textproduct: Honolulu

SYNOPSIS

Deepening low pressure to the west of the islands will draw up tropical moisture from the south through the week. This will lead to periods of heavy rainfall, thunderstorms, and an increased flooding risk. Strong and gusty south to southeasterly winds are also expected Thursday and Friday. Unsettled conditions could continue through the weekend and next week as surface moisture and instability lingers.

DISCUSSION

Current satellite imagery this morning shows an area of low pressure beginning to develop west of the islands. A potent upper level trough to the northwest is currently moving southeast and will enhance the surface trough to the west through the next few days. Heavy showers and thunderstorms can be seen mostly around east Maui and southeast Big Island, as well as the waters south of Maui.

South to southeast winds will strengthen today, pulling deep tropical moisture northward over the islands with precipitable water values around 2". An upper level trough will also move closer to the state and enhance large- scale lift. As a result, widespread heavy rain is expected to gradually increase through the day statewide. A Flood Watch is in place for all islands from this morning through Friday afternoon. A Winter Weather Advisory is also in effect this morning for a combination of freezing rain and snow on the summits of the Big Island above 12,500 feet, lasting through Friday morning.

There has been increasing focus on the need for a Wind Advisory or a High Wind Watch beginning Wednesday night as southerly winds begin to ramp up. These winds will be especially strong strong in and around heavier showers and thunderstorms, and in the vicinity of mountain ranges where downsloping gusts over northern and windward slopes can occur. Severe wind gusts (60+ mph) look increasingly likely with latest high resolution model runs. Late tonight through Thursday morning could see advisory level winds over Kauai and Oahu (30-39 mph) and then possibly high wind warning levels (40+ mph) by Thursday afternoon through Friday morning. Future shifts will monitor and refine the situation as the time period approaches.

Thursday night through Friday night. Intense upper-level forcing develops. The combination of right entrance dynamics and curved flow will maximize upper-level divergence over the western end of the state by Thursday evening. A mature jet-front system takes shape west of Kauai as this forcing rapidly organizes and ramps up and showers and thunderstorms along the frontal zone begin to intensify. The heavy rain then surges eastward Thursday night bringing the heaviest rainfall to Kauai late Thurs night into early Friday, Oahu during the first half of Friday, and Maui County and the Big Island during the second half of Friday.

High-end advisory winds are likely on the Big Island Summits and possibly Haleakala Summit on Maui Thursday night through Friday. Snow potential on the Big Island Summits will be limited by marginal temperatures.

Saturday onward. The resident band of tropical moisture will likely linger over the islands for the foreseeable future, although a short break in rain is anticipated by Saturday as drier air briefly moves in. More periods of rain are possible through the remainder of the weekend into next week as more upper level disturbances track over the state. This may yield periods of localized heavy rain, but that remains to be seen.

AVIATION

Latest radar imagery continues to show rain showers affecting mainly Maui and the Big Island early this morning. While dry conditions prevail across the islands west of these two, some low clouds have been observed thanks to recent rainfall and light winds. Expect mixed conditions for most of the period as a result. Weather models suggest another round of widespread showers is expected to arrive around late afternoon / early evening across the islands.

AIRMET Sierra is in effect for IFR conditions for Lanai, which has reported ceilings as low as a couple hundred feet most of the overnight period. This is expected to improve during the day Wednesday, however. Mountain obscuration continues to impact Molokai, Maui, and the southeast portions of the Big Island as well, with some improvement expected after sunrise, before deteriorating once again once this next round of rain arrives.

MARINE

South to southeast winds are expected to steadily increase late today into Thursday, as a low pressure system develops just west of the state. A Small Craft Advisory (SCA) is now in effect for the waters around the Big Island and the SCA will expand in coverage tonight as the winds continue to strengthen. Showers, some locally heavy, will be possible through Friday as a result of the southerly winds bringing tropical moisture to the north over the islands. As the low pressure system further deepens Thursday into Friday, we will likely see a greater coverage of thunderstorms and winds are expected to increase to gale force across many coastal waters. A Gale Watch has been issued starting Thursday for the waters around Kauai and Oahu. Depending on how the low pressure system evolves, Gale force winds could be possible across all Hawaiian Waters around Thursday night. Very rough boating conditions are expected late Thursday into Friday as seas quickly rise in response to the strong to gale force southerly winds.

The current south-southwest swell continued to fill in overnight and latest near-shore buoys are at around 4 feet 17 to 18 seconds this morning. We should see this swell continue to fill in a bit more throughout the day today and slowly drop tonight through the rest of the week. A High Surf Advisory has is in effect for all south facing shores of the Hawaiian Islands through tonight.

Along north facing shores, a new moderate long-period northwest (310-320 degree) swell continues to fill in this morning and will steadily rise throughout the day today. This swell should peak just below advisory thresholds later today. A smaller short-period northwest swell is expected over the weekend into early next week.

As the long-period south swell declines on Thursday, strong to gale force southerly winds are expected to develop on Thursday and strengthen even further Thursday night into Friday. This increase of southerly winds will bring a rapid increase of short- period energy Thursday into Friday and will once again bring surf heights above the advisory thresholds for south facing shores. Large disorganized surf is expected along many south facing shores Thursday into Friday.

Surf along east facing shores could see an increase later this week due to the strengthening southeasterly winds especially for southeast exposures.

HFO WATCHES/WARNINGS/ADVISORIES

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

Flood Watch through Friday afternoon for all Hawaii islands-

Winter Weather Advisory until 6 AM HST Friday for Big Island Summits.

Small Craft Advisory from 6 PM this evening to 6 AM HST Thursday for Kauai Northwest Waters-Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel-Oahu Leeward Waters.

Gale Watch from Thursday morning through late Thursday night for Kauai Northwest Waters-Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel-Oahu Windward Waters-Oahu Leeward Waters.

Small Craft Advisory until 6 AM HST Thursday for Big Island Windward Waters-Big Island Leeward Waters-Big Island Southeast Waters.