Severe Weather Reports

A highly unstable cold front associated with an intense low pressure over northern Algeria traversed the central Mediterranean in the very early hours of Thursday 11th November. This produced heavy thunderstorms across the Maltese Islands. Gozo bore the brunt of the stormy weather. 115.3 mm of rain were measured in Għajnsielem, the hardest hit locality. Several other localities in Gozo measured around 100 mm of precipitation. That is just over a month’s worth of rainfall. This fell in a matter of just over an hour. The wind experienced a sudden shift to the West as soon as the thunderstorm made landfall. Gusts of up to 70 km/h were recorded by exposed weather stations around the Maltese Islands. Valley systems burst and roads flooded. There were collapsed rubble walls and downed power lines.

What is a cold front? A cold front develops when a mass of comparatively colder air moves into an area where warmer air is present. In our case, a southerly wind had been advecting very warm air across the central Mediterranean for days. This warm air was constantly picking up moisture from the very warm sea locally. This caused the atmosphere above our region to become saturated. This was evident from the episodes of misty weather that had characterized our meteorological scene for days on end. An outbreak of colder air that had penetrated the Mediterranean earlier in the week reached the Maltese Islands late on Wednesday 10th November. Drier, colder air pushes the warmer, moister air upwards. This forces the rising air to cool rapidly. As the moisture-laden air cools, the water vapor within condenses, forming rain-bearing clouds. The greater the contrast between the two, the more intense the rain-bearing clouds that form are. The chart below is a simplified representation of what happens at a cold front.

What type of thunderstorm was it? There are four main types of thunderstorms: single-cell, multi-cell, squall line and supercell. Which type forms is dependent on atmospheric conditions, namely instability and wind shear (wind conditions at different layers of the atmosphere), at the time. In our case, we experienced a squall line. A squall is defined as an elongated and narrow line of thunderstorms that form along a cold front. A squall line is typically with a brief but extremely intense episode of torrential precipitation, hail, incessant lightning and damaging wind gusts. When wind in the upper levels of atmosphere is relatively weak, squall lines move more slowly, exacerbating their impact on affected areas. Since a squall line is comprised of multiple small cells of thunderstorms, it is difficult to predict which localities will be hit worst well in advance.

Video: Terence Farrugia

Video: Michael Vella

The sea surface temperature around the Maltese Islands this summer peaked at 30⁰C, a full 3⁰C warmer than the average. The high sea surface temperature contributed to increased rates of evaporation, which in turn, lead to an increase in humidity levels. From 1/10 through to 3/10, a series of heavy thunderstorms occurred around the central Mediterranean. A short-wave upper-level low pressure system passed across our region bringing relatively cold air over a very warm sea, creating a great deal of instability. In addition to this, a greater amount of moisture was available to be advected into these thunderstorms. The end product were thunderstorms with intense rainfall.

Most thunderstorms were localized, affecting a few localities at a time. At least two main thunderstorms affected the Maltese Islands. The first came in the early hours of Saturday 2/10, whilst the second hit some 24 hours later, in the early hours of Sunday 3/10. Gozo got the most rainfall from both thunderstorms. The second thunderstorm was the more intense one. It packed rainfall rates of over 300 mm / hr, winds of up to Force 7 and dangerous cloud-to-ground lightning. The coastal town of Xlendi, notable for frequent flooding in storms, bore the brunt of it all. The valley system that empties in Xlendi collects run-off from the localities which received the most rainfall from these thunderstorms. Cars were carried to sea and debris littered the roads. Rainfall during the second thunderstorm was most intense between 3:28 a.m. and 3:39 a.m. when a staggering 37.8 mm of rain were dumped in just 11 minutes. The temperature dropped from 23.6°C to 17.9°C during the thunderstorm. The wind gusted up to 56.3 km/h. In addition to these two main storms, a few other showers and isolated thunderstorms also affected the islands over a period of some 48 hours. The absence of wind at the upper levels of the atmosphere meant that the entire zone of instability, along with all the individual thunderstorms within it were extremely slow-moving. This exacerbated further the amount of rainfall dumped by the storms. Below are values for total rainfall gathered in several localities around the Maltese Islands:

