Heavy precipitation and strong windy in the northern half of Portuguese mainland 15th and 16th, September 2015

2015-09-17 (IPMA)

In September 15^th and 16^th a low pressure system has affected Portugal mainland. This system moved from Atlantic region at north of Azores islands to the north of Iberia, deepening along the trajectory. In september15^th at 12 UTC (13h local time) the low was centered north of Corunha with 990 hPa of minimum pressure value, moving to the Bay of Biscay (Figure 1).
This weather system brought heavy and persistent rainfall in the North and Central regions, mainly in north of the river Mondego. Between 00UTC September 15^th and 12UTC September 16^th, high values of accumulated rainfall were occurred. Values of accumulated rainfall in 24 hours, reached more than 100mm in Minho, Trás – os - Montes, Douro Litoral and Beira Litoral, the highest values were recorded in Cabril, with 171 mm, Luzim with 142 mm, Cabeceiras de Basto with 125 mm, Arouca e Viseu with 122mm and Montalegre with 111mm.

In contrast, in the regions to the south of the Mondego River and, especially, in the Alentejo and Algarve, 24 hours rainfall values were much lower (Figure 2), and in most southeast part no rain has occurred until 9:00 UTC September 16^th.

Values of accumulated rainfall in 6 hours in north-western part was widespread over 50 mm/ 6h, locally greater than 60 mm, occurred (Figure 3).

The Figure 3 shows the map of accumulated rainfall in 6 hours (10 to 16 UTC, September 15th) obtained from Radar located in Arouca (A/PG). This field was obtained based on observations made by the radar beam to the lowest possible altitude above ground and with temporal integration of 5 in 5 minutes. In this image we can see the highest values of accumulated rainfall are in the Braga and Porto districts. Radar estimated values of accumulated rainfall in 6 hours in those districts suggest that the maximum values could have reached 110 mm in 6 hours at some locations.

The southerly wind was strong and the highest values were observed from the evening September 15^th, with 40 to 55 km/h recorded in the highlands and in some places of the west coast north from Lisbon. The maximum values of gusts occurred from late Tuesday, September 15^th, until early morning of September 16^th, with values about 90 km/h or higher in several places, like Mogadouro, with 100 km/h, Pampilhosa da Serra with 99km/h, Cabeceiras de Basto and Penhas Douradas with 92 km/h.
On the west coast, the height of the waves in Leixões (observation, Instituto Hidrográfico) was 4.5 meters significant height and 7 m of maximum height.

• Figure 1- Images resulting from the combination of Water Vapor 6.2 μ and 7.3 μ and infrared 9.7 μ and 10.8 μ channels from MSG satellite of 2015 September 15th at 12:00 UTC.

• Figure 2- Map of accumulated rainfall in 24 hours, from 09 UTC, September 15th, to 09 UTC, September 16th, Portugal.

• Figure 3 - Map of accumulated rainfall in 06 hours, from 10 to 16 UTC, of September 15th (A/PG radar).

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