textproduct: Honolulu

SYNOPSIS

Surface high pressure strengthens to the northeast of the state today and will bring an increase in trade wind flow. In addition, expect an uptick in low cloud cover and shower activity through early Monday over windward portions of Maui, Molokai, and Oahu as an area of enhanced moisture moves in from the east. From late Monday through Friday, light easterly winds will return to the region. Afternoon coastal sea breezes should help to focus scattered clouds and isolated showers over mainly interior and mountainous locations. For the latter part of the week, deeper moisture moving in from the southeast could bring increased precipitation chances to the Big Island.

DISCUSSION

Latest RADAR and IR satellite imagery loop from this morning showed a band of enhanced low clouds and showers moving into Maui County from the east. For the most part, this activity has been light, though some brief pockets of heavier showers have been noted. A few stations on the West Maui mountains picked up just over half an inch of rainfall accumulation over the past 6 hours, but most windward sites reported less than a quarter inch. Through the remainder of this morning, surface high pressure will gradually build well northeast of the state and allow trade winds to strengthen into the moderate range. By this afternoon, these trades should help to drive the aforementioned area of enhanced low clouds and showers westward over Oahu. A model time-height cross-section over Oahu shows the capping marine boundary temperature inversion height deepening to around 8000 to 9000 feet. Though most activity will affect windward sides of the island, the moisture level should be deep enough to allow for some showers to briefly spill over the terrain and onto leeward sides. For the Big Island, expect leeward showers to once again develop over the South Kona slopes in this afternoon during peak diurnal heating. By tonight, showers should be confined to windward areas on most islands as moderate trades and residual moisture from the earlier band begin to exit the region.

Late Monday through Wednesday afternoon, the surface high weakens and moves north, once again allowing a return of light to locally moderate easterly background flow. Scattered afternoon low clouds and isolated showers will likely be limited to the interior of islands and over elevated terrain as diurnally driven sea breezes prevail. During the nights isolated showers may affect windward locations, but nothing of significance.

Late Wednesday through Friday, both the GFS and ECMWF have been consistent over the last several model runs in advecting an area of deeper moisture up from the southeast towards the Big Island. This moist layer, roughly 10000 to 12000 feet deep by current model estimates, would help enhance shower activity initially over the Big Island, then Maui county if it holds together.

AVIATION

Moderate and rather stable trade winds will persist today. A small area of low level moisture within the trades is producing MVFR conditions over windward slopes of Maui, Molokai, and Lanai, and AIRMET Sierra for mountain obscuration remains in effect on these islands. The moisture will likely spread to windward Oahu this morning and require an expansion of the AIRMET, while conditions improve on Maui. VFR conditions will prevail elsewhere, except for isolated MVFR across leeward terrain in the afternoon.

A sharp, though weak, upper level trough is producing moderate turbulence aloft over the Big Island, where AIRMET Tango is in place above FL300.

MARINE

A long-period south-southwest (200 degree) swell will continue to fill in today and peak tonight into Monday before gradually declining through the rest of the week. Buoy 51002, south of the islands, is observing around a 4 foot swell at 19 seconds this morning, with a peak observation exceeding 5 feet. As a result, surf heights along exposed south facing shores are forecast to hover near high- end advisory levels today, while west-facing shores see moderate surf. There is potential that as the swell reaches its peak tonight that surf could reach warning levels, along with advisory level surf for west facing shores. South shore surf will remain elevated through much of next week due to the slow decline of this large south swell and a series of continued overlapping southerly swells.

This swell will also coincide with the peak monthly tides which will add the potential for significant wave runup on top of minor coastal flooding potential during the monthly peak tides through the first half of next week. Once the peak swell passes, south shore surf will remain elevated through much of the week due to the slow decline of this large south swell and a series of continued overlapping southerly swells.

Very little swell energy is arriving along north facing shores, keeping surf tiny to flat. A small bump in surf is possible for north- facing shores on Tuesday with the arrival of a small northwest swell and a smaller north swell. Gentle to moderate east-northeasterly winds expected through next week will keep surf along east facing shores below seasonal average.

A Coastal Flood Statement remains in effect for minor coastal flooding of vulnerable low-lying coastal roadways, docks, boat ramps and other coastal infrastructure. King Tides combined with water levels that are running higher than predicted will lead to minor flooding potential around the daily peak tide, which will be during the late afternoon and early evening hours, through at least Tuesday. While minor coastal flooding is expected along all shorelines, the combination of the south-southwest swell, King Tides and the daily high tide will make low-lying coastal areas along south and west shores susceptible to more widespread coastal flooding through the first half of next week.

HFO WATCHES/WARNINGS/ADVISORIES

High Surf Advisory until 6 AM HST Tuesday for Niihau-Kauai Leeward-Waianae Coast-Kahoolawe-Maui Leeward West-Kona-Kohala- Kauai South-East Honolulu-Honolulu Metro-Ewa Plain-Molokai Southeast-Molokai Leeward South-Lanai Leeward-Lanai South-Maui Central Valley South-Kipahulu-South Maui/Upcountry-South Haleakala-Big Island South-Big Island Southeast.