moderate in the west, but as rainfalls cross the region, they gradually lose intensity (see UTC with an intensity of 209.6 kA. The most intense negative one is -81.7 kA. Fig.7). During the event, 539 lightning strokes are recorded, of which the most intense positive lightning is 104.4 kA and the most intense negative one is -81.8 kA (see Fig.8). On 31, it stops raining around 00 UTC. However, there are still some weak rainfalls in the northeast, then they stops for a few hours. After precipitation, temperature drops between 5 and 10°C and also the wind calms down in Figure 8. Time and spatial distribution of the lightning the 30 and 31/08/2015. most of the region. Max Total 24 Station Maximum SW gust (km/h) Hour UTC Hour Hourly max Hour Station 10 min h (UTC) (mm) (UTC) (mm) (mm) Orduña 126.3 20:20 Añarbe 18,4 14:10 36,5 14:30 78,9 Oiz 113.9 21:10 Kanpezu 15,6 19:50 29,6 20:30 37,8 Navarrete 15,5 13:10 17,7 14:00 26,9 Punta Galea 111.5 20:50 Amundarain 13,4 13:50 22 14:30 31 Matxitxako 109.7 21:10 Páganos 12,4 13:00 16,8 13:50 30,8 Alegria 107.6 20:50 Ordizia 10,3 13:40 18,7 14:10 29,4 Zaldiaran 104.8 20:20 Zaldiaran 7,3 19:00 18,4 19:20 25,2 La Garbea 104 20:20 Figure 9. Evolution of the downpours from 13:10 to 14:10 UTC and from 18:40 to 21:10 UTC. Table 2. Maximum rainfall registered in one hour and 10 minutes. ..

Table 1. Maximum southwest gusts registered..

Figure 7. Evolution of the downpours and the maximum wind gusts from 20:30 to 22:30 UTC. Summary and conclusions V This is an event of two successive days, with typical storms of warm season in this area. During the first day, the highlight is the wind intensification, as the squall line enters in the territory. Whereas the second day, the highlight is the rainfall intensity. V Trajectory of storms is the usual one in this zone, they come from the southwest and move to northeast. V Although these storms are common, occasionally they may cause problems due to strong wind gusts, hail and a lot of precipitation in a very short period of time, especially when storms affect a populated area, such as a city, and when rainfall exceeds 10 mm in 10 minutes or 30 mm in 1 hour. Figure 10. Consequences of the heavy downpours and the strong wind in Rioja Alavesa and Vitoria-Gasteiz during the event. 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