textproduct: Honolulu

SYNOPSIS

Showers will continue to decrease and breezy easterly trade winds diminish tonight and Wednesday as low pressure west of Hawaii fills and high pressure to the northeast weakens. A deep ridge settling over the islands on Thursday will lead to light winds and stable conditions with light interior showers. A dissipating front will bring an increase in showers to Kauai and Oahu Friday into the weekend, followed by another front early next week.

DISCUSSION

A gradual drying trend is underway as breezy easterly trade winds continue to focus showers over windward and southeast slopes. An upper level low is filling several hundred miles southwest of Hawaii as it drifts away, limiting instability locally. Its associated surface trough, combined with strong high pressure far northeast of the area, continues to produce breezy easterly trade winds over the islands, but the broad zone of convergence that is producing the heaviest showers has shifted west of Hawaii. As a result, rain rates have dropped considerably over the state, and the wettest windward slopes on Kauai and Big Island have measured generally between a third to an inch of rain during the past six hours. Most leeward areas have been dry, but as the high clouds thinned, interior Big Island has experience a few heavy showers over the interior and South Kona slopes due to daytime heating. Expect all areas to continue to experience as gradual drying trend overnight as trades begin to decline.

Trades will continue to ease on Wednesday and will shift out of slightly south of due east as the high to the northeast drifts away. Drier and increasingly stable mid level air pushing in from the east will lead to a decline in rainfall over windward and southeast slopes. Sea breezes will be more extensive across leeward areas, but aside from the Big Island, showers will be isolated.

A ridge at the surface and aloft will move over the island chain on Thursday. Light and variable winds will give way to sea breezes in the afternoon, triggering interior clouds and a few showers in the afternoon. The atmosphere will be stable, keeping rainfall accumulation minimal.

A shallow and weakening front will advance to Kauai on Friday, then stall and slowly dissipate over the western end of the island chain during the weekend. As shower chances increase for Kauai on Friday and possibly as far south as Oahu on Saturday, the rest of the state will experience rather dry conditions dominated by light land and sea breezes. Chances for showers will decrease on Sunday as the remnant moisture band dissipates and lift northward ahead of another advancing front. This potentially stronger front may move down the island chain early next week, though uncertainty regarding this feature remains high at this time.

AVIATION

A slow drying trend is underway as breezy easterly trade winds continue to focus moisture over windward and southeast slopes. Stability will gradually increase as an upper level low moves away from the state tonight and Wednesday, and easterly trade winds will slowly decrease as the high to the northeast moves eastward. MVFR conditions will be focused over windward and southeast slopes, especially on the Big Island and Kauai where AIRMET Sierra for mountain obscuration is in place. Expect the AIRMET to be dropped by Wednesday morning as drier air moving in from the east leads to a decrease in rainfall. Expect spotty heavy showers over leeward and interior Big Island to diminish rapidly around sundown, while VFR prevails over other leeward areas. AIRMET Tango remains in effect for moderate turb over and west of terrain due to the gusty trades. This AIRMET will likely be taken down as winds ease overnight.

MARINE

Strong east to southeast winds will continue to ease tonight as a trough west of Hawaii weakens and tracks further away from the state. Moderate to locally fresh east-southeast winds will develop Wednesday as a ridge builds north of the region. A front approaching the state from the northwest will further weaken the winds to light to moderate on Thursday as they shift southerly. The frontal boundary will weaken as it enters the coastal waters Friday, then dissipates near the central islands by Saturday. Light to gentle winds expected on Saturday as another stronger front approaches the state from the northwest Sunday, veering winds more southerly and gradually becoming more moderate before pushing south over the island chain next week.

A Small Craft Advisory (SCA) for both winds and seas remains in effect for all coastal waters, except the Oahu and Maui County Leeward Waters and Maalaea Bay. Wind and seas will then fall below advisory criteria for all waters except the waters surrounding the Big Island early Wednesday morning. Wind and sea then fall below advisory criteria for all waters by Wednesday evening.

A moderate, medium period, north-northeast (040-060 degree) swell will continue to veer easterly and decline tonight. A small, long period, northwest (310 degree) swell will build late tonight, peak Wednesday, then slowly decline through Friday. Forerunners for the next significant large, long period north-northwest swell (330 degree) is expected to arrive Friday night and peak Saturday. This swell will likely produce warning level surf along exposed north and west facing shores. Another, large to extra large, north-northwest swell may fill in early next week with surf heights peaking well into warning levels.

A High Surf Advisory remains in effect for exposed east facing shores through tonight due to the combination of the northeast swell mentioned above and rough easterly swell generated by the local and upstream easterly winds. Surf should drop below advisory levels Wednesday.

HFO WATCHES/WARNINGS/ADVISORIES

High Surf Advisory until 6 AM HST Wednesday for Olomana-Kauai East-Kauai South-East Honolulu-Koolau Windward-Molokai Windward- Molokai Southeast-Windward Haleakala-Kipahulu-South Haleakala- Big Island Southeast-Big Island East-Big Island North.

Small Craft Advisory until 6 AM HST Wednesday for Kauai Northwest Waters-Kauai Windward Waters-Kauai Leeward Waters- Kauai Channel-Oahu Windward Waters-Kaiwi Channel-Maui County Windward Waters-Pailolo Channel.

Small Craft Advisory until 6 PM HST Wednesday for Alenuihaha Channel-Big Island Windward Waters-Big Island Leeward Waters-Big Island Southeast Waters.