Alaska’s cruise season runs from roughly May to September, and the weather across those five months varies more than the temperature range suggests. Rainfall is what separates them.
This guide uses long-run averages for Juneau, Ketchikan and Sitka, the three ports most Inside Passage itineraries call at. The pattern is consistent across all three.
The Season in Short
Temperatures through the season sit between roughly 9C and 14C at all three ports, so the difference between the coolest month and the warmest is only about five degrees.
Rainfall moves far more, with the wettest month of the season delivering close to three times what the driest does. That pattern holds across all three ports, so choosing a month is really a choice about how wet you are prepared to get.
Month by Month
Average temperature and average rainfall for each month of the season, taken at Juneau, Ketchikan and Sitka in turn.
- May. Juneau 9.2C and 86mm, Ketchikan 9.2C and 208mm, Sitka 8.8C and 107mm. The coolest month of the season, and already dry at Juneau.
- June. Juneau 12.6C and 82mm, Ketchikan 12.3C and 168mm, Sitka 11.6C and 73mm. The driest month at all three ports, with long daylight.
- July. Juneau 13.8C and 117mm, Ketchikan 14.3C and 167mm, Sitka 13.4C and 105mm. Warm and still relatively dry.
- August. Juneau 13.3C and 146mm, Ketchikan 14.4C and 249mm, Sitka 14.0C and 174mm. The warmest month at two of the three, and noticeably wetter.
- September. Juneau 10.0C and 219mm, Ketchikan 11.4C and 350mm, Sitka 11.7C and 298mm. Cooling, much wetter and with daylight shortening quickly.
The figures are long-run monthly normals recorded by NOAA at Juneau, Ketchikan and Sitka. They describe the average, not any particular year.
The Daylight Swing
On an itinerary built around glaciers, whales and scenery, how long the light lasts matters as much as whether it rains. Alaska swings hard across the season.
At Juneau the June solstice brings 18 hours and 16 minutes between sunrise and sunset, with first light before four in the morning. By the equinox in late September that has fallen to 12 hours and 18 minutes, and at that point the day is shortening by around five minutes every twenty-four hours.
Six hours is a lot of scenic cruising. An early-season sailing can be passing a glacier at nine in the evening in full daylight, where the same passage in September happens in the dark.
Scenic cruising is scheduled around the route, not around sunset. In June that rarely matters. Later in the season it decides whether you see the thing you came for.
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Why June Is the Driest
June records the lowest rainfall of the season everywhere, and it is not close. The two driest figures of the whole season, 73mm and 82mm, both fall in June, against roughly 300mm and 220mm at the same places in September.
It also brings the longest daylight of the season, which matters on an itinerary built around scenery and wildlife. Temperatures at 11.6C to 12.6C are within a degree or two of the July and August peak.
If you have flexibility, June combines the least rain with the most daylight and temperatures close to the summer peak. Nothing else in the season matches that combination.
Not All Three Ports Are Equal
The three main ports are not equally wet, and the gap between them is wide enough to change how you pack.
- Ketchikan averages around 3,600mm a year, which puts it among the wettest inhabited places in North America.
- Sitka records roughly 2,200mm.
- Juneau is comparatively dry at about 1,600mm.
Even Ketchikan’s driest month of the season averages 168mm, which is more than double what Juneau gets in June. The town leans into the reputation rather than apologising for it.
The September Trade
September cools by two to three degrees and gets dramatically wetter. Ketchikan averages 350mm, Sitka 298mm and Juneau 219mm, and daylight shortens through the month.
What it offers instead is autumn colour, quieter ports, lower fares and a better chance of the northern lights as the nights lengthen. It is a real trade rather than a lesser month, but it needs packing for properly.
What to Pack
Waterproofs matter more than warm layers in every month of this season. A jacket that keeps rain out will do more for a day ashore than an extra jumper.
Layers still earn their place, because glacier viewing on an open deck is markedly colder than the town temperature suggests, and the wind off the ice does the work. Waterproof footwear handles the wet quaysides that come with the territory.
A dry September and a wet June both happen. These figures describe what to prepare for, not what any single sailing will get.
Frequently Asked Questions
June, at all three main Inside Passage ports. Sitka averages 73mm, Juneau 82mm and Ketchikan 168mm, which are the lowest figures of the season at each.
June combines the lowest rainfall of the season with the longest daylight and temperatures within a degree or two of the July and August peak.
Between roughly 9C in May and 14C in July and August at the main ports. The range across the whole season is only about five degrees.
Ketchikan, averaging around 3,600mm a year. Sitka records roughly 2,200mm and Juneau about 1,600mm.
It is much wetter, with Ketchikan averaging 350mm and Sitka 298mm, and daylight shortens quickly. It brings autumn colour, quieter ports and a better chance of the northern lights.
At Juneau the June solstice gives 18 hours and 16 minutes between sunrise and sunset. By the late September equinox that is down to 12 hours and 18 minutes, losing around five minutes a day.
Roughly May to September. Outside that window most itineraries do not operate.
Yes, in every month of the season. Rainfall is the defining feature of the climate and matters more than heavy insulation.
Noticeably colder than the town temperature suggests, because wind off the ice drops the effective temperature on an open deck. Layers handle it better than one thick coat.
The driest month of the season at every main port, with the longest daylight and near-peak temperatures. If your dates are flexible, that is the one to aim at.
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