textproduct: Honolulu

SYNOPSIS

Moderate to breezy stable trade winds will continue today through tomorrow afternoon with light showers expected over mostly windward and mountain areas. A plume of tropical moisture is then forecast to increase rain chances over the Big Island and to a lesser extent, the rest of the islands late Saturday through Monday. Drier trade winds should then return from Tuesday through the end of next week.

DISCUSSION

High pressure to our north will drive moderate to breezy trade winds through much of next week. An upper level ridge is currently moving overhead today along with drier air, allowing showers to stay light and isolated mostly to windward and mountain areas. This dry summertime pattern is expected to persist through tomorrow afternoon with CIMSS MIMIC precipitable water values estimated at around 1.2-1.3 inches. The 12Z balloon launch from last night showed precipitable water values around 1.3 inches as well.

Current satellite imagery depicts a weak tropical low with an associated tropical wave approximately 560 nm east- southeast of Hilo. Model guidance projects this low to weaken into a trough as it moves west- northwest. This trough will have a decent moisture profile associated with it when it arrives. Current CIMSS MIMIC precipitable water shows values of around 1.8 to 2 inches associated with it and it is expected to reach the Big Island by Saturday evening. Rainfall chances and humidity will increase for the Big Island and Maui County, then to a lesser extend, Oahu and Kauai Sunday into Monday. The most recent GFS model time- height cross section depicts the boundary layer inversion weakening, as conditions become more unstable, and moisture deepening up to at least 15,000 feet. Thus, from Saturday afternoon through Monday, embedded heavy rainfall will be possible for the Big Island and Maui County, especially for windward locations.

Though it is still too early to fine tune the forecast storm total amounts, typically wetter windward locations on Maui and the Big Island could pick up a couple inches of rainfall accumulation. Sufficient instability may exist for a rumble of thunder Sunday afternoon over the south or southwest slopes of Mauna Loa on the Big Island, but confidence is too low at this time to add to the current forecast package.

Typical summertime stable trade wind weather will return Tuesday through the end of the week with scattered showers mainly windward and mauka areas during the nights and early mornings.

Looking into the long range forecast for late next week, both google deepmind and global dynamic models are showing potential Tropical Cyclone development in the central Pacific. Details are far too early at this time but warrants further monitoring.

AVIATION

Trade winds are forecast to increase through tomorrow as a surface high pressure system to our northeast strengthens and moves slightly closer to Hawaii. A moist band is moving westward across Maui County this afternoon, and is expected to become more diffuse (showers will decrease) as it heads toward Oahu this evening. Leeward turbulence will increase tonight into Saturday, so an AIRMET Tango may become necessary. Long range forecast calls for continued trades through the next seven days.

MARINE

Moderate trades will continue to strengthen to fresh to locally strong speeds by this evening as surface troughing to the northeast weakens and a trough approaching from the southeast enhances the local pressure gradient. A Small Craft Advisory is in effect for locally strong trades across the typical windy channels and waters around Maui County and the Big Island. Fresh to locally strong trades will persist through at least the weekend, with the strongest winds over the southern portion of Hawaiian coastal waters as the surface trough nears.

Background medium- and long-period energy will continue to produce small surf along south-facing shores through tonight. A series of larger, medium- to long-period south swells will cause surf to trend upward along south- and west-facing shores from this weekend into early next week. A moderate, long-period south swell will peak near High Surf Advisory thresholds by midweek.

Along east-facing shores, trade wind swell will remain slightly below seasonal averages today before building this weekend in response to strengthening trades. Surf along north-facing shores will remain flat to tiny through the period, though a small, medium-period north swell could bring a slight bump on Sunday.

Due to a combination of peak monthly high tides and water levels running higher than predicted, minor coastal flooding is possible around the daily peak tide during the afternoon hours late this weekend into early next week along vulnerable shoreline areas.

HFO WATCHES/WARNINGS/ADVISORIES

Small Craft Advisory until 6 PM HST Saturday for Maalaea Bay- Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big Island Southeast Waters.