textproduct: Honolulu

SYNOPSIS

An upper-level feature moving through the islands through Monday will bring high clouds and the possibility for isolated heavy showers and thunderstorms. Winds will weaken, leading to afternoon showers Tuesday and Wednesday, before trade winds build back in Thursday and Friday.

DISCUSSION

A surface trough, and an associated shortwave upper level trough are found a few hundred miles northwest of Kauai this afternoon. The shortwave is helping to trigger thunderstorms between 200 and 400 miles northwest of Kauai. As the shortwave swings to the east over the next day, thunderstorm activity will move with it. The GFS and ECMWF global models are in general agreement with the timing of these feature, however the GFS is stronger with the divergence associated with the upper level feature. Thuderstorm probabilites with the National Blend of Models (NBM), and climatological based thunderstorm probabilities with both the GFS and ECMWF keep the bulk of the instability to the north of the islands, but enough of it sinking southward tonight that its possible for some thunderstorms to make it into the islands. While the highest chance for thunderstorms will be over/near Kauai and Oahu, upslope thunderstorms on the Big Island are also possible. Have made some adjustments to the thunderstorm areas based on the latest guidance from the NBM, GFS and ECMWF.

The surface trough is expected to move over the islands behind the shortwave feature, and will lead to lighter winds Tuesday and Wednesday. Under this lighter wind regime, we are likely to see some showers develop over interior areas of the islands during the afternoon. Trade winds are expected to return to the islands Thursday, with more typical trade winds showers riding in on the trades by the weekend. Little change has been made to the PoPs and associated forecast fields from Wednesday onwards.

AVIATION

A light easterly background trade wind flow remains over the state today. The light winds have allowed some land and sea breezes to develop over terrain sheltered areas. Expect these winds to remain light and veer more ESE starting tomorrow. Showers remain possible for windward and mountain areas, and over island interiors during afternoon hours through tomorrow. An upper level disturbance moving eastward through the state combined with the light winds at the surface could increase afternoon shower coverage statewide tomorrow. MVFR conditions should be expected within showers, and IFR conditions are possible within heavier isolated downpours. VFR will occur elsewhere.

No AIRMETs are currently in effect.

MARINE

A trough of low pressure passing northwest of the state will act to weaken and veer winds east southeasterly through mid-week across the local waters. Light flow should allow for afternoon sea breeze development near the shores. As conditions become increasingly unstable aloft, isolated thunderstorms will be possible tonight into Monday over the waters surrounding Kauai and Oahu. By Thursday, high pressure ridging builds back in north of the state with fresh northeasterly trade winds returning to the region lasting through next weekend.

Swell and surf will remain small along all shores through late Monday.

A small, medium period, northwest swell will move across the waters Monday night and continue through Wednesday. Another small, long period, west northwest swell will enter the waters Wednesday night and provide a slight bump up for surf along west and northwest facing shores. A moderate, medium to long period, northeast swell arrives on Tuesday and peaks on Wednesday, bringing moderate surf to north- facing shores, before slowly declining through the end of the week.

Surf will remain small along south facing shores through the period. Surf heights along east facing shores also remain small due to weaker trade winds until the arrival of the aforementioned moderate, medium to long period, northeast swell building into the Hawaii region from Tuesday through Thursday.

HFO WATCHES/WARNINGS/ADVISORIES

None.