textproduct: Honolulu

SYNOPSIS

A strong high pressure system will build north of the Hawaiian Islands today on a slow eastward drift. Strong gusty winds will develop over and down stream of island mountain ranges today. Wind speeds will exceed advisory thresholds in the windier areas of all islands except for Kauai and Niihau. Passing showers are possible over windward and mountain areas through much of this week, higher rainfall amounts will favor the overnight to early morning hours.

DISCUSSION

A combination of satellite and radar imagery shows unsettled 'open cell' cumulus clouds riding in on the trade winds producing periods of rain showers across much of the state. Higher rainfall amounts will favor windward mountain areas of all islands, especially in the overnight to early morning hours. Upper air weather balloon soundings from Lihue and Hilo at 2 AM HST (12Z) observed subsidence temperature inversion heights around 6,000 feet, high enough for periods of passing showers to remain in the forecast. Stronger low level winds will also help to lift unstable clouds up over island mountains orographically forcing enhanced rainfall activity.

A satellite scatterometry pass from last night around 9:40 PM HST, measuring wind speeds through microwave scattering of wind blowing over the ocean water, reported near Gale Force winds in the Kaiwi and Alenuihaha channels last night. This satellite wind report and our forecast for increasing winds through the day led to issuing Gale Warnings for the windiest channels and Wind Advisories through 6 PM HST for windier land areas over the down wind of island mountain ranges from Oahu eastward to the Big Island of Hawaii. Kauai and Niihau will likely remain just below our Wind Advisory thresholds today.

Enhanced showers will likely continue through much of this week. Initially, we see a low level cloud band moving across the islands from the north from Monday afternoon into Tuesday. Then a passing weak upper low tracks from west to east across the islands on Wednesday and Thursday lifting cloud heights, and finally we see yet another low level cloud band riding into the islands on the trade winds by Friday. In essence, these passing weather systems will keep periods of enhanced showers in the forecast into next weekend. Light to moderate beneficial rainfall amounts are expected in this weeks wet trade wind weather pattern.

AVIATION

Breezy trade winds will continue today with brief periods of showers over windward and mountain areas, spilling into leeward areas at times. Localized afternoon sea breezes along leeward Big Island will lead to scattered showers developing along the Kona slopes. Brief periods of MVFR conditions may accompany some of the more robust activity.

AIRMET Sierra has been cancelled due to the improvement of ceilings and visibility.

AIRMET Tango remains in effect for low level moderate turbulence over and downwind of island mountains.

MARINE

Strong trade winds expected today as a strong high builds far north of the islands. Some zones have been upgraded to a Gale Warning today, with the remaining coastal waters in the Small Craft Advisory (SCA). Winds will diminish gradually tonight and Monday as the high moves to the northeast, but a new north swell will push combined seas above 10 feet for exposed waters, likely prompting an extension of the SCA for most zones. Expect moderate to locally strong trade winds to require a SCA for the typically windy waters Tuesday through the remainder of the week.

A moderate, long-period north swell will build today and produce surf near High Surf Advisory thresholds along north-facing shores, as well as some exposed eastern shores into Monday morning. The north swell will gradually decline late Monday through Wednesday. A tiny pulse of northwest swell will be possible late next week, followed a moderate swell next weekend.

Rough surf along east-facing shores will remain above seasonal average over the next couple of days due to elevated trade wind swell and some wrapping north swell. East shore surf is expected to be around seasonal average Tuesday through the rest of the week. Mainly small background level surf will persist along south- facing shores through much of the week, though a small long- period swell will be possible on Friday.

FIRE WEATHER

Strong winds will develop today for most higher elevation mountain areas. Relative humidity levels in these areas will remain above critical fire weather thresholds. Wet weather conditions will also help to mitigate fire threats by increasing humidity levels in lower elevation areas. Temperature inversion heights near Maui and the Big Island will range from 6,500 to 7,500 feet elevation today.

HFO WATCHES/WARNINGS/ADVISORIES

Wind Advisory until 6 PM HST this evening for portions of Oahu, Molokai, Lanai, Kahoolawe, Maui and the Big Island of Hawaii.

Small Craft Advisory until 6 AM HST Monday for all Hawaiian coastal waters and channels outside of the Gale Warning area.

Gale Warning from 6 AM this morning to 6 PM HST this evening for Kaiwi Channel-Maalaea Bay-Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big Island Southeast Waters.

Small Craft Advisory from 6 PM this evening to 6 AM HST Monday for Kaiwi Channel-Maalaea Bay-Pailolo Channel-Alenuihaha Channel- Big Island Leeward Waters-Big Island Southeast Waters.