Kalama Valley, HI Weather

Synopsis

Light trade winds will produce rather dry conditions over most areas into tonight. Chances for heavy showers will increase during the day on Tuesday as developing low pressure and deep tropical moisture approach the islands from the southwest. The trough, or possibly a weak low, will pass over the island chain Tuesday night through Wednesday night, bringing the threat of heavy flooding rainfall and thunderstorms. The trough will weaken Thursday and Friday, though humid conditions with a chance of heavy showers may linger. Trade winds may briefly redevelop Saturday, though unsettled conditions could return Sunday.

Discussion

One last day of fair trade wind weather is in store. Trade winds will be rather weak as a broad and deep surface low meandering about 950 miles to the northeast keeps surface ridging displaced far northwest of the state. As a result, the local pressure gradient and the easterly trades will be weak. The atmosphere remains quite stable due to a ridge aloft centered just south of the islands. Dew points are comfortably hovering in the mid 60s, and the overnight soundings revealed an inversion based between 5,000 to 6,500 feet. Most islands will experience minimal windward rainfall today, with afternoon sea breezes leading to clouds and isolated showers over leeward terrain. The exception continues to be Big Island, where a small pocket of moisture remains hung up along the windward coast and should produce slightly more showers.

Little will change overnight, though increasing high clouds will signal a developing heavy rain threat. While Tropical Depression Olana will remain far to the west and not directly impact the area, a trough extending eastward from system will slowly drift to within 300 miles southwest of Hawaii tonight. Meanwhile, an upper level trough, now centered over 1,000 miles west of Kauai, will advance eastward, tapping moisture aloft from thunderstorms along the surface trough and sending increased high clouds overhead.

Chances for heavy flooding rainfall and thunderstorms will increase during the day on Tuesday. The upper level trough will edge closer to the state, and the entrance region of a jet stream aloft along the eastern flank of the feature will steadily erode the ridge aloft early Tuesday and then provide large scale ascent over the region later in the day. This will pull the surface trough toward the islands, spreading a deep tropical airmass with precipitable water in excess of 2 inches throughout the day. Chances for widespread rain should remain relatively low Tuesday morning as the Flood Watch goes into effect, but as moisture increases later in the day, the flood threat will slowly increase.

The flood threat will be greatest Tuesday night through Wednesday night. The jet stream aloft will provide its strongest ascent over the region, pulling the surface trough over the state and possibly generating an elongated surface low near or over the islands. A deep tropical airmass with precipitable water in excess of 2 inches will accompany this system and provide ample fuel for heavy rainfall. The GFS and ECMWF show the surface trough stretching across the central portion of the island chain, favoring Oahu and Maui County for the greatest rainfall. However, all islands will have a heavy rain threat.

The threat of heavy rainfall will diminish Thursday and Friday. The jet stream aloft will lift to the north, and weak upper level anticyclonic flow will develop, ceasing the large scale ascent over the islands. A weak surface trough and ample moisture will likely linger, providing some chance for heavy showers that could require the Flood Watch to be extended.

Aviation

A gentle trade wind pattern is expected today with localized sea breezes developing across many leeward areas. An area of moisture lingering around the Big Island will bring some showers throughout the day today with periods of MVFR conditions possible mainly over windward Big Island. Otherwise, VFR conditions should prevail.

A band of deep tropical moisture will arrive from the southwest Tuesday and will likely bring periods heavy rain and isolated thunderstorms. MVFR to IFR conditions should be expected at many areas across the state starting as early as Tuesday afternoon and persisting into Wednesday.

No AIRMETs in effect, but we may need AIRMET Sierra for mountain obscuration over parts of windward Big Island later today.

Marine

Issued at 311 AM HST Mon Oct 25 2026 A surface low far northeast of the Hawaiian Islands will stall roughly 800 nm northeast of the state today. A low pressure trough is forecast to approach the area by late tonight and bring heavy showers with embedded thunderstorms to the waters over the next several days. Relatively weak wind flow will persist through the forecast period.

A short period north northwest swell will gradually lower today. Meanwhile, an overlapping, moderate, mid- period northwest swell is will build today and bring surf to near advisory levels along north and west facing shores through Wednesday. A small north northeast swell should arrive during the second half of the week.

Overlapping south-southwest swells will keep surf along south-facing shores to around seasonal averages through Tuesday. A larger south swell may push surf up to near advisory levels Thursday and Friday.

Surf along most east-facing shores will be small through early this week, but areas exposed to wrapping northerly swells will experience periods of moderate surf today. East- facing shores should also see a bump in surf if the north- northeast swell materializes during the latter half of this week.