Westerlies at sea, afternoon storms over the lakes. Sailing weather for the end of July

The final days of July bring a pattern over the Baltic that means one thing to a beachgoer and another to a crew. The sea is markedly colder than the land, the wind blows from the western sector, and in the afternoon the sea breeze reinforces it. Over the Great Masurian Lakes the situation is reversed and in one respect harder: warmer, calmer in the morning and more violent after midday. Mariusz Jasłowski, meteorologist and editor-in-chief of Meteo24.com.pl and an expert contributor to Gazeta Morska, sets out what to expect region by region.

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25 july 2026 | 17:31 | Source: Gazeta Morska | Prepared by: Kamil Kusier | Print

fot. Mariusz Jasłowski

One pattern, two different consequences

The last days of July are governed by a single relationship: the Baltic is colder than the heated land. This is the inverse of the arrangement that drives autumn storms at sea, and that inversion determines everything that follows.

Over the water, a cold sea stabilises the lower layer of air, so convection develops more weakly and there are fewer thunderstorms over the sea itself. But the same temperature difference sets off the sea breeze circulation, which by afternoon can add several knots to the gradient wind. Inland, a few dozen kilometres from the coast, the same difference works in the opposite direction: a heated surface drives convection, and over the Masurian lakes it is supplied with moisture from the numerous water bodies.

In short, at sea the problem will be wind. Over the lakes it will be thunderstorms.

The western coast: westerly and north-westerly sector

The calmest stretch of the Polish coast in this period.

- The last days of July on the western coast promise to be decidedly calmer than the first half of the month, says Jasłowski. - We will still see inflows of cooler air from over the Baltic, so temperatures will rarely exceed 22 to 24 degrees, but at the same time there will be more and more sunshine. Residents of Świnoujście, Międzyzdroje, Dziwnów and Kołobrzeg can count on several genuinely pleasant days, though mornings will remain quite brisk.

Calm, however, does not mean settled.

- The Baltic is currently much colder than the heated land, so even in sunny weather local cumulus clouds may develop, bringing brief showers or isolated thunderstorms, he notes. - A moderate and at times fairly strong wind from westerly and north-westerly directions will also be characteristic, and it may lower the apparent temperature by several degrees.

That last passage matters most to a crew and needs translating into geography. The Polish coast runs broadly east to west, so a westerly runs along the shore while a north-westerly carries a clear onshore component. When the wind veers into the north-west along this stretch, swell builds over the shallows and conditions at harbour entrances deteriorate. Kołobrzeg, Darłowo and Ustka have narrow entrances oriented such that a sea from this sector meets the heads obliquely. The decision to return is taken before the wind rises, not after.

The central coast: a sea breeze adding to the gradient

Jasłowski describes the stretch from Ustka to Łeba as the most changeable over a span of days.

- In the central part of the coast, taking in Ustka, Darłowo, Mielno and Łeba, the end of July will be a period of highly variable weather, he says. - Atmospheric fronts will move relatively quickly, so one day may be dominated by full sunshine and the next bring passing showers and stronger gusts.

His observation on the breeze mechanism is the key one for passage planning.

- Air temperature will mostly hold between 20 and 23 degrees, while the temperature of the Baltic will remain distinctly lower. As a result the temperature differences will favour the formation of a sea breeze, which will noticeably strengthen the wind particularly in the afternoon.

The daytime sea breeze is a local circulation driven by exactly that difference. Land heats faster, air above it rises, and cooler air from over the water moves in to replace it. It typically begins in the late morning, peaks in the early afternoon and dies away in the evening, extending a few to a dozen or so miles offshore.

The practical consequence is that the breeze does not replace the gradient wind but adds to it. Where the synoptic wind blows from the same sector as the breeze, the afternoon reinforcement can amount to several knots and shift conditions by a full Beaufort force. Where it opposes, a zone of light and variable winds appears near the shore during the afternoon while conditions further offshore remain unchanged. A morning assessment from an anchorage therefore says nothing about three in the afternoon.

The eastern coast: differences across a dozen kilometres

The stretch hardest to forecast begins beyond Władysławowo.

- On the eastern coast, from Władysławowo through Hel and the Tricity to the Vistula Spit, the weather will remain the most dynamic, says Jasłowski. - This region often lies at the junction of different air masses, so the end of July may bring both very sunny days and periodic rainfall or thunderstorms developing in the afternoon hours.

He draws attention to something sailors on the Gulf of Gdańsk know from experience.

- Conditions on the beaches may differ even between places a dozen or so kilometres apart, he says, adding that the passage of fronts should be expected to bring sudden increases in wind speed and brief but intense rainfall.

The reason for that unevenness is geographical. The Hel Peninsula separates two bodies of water with entirely different characteristics. On the open sea side, waves build over full fetch; on the bay side they are short and steep; and in the lee of the peninsula it can be almost flat in the same wind. Add the built-up area of the conurbation and the moraine uplands, which modify the flow over the Gulf of Gdańsk itself. An area forecast, under these conditions, is an indication rather than an answer.

