The brochures suggest unbroken sunshine for the Costa del Sol in the summer months, and the pictures show cloudless skies. but reality is somewhat different. Although widespread unsettled weather is rare in August, and rain caused by weather fronts is even rarer, it does occur, and on 17th August 2006 significant frontal rain occurred over most of Andalucia.
Introduction
On average, measurable rain falls in August, in about 1 year in 2 on the Costa del Sol, but only 1 in every 3 or 4 years north of Almeria. In the hills and mountains behind Malaga 1 or 2 days of rain are likely in most Augusts. These statistics hide an interesting fact. Most of the rain falls in short, sharp bursts from unstable medium-level clouds (above about 2000 m/6,500 ft). Often, these clouds are considerably higher (4,000 m/13,000 ft), and although the instability is deep enough to cause rumbles of thunder, the air beneath the cloud is too dry to allow more than a few spots of rain to reach the ground.
Historical perspective
In August 1996 there was some quite widespread non-frontal thundery rain on the 15th with several places in Andalucia receiving over 10 mm (0.4 inches) of rain. In 1997, while much of northern Europe was enjoying one of the hottest Augusts of the century, Iberia was less fortunate. Several thundery outbreaks occurred and at Granada and Nerja there was measurable rainfall on 5 days. However, the rain was generally associated with disturbances in the upper air rather than surface discontinuities. The exception was on the 25th and 26th when a weakening cold front gave between 1 and 3 mm (0.04-0.12 inches) of rain across much of Analucia as it moved slowly southeast. The rain that fell on the 17th August 2006 was of a completely different magnitude.
The early developments
Figures 1 and 2 (below) show the surface pattern and the infra-red satellite imagery for the early hours of the 16th August 2006.
Figure 1 UK Met Office Surface Analysis 00 UTC 16th August 2006
The area of low pressure to the south of Ireland, unusually deep for mid August, had a sphere of influence that extended well south into Iberia. The marked upper trough (denoted by the front with open triangles northwest of Spain ahead of the surface cold front) was responsible for a broad band of high cloud advancing into northern Spain. The semi-permanent Spanish heat low of summer months had been displaced eastwards to near Valencia and that allowed relatively cool low-level Atlantic air to cross much of the peninsular. Inland temperatures across southern Spain regularly reach the high 30s or low 40s (100-109F) in August but the Atlantic influence had helped to reduce maxima on the 15th to 30 Celsius (86F) at Cordoba and 28 Celsius (82F) at Granada. Even in Malaga, where a brief interlude of northwest winds in the summer months usually brings searing heat, the temperature only rose to 34 Celsius (93F). Figure 3 (below) shows the upper-air sounding for Gibraltar. The marked low-level inversion, the result of air cooling over the relatively cool sea, is noticeable and can be faintly seen as dark grey stratocumulus cloud on the satellite imagery (Figure 1). Although the cloud is easier to see over water, areas of cloud are apparent over many parts of Spain. Also there is the first hint of moistening at medium levels on the ascent as the white altocumulus cloud to the southwest of Spain spreads east-northeastwards.
Figure 3. 00 UTC 16th August 2006 Gibraltar Skew-T Upper-Air Sounding
This was a relatively cool and windy day across southern Spain. At Almeria Airport, close to the coast and just to the east of the city, the southwesterly wind was force 6 or 7, 25-30 knots, for much of the day. Gusts exceeded 40 knots and duststorms occurred on the desert roads north of the city. In Malaga, a moderate or fresh northwest wind persisted all day. Dewpoints were fairly low and, even though temperatures reached 29 Celsius (84F) it felt cool in the shade. A sea breeze tried to develop along the coast during the afternoon and it then became apparent that the windy weather had helped to trigger a huge upwelling of cold water along the coast that affected beaches either side of Malaga, with a similar drop in sea temperature assumed at Almeria based on air temperatures recorded at the coastal airport. In the sea immediately adjacent to the beach west of Malaga the sea temperature dropped from 25 Celsius (77F) to 20 Celsius (68F) in just 24 hours.
The day the rain came
The synoptic situation (Figure 4) and the infra-red satellite image (Figure 5), both for 00 UTC, show the complex nature of the developments leading up to the widespread, but short-lived rainfall.