Għarb – 32.1 mm + 69.5 mm = 101.6 mm

Victoria – 26.3 mm + 59.7 mm = 66.0 mm

Xewkija – 21.8 mm + 37.4 mm = 59.2 mm

Nadur – 26.9 mm + 60.9 mm = 87.8 mm

Marsalforn – 38.1 mm + 55.5 mm = 93.6 mm

Mellieħa = 8.8 mm + 32.4 mm = 41.2 mm

Buġibba – 15.0 mm + 25.2 mm = 40.2 mm

Mġarr – 13.7 mm + 31.0 mm = 44.7 mm

Naxxar – 12.2 mm + 26.4 mm = 38.6 mm

Mosta – 9.8 mm + 14.7 mm = 24.5 mm

Dingli – 24.3 mm + 28.6 mm = 52.9 mm

Msida – 22.4 mm + 21.1 mm = 43.5 mm

Valletta – 19.8 mm + 23.6 mm = 43.4 mm

Sliema – 23.4 mm + 18.3 mm = 41.7 mm

Mqabba – 22.6 mm + 24.9 mm = 47.5 mm

Żabbar – 17.5 mm + 16.3 mm = 33.8 mm

Birżebbuġa – 12.2 mm + 12.0 mm = 24.2 mm

Thanks to these thunderstorms, the total rainfall all round the Maltese Islands for October 2020 is already more than half than the climate average for the same month.

A series of waterspouts were observed, particularly on Friday. These start their existence as a funnel cloud below a cumuliform cloud when a column of swirling air begins to rotate, causing the surrounding moisture to condense into water droplets. This vortex begins to move downward, creating a funnel shape which protrudes for a distance below the cloud. Often, it is too weak to extend all the way down and make contact with the ground. When it does manage contact with the ground, it takes the form of a weak waterspout or landspout. Most funnels which form locally are of this kind, and rarely of their stronger relatives, the tornadoes.

A moist and unstable atmosphere was present over the central Mediterranean, together with some wind shear (a difference in wind speed or direction over a relatively short distance in the atmosphere). A thunderstorm with an intense updraft (a current of rising air) ingested and tightened a rotating layer of air. This led to the formation of a confirmed funnel over fields near Għarb in Gozo. The funnel developed at around 10 a.m. and lasted for less than 5 minutes. It whipped dust and lose debris. The same cloud produced a downpour along the northeast coast of Gozo and Malta. Several other funnel clouds were reported over the past few days. The instability that led to the formation of this funnel was also responsible for the series of isolated thunderstorms that affected the Maltese Islands from Saturday 4/9 through to Sunday 12/9.

Thanks to Andrea Muscat and Joseph Casha for the photos and video!

The sea surface temperature around the Maltese Islands this summer peaked at 29⁰C, a full 2⁰C warmer than the average. The high sea surface temperature contributed to increased rates of evaporation, which in turn, lead to an increase in humidity levels. From 12/09 through to 16/09, a series of heavy thunderstorms occurred around the central Mediterranean. A short-wave upper-level low pressure system passed across our region bringing relatively cold air over a very warm sea, creating a great deal of instability. In addition to this, a greater amount of moisture was available to be advected into these thunderstorms. The end product were thunderstorms with intense rainfall.

Most thunderstorms were localised, affecting a few localities at a time. At least four main thunderstorms affected the Maltese Islands. The first came in the early afternoon of Saturday 12/09. This drenched central and southern Malta. With 67.2 mm of rain, Luqa was the wettest locality. The second thunderstorm occurred in the morning on Sunday 13/09. This affected areas further north. Nadur and Mellieħa found themselves at the centre of this storm, with rainfall values of 40 mm each. The third and most intense storm struck at noon on Monday 14/09. It packed rainfall rates of up to 167 mm / hr, gale force winds and dangerous cloud-to-ground lightning. The island of Gozo bore the brunt of it all. It had to endure nothing less than four hours of torrential rain. In the end, the localities of Għarb and Xagħra had received the most rainfall, with 112.1 mm and 102.0 mm respectively. The 112.1 mm total from Għarb is equal to 1/5 of a typical year’s rain and almost three times the typical September’s rain. Some areas around Malta experienced the third thunderstorm too, with hardest-hit Attard recording 68.5 mm of rainfall. A final thunderstorm formed in the evening of Wednesday 16/09 and swept across parts of the Maltese Islands, affecting central and southern particularly intensely clique neste link. 37.0 mm of rain were measured in Msida. In addition to the four main storms, a number of showers and isolated thunderstorms also affected the islands. The absence of wind at the upper levels of the atmosphere meant that the entire zone of instability, along with all the individual thunderstorms within it were extremely slow-moving. This exacerbated further the amount of rainfall dumped by the storms. Below are values for total rainfall gathered in several localities around the Maltese Islands:

Għarb – 129.9 mm

Victoria – 121.1 mm

Nadur – 117.0 mm

Xewkija – 84.6 mm

Mellieħa – 68.4 mm

Buġibba – 63.3 mm

Mġarr – 44.1 mm

Attard – 132.8 mm

Għargħur – 80.5 mm

Msida – 123.4 mm

Rabat – 40.3 mm

Valletta – 77.8 mm

Luqa – 134.7 mm

Żabbar – 136.7 mm

Birżebbuġa – 70.6 mm

Thanks to these thunderstorms, the total rainfall all round the Maltese Islands for September 2020 is already higher than the climate average for the same month.

