textproduct: Honolulu

SYNOPSIS

A weak high pressure ridge will hover over the Hawaiian Islands through the weekend. A combination of large scale stability under the ridge and southeasterly wind flow will keep the smaller islands in the leeward rain shadow of the Big Island mountains; leading to minimal shower activity statewide. A weak cold front approaches the islands from the northwest and stalls out near Kauai by next week Tuesday, increasing shower trends over the western half of the state.

DISCUSSION

Fairly stable and shallow cumulus clouds appear over the Hawaii Region in this morning GOES IR satellite imagery. A hot dark spot shows up over the Kilauea Volcano due to the large heat emissions from the recent volcanic eruption cycle. Farther to the northwest of the islands a cold front passes through the Central Pacific basin, keeping a weakened ridge firmly in place over the island chain. Recent eruptions of Kilauea will pull up VOG (Volcanic smOG) today, appearing as hazy skies for all islands. University of Hawaii VOG model shows these hazy skies lingering through at least Thursday.

The weak high pressure ridge over the islands will produce strong and stable subsidence, downward vertical atmospheric motions, across the entire state through the weekend. Subsidence driven temperature inversion heights were measured this morning at 2 AM HST (12Z) from our upper air weather balloon soundings at around 5,000 feet at both Lihue and Hilo. At these low levels the temperature inversion will tend to cap vertical cloud growth, significantly reducing clouds and limiting showers. The position of this ridge over the islands will also keep the state in a light southeast wind flow pattern. These light large scale winds will allow for local scale daytime onshore sea breezes to develop over each island.

The larger scale southeasterly winds will also produce a rain shadow effect, as clouds approaching the state will flow around the Big Island and clouds will drift more parallel to the smaller island mountain ranges. This means less clouds lifting orographically over island mountains, resulting in even lower shower activity. A few stray showers are still possible, however any showers that develop will likely be brief and amounts will be light. Once exception to this rule will be the eastern slopes of the Big Island and eastern slopes of Maui where scattered shower activity remains in the forecast, mainly during the overnight to early morning hours.

The weather pattern changes early next week, as a cold frontal system approaches the state from the northwest. The forward motion of this frontal cloud band will stall and diminish near Kauai from next week Tuesday to Wednesday. Southerly winds ahead of this frontal band will pull up very humid air over the western islands, shower activity will increase over Niihau, Kauai and Oahu during this time period. Wet weather trends will favor the southern and western slopes of Kauai and Oahu, as the southerly winds will lift the tropical moisture over the island mountains. Stay tuned as this pattern remains in the extended outlook forecast period and these island by island weather impacts will likely evolve over time.

AVIATION

Light to moderate E-ESE trade winds will continue into Thursday. This will allow periods of daytime sea breezes to occur over most terrain sheltered leeward areas. MVFR conditions may briefly develop within passing showers over exposed windward and mauka regions, and along sheltered leeward and interior areas during daytime seabreezes. Otherwise, VFR will prevail.

No AIRMETs in effect, and none are expected.

MARINE

A surface ridge, parked just north of the state, will allow for mainly gentle to moderate east to east southeast flow across the waters through the end of this week. However, winds are forecast to ramp up for the typical windy areas around Maui and the Big Island late Thursday into Friday and a Small Craft Advisory may be needed. Waters located in the of the lee islands will be more sheltered, and thus lighter flow with a nocturnally driven land/sea breeze pattern close to the coasts. Similar conditions prevail through the weekend as a frontal system stalls north of the islands.

A series of rather impressive long period northwesterly swells will affect the waters through the forecast period. Forerunners from the first, medium to large, northwesterly (320-340 deg) swell showed up at Waimea Bay buoy early this morning. Swell energy will ramp up through the day, peak tonight, then slowly ease during the day Thursday. Thus, a High Surf Advisory will remain in effect through Friday morning for most exposed north and west facing shores of the smaller islands. Combined seas during the peak of the swell will approach 10 ft and a Small Craft Advisory (SCA) for rough seas may warranted for the waters northwest of Kauai and Oahu.

Over the next several days, models rapidly deepen a low in the north Pacific, roughly 1,500 to 2,000 nm northwest of Kauai. A significant fetch of gale to storm force winds associated with this low will be pointed down the great circle route towards the island chain and produce the next big swell event. Guidance depicts this long period, northwest swell first reaching the waters by mid- day Saturday, then ramping up quickly thereafter. The peak of the event currently appears to be Saturday night into Sunday with the potential for the first of the season extra-large northwest (310-320 deg) swell. If current trends come to fruition, a High Surf Warning for exposed north and west facing shores would be needed, in addition to a SCA for rough seas.

Surf along east facing shores will remain small through the weekend due to the lighter winds. Expect manly tiny surf along south facing shores with minor background energy.

FIRE WEATHER

Wind speeds and humidity levels remain below critical fire weather thresholds through the weekend. Only minimal shower activity lingers in the forecast. Temperature inversion heights near the Big Island and Maui will range from 5,000 to 6,000 feet today.

HFO WATCHES/WARNINGS/ADVISORIES

High Surf Advisory until 4 AM HST Friday for north and west facing shores of Niihau, Kauai, Oahu, Molokai, and north facing shores of Maui.