Photo Credit: Tyler Rooke

West Side

Today: Scattered showers, mainly after 4pm. The rain could be heavy at times. Mostly sunny, with a high near 83. Southwest wind 3 to 8 mph. Chance of precipitation is 30%. New precipitation amounts of less than a tenth of an inch possible.

Tonight: Scattered showers, then frequent showers and possibly a thunderstorm after 1am. Some of the storms could produce heavy rainfall. Low around 70. Breezy, with a northwest wind 5 to 10 mph becoming north 16 to 21 mph in the evening. Winds could gust as high as 29 mph. Chance of precipitation is 80%. New rainfall amounts between a quarter and half of an inch possible.

M.L.King Day: Frequent showers and possibly a thunderstorm. Some of the storms could produce heavy rainfall. High near 76. Windy, with a north northeast wind 23 to 30 mph, with gusts as high as 44 mph. Chance of precipitation is 100%. New rainfall amounts between three quarters and one inch possible.

South Side

Today: Scattered showers, mainly after 3pm. The rain could be heavy at times. Partly sunny, with a high near 83. Calm wind becoming west southwest 5 to 7 mph in the afternoon. Chance of precipitation is 30%. New precipitation amounts of less than a tenth of an inch possible.

Tonight: Scattered showers, then showers likely and possibly a thunderstorm after midnight. Some of the storms could produce heavy rainfall. Mostly cloudy, with a low around 69. Breezy, with a west northwest wind 5 to 10 mph becoming north northeast 12 to 17 mph after midnight. Winds could gust as high as 24 mph. Chance of precipitation is 60%. New rainfall amounts between three quarters and one inch possible.

M.L.King Day: Frequent showers and possibly a thunderstorm. Some of the storms could produce heavy rainfall. High near 76. Breezy, with a north northeast wind 17 to 22 mph, with gusts as high as 31 mph. Chance of precipitation is 80%. New rainfall amounts between 1 and 2 inches possible.

North Shore

Today: Scattered showers, mainly after 3pm. The rain could be heavy at times. Mostly sunny, with a high near 84. Calm wind. Chance of precipitation is 30%.

Tonight: Scattered showers, then frequent showers and possibly a thunderstorm after midnight. Some of the storms could produce heavy rainfall. Low around 70. Breezy, with a light northwest wind becoming north northeast 13 to 18