A series of waterspouts were observed, particularly on Sunday. These start their existence as a funnel cloud below a cumuliform cloud when a column of swirling air begins to rotate, causing the surrounding moisture to condense into water droplets. This vortex begins to move downward, creating a funnel shape which protrudes for a distance below the cloud. Often, it is too weak to extend all the way down and make contact with the ground. When it does manage contact with the ground, it takes the form of a weak waterspout or landspout. Most funnels which form locally are of this kind, and rarely of their stronger relatives, the tornadoes.

At least two thunderstorms drenched Malta. The second of these, which struck in afternoon, was confirmed as having been severe. It produced wind gusts of up to Force 8 (over 62 km/h), hailstones with a diameter of around 2.5 centimetres (the size of a €2 coin) and a confirmed waterspout (dissipated when over the beach) close to Armier Bay. Total rainfall ranged from 6.4 mm (Birżebbuġa) to 41.2 mm (Għargħur). Another independent weather service provider reported close to 70 mm in St.Paul’s Bay. A separate shower occurred over parts of Gozo. The western half of the island remained completely dry.

An upper-level low pressure system over the central Mediterranean brought relatively cold air from the Balkans over a warm central Mediterranean, creating a great deal of instability with towering cumulus and cumulonimbus clouds that sparked heavy showers and thunderstorms. The wind was very light at all levels of the troposphere, allowing the storms to build up gradually as a result of diurnal heating. Colder air in the upper levels of the atmosphere increases instability. This is because the warmer air at ground level, being lighter and more buoyant, tends to rise. This vertical movement cools the rising air, causing the moisture within to condense and form clouds.

Wednesday 27th May was expected to be a fairly unstable day, but admittedly, this instability was much greater than initially predicted.

Thanks to Andrea Muscat, Steve Cutajar, Norbert Vella, George Carl Camilleri, Josephine Debrincat and Chrystal for the photos!

Parts of the Maltese Islands, especially Gozo, experienced a probable heat burst in the very early hours. Starting at 02:19, the air temperature rose rapidly by 6.1°C (from 20.6°C to 26.7°C) in a space of just 20 minutes. Levels of relative humidity also experienced a sharp drop, from a humid 80% to an extremely dry 37%, all in a gap of a few minutes. In typical heat burst fashion, the wind increased dramatically from a complete calm to 40.2 km/h (Force 6) in a matter of minutes. The behaviour of air pressure also reveals tell-tale signs of a heat burst. It climbed by 2hPa, from 1009.5hPa to 1011.5hPa, over the same time period. This lasted for some time, with the air temperature and wind subsiding gradually afterwards.

A rare phenomenon, heat bursts tend to be more likely in the late spring and more specifically at night. Lasting anywhere from a couple of minutes to several hours, the main distinguishing features of heat bursts are sudden strong gusty winds, extreme temperature changes and a drop in humidity.

This probable heat burst can be blamed on a cut-off low pressure system which traversed our area during the night between Wednesday and Thursday. This cut-off low had several embedded isolated thunderstorms within it. Most were in an advanced stage of dissipation. One of these isolated thunderstorms passing just off Gozo’s west coast is the likely culprit. As a thunderstorm starts to dissipate, its clouds rise, leaving a layer of cold air beneath. At this stage, much of the moisture has been eliminated from the air by earlier precipitation. Due to gravity and the rapidly disappearing updrafts (currents of rising air within clouds), the dense air starts to sink to the ground. As its descent accelerates down, the air is subjected to increasing air pressure and friction. The increased atmospheric pressure causes the air to warm under compression ed-hrvatski.com. The friction between falling and stationary air particles creates further heat. All this contributes to the thermal buildup of the sinking air. As the air heats up, any remaining moisture is forced to evaporate. Momentum allows the layer of air to continue to speed up its descent. The result is a heated, dry pocket of air that hits the surface, forcing warm gusty wind to disperse away from this point of impact. A heat burst has been likened to someone blowing down on a puddle of water.

A depression (980hPa) formed over the Gulf of Genoa, and migrated across Italy and into central/southern Adriatic Sea. Simultaneously, a ridge of high pressure developed to its east, over France. The pressure difference generated a severe gale from the Northwest across the central Mediterranean. The depression was derived from one which had produced a windstorm across the British Isles and northwest Europe some days before.

This was only the second gale in an otherwise unusually calm autumn. A maximum wind gust of 82.1 km/h (Force 9) from the Northwest was measured by our weather station in Għarb. Even stronger gusts of around 100 km/h (Force 10) were measured on high ground. The severe gale-force mistral winds caused minor disruption and also some light damage.

