Weather news

According to recently published data from ECMWF's Reanalysis v5 dataset (ERA5), July 2023 was, by quite some margin, the hottest month globally on record, with an average temperature of 16.95°C. This comfortably exceeds the previous record of 16.63°C, set only as recently as July 2019. The 29 days from 3 July to 31 July were the hottest 29 days on record. The ERA5 dataset is produced by ECMWF for the European Union's Copernicus Climate Change Service (C3S), and goes back 73 years to January 1940, but some leading climate scientists have suggested that the records could plausibly apply to at least the past 120 000 years. It is in this context that heatwaves, both on land and sea, and albeit more indirectly flooding, have tended to dominate the weather news agenda in recent months, as many of the following stories will attest to! (Sources: Copernicus Climate Change Service, Professor Michael Mann.) A prolonged and at times extreme heatwave affected the Greater Mediterranean region for much of July. Unofficially dubbed Cerberus and Charon, it initially began across the western Mediterranean, with temperatures in southern and eastern Spain peaking in the low to mid 40s Celsius, setting new records in Catalonia. As the heat extended east, Jerzu in Sardinia provisionally peaked at 48.2°C on 24 July, exceeding the previous maximum July temperature recorded in Europe (48.0°C in Athens, 1977). Meanwhile, Palermo set a new all-time daily maximum and minimum temperature of 47.0°C and 36.2°C respectively, within 24 hours of each other – the latter a new European daily minimum record. New national records were set in Albania (44.0°C at Kucova on 25 July) and Izmir, western Turkey (43.2°C on 26 July). The Algerian and Tunisian capitals also set all-time records of 48.7°C and 49.0°C respectively. In North America, prolonged extreme heat gripped many southern areas, especially the desert southwest, throughout most of July, extending into early August. Phoenix, Arizona, recorded an average temperature of 39.3°C in July, with 31 days, up to and including 30 July of daytime maxima at or above 43.3°C, making it the hottest month in the city's history. The temperature at the infamous Furnace Creek, California provisionally peaked at 53.3°C on 16 July. Parts of northern Mexico also saw temperatures exceed 50°C with at least 200 deaths attributed to heat. Extreme heat made its mark in China too, with a new all-time national record being set on 16 July as 52.2°C was recorded in Turpan City, Xinjiang Province. In the southern hemisphere, despite it being late winter, temperatures here were unprecedentedly high too. August opened with temperatures in the high 30s Celsius in northern Argentina and Paraguay, more than 10 degC above the average and likely challenging records. Tasmania and Sydney, Australia, have also experienced their warmest July on record, with an average maximum temperature of 19.91°C for the capital of New South Wales. With so much hot weather, wildfires have been especially numerous; we will provide a full summary of the major events in next month's news. (Sources: ECMWF, Copernicus, WMO, Relief Web, Reuters, Government of Canada, National Weather Service, Weatherzone.) Prolonged and extreme heat across southern Europe and North Africa led to record sea surface temperatures (SSTs) in the Mediterranean basin in July. On 24 July, the average SST was 28.4°C, toppling the record previously set on 23 August 2003, according to C3S. In the tropical Atlantic, the shallow coastal waters around Florida are especially warm. A buoy forty miles south of Miami posted a temperature of 38.4°C challenging SST world records, though we note that this was only at a depth of five metres and is likely a very localised anomaly. Whether this reading is wholly representative or not, the wider oceanic heat has severely stressed the delicate corals around the Florida Keys, leading to what is known as bleaching – the expulsion of algae living in their tissues – causing the coral to turn white. Short periods of high temperatures are not fatal, but prolonged heat can be, as the algae cannot grow back and thus the coral dies. NOAA predict a 70–100% chance of the extreme heat in the wider North Atlantic persisting into the autumn. Antarctic sea ice continues to expand slowly, but as of early August, it remains at record (since 1979) low levels, likely in part a consequence of anomalously high SSTs. And let's not forget the elephant in the room – El Niño. The central and eastern tropical Pacific continues to warm, with general model consensus that this will continue until at least the end of 2023. Generally, the stronger the SST anomaly, the stronger the atmospheric response, but the already high ocean temperatures may complicate the wider impacts of El Niño on weather patterns further afield. (Sources: Copernicus, NOAA, NSIDC, WWF, Emily Becker (blog).) In contrast, the UK experienced a much wetter and much duller than average (c/f 1991–2020 LTA) month, with only the Julys of 1936, 1939, and most recently 1988 both wetter and duller. With a total of 140.1 mm July 2023 was provisionally the 6th wettest on record for the UK, however both Northern Ireland (185.4 mm) and the ‘Northwest England and North Wales’ region (196.2 mm) provisionally recorded their wettest ever July, eclipsing previous record totals set in 1936 and 1939 respectively. Both of these were very close, and it's possible that these rankings could slightly change as more data becomes available. Our nearest neighbours Ireland and France also have similar stories to tell; it was provisionally the wettest July on record for Ireland (since 1940), whilst in France overall July 