textproduct: Honolulu

SYNOPSIS

A stable subtropical ridge over the region will keep cloud cover and shower activity to a minimum until this weekend. Southeasterly wind flow will allow sea breezes to expand over all islands. On Saturday and Sunday a shallow cold front and upper level disturbance will move into the islands and combine forces, as high pressure builds in from the northwest. Expect periods of enhanced trade wind showers across the Hawaiian Islands this weekend, favoring the overnight to early morning hours along windward and mountain slopes.

DISCUSSION

Local radar imagery shows southeasterly winds are blowing across the Hawaii region. In this stable environment little clouds or showers show up in either radar or satellite imagery this evening. Infra-red satellite imagery clearly shows a small thermal hot spot on the Big Island over Halemaumau Crater associated with the ongoing Kilauea volcanic eruption. Sulfur dioxide (SO2) emissions from the Kilauea eruption are producing a plume of VOG (Volcanic fOG or smOG) over large areas of the Big Island, drifting as far north as Waimea and Kohala Ranch and as far east as Hilo, based on air quality surface observations this evening. This VOG plume also extends north and west away from the Big Island, riding the southeasterly wind flow into the smaller islands in Maui County, Oahu, Kauai and Niihau. VOG impacts will persist over the smaller islands through Friday, and will mainly affect people with breathing difficulties and the VOG plume. Returning trade winds this weekend should blow much of this VOG plume towards the south, away from the most populated areas.

The surface ridge north of the state remains in a weakened state with light to moderate large scale southeast winds blowing across the region in response to the fading surface ridge. This wind direction shift places the smaller islands from Kauai to Maui in the leeward wind and rain shadow of the Big Island, allowing local scale land and sea breeze winds to strengthen and expand in coverage across all islands through Friday. Upper level stability from the subsidence (downward moving air) under the descending limb of the subtropical ridge and the rain shadowing effects for islands in the lee of the Big Island will limit cloud cover and rainfall activity until this weekend.

By Saturday morning, a shallow cold front slowly drifts into Kauai and Oahu. The forward motion on this front will stall out at some point, perhaps as far east as Oahu, and the shallow frontal cloud band will start to break apart. High pressure will build in rapidly across the dissipating front with moderate to locally breezy trade winds blowing across the region. An upper level trough will also be riding in above this shallow front, colder air aloft and wind divergence ahead of this upper trough will help to lift the low level boundary layer and over- ride the stabilizing effects of the large scale subtropical ridge. This means low level cloud bands from the dissipating front and upstream moisture will produce periods of enhanced showers moving into the windward and mountain slopes of each island likely lasting into the first half of next week.

AVIATION

Weakening trades will allow development of a land/sea breeze regime for the next few days. Low cigs and -SHRA may develop across mauka areas each afternoon and evening, then decrease overnight. While SHRA should be light ISOL MVFR conds may occur within them. That said, VFR should generally prevail. Due to the continuing eruption of Kilauea, leeward portions of the Big Island may see MVFR vsbys due to vog.

No AIRMETs are in effect.

MARINE

A return of hazardous marine and coastal conditions is expected across exposed Hawaiian waters as another long-period west- northwest (290-315 degrees) swell arrives late tonight and builds down the island chain Thursday. Swell observations at the offshore buoys northwest of Kauai are quickly rising this evening, with the peak energy centered around 19 seconds. This new swell is expected to drive surf heights to warning levels from early Thursday through Friday, prompting an issuance of a High Surf Warning for exposed north and west-facing shores of the smaller islands. Either an advisory or warning will be needed for west- facing shores of the Big Island in the early Thursday morning package.

Following a series of extra-large northwest swells last week, this event is expected to exacerbate ongoing erosion issues, including the potential for periodic overwash onto low-lying properties and roadways, particularly during high tide cycles. In addition to the coastal impacts associated with the surf, a Small Craft Advisory will be issued to account for seas exceeding 10 ft over exposed waters this evening. This swell will slowly lower over the weekend and into early next week. An upward trend is possible Monday through midweek, as a couple of smaller northwest swells arrive.

Surf along south and east facing shores will remain small through early next week. The exception could be for south facing shores exposed to more westerly swells, which could see a slight rise through the day Thursday as the aforementioned west-northwest builds down the island chain.

Trades will steadily weaken and shift out of the east-southeast direction overnight and into Thursday as the surface ridge weakens in response to a cold front passing to the north over the next couple of days. This pattern will translate to a return of a land and sea breeze regime for most coasts Thursday through Friday. Breezy trades are forecast to return over the weekend as the ridge strengthens to the north in the wake of the front stalling near/north of the western end of the state.

FIRE WEATHER

No critical fire weather conditions are expected for the next seven days. Mostly dry weather remains in the forecast across the state with limited shower activity as the dry spell continues through Friday. Shower trends increase starting Friday night with enhanced trade wind showers lasting into early next week.

HFO WATCHES/WARNINGS/ADVISORIES

High Surf Warning from midnight tonight to 6 AM HST Friday for Niihau-Kauai Leeward-Waianae Coast-Oahu North Shore-Maui Windward West-Kauai North-Molokai Windward-Molokai North-Molokai West-Maui Central Valley North-Windward Haleakala.

Small Craft Advisory from midnight tonight to 6 AM HST Friday for Kauai Northwest Waters-Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel-Oahu Windward Waters-Oahu Leeward Waters- Kaiwi Channel-Maui County Windward Waters.