Extreme weather – heat, cold, precipitation and high winds – marked the first week of 2018, with widespread impacts on transport, infrastructure and health. National meteorological and hydrological services have issued regular and reliable forecasts and warnings to alert to the public to weather-related hazards.
When one part of the world is cold, there is likely another part of the world experiencing abnormally warm conditions. This is because the day-to-day weather of Northern Hemisphere features undulations or wave patterns, which are characterized by "valleys" (troughs) allowing cold air, and by "hills" (ridges) being typically associated with warm air. This is part of natural climate variability, which continues to play a major role on monthly and seasonal timescales.
North America
A major winter storm hit the eastern coast of the United States on 4 January, causing major travel disruption and flight cancellations. It battered coastal areas with heavy snow, blizzards and strong winds, from Florida in southern USA to Maine. Satellite images showed a long line of clouds stretching over a thousand miles south of the storm, which drew in moisture all the way deep in the Caribbean. Talahasee, in Florida, had its first snow in 28 years, according to the US National Weather Service. The city of Boston suffered coastal flooding after it saw the highest ever recorded tide since 1921. Some flood waters froze in frigid temperatures, prompting NWS Boston to warn people against “going out and floating on icebergs..”
The mid-latitude cyclone was widely known as “bomb cyclone” because it underwent rapid intensification over a very short period. Official NWS surface analysis showed a central pressure of 951 millibars – a pressure drop of 59 mb in 24 hours. The low was located about 300 miles south of Boston. “Although no unprecedented, the explosive deepening of mid-latitude cyclones over this part of the Western Atlantic during winter (where central pressure drops more than 50 mb in 24 hours) is rare. Detailed records over the past 45-50 years suggest a return interval for this event, in this area, may only occur ever 25-30 years,” said NWS.
The “bomb cyclone” was followed by a further drop in temperatures, with a number of new daily cold record temperatures being set from 5-7 January. NWS warned that the Arctic air mass and dangerously cold wind chills expected across much of the eastern two-thirds of the country. After the chill, a widespread January thaw is expected for much of the USA with above normal temperatures for mid-month.
Whilst it has been frigid in much of Canada and the Midwestern and Eastern United States, this has not been the case in the Western USA and Alaska. NASA issued a temperature anomaly map based on data from NASA’s Terra satellite, showing land surface temperatures from December 26, 2017 to January 2, 2018, compared to the 2001–2010 average for the same eight-day period.
“The map of North America underscores one of the realities of weather—when a cold snap hits one region, warmth often bakes another one. A giant meander (or Rossby wave) in the jet stream is the common thread that connects the warm weather west of the Rockies with the chill east of them. As the crest of a Rossby wave—a ridge—pushed unusually far toward Alaska in December, it dragged warm tropical air with it. In response, the other side of the wave—a trough—slid deep into the eastern United States, bringing pulses of dense, cold Arctic air south with it. The Rocky Mountains have boxed in much of the coldest, densest air, serving as a barrier between the cold and warm air masses.,” said NASA.
Europe and North Africa
Europe also experienced extreme weather, with a powerful storm which brought strong winds and heavy rain to Ireland, UK, Germany, France, Switzerland, Austria and Italy. During 3 January, the storm – known as Eleanor - passed from Ireland and UK down through the Alps and reached Corsica in the evening. Wind gusts of 155 km/h were recorded at Knock Airport, Ireland. Meteo France said one quarter of France had winds of more than 100 km/h.
At the same time, January maximum temperature records tumbled in France, which saw a new national record for 3 January of 11.5°C (+6.3°C/norm). Southeastern France saw new monthly records, including 22.9°C on 3 January in Cannes.
Heavy snowfall and mild temperatures led to a heightened avalanche risk in many parts of the Alps. Snowmelt combined with sustained precipitation caused flood alerts in parts of France, Germany and Switzerland.
A sustained large-scale low pressure system brought cold, stormy weather to much of the western Mediterranean region, including Algeria, Morocco, Spain, Portugal and south of France over the weekend of 6-7 January. Snow was reported on high-altitude areas of the Sahara desert.
China
From January 2 to 4, central and eastern China saw heavy rain and snow in Shaanxi, Henan, Hubei, Anhui and Jiangsu provinces. There was cumulative precipitation of 10-30 mm in many areas, and 40-60 mm in south-eastern Henan, eastern and northern Hubei, central Anhui, and portions of central Jiangsu. “Monthly rainfall extremes have been broken by daily rainfall in 92 counties and cities in areas like Henan and Shaanxi,” according to the China Meteorological Administration.
Southern hemisphere
Parts of the southern hemisphere are witnessing extreme heat.
The South African Weather Service issued warnings for a heatwave “with persistently high temperatures” over many parts of the country from 5-8 January. The Indian ocean saw the first tropical cyclone of the season, with tropical cyclone Ava impacting Madagascar on 6-7 January, causing casualties and disruption.
Widespread parts of Australia were gripped by intense heat from 5-7 January. The weather station at Penrith, Sydney, reached 47.3°C on 7 January (Sunday) at the height of the Sydney heat, according to Australia's Bureau of Meteorology. Hot, dry and gusty conditions led to an elevated fire risk through parts of South Australia and Victoria.