Local Hazardous Weather Conditions

• Wind and Weather: Koper's climate is dominated by the Bora wind, which can occur anytime during the year. However, the peak frequency occurs in the cold season (November - March). To a lesser extent, the Scirocco wind affects Koper but is not nearly as strong or as frequent as the Bora. Gulf of Genoa lows have an influence on weather in the northern Adriatic Sea as they either move toward Koper causing stormy weather with clouds and rain, or they move southeastward causing a steep pressure gradiant over at Koper, initiating a Bora outbreak.

Bora: Bora winds result when cold air accumulates over the Balkan Peninsula, especially the region between Albania and Austria. When the depth of the cold air pool reaches the height of mountain passes, the Bora will commence.

• There are two primary weather patterns associated with the Bora:

1. Anticyclonic pattern: A large high pressure cell is present over central Europe without a well defined low pressure center to the south.
   

2. Cyclonic pattern: A low pressure center is present in the southern Adriatic Sea or in the Ionian Sea. In either case, the pressure is higher on the European side of the mountains and lower on the Mediterranean side.

• The Bora is most common in the Adriatic Sea where it flows mainly from the northeast through gaps in the Dinaric Alps. One of these gaps is near Trieste and is known as the Trieste Gap. The gap east of Koper is not as prominent as the Trieste Gap. On occasion, the Bora can be very localized, extending only a few miles offshore. At other times, the Bora will dominate the entire Adriatic Sea and, when the area of steep pressure gradiant is large enough, the Bora can extend as far south as Malta.

• In the northern Adriatic Sea, the wind direction associated with the Bora is generally northeasterly but can vary in local areas due to the terrain. The Bora at Koper and Trieste is east-northeasterly. It is more northerly farther south and even northwesterly along Italy's southeast coast.

  The strongest winds occur along the eastern shore of the Adriatic Sea from Trieste to the Albanian border.

  Anticyclonic pattern Bora winds are most intense to the north, decreasing somewhat moving southward.

  The greatest intensity of the Bora occurs where the mountain peaks are at least 2000 ft above sea level and not more than two or three miles inland.

  Winds are usually less intense over the open water of the Adriatic Sea, but gale force winds (greater than or equal to 34 kt) are common. The frequency of the gale force Bora in the open sea is greater for the cyclonic type of pattern than for the anticyclonic pattern.

  During the cyclonic pattern, the strongest winds are usually found in the southern Adriatic Sea.

  Bora winds are most common during the cool season (November through March). In general, the frequency of gale force winds varies from one day per month, or less, in the summer to six days per month during winter months.

  The average duration of a continuous gale force Bora over the Adriatic Sea is about 12 hours but the winds sometimes will last up to two days.

  The average duration of a Bora that reaches gale force some time during its history is 40 hours with a maximum duration of 5 days.

  At Koper, the average duration of a gale force Bora varies from three days in winter to one day in summer. Local mariners state that the Bora will last an odd number of days; 1, 3, 5, etc. However, the Bora has been known to last for up to 30 days at Trieste without a significant lull.

  In 1956 a gust of 125 kt was recorded at Trieste.

  The Bora does not usually start with a sudden blast but will build up at a relatively moderate pace. A 60 kt Bora will not reach peak intensity during the first 3 or 4 hours. This may allow time for some protective measures to be assessed and conducted.

  Wave heights near the port of Koper are normally not high with a Bora because they are fetch limited.

  There is a noticeable diurnal variation at stations along the eastern coast of the Adriatic Sea during Bora conditions. During the day, the sea breeze counteracts the offshore flow of the Bora, which decreases the strength of the Bora between 1200L and 1800L.

  With the anticyclonic pattern, the Bora is basically a dry wind due to its katabatic nature. Clear skies and good visibilities are found in the lee of the mountains while thick clouds associated with up-slope motions are found on the mountain crests. These clouds subsequently dissipate in the descending air on the lee side, and appear as "cap clouds" to an observer on their west side.

  With the cyclonic pattern, the Bora is often accompanied by low clouds and reduced visibilities associated with rain and/or drizzle. These conditions are more noticeable over the open water areas than along the coastal zone.

• Scirocco: The Scirocco is a southeasterly to south-southwesterly wind over the Mediterranean originating over North Africa, sometimes affecting the Adriatic Sea area.

  The Scirocco tends to occur year-round without a favored month or season.

  The Scirocco normally occurs within the warm sector of a cyclone passing either north or west of the region. These cyclones originate either over North Africa or south of the Alps, primarily in the Gulf of Genoa in the latter case.

  Scirocco conditions occur in the Gulf of Genoa case when the circulation extends far enough southward to draw air from the North African region.

  The onset of the Scirocco is more gradual than the onset of a Bora. It occurs more frequently in the southern part of the Adriatic Sea with a decrease in frequency northward.

  Although the Scirocco is not as strong as the Bora, winds can reach gale force (greater than or equal to 34 kt), especially in winter and spring.

  The average duration of continuous gale force winds during a Scirocco is 10 to 12 hours and occasionally as long as 36 hours. The maximum wind speed likely during a Scirocco is about 55 kt. Local terrain features alter the effect of the Scirocco. Winds will parallel the coast in general.

  As a cold front approaches from a relatively flat, low surface, such as the ocean, toward an elevated land mass, super-gradient winds will occur due to the Venturi effect. These winds are restricted to a narrow band between the front and the landmass and do not normally sustain for long periods of time. Seas are usually not high with strong southeasterlies due to protection from the terrain, but sustained Scirocco winds can raise seas of 6.6 to 9.8 ft (2 to 3 m) in the open water west of Koper. Waves in the harbor are fetch limited.

• Genoa Lows: Genoa lows are low-pressure systems which develop to the south of the Alps in the region incorporating the Gulf of Genoa, Ligurian Sea, Po Valley, Gulf of Venice and northern Adriatic Sea. Although several factors are important in cyclogenesis, the development of the cyclone near the Gulf of Venice--as opposed to the west near the Gulf of Genoa--depends on the amount of cold air penetrating the Po Valley from the northeast. If there is little or no cold air entering the Po Valley, the low will probably form in the Gulf of Venice; otherwise, cyclogenesis will occur to the west. Genoa cyclones usually remain stationary (or at least leave a residual trough) south of the Alps throughout their life history. If the lows do move, they generally follow one of two tracks.

  The first track, common for cyclones developing in the Gulf of Venice, is a northeasterly to north-northeasterly direction across the Alps. This track is associated with strong southwesterly flow aloft. In this case, Scirocco conditions are likely if the circulation of the low extends southward into North Africa, allowing air from the desert source to move northward.

  The second track, associated with a strong anticyclone over the Balkans, Turkey and the Black Sea, is in a southeasterly direction from the Gulf of Genoa towards the Ionian Sea. In this case, a gale force Bora is extremely likely by the time the depression moves into the Ionian Sea.