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April 11, 2021

How to spot a weather window – part two

Weather windows and thunderstorms
Summary
On how to improve uptime in offshore operations


It is common knowledge, that weather forecasts can be uncertain, and that this uncertainty in general will increase with time. A forecast can also perform poorly for a given location and with certain weather conditions (for example due to challenging topography).
Furthermore, in some weather situations certain meteorological parameters can be below your working limits while other parameters may be unfavorable - for example a weak southerly flow of moist and warm air often means weak winds and low wave height but can also mean high risk of thunderstorms.
This article is aiming for an increase in the understanding of these difficulties, and how to optimize the duration and number of weather windows. In part 1 we focused on winds and waves where this part will focus on work interrupting thunderstorms.

The diurnal variation of thunderstorms

During summer, the peak season for thunderstorms, the showers/thunderstorms will typically form around noon and then culminate during late afternoon. However, this is only true for land areas and areas close to the coast if the wind direction is offshore. If you want to read more about the different season for lightning, then just click here.
For offshore wind farms located far from the coast or with the closest coast to the east, this diurnal variation will be less distinct, as the showers/thunderstorms typically will be weakening over the colder sea surface.
This diurnal variation can be seen in the graphs below (figure 1 and 2), showing the distribution of lightning alerts issued by Vento Maritime during the day (06-18 LT). The data is for wind farms in the Baltic Sea, south of the island Lolland and in the Danish water Kattegat and is representative for many offshore wind farms in Northern Europe.

Graph for lightning distribution
Figure 1: The distribution of lightning alerts (high risk of lightning) between 06 and 18 LT, the whole year for a site in the Baltic Sea just south of the island Lolland.
Graph for lightning distribution
Figure 2: The distribution of lightning alerts (high risk of lightning) between 06 and 18 LT, the whole year for a site in Kattegat.
Graph for lightning distribution
Figure 3: The distribution of lightning alerts (high risk of lightning) between 06 and 18 LT, the whole year for a site in the easternmost North Sea.

 

As seen in the figures 4 and 5, the diurnal variation becomes less distinct during the winter.
For a wind farm in the Danish part of the North Sea, figure 3 and 6, there is no clear diurnal variation and no significant difference in the winter months.

Graph for lightning distribution
Figure 4:  The distribution of lightning alerts (high risk of lightning) between 06 and 18 LT, in the winter months Dec-Feb in the westernmost Baltic Sea.
Graph for lightning distribution
Figure 5: The distribution of lightning alerts (high risk of lightning) between 06 and 18 LT, in the winter months Dec-Feb in Kattegat.
Graph for lightning distribution
Figure 6: The distribution of lightning alerts (high risk of lightning) between 06 and 18 LT, in the winter months Dec-Feb in the easternmost North Sea.
 
The direction of thunderstorms

The figures 7-9 shows the direction of the potential thunderstorms (approaching from) compared with the general wind distribution. The data for potential thunderstorms is derived from lightning alerts sent by Vento Maritime.
It is clearly seen that thunderstorms almost never move from east towards west. This is because an easterly flow often is a result of a high pressure over Scandinavia or western Russia, and the high will suppress the buildup of thunderstorms.
In Northern Europe, the southwesterly direction for thunderstorms is much more dominant than the general wind distribution. An air mass from southwest, is typically moister than airmasses from west, and the moisture is an important factor for the development of thunderstorms.
For the sites in the North Sea and westernmost Baltic Sea, it is also more common with thunderstorms from south, compared with the general wind distribution. This is due to the shorter distance to the continent, where heavy showers and thunderstorms often form if there is a southerly flow of warm and moist air.
For the Kattegat site, it is very rare with thunderstorms from northwest. This is due to the Norwegian mountains, that will “dry out” the air (föhn effect) and prevent significant thunderstorms to develop.

Rose for thunderstorm direction
Figure 7: Site in westernmost Baltic Sea: To the left, a wind rose showing the distribution of the wind directions. To the right, the distribution of the directions from which potential thunderstorms are approaching.
Rose for thunderstorm direction
Figure 8: Site in Kattegat: To the left, a wind rose showing the distribution of the wind directions. To the right, the distribution of the directions from which potential thunderstorms are approaching.
Rose for thunderstorm direction
Figure 9: Site in easternmost North Sea: To the left, a wind rose showing the distribution of the wind directions. To the right, the distribution of the directions from which potential thunderstorms are approaching.
 
Rules of thumb for thunderstorms

Based on the above graphs and diagrams, there are some basic rules for Northern Europe, that can help you plan the day and maximize the working hours:

- In the summer, when thunderstorms are most frequent, the thunderstorm activity will often be concentrated around the afternoon.

- If your location is relatively close to the shore, and the wind is directly offshore, there is an increased risk of thunderstorms. This is especially true during the summer but may also be the case during the autumn and spring.

- During easterly winds, there will rarely be heavy showers/thunderstorms.

- If any significant topography is near the site, you will seldom experience thunderstorms with a wind direction from these mountains as the air typically will be too dry for thunderstorm formation.

- With a flow from between south and southwest, and with forecasted showers over your site, there is an increased risk of thunderstorms.

If you like even more rules of thumbs regarding thunderstorms, we have written this short article on how to differentiate between thunderstorm clouds and regular stratocumulus clouds.

Lastly, it is also important to understand the synoptic situation. Are the conditions generally stable (the air temperature is rising with height) or unstable (air temperature is falling with height)?
A warm air mass behind a warm front, typically means no or low risk of lightning, while a cold air mass typically means showers and an increased risk of thunderstorms.
High pressure weather will always reduce the risk of thunderstorms significantly, while low pressure/front passages in some cases could trigger thunderstorms.

If you want to read more about how, when and where thunderstorms form, you are very welcome to read this guide.