Storm Aila : An unusually strong autumn storm in Finland

Satellite image of storm Aila on 17 September 2020 at 0900 utc. Storm was a high-impact autumn in Finland which well predicted by medium-range forecasts. The extreme winds caused established new Finnish records for the month September. (Source: Meteorological Institute/EUMETSAT.) Extratropical cyclones (ETCs) are responsible most day-to-day variability weather mid-latitudes. ETCs occur frequently winter season, and favourable atmospheric conditions they can strengthen into powerful windstorms cause significant damage to society due their associated winds. Preparing is crucial many domains society, such as forestry, insurance companies, energy sector power grid operators. Thus, accurately predicting track intensity remains task forecasters. Although located tail end North Atlantic track, experiences several each year (Gregow et al., 2020). They all seasons but typically strongest observed late winter. For instance, Aapeli January 2019 windstorm record with maximum 10min wind speed 32.5ms−1 (63 kn) gust 41.6ms−1 (81 kn; Tollman 2019). Tapani (also named Dagmar other Nordic countries) subsequent Hannu next day December 2011 belong category greatest impact left about 570 000 customers without electricity (Kufeoglu & Lehtonen, 2014). Windstorms also lead rise sea level via coastal flooding, during Gudrun 2005. least storm-force (>24.5ms−1 Beaufort scale) northern Europe usually rare, proportionally them post-tropical (Sainsbury One example these Mauri, developed from remnants Hurricane Debby 1982 (Laurila In contrast, Aila, topic this paper, classic baroclinic no tropical origins. an exceptionally strong storm. traversed central approximately 63°N towards east thus followed earlier notable (see Figure 1 Valta Institute (FMI) issued highest warning (red) gusts rough seas western Bothnian Sea. first red warnings were given three time, indicated confidence high predictability At time winds, FMI encouraged people stay indoors areas. 10-minute 29.4ms−1 (57 35.3ms−1 (69 2). Furthermore, strong-gale-force (>20.8ms−1 lasted over 15 hours Finland. inland station 26.8ms−1 (52kn; FMI, 2020), Forest Centre (FFC) estimated volume forest be 0.4–0.7 million m3 (FFC, Based preliminary estimations, 160 households altogether 2950 emergency call outs (Láng submitted). Additional impacts arose unusually precipitation totals, amounted 66mmday−1 (FMI, speeds fact that could relatively early, main motivation study. objective give overview Aila’s synoptic development. This performed analysing reanalysis data. second quantify Aila: how early signal visible forecasts? Finally, third put climatological larger context: unusual Aila? aim conducted long-term observations We used ERA5 (Hersbach 2020) European Medium-Range Weather Forecasts (ECMWF) describe evolution verify 6-hourly temporal 0.25° horizontal resolution. both surface pressure fields ERA5. downloaded Copernicus Climate Data Store (cds.climate.copernicus.eu). studied using high-resolution (HRES) deterministic forecasts Integrated Forecast System (IFS). IFS operational forecast model ECMWF. retrieved ECMWF Archival Retrieval (MARS). initialised 0000 utc 1200 7–18 2020. original resolution HRES 9km, corresponds 0.1° data regridded spatial (31 km) same used. analysed FMI's stations across investigation, we analyse distributions two where observed. observational dataset includes 1-hour values 2004 Before 2004, only recorded instantaneous likely missed hence, use here maximums available onwards. section gives based reanalysis. situation 15–17 low-level upper-level meteorological variables 3(a) shows initial when started develop. There ridge present (Figure 3(a)). Due ridge, jet stream shifted northward oriented northwest southeast Scandinavia. Owing southerly flow side air mass northwestern very warm. A weak frontal boundary north British Isles, evident vorticity shown 3(a), separated warm colder, polar Norwegian Sea 3(c)). Because temperature difference, edge highly vulnerable cyclogenesis. cyclogenesis trough approached pre-existing, zone west. axis between Iceland Scotland. Vorticity advection ahead forced ascent vicinity front (not shown). mid-level convergence pre-existing rapidly increase causing occur. rapid spin up process understood physically mathematically considering ‘stretching’ term full equation Lackmann, 2011, 5.3.1 more details). took place right entrance region streak 3(a)) enhanced cyclone development known location divergence. As result, low area formed 1800 minimum 1013hPa (between times Figures 3(c) (d); not On 16 2020, utc, longer linearly, featured clearly identifiable wave southern Norway 3(b)). Compared 12 before 