textproduct: Honolulu

SYNOPSIS

An approaching weak cold front will stall out just Northwest of Kauai and continue to keep the high pressure ridge entrenched directly over the Hawaiian Islands. Large scale stability under this ridge and drier southeasterly wind flow will minimize shower activity into Tuesday. Another cold front will approach the islands from the northwest on Tuesday. Clouds and showers along the frontal band may stall over Kauai by Wednesday, increasing shower activity over the Garden Isle. Trade winds will build back into the region and strengthen from Thursday onward, bringing back periods of passing showers to windward and mountain areas, favoring the typical overnight to early morning hours.

DISCUSSION

This mornings satellite picture shows a weak cold frontal system approaching the islands from the northwest and a weak high pressure ridge over the Hawaiian Islands. Bands of high level cirrus ice crystal clouds associated with this approaching system are passing over the island chain. The low level stratocumulus clouds near the islands appear very stable with little rainfall expected under the stable subsidence of the ridge anchored directly over the state. The subsidence temperature inversion heights, as measured by weather balloon observations from Lihue and Hilo at 2 AM HST (12Z) this morning, are hovering between the 5,000 and 6,500 feet elevation level. This temperature inversion will continue to serve as a stable capping force for cloud development, reducing shower potential.

Additional stable dry trends are also due to southeasterly wind flow that places the smaller islands in a drier rain shadow of the Big Island with clouds tracking more parallel to island mountain ranges, limiting orographic lifting of clouds over windward mountains. Lighter large scale winds will also keep daytime onshore sea breezes going for most areas through Tuesday. VOG (Volcanic smOG) from the Kilauea Volcano will also ride into all islands on these drier southeasterly winds. This VOG will appear as haze affecting all islands today, with some decreasing trends from Monday onward as the low level steering winds begin to change.

This dry southeasterly wind pattern will continue for most areas through Tuesday or Wednesday. On Wednesday, medium range model solutions are showing changes in our local weather pattern as a weakening cold front moves into Kauai and stalls. Shower activity will likely increase over Niihau and Kauai for 24 hours, until this front breaks apart and drifts westward away from the islands. Differences between the main operational American (GFS) and European (ECMWF) remain, with the ECMWF model showing wetter trends for the islands in Kauai County.

A high pressure system will build in north of the state for the second half of the week, increasing easterly trade winds across the region. Expect some southeasterly winds to linger on Thursday, with winds becoming more easterly and stronger from Friday through next weekend. As the ridge over the islands lifts northward, our typical trade wind shower activity returns with brief passing showers forecast along windward and mountain areas, favoring the typical overnight to early morning hours.

AVIATION

Light southeasterly large scale winds will produce a stable land and sea breeze weather pattern across the state. Upslope sea breezes each day will turn into lighter downslope land breezes each night. Stable conditions aloft will keep VFR conditions firmly in place across the state through at least Monday.

There are currently no AIRMETs in effect, and none are expected.

MARINE

A high pressure ridge will remain over or just north of the state through Tuesday and maintain light to moderate southeast background flow. This pattern will allow for nocturnally driven sea breeze/land breezes along waters adjacent to the islands coasts. By Wednesday, a front will approach Kauai from the northwest, but models show it weakening into a trough then retreating to the west. Typical easterly trades are finally progged to return late this week into next weekend as surface high pressure builds to the northeast of the area.

A series of large to extra large west-northwest to northwest swells will pass through the waters over the next several days. The current long period west-northwest swell (300-320 degree) continues to peak this morning at an impressive 15-17 feet, as noted on the latest Waimea Bay Buoy observations. A Small Craft Advisory will remain in effect through noon on Monday for marine zones with exposed waters for combined seas over 10 feet. This swell is expected to slowly lose energy this afternoon through tonight, but continue to produce surf above warning criteria. Thus, the High Surf Warning will remain in effect through noon on Monday for exposed north and west facing shores of most islands. Surf should drop down to advisory levels by Monday afternoon as the swell energy eases. Another large, long period northwest swell (310-330 degree) is forecast to fill in on Wednesday, peak Thursday, then slowly subside on Friday. Surf may once again approach warning levels.

East shore surf will remain small through the weekend due to weak winds. Select south facing shores could experience westerly wrap from this weekend's large west northwest swell passage.

FIRE WEATHER

Rather dry conditions are expected to persist through Tuesday. Winds will be light and variable under a stable land and sea breeze pattern, preventing critical fire weather thresholds from being approached. With the inversion holding around 6,500 ft, very dry conditions will persist on the upper elevations of the Big Island and Maui.

HFO WATCHES/WARNINGS/ADVISORIES

High Surf Warning until noon HST Monday for Niihau-Kauai Leeward- Waianae Coast-Oahu North Shore-Maui Windward West-Kona-Kohala- Kauai North-Molokai Windward-Molokai North-Molokai West-Maui Central Valley North-Windward Haleakala.

Small Craft Advisory until noon HST Monday for Kauai Northwest Waters-Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel- Oahu Windward Waters-Kaiwi Channel-Maui County Windward Waters- Big Island Windward Waters.