|Above: A file photo of aircraft at Heathrow Airport in front of the London skyline on October 11, 2016. On Sunday, January 19, 2020, Heathrow recorded the highest mean sea level pressure in more than 300 years of London-area weather records. Image credit: Photo by Jack Taylor/Getty Images.|
With only a few days left until the United Kingdom’s fraught departure from the European Union, a different kind of extreme pressure has suddenly hit London. According to UK climatology expert Stephen Burt (Department of Meteorology, University of Reading), the mean sea level barometric pressure at Heathrow Airport reached 1049.6 mb (30.99”) at midnight GMT Sunday night (7 p.m. EST).
Not only is this a record for Heathrow, beating 1047.3 mb from February 7, 1964, but it’s also the highest barometric pressure recorded in London in more than 300 years of recordkeeping, Burt said in a summary. The previous London record was 1049.1 mb at Kew Gardens at 11 a.m. GMT on January 18, 1882, as reported by Burt in a 2007 article for the journal Weather, “The Highest of the Highs… Extremes of barometric pressure in the British Isles, Part 2 – the most intense anticyclones.” Burt also cited a 2011 paper in the International Journal of Climatology that analyzed near-continuous daily data for the London area from 1692 to 2007.
Even higher values were notched on Sunday elsewhere in the British Isles. Liscombe, Devon, recorded 1050.3 mb (31.02”), and several other stations topped 1050 mb. The last time values this high were recorded anywhere in the British Isles was in the Hebrides on January 16, 1957, Burt said. The all-time UK record remains the 1053.6 mb recorded at Aberdeen Observatory on January 31, 1902.
Before this weekend, only nine weather systems since 1800 had yielded a pressure of at least 1050 mb somewhere in the British Isles, according to Burt.
“Slightly higher values may yet be reported between exact observing hours,” Burt added. A station at Mumbles, Wales, reported 1050.5 mb, but Burt believes that reading may be in error on the high side by a few tenths of millibars.
Extreme high pressure wasn’t limited to the British Isles. Etienne Kapikian (Meteo-France) reported on Twitter that the 1049.7 mb recorded at Abbeville, France, on Monday morning appears to be that nation’s highest mean sea level pressure in at least a century, topping 1048.9 mb from Chemoulin (L-Atl) on March 3, 1990. Preliminary data from Belgium also saw several stations approach 1050 mb, although a reported 1051.5 mb in Chièvres (which would be a new national record) appears to be in error on the high side, according to Burt. In the Netherlands, de Bilt reached 1048.2 mb, not far from that city's national record of 1050.4 hPa from January 26, 1932. For more on the pressures reported in Europe, see Jérôme Reynaud's Géoclimat Facebook page.
As is typical with high pressure in winter, the weather was mainly calm but chilly where the records were set, as opposed to the tempestuous conditions that typically accompany low-pressure records.
What put the pressure on?
There’s a straight line—or at least a dotted, meandering line—between the mammoth blizzard that paralyzed parts of Newfoundland on Friday and Saturday, the exceptional high pressure over the British Isles and Northwest Europe, and an extratropical storm (named Gloria by European weather agencies) that’s bringing high winds, heavy rain, and mountain snows to parts of Spain.
|Figure 1. The height of the 500-mb pressure surface (colors, in decameters) and mean sea level pressure (black contours) as analyzed by the GFS model at 12Z (7 am EST) Friday through Monday, January 17-20, 2020 (upper left to lower right). Image credit: tropicaltidbits.com.|
All three are part of a major readjustment of the jet stream over the last several days across the North Atlantic. Just as strong high pressure often develops on the periphery of a hurricane or typhoon, the powerful storm over Atlantic Canada helped generate the intense high-pressure cell that developed over London near the surface and aloft. In basic terms, if air is strongly rising in one region, it must be sinking somewhere else, usually not too far away.
The process gets more complex when the wintertime jet stream is involved. In this case, a jet-stream phenomenon known as wave breaking led to the Newfoundland storm system diving southeast and intensifying as the high-pressure cell downstream strengthened while building to the northeast.
Last night's extreme #highpressure was close to the UK record and forecasted days in advance. One contributory factor was the #jetstream buckling ahead of a winter storm in Newfoundland. An upper ridge formed, causing air to converge, accumulate and raise surface pressure. pic.twitter.com/sivKV61PRx— Met Office Science (@MetOffice_Sci) January 20, 2020
Even further downstream, a storm system dove southward across France over the weekend and is now stalling out as Gloria over the Iberian Peninsula and northwest Africa. The winds, rain, and snow associated with this slow-moving system have taken at least three lives and cut power to more than 10,000 people, according to the Associated Press.
Lots of damage in Calpe, Alicante, spain this afternoon 20th January from rough seas from #StormGloria Video reported to us with permission by https://t.co/BBc0iGI7MM thanks; #severeweather #ExtremeWeather pic.twitter.com/1LSBJbQlXj— WEATHER/ METEO WORLD (@StormchaserUKEU) January 20, 2020
Conditions were much calmer on Monday in Newfoundland and Labrador, where Friday’s storm brought sustained hurricane-force winds and massive snows to parts of the province. The storm’s mean sea level pressure dropped by 40 millibars in 24 hours, more than meeting the 24 mb/24 hr definition of a meteorological bomb even after adjustments for latitude. Although the wind sensor failed at the provincial capital of St. John’s, the city likely experienced winds of minimal hurricane strength (sustained at 74 mph or more) for several hours. According to Environment Canada, and as reported by weather.com, the city’s airport measured 76.2 cm (30”) on Friday, making it the snowiest calendar day in St. John’s records that date back to 1942.
In case you’re wondering: What is mean sea level pressure?
Because atmospheric pressure normally goes down with altitude, barometric readings taken at heights above sea level are adjusted to how they would read at sea level through well-established procedures based on recent conditions as well as typical temperature profiles at each station. The result, “mean sea level pressure” (MSLP), allows for apples-to-apples comparisons and a more useful mapping of surface features, including highs, lows, and fronts. For stations that sit close to sea level, such as London, the MSLP adjustment is minimal, which adds heft to the case that the high pressure value observed in London this weekend is unprecedented.
See the National Weather Service explainer for more on the differences between station pressure, altimeter setting, and mean sea level pressure.