The damage included uprooted trees, broken branches and collapsed walls. The ferry linking the two islands of Gozo and Malta had to be rerouted throughout the day. Catamaran journeys between Malta and Sicily were cancelled. The Ħaġar Qim temple complex was closed off to the public briefly after a protective shelter was damaged by the wind. A stand shelter at the Centenary Stadium was damaged, leading to two matches being called off https://polska-ed.com/kupic-generic-cialis/.

Thanks to Andrea Muscat for the photos!

This was the strongest wind in months. A maximum wind gust of 78.9 km/h was measured in Nadur. The severe gale-force winds and heavy rain caused disruption and some light damage. In this report, we will describe what gave rise to this intense system and how it affected the Maltese Islands.

The intense system classified as a Mediterranean Tropical-Like Cyclone (or Medicane). Being the first one since February 2019, it formed early on Monday 11/11 and dissipated by late on Tuesday 12/11. A strong jet-stream blew a stream of very warm and humid air from over the Sahara Desert towards the western and central Mediterranean. Simultaneously, cold air from northern Europe penetrated the western Mediterranean. Both air masses clashed together, initiating the process of Mediterranean Tropical-Like Cyclone development. As a result of the clash, an existing low pressure system started intensifying very quickly, in a process referred to as cyclogenesis in meteorology. Atmospheric pressure at its centre dropped drastically, and a Mediterranean Cyclone was officially formed. The system made landfall in Algeria and lost most of its strength. Once it emerged over the very warm sea of the Gulf of Gabes, it regained strength. Its ‘eye’ was over the Sicilian Channel, close to the islands of Lampedusa and Linosa, at its closest point to the Maltese Islands.

The cold air mass within the stormy system caused temperatures to dip all around Malta and Gozo. In fact, the temperature at our weather station in Għarb dropped by some 7°C in one hour. An outer band of convection associated with the storm system affected parts of the Maltese Islands, especially Gozo, in the mid-afternoon on Monday 11/11. At its closest point, the Mediterranean Tropical-Like Cyclone drenched the Maltese Islands with a series of heavy rain and hail showers. These lasted all day Tuesday 12/11. The storm generated waves of up to 8.0 metres.

72.5 mm of rain were measured in the 48 hours that the storm system affected us. Għarb was the wettest locality. Thanks to this heavy precipitation, Għarb reached the monthly average for November before mid-month. The severe gale-force winds produced some light damage. The damage included uprooted trees, collapsed walls and downed power lines. A number of coastal areas suffered extensive flooding from the high waves. The ferry linking the two islands of Gozo and Malta was suspended for over 12 hours, as the very rough sea made crossing the channel too dangerous. Similarly, several catamaran journeys between Malta and Sicily were cancelled.

Thanks to Andrea Muscat for the photos!

A series of heavy thunderstorms were caused due to an short-wave upper-level low pressure system which passed rapidly across the central Mediterranean bringing relatively cold air over a very warm sea, creating a great deal of instability. There was plenty of moisture in the atmosphere. This produced a Mesoscale Convective System (MCS) which passed over the Maltese Islands from the late afternoon till late at night. This caused a series of heavy thunderstorms with blustery spells and also some hail, especially over Gozo. An MCS is a mass of organised thunderstorms. They take on a round or linear shape with clear-cut edges. These features distinguish an MCS from other thunderstorm structures. Highest rainfall totals from this storm were: 50.1 mm in Għarb, 40.2 mm in Xagħra, 39.7 mm in Rabat (Malta), 32.0 mm in Victoria and 25.0 mm in Nadur. Thanks to this MCS, the total rainfall in Gozo for September 2019 is already higher than the climate average for the same month. With a total of 77.7 mm, Għarb is the wettest locality so far since September 1st.

Thanks to Andrea Muscat and Miklós Rajczi for the photos!

A moist and unstable atmosphere was present over the central Mediterranean, together with some wind shear (a difference in wind speed or direction over a relatively short distance in the atmosphere). A convective cloud with an intense updraft (a current of rising air) ingested and tightened a rotating layer of air. This led to the formation of a confirmed waterspout off Gozo’s northeast coast. The waterspout developed at around noon and lasted for less than 15 minutes. The same cloud produced a downpour over parts of Gozo. At least two funnel clouds were reported over the past few days. The instability that led to the formation of this waterspout was also responsible for the series of isolated thunderstorms that affected the Maltese Islands from Sunday 01/09 till Friday 06/09.

Thanks to Andrea Muscat, Audrey Marie Bartolo, Nyam Cassar and Jonathan Debono for the photos!