textproduct: Honolulu

SYNOPSIS

A front will stall and dissipate north of the Big Island this evening, maintaining shower chances for Maui County through tonight. Limited rainfall is expected Friday. Moderate southwesterly winds ahead of a stronger front will increase shower potential along leeward areas Saturday. The highest potential for heavy rainfall and thunderstorms will develop over the western end of the state Sunday through Tuesday.

DISCUSSION

Afternoon satellite and radar imagery shows that the band of low level clouds and embedded light to moderate showers, associated with a weakening front moving over the state today, is currently located across primarily the islands of Maui County. Rainfall amounts today have been modest at best, however, with the most rain falling over portions of Oahu and Molokai (generally receiving up to a little more than half of an inch in the heaviest spots over the past six hours). Meanwhile, moderate northerly winds have filled in behind the front, with increasingly stable and dry post-frontal conditions observed for Kauai today. Expect a drying trend for Oahu tonight, though shower chances will continue into tonight for Maui County as the front stalls and dissipates north of the Big Island. Additionally, dewpoint temperatures will dip in the upper 50s tonight around Kauai and eventually Oahu as drier more stable air filters in behind the front. Winds will ease to light speeds tonight as the front dissipates, allowing land breezes to develop. For the Big Island summits, strong southerly winds are mixing down to summit level. These should gradually decrease throughout the night as well, however. In the meantime, a Wind Advisory has been issued for borderline advisory-level winds on the Big Island summits through tonight.

On Friday, lingering moisture leftover from the front, along with afternoon seabreezes, could trigger a few showers along leeward and interior areas, especially around Maui County. Moderate southerly winds will develop across the state on Saturday as a stronger front approaches the islands from the northwest. By Saturday night, a pre-frontal convergence band is expected to set up over the western half of the state. This will increase chances for showers and thunderstorms, particularly near Kauai and its surrounding waters for Saturday night.

Little change has been made to the forecast for Sunday through Tuesday with this afternoon's forecast package, as it remains on track compared to the latest model guidance. An upper level trough will become negatively tilted and pinch off into a low supporting strong cyclogenesis at the surface northwest of the state. A band of heavy rain and potential thunderstorms associated with the surface low will settle somewhere over the western end of the state Sunday through Monday night, with the heaviest rainfall potential Sunday night into Monday. Upon the departure of supporting upper dynamics on Tuesday, the surface low will move far north of the state and weaken, leaving an orphaned frontal moisture band over the area through a least mid-week. Expect lingering showers over the the islands through at least mid-week. Southerly winds will prevail throughout this time.

AVIATION

Light showers and some low clouds will continue to slowly progress southeastward along a weak front which will stall and weaken over Maui County. For the Big Island, dry and mostly clear conditions will persist into Friday.

AIRMET Sierra for mountain obscuration is now focused on Molokai and Maui where the highest likelihood of lower clouds and showers will persist tonight into Friday morning.

Finally, with north winds behind the front at the surface and west southwest flow aloft, will be watching for continued low level wind shear of up to 20kt PHOG. This shear is expected to diminish by tonight.

MARINE

A weak cold front is slowly progressing southeastward across the central waters around Maui this afternoon, where it is expected to stall this evening and dissipate by Friday. Gentle to moderate northerlies are filtering in behind the front with light and variable winds downstream. Another front will approach and possibly enter the northwestern coastal waters around Kauai late Saturday into Sunday, where it will then stall. Light southeasterlies will veer more southwesterly and strengthen on Saturday as the front approaches, becoming more westerly behind the front. Numerous showers with locally heavy rain and isolated thunderstorms are possible in association with this frontal boundary. The proximity of this feature will also maintain a southerly flow across most of the region through the rest of the forecast period.

A moderate size, medium period northwest swell peaked this morning and will continue to gradually decline this evening through Friday. Therefore, the High Surf Advisory has been cancelled. Another moderate, long period northwest (310-320 degree) swell that will be generated from a developing gale low just northwest of the offshore waters late Friday will move into the region Sunday and Monday. In addition, a mix of small to moderate size, medium period north (350-010 degree) swells will arrive this weekend, with a stronger pulse expected late Sunday into Monday, keeping surf heights elevated into early next week. Due to the mix of these swells, surf heights may approach advisory levels from late Sunday into early next week.

East shore surf will remain small through the forecast period due to the absence of trade winds, though locations exposed to north swells could see an uptick early next week. Expect minimal background surf for south facing shores, with the minor south- southeast pulse that arrived earlier today gradually fading into the weekend. Southwestern exposures on primarily Kauai and Oahu may receive small to moderate choppy windswell this weekend from strengthening southwest winds ahead of an approaching front.

FIRE WEATHER

Conditions will remain below critical fire thresholds into the weekend. Increasing shower chances associated with an incoming front, combined with light to moderate winds, will prevent us from reaching critical fire weather thresholds over the next several days.

HFO WATCHES/WARNINGS/ADVISORIES

Wind Advisory until 6 AM HST Friday for Big Island Summits.