The meteorologist's own recommendation is unambiguous here.

- Anyone planning time on the Gulf of Gdańsk should follow short-term forecasts regularly, he says.

Warmia and Masuria: warmer, but more dangerous in the afternoon

Over the lakes the pattern differs and demands a different discipline.

- Warmia and Masuria are ending July with distinctly more summery weather than the coastal regions, although passing atmospheric fronts will make their influence felt here too, says Jasłowski. - Daytime temperatures will often reach 23 to 26 degrees, and locally may be higher still. That also means a greater probability of afternoon thunderstorms, especially over the lakes.

He sets out the mechanism directly.

- The heated land surface and the numerous bodies of water create conditions favouring rapid development of storm clouds. Even if the morning is entirely sunny, the situation can change very dynamically in the afternoon.

And a warning addressed to sailors specifically:

- Sailors and users of watercraft should pay particularly close attention to meteorological bulletins, because summer storms in Masuria often appear suddenly and are accompanied by strong gusts of wind.

It is worth setting out why a storm over a lake can be more dangerous than one at open sea, despite waves being incomparably smaller. Four things combine.

First, warning time. At sea a storm cloud is visible above the horizon from a long way off. Over a lake ringed by forest the front hides behind the treeline and becomes visible only when it is close.

Second, the absence of sea room. At sea there is the option of standing off the shore and riding out a squall on open water. On Śniardwy or Mamry every manoeuvre ends at a shore, and a shore in strong wind is a hazard rather than shelter.

Third, the nature of the gust. The downdraught from a storm cell strikes the surface and spreads horizontally, so the wind shifts abruptly and usually arrives ahead of the rain. A boat carrying full sail with the wind aft takes that gust on the beam.

Fourth, the composition of the fleet. The Great Lakes carry many chartered boats crewed by people of varying experience, often without the habit of reefing early.

Jasłowski stresses that no prolonged breakdown is in prospect.

- Between successive fronts there will be plenty of sunshine, and the end of July should provide many hours of very good weather for time spent by the lakes, he says. - This is the classic summer pattern for north-eastern Poland: warm, humid and periodically stormy.

Seventeen masts worth watching

The Great Masurian Lakes have a tool found nowhere else in Poland, and one that may prove useful this week.

The Warning Signalling System was commissioned in July 2011, following the 2007 disaster in which a squall with winds of around 130 kilometres per hour and waves reaching three metres claimed twelve lives. It comprises seventeen masts between fifteen and twenty-five metres tall, distributed along the whole waterway, among others on Mamry, Kisajno, Niegocin, Jagodne, Tałty, Mikołajskie, Bełdany, Śniardwy and Roś. An eighteenth, prototype mast stands at Giżycko by the entrance to the Łuczański Canal from Niegocin.

The signal is yellow, pulsing and visible from eight to nine kilometres even in full sunshine. Two states apply. Forty flashes per minute means CAUTION, that is thunderstorms and wind expected, without indication of time or place. Ninety flashes per minute means DANGER, an immediate threat requiring crews to seek shelter at the shore without delay.

The system is triggered automatically on a bulletin from the Institute of Meteorology and Water Management forecasting office in Białystok, and the whole procedure from message to illuminated lamps takes under a minute. The waterway is divided into three sections, northern from Węgorzewo to Giżycko, central between Giżycko and Mikołajki, and southern below Mikołajki, allowing each to be warned independently. The lamps operate from the beginning of May to the end of September, from sunrise to sunset, and switch off automatically when a warning expires. By 2023 they had been activated more than a thousand times.

The system's limitation follows from its design. It runs from sunrise to sunset, so it does not operate at night, and the CAUTION state gives no indication of when or where a phenomenon will arrive. It is a prompt to heightened vigilance and observation of the sky, not a forecast.

Thresholds and sources

Forecasts and warnings for Polish sea areas are issued by the Central Office of Maritime Meteorological Forecasts of the Institute of Meteorology and Water Management in Gdynia, with the full set of material, including wave and water temperature forecasts, available at baltyk.imgw.pl. The Baltic is divided into forecast zones, of which the Western, Southern and South-Eastern cover Polish waters. Warnings for sea zones and for the coastal strip are issued separately, so an alert for the coast does not necessarily imply a warning for open water, or the reverse.

In Polish terminology strong wind means force 6 to 7 on the Beaufort scale, while a storm warning covers force 8 to 12. English-language bulletins divide this into a strong wind warning for 6 to 7, a gale warning for 8 to 9 and a storm warning for 10 to 12. Vessels equipped with NAVTEX receive bulletins automatically.

For the Masurian lakes, warnings are issued by the Institute's forecasting office in Białystok, and one of their effects is the activation of the mast system described above.

None of these tools will answer the question that will matter most over the lakes this week: whether the cell that develops at three in the afternoon passes over you or two kilometres away. Only observation of the sky answers that, and a decision taken early enough.

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Kamil Kusier
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