2023 was very slightly drier than average but this masking a stark north/south divide with the northern two-fifths of the country reporting up to 200% of average. The unsettled pattern which typified July was widely attributed to the position of the jet stream, and indeed an anomalously south-shifted jet did indeed generate an abundance of cyclonic weather over the UK. However when looking at the bigger picture one can see that this was part of a larger train of anomalous and persistent tropospheric troughs and ridges across the western half of the northern hemisphere, around which the jet was preferentially directed. As can be seen from the Hovmoller diagram of 500 hPa gph (Figure 1), there was a marked pattern switch towards the end of June, as persistent troughing near 40–50°W was abruptly replaced by an apparently shorter wavelength pattern with persistent ridging in the same place. The net effect was a change from predominantly anticyclonic conditions (settled June) to cyclonic conditions (unsettled July) over the UK, however the root cause of such a shift remains unclear and would benefit from further research. (Sources: Starlings Roost Weather, Met Office, Met Eireann, MeteoFrance.) The boundary between the heat in southern Europe and notably suppressed temperatures across much of northwest Europe was the focus for several rounds of severe thunderstorms, bringing flash-flooding and spectacular hail. The European record for giant hail was broken twice in five days in northeast Italy; on 19 July, a hailstone measuring 16 cm in diameter fell at Carmignano di Brenta, but this was surpassed on 24 July when a hailstone of 19 cm diameter landed in Azzano Decimo. The latter comes close to the world record hailstone discovered in South Dakota in 2010. In neighbouring Slovenia, Storm Petar brought catastrophic flooding on 6 August, as more than a month's worth of rain fell around ten hours in places. Slovenia's Prime Minister described the situation as the worst natural disaster to affect the country since its independence in 1991. (Source: ESSL, BBC News.) The first windstorm of the 2022/2023 UK storm season to be named by the Western Europe group (which consists of the Met Office, Met Éireann, and the Royal Dutch Meteorological Institute KNMI), developed and affected the UK on 5 August. It was the latest such windstorm of a season since the storm naming system was established, although not the first named storm to affect the UK this year with both Otto and Noa (named by other pan-European storm naming groups) blowing through in February and April, respectively. Whilst named for the potential of requiring an Amber National Severe Weather Warning for wind (which was indeed subsequently issued), rainfall was just as impactful in some places with widespread falls of 30-50 mm across northeast England resulting in some property flooding. Unofficial reports of up to 70 mm both here and in thundery downpours across East Anglia later in the day were also received. Overall the winds were perhaps slightly below expectations, with only very exposed official sites recording gusts in excess of 50 knots, with the main swathe of strongest winds also ending up a little further west to be focussed over Cornwall, rather than Devon. That said, localised effects downwind of Dartmoor did mean that Berry Head recorded its strongest August gust since its record began, in 1993. Multiple transport impacts occurred, along with a smattering of power cuts, overall low-level but as is always the case with event cancellations and people changing their plans due to the warning, it is difficult to know the true extent and severity of any potential impacts had the strong winds arisen unexpectedly. (Sources: Met Office, National Grid, RAC, National Rail, Twitter.) Readers of Weather cannot have failed to notice the recent flurry of activity in the sphere of Artificial Intelligence (AI), with widespread coverage in mainstream media of systems such as ChatGPT. AI developments will have far-reaching consequences in many aspects of our lives, and the realm of meteorology is no exception. Since around 2018, the number of research teams and published academic papers concerning weather forecasting via AI has substantially increased, and very recently a step-change occurred with the development and public release of two AI global forecasting models – NVIDIA's FourCastNet and Huawei's Pangu-Weather. Whilst these models are now capable of achieving similar forecast accuracy as the world's best NWP model (ECMWF IFS) in terms of large scale continuously varying fields such as air pressure or geopotential height, due to their construction methodology they also suffer from a number of drawbacks such as not being able to as accurately represent all-important weather extremes. It is expected that continued research and development will continue to improve these weaknesses, much as traditional models have continued to evolve since the inception of numerical weather prediction. The real game changer with these models however is that they run orders of magnitude faster than NWP models, using just a fraction of the computing power, removing the explicit requirement for supercomputing and opening up the possibility of hyper-ensembles with many thousands of members. The future of AI weather modelling is very exciting and something that we will be certainly be keeping tabs in the months and years to come! Real-time forecasts generated by both FourCastNet and Pangu-Weather can be visualised via the ECMWF website – https://charts.ecmwf.int/ – and searching for “machine”. (Sources: Nature Volume 619 Issue 7970, ECMWF.)

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