textproduct: Honolulu

This forecast discussion was created in the public domain by the National Weather Service. It can be found in its original form here.

SYNOPSIS

Moderate trade winds will continue through through today, with clouds and passing showers affecting mainly windward and mauka areas. A plume of tropical moisture will bring the potential for locally heavy rain and a few thunderstorms early next week, particularly over Maui and the Big Island. A cold front then appears to stall over or just north of the islands around the middle of the week while a new high builds to its north. This is expected to bring a period of wet trades and breezy to locally windy conditions to the islands for the latter half of the week.

DISCUSSION

Currently, a surface ridge centered about 1500 miles northeast of Honolulu is helping to produce light to moderate trades across the state this morning. As can be seen on radar, low level moisture filtering in on the trades is bringing isolated to scattered showers to mainly windward coasts and slopes, with a few spillovers to leeward areas at times. Moderate trades will continue through today as high pressure remains to the distant northeast of the islands and a weakening front to the northwest dissipates. This lighter tradewind pattern could allow a few showers to develop over leeward terrain with the potential for localized afternoon sea breezes.

The main forecast concern in the short-term is the potential for heavy rain and flooding, particularly for windward Big Island and Maui, starting tonight through Tuesday. A surface trough about 500 miles southeast of Hilo this morning will continue to advance towards the eastern end of the state through the day. CIMSS MIMIC Precipitable Water imagery shows a broad area of 2+ inches of precipitable water around this trough. As this trough moves by to the south and its plume of moisture moves west-northwestward across the island chain tonight through Tuesday, a weak shortwave trough aloft will also set up over the state and bring increased atmospheric instability. The low level trough, deep tropical moisture, and shortwave trough aloft look to all line up and bring the best chance of heavy rain even a slight chance of thunderstorms to windward Big Island and Maui Monday through Tuesday. As such, a Flood Watch is in effect for Maui and the Big Island from late tonight through Tuesday. However, forecast confidence in the potential for flooding is only moderate due to model discrepancies. Otherwise, the trades should strengthen a bit tonight and Monday as the surface trough slides by to the south of the state, then weaken again with the approach of the next cold front...which is the next main focus of the forecast.

Model guidance suggests that a cold front will approach and potentially pass through the state on Wednesday through Thursday. A new high building in north of the front Tuesday night is expected to lead to a resurgence in the trades, with breezy to locally windy conditions developing by late Wednesday and holding into next weekend. The ECMWF is the most aggressive with this front and brings it all the way through the Big Island by Thursday morning. In contrast, the GFS brings it to Kauai's doorstep on Thursday morning and then has it dissipate. With these two significantly different solutions, forecast confidence decreases Wednesday and beyond. For now, will continue to lean towards the GFS's more conservative solution of wet trades through the rest of the week into next weekend, but will likely need to make some adjustments as the details become more clear.

AVIATION

Moderate trade winds will continue today as high pressure remains to the far northeast of the state and a weakening front dissipates to the northwest. Latest satellite and radar imagery shows isolated to scattered light showers moving into windward slopes and coasts embedded within the trade winds. VFR conditions are prevailing across most of the state. Expect more of the same for the rest of today, with moderate trades and typical trade wind showers over windward and mountain areas.

Tonight into Monday, trades will strengthen a bit as a trough moves south of the state. This trough will bring an influx of moisture across the eastern end of the state beginning during the early morning hours of Monday, increasing showers and low cloud cover over at least portions of the Big Island and Maui. Some showers could be heavy at times, and there will be a slight chance of thunderstorms in the afternoon over the eastern islands and surrounding waters.

There are currently no AIRMETs in effect. However, mountain obscuration could become an issue late tonight or early Monday as the trough approaches closer to the Big Island.

MARINE

Moderate to fresh trades will briefly strengthen tonight into Monday as a surface trough approaches the state from the east potentially warranting a short-lived Small Craft Advisory for the windier waters of Maui County and the Big Island. This trough will also bring the threat of thunderstorms to the waters around the Big Island and Maui Monday night through Tuesday. Strong trades will then develop Wednesday into Wednesday night. Confidence is lower than normal regarding the potential for this episode of strong trades to persist into the weekend.

Current observations at offshore NDBC Buoy (51101) have begun to pick up the forerunners of the reinforcing swell through the early morning hours. This moderate long period NW (320-330 degrees) swell is expected to fill in during mid-morning with surf likely peaking this afternoon into the evening. For a brief period this morning, the surf may drop just below High Surf Advisory (HSA) criteria but will quickly fill back in during the day and has been extended for north and west facing shores through tonight. The reinforcing swell should steadily decline over the next few days. A series of moderate NW to NNW swells are possible during the second half of the week into next weekend.

Surf along east-facing shores will remain small and choppy with some wrap possible from the NW swells at select exposures. An upward trend is expected later in the week as the trades strengthen. Surf along south-facing shores will remain small with mainly background energy.

HFO WATCHES/WARNINGS/ADVISORIES

High Surf Advisory until 6 AM HST Monday 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.

Flood Watch from late tonight through Tuesday afternoon for Maui Windward West-Maui Leeward West-Haleakala Summit-Kona-Kohala-Big Island Interior-Maui Central Valley North-Maui Central Valley South-Windward Haleakala-Kipahulu-South Maui/Upcountry-South Haleakala-Big Island South-Big Island Southeast-Big Island East- Big Island North.


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