3(a)), had increased. system decreased 1008hPa 3(d)). deepening continued phasing trough. moved coast 4(a)). point, still Swedish Gulf Bothnia. intensification progress 500hPa situated west structure vertically tilted generally characteristic strengthening system. cold-air northeasterly isobars below 500-hPa 4(a)) further contributed cooling decrease thickness causes geopotential heights fall (decrease) altitudes above amount cold place, hence upper During night September, reached its upstream mean (MSLP) greater than 1032hPa 4(b)). gradient resulted northerly airflow along Bothnia areas, night. parallel Bothnia, presumably helped formation because large fetch open water blew obstruction. already mature phase. detached closed circulation 4(c)). upper- lower-level lows almost aligned, implying ceased. seen pressure, did 4(b) (c). 4(c)), gusty blowing northeast, especially land areas According ERA5, 995hPa 0600 (c), 24-hour rate 16hPa gale-force (21ms−1) morning 13 4 days advance. days. it quite captured numerical prediction models, so early. mostly model, why compare different initialisation 5, MSLP every 11 shown. valid analysis 5(l)), have ranging 144 5(a)) 24 5(i)). impression 5 centre varied hundred kilometres consecutive T+108 T+144 hour (Figures 5(a–e)), after 96 increased consistency emerged 5(f–k)). However, regardless centre, present. Most too southwestern eastern northeastern dipole type difference appear majority 5). Consequently, forecasts, far reality 5(l)). magnitude errors hPa T+96 T+ 84 error less 10 5(f)–(k)). consistently lower actual value (996hPa, Only weaker 5(a, d)). 6 10-metre specific x-axis panels describes y-axis case accurate forecast, (red shading) would agree (shown bottom rows 6). allows us determine degree means multiple, vertical lines colour. 6(a)) Finland, including big fraction average 6(b)) considered forecast. Some indications T+216 T+240 yet consistent, partly disappeared 192–204 time. After that, starting 180 stormy began became consistent. Nevertheless, small timing 0 form invariant lines, remain fixed good. hours, tend drift rightward later times, meaning 12–24 reality. 6a), compared slightly overestimated short (12–72 hour) critical frame preparations communication. well, some modest underestimation 48–60 times. order assess represent real, 7 series stations: Pietarsaari Kallan 7a) Rauma Kylmäpihlaja 7b). locations marked 2. selected Aila. underestimated windiness 7(a)), while peak line 5(i), west, indicating stronger near reality, 5l) short-term one 7b), 6a). although having magnitude, fairly stations. fully comparable reasons. First, point values, averages cell nearest Secondly, made isolated islands, top lighthouses, 30 metres altitude (Pietarsaari Kallan, 7(a)) 38m (Rauma Kylmäpihlaja, 7(b)) 10-metres values. land-sea mask 0.43 0.23. interprets half pure marine conditions. reasons, 3–7ms−1 higher modelled gusts, 7(a)). Out stations, maxima (57kn) 34.8ms−1 (68kn) 28.1ms−1 (55kn) (69kn) investigated historical detail context. comparison against climatology, occurred middle 8 histograms Septembers 2004–2020 addition, ever blue 8. entire 17-year period (2004–2020) Similarly, closest points (grey 8). 1979–2020 shown) similar result found. Therefore, conclude out observation 1979–2020. (excluding mountainous 200 m level) whole history considered, found 2, 8), country. caveat conclusion network twentieth century sparse certain day. historic biased must interpreted caution. When comparing skewed (i.e. narrower distribution) observations. may coarser (10m 30–38m stations) local features resolved Moreover, captures distribution better distribution. IFS, parameter calculated summing terms: speed, represents roughness layer stability, convection (ECMWF, 2013). suggest underestimated, parametrisation, term, overestimated, (of opposite sign) compensate other. These are, however, or land. issue needs research scope our finding negatively speeds) agreement Sweden (Minola severe affecting mainly part destroyed cubic services. typical cyclones. under forcing stream. 18 hit arrived exceptional terms substantial impacts, meteorologically deep cannot classified explosive. meteorologists able moderately long example, (the announced advance, indicates event. What remarkable itself hitting strength, potential speculate scale developing dominance adiabatic contributions (e.g., advection) diabatic processes largely reasons predictability. year. readings observed, been stronger, belongs Luckily, communicated adequate thank Ville Siiskonen retrieving use. acknowledged making available. work supported Ministry Environment (SUOMI-hanke), Agriculture Forestry (MONITUHO), Atmosphere Competence Center (ACCC).