Weather conditions at Brooks Camp, the most visited spot within Katmai National Park and Preserve, are characterized by overcast skies and frequent rain storms or drizzle during most of the summer. Heavy storms accompanied by intense winds known as williwaws occur and may persist for several days. Summer high temperatures average 63ºF (17°C) and low temperatures average 44ºF (7°C). The sky is clear or partly cloudy about 20 percent of the summer. During late spring and early autumn, skies are usually clear about 40 percent of the time and winds are generally moderate. During this time, high temperatures average 56ºF (13°C) and low temperatures average 36ºF (2°C).

Winter weather at Brooks Lake from October through April is variable, average maximum daily temperatures from 19º to 48ºF (-7° to 8°C), and minimum temperatures between -3º and 40ºF (-19 and 4°C). All months usually have individual daily highs exceeding 40ºF (4°C) and less than one-third of the days had maximum temperatures below freezing.

Total annual precipitation varies from year to year but can average from 20 to 40 inches. Snowfall can average one to 14 inches per month from October to March.

Spring, summer, and fall temperatures at King Salmon are similar to those at Brooks Camp. The average winter temperature at King Salmon ranges from a high of 21ºF (-6°C) to a low of 6ºF (-14°C). Annual precipitation is about 60 inches along the Shelikof Strait, 25 inches in the lake region, and 19 inches at King Salmon. About 60 percent of the precipitation at King Salmon occurs from May through September as rain.

Due to cool temperatures, frequent rains, and occasional strong winds, reliable equipment and clothing are necessities for all recreational activities at Katmai.