Forecasters expect an extra-large, northwest swell to arrive this evening, bringing potential surf of 40 to 50 feet Saturday, and prompting a high surf warning for islands from Niihau to Maui from 6 p.m. today to 6 a.m. Sunday.

“This new northwest swell will potentially become the largest surf event of the season so far,” according to the National Weather Service.

The high-surf warning covers the north and west shores from Niihau to Molokai, and north-facing shores of Maui.

Forecasters expect surf of 15 to 20 feet on north shores under the current high-surf advisory to lower this evening to 10 to 15, then rapidly build to 40 to 50 feet on Saturday on northern shores.

Surf on west shores is expected to lower to 8 to 12 feet this evening, then rapidly build to 25 to 35 feet on Saturday.

Impacts under the current advisory include strong, breaking waves and rip currents, which make swimming difficult and dangerous, forecasters said.

Impacts under the warning that begins at 6 p.m. are high, they said, noting that the public should beware of ocean water occasionally sweeping across normally dry portions of beaches and that anyone entering the water could face “significant injury or death.”

Surf on other shores, meanwhile is expected to remain small — at 1 to 3 feet today and Saturday for south shores, and at 1 to 3 feet today to 2 to 4 feet Saturday for eastshores.

Partly to mostly sunny skies are expected today, with scattered showers and highs from 79 to 84 degrees Fahrenheit. Lows tonight dip to 61 to 69.

Variable winds are expected at 5 to 15 mph today and tonight but should strengthen on Saturday.

A shift in the weather pattern is not expected until Sunday, when a strong front is forecast to move in, bringing possible thunderstorms and locally heavy rain.

A small craft advisory has also been issued for Hawaiian waters from northwest of Kauai to windward of Hawaii island through 6 a.m. Sunday.

Forecasters expect yet another extra-large, northwest swell to build on Monday, prompting another round of warning-level surf early next week.