Figure 4 UK Met Office Surface Analysis 00 UTC 17th August 2006
The 17th started fine across Andalucia, but the analysis for midnight still shows the cold front driving steadily southeast over Iberia. The front was well-marked on satellite images as a band of medium to dark grey cloud stretching out into the Atlantic from west of Portugal and appeared to pass through Lisbon between 03 and 04 UTC evidenced by a temporary increase in stratocumulus cloud, a veer in wind direction and a lowering of the dew point. However, no precipitation fell. The trough line shown over southern Portugal was marked by the broad band of medium-level cloud, which gave some rain in Portugal and western Spain, 8mm (0.3 inches) at Sines on the west coast of Portugal, and was probably the remnants of the upper trough that appeared ahead of the cold front in northern latitudes in Figure 1. The main feature of interest in Figure 5 is the lines of convection developing southwest of Portugal and the hint of some form of wave forming as shown by the water vapour imagery in Figure 6.
Figure 6. The water vapour imagery for 00 UTC 17th August clearly shows the circulation around the twin-centred upper vortex lying northwest of Spain. The cold front, verified by surface observations, lies from northwest Spain to south central Portugal, but the main area of interest is the dry slot (dark grey area) just beginning to form southwest of Portugal, and this is indicative of development as the low-latitude jet interacts with the weather system sinking south over the Iberian Peninsular.
The midnight upper-air sounding from Gibraltar on the 17th ( Figure 7) shows moistening at most levels when compared to 24 hours earlier (Figure 3). Much of the cloud appears to be layered medium-level cloud and the lumpy cloud showing up east of the 'dry slot' in Figure 6 could well be nothing more than cirrus floccus as hinted at on the ascent in Figure 7.
Figure 7. 00 UTC 17th August 2006 Gibraltar Skew-T Upper-Air Sounding Reproduced permission of Department of Atmospheric Science, University of Wyoming
The early morning of the 17th
As the morning progressed, and the surface cold front continued to move southeast, so the bands of cloud began to merge and a distinct wave began to develop. At 03UTC (Figure 8) very little rain was falling across southern Spain but Sines (Portugal) had another millimetre between 00 and 03 UTC. Of special interest in Figure 8 is the line of convective lumps of cloud running southwest from the southwest tip of Portugal. These produced a few thunderstorms, surprisingly the only known thunderstorms observed in this rainfall event.
Later in the morning
By 06 UTC the trough and cold front, shown in Figure 4, had effectively merged to produce a marked cold front. At Sines this was shown by a pressure kick, wind veer and lowering of the dew point between 03 and 06UTC. At Faro, on the Algarve, the front was even more well-marked with a pressure fall of 3mb in 3 hours up to 06UTC followed by a similar rise from 06 to 09UTC. Some heavy rain fell, too, with 13mm recorded between 04 and 06UTC. Ahead of this band of weather the ribbon of high medium-level cloud and cirrus stretching from south of the Balearics across to Morocco and beyond, remained largely innocuous although it did produce a few spots of rain in Almeria, and, for that area, the only rain of the event.
The main event
The most widespread of the rains in Andalucia occurred between 06 and 12 UTC. The satellite sequence, Figures 9,10 and 11, shows quite a broad band of thick cloud moving southeastwards. By midday, though, there is just a hint of darkening of the 'colours' on the satellite image, suggestive of a warming of the cloud tops. This is borne out by the dry zone appearing at 350hPa on the midday Gibraltar ascent (Figure 13.). The sampling of the air through the front shows near saturation from 400 to nearly 8,000 metres (1500 to 25,000 feet) and the steady moderate to heavy rain reported was a reflection of this. Not surprisingly, the heaviest rain occurred in the west and north of Andalucia where the band of cloud was widest. At Seville, there were periods of rain, heavy at times, from 04 to 09 UTC, but unfortunately the total of 29 mm (1.1 inches) only represented the period 06 to 09 UTC. At Cordoba the rain fell from 05 to 11UTC with a total of 48 mm (1.9 inches) and at Granada there was rain from 11 to 15 UTC with a total of 14 mm (0.5 inches). A full list of the rainfall totals, and August averages where available, can be found at the end of the article.
Figure 8. IR Imagery 03 UTC 17th August copyright 2006 EUMETSAT
Figure 9. IR Imagery 06 UTC 17th August copyright 2006 EUMETSAT
Figure 10. IR Imagery 09 UTC 17th August copyright 2006 EUMETSAT
Figure 11. IR Imagery 12 UTC 17th August copyright 2006 EUMETSAT
Figure 12. 12 UTC 17th August 2006 Gibraltar Skew-T Upper-Air Sounding Reproduced permission of Department of Atmospheric Science, University of Wyoming
Figure 13. 12 UTC 17th August 2006 Murcia Skew-T Upper-Air Sounding Reproduced permission of Department of Atmospheric Science, University of Wyoming
The afternoon of the 17th
After midday on the 17th the front began to weaken quickly in the south and rain became much lighter as it moved eastwards. The 12UTC upper-air sounding for Murcia (Figure 13.) shows much drier low-level air compared to the Gibraltar ascent (Figure 12). In this area, to the lee of the Sierra Nevada, and not far from semi-desert north of Almeria, rain from a westerly direction is always hard to come by. For example the average annual rainfall at Murcia is 284 mm, compared to 760 mm at Gibraltar.
The passage of the cold front
The 15UTC satellite image (Figure 14) shows the wave making steady northeastwards progress over northern Spain with the front weakening further as it leaves the south coast of Andalucia. In Malaga, 2mm of rain fell between 11 and 13 UTC, with the front passing through around 13 UTC. At Almeria the temperature fell 3 degrees Celsius between 1500 and 1530UTC but otherwise clouds remained broken with no rain falling. Murcia/San Javier Airport on the coast experienced a wind veer between 15 and 16UTC but no rain, and Murcia/Alcantrilla had 0.2 mm (0.01 inches) of rain at 16 UTC , a couple of hours after the front passed through with a wind change from 220/10kt to 320/17kt.
Figure 14. IR Imagery 15 UTC 17th August copyright 2006 EUMETSAT
Figure 15. IR Imagery 18 UTC 17th August copyright 2006 EUMETSAT
Figure 16. IR Imagery 21 UTC 17th August copyright 2006 EUMETSAT
Figure 17. IR Imagery 00 UTC 18th August copyright 2006 EUMETSAT
The satellite sequence in figures 14 to 16 shows the rapid progress northeastwards of the front in more northern latitudes but the very weak front has almost ground to a halt south of Spain by 18UTC. The midnight analysis from the UK Met Office showed that the wave had moved northeastwards, then northwards, to be near southeast England. The associated surface cold front had been truncated east of the Pyrenees but shallow cold air near the surface on the well-dried out Gibraltar sounding (Figure 19), plus earlier surface observations in southern Spain, maybe suggested that the front was omitted prematurely. The Murcia sounding (Figure 20), at first glance, shows no cooling of the air, but low-level winds, katabatically warmed over the 'sierras' to the north and west had modified the lowest levels of the atmosphere.
Figure 18.UK Met Office Surface Analysis 00 UTC 18th August 2006
Figure 19. 00 UTC 18th August 2006 Gibraltar Skew-T Upper-Air Sounding Reproduced permission of Department of Atmospheric Science, University of Wyoming
Figure 20. 00 UTC 18th August 2006 Murcia Skew-T Upper-Air Sounding Reproduced permission of Department of Atmospheric Science, University of Wyoming
Conclusion
The rainfall over Andalucia was not record-breaking in the amounts that were recorded, and it is not unknown to have frontal rain during the summer months. However, it is very unusual to have such a widespread area of rain in southern Spain in August, especially given the fact that the event was almost entirely without thundery activity.
Table of Rainfall Observing Sites mentioned in text
Name of Place
Time of Rain
Amount of Rain
August Average
Lisbon
Nil
Nil
5
Sines
n/a
9
3
Faro
04-06
13
1
Huelva
n/a
20
4
Gibraltar
09-14
6
3
Jerez
07-11
n/a
6
Moron
04-10
7*
6
Seville
04-09
29*
6
Badajoz
03-07
8
5
Cordoba
05-11
48
3
Cuidad Real
04-14
9
7
Granada
10-15
14
3
Malaga
10-13
2
3
Almeria
05-06
Trace
1
Murcia/San Javier
Nil
Nil
1
Murcia/Alcantarilla
16-17
0.2
1
* Denotes total rainfall figure unavailable
Map of rainfall recording sites mentioned in text.
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