textproduct: Honolulu

SYNOPSIS

A weakening high pressure ridge north of the Hawaiian Islands will keep trade wind speeds on the decreasing trend lasting into Saturday, then ridge and wind speeds strengthen from Saturday afternoon into early next week. Passing showers are possibly over windward and mountain areas during this time period, mainly during the overnight to early morning hours. The next best chance for statewide rainfall may develop from Monday into Tuesday of next week, as the cloud remnants from an old cold front drift across the islands on the trade winds.

DISCUSSION

A combination of satellite and radar imagery this morning paints a wetter than normal picture for the windward mountain slopes of all Hawaiian Islands this morning. Two parts to this passing shower story are upper air soundings from 2 AM HST (12Z) this morning measuring trade wind temperature inversion heights in the 6,000 to 7,000 foot range and deep easterly trade winds above the 30,000 foot level. Cloud heights remain elevated allowing passing showers to continue mainly during the overnight to early morning hours, favoring the windward mountain areas.

In the large scale pressure pattern forecast model guidance continues to show the high pressure system north of the islands weakening into early Saturday. Trade wind speeds will trend slightly lower during this time period. A stronger high pressure system builds in north of the state from late Saturday into the first half of next week with more breezy trade winds blowing across the region. The difference in wind speeds will be subtle.

Weather wise a band of clouds associated with an old dissipated front will drift over the islands early next week from Monday night into Tuesday, increasing clouds and shower coverage. Long range model solutions are showing a weak upper low dropping into position just west of Kauai from Tuesday to Wednesday. Southeasterly winds aloft are pulling up remnants of this old frontal cloud band back over the islands along with added upper level instability. If this pattern develops as predicted by the global models, then an extended wet weather pattern may linger into next week Thursday, especially over the windward mountain slopes of Maui and the Big Island. Both the American (GFS) model and the European (ECMWF) model are showing a flavor of this extended wetter pattern with the GFS being the wetter of the two solutions. This pattern would likely produce another round of beneficial light to moderate rainfall rates.

AVIATION

Stable clouds and -shra will continue across many windward areas this morning with a few showers spilling over to leeward areas. Trade winds have eased slightly but will remain breezy today. Light showers will ride in with the breezy trades and will produce brief MVFR ceilings at times mainly across windward and mountain areas. Otherwise VFR conditions will prevail.

AIRMET Tango is in effect for low level moderate turbulence downwind of terrain. AIRMET Sierra is in effect for mountain obscuration over windward areas of Oahu Molokai and Maui. Mountain obscurations should generally decrease during the day today.

MARINE

A high pressure system north of Hawaii will slowly weaken today decreasing wind speeds across the marine areas. The Small Craft Advisory (SCA) has been trimmed back slightly and no longer includes the Northwest Kauai, Leeward Kauai, and Leeward Oahu zones as winds have begun to diminish in response to weakening of the high pressure and seas have fallen below 10 ft with a fading north- northwest swell. These areas will still see conditions near, but not meeting, the small craft criteria this morning. Otherwise, the SCA is in effect for the remaining coastal waters. By tonight, the winds diminish and seas subside enough to drop all but the typical windy waters near Maui County and the Big Island from the SCA. Late this weekend, high pressure restrengthening northeast of the islands may bring strong winds back to all waters Sunday into Monday.

A moderate, north-northwest swell will continue to decline today into the weekend. Moderate surf for north and west-facing shores will decrease to small levels by tonight. A new small, north-northwest swell will support continued small surf into Saturday before the arrival of a more significant north swell. This moderate, long-period north swell arriving Saturday night may push surf back to advisory levels on Sunday for north and west-facing shores. Surf to at least moderate levels will last into Monday before the swell really begins to fade on Tuesday.

Surf along east-facing shores will remain rough and choppy due to the strong trade winds today. East-facing shores may decrease slightly tonight and Saturday as trades ease early this weekend, but will increase again Sunday into early next week as trades restrengthen. South shore surf will stay tiny to small for the next few days.

FIRE WEATHER

Wind speeds will trend lower today as the high pressure ridge north of the state weakens. Critical fire weather conditions will be limited by wind speeds for most areas and by higher minimum humidity levels for the higher elevation zones that reach wind thresholds. Wind speeds will increase into the moderate to breezy levels by late Saturday with these slightly higher winds lasting into early next week. Temperature inversion heights near Maui and the Big Island today will range from 6,500 to 7,500 feet elevation.

HFO WATCHES/WARNINGS/ADVISORIES

Small Craft Advisory until 6 PM HST this evening for most Hawaiian coastal waters and channels.

Small Craft Advisory until 6 PM HST Saturday for windier waters and channels around Maui and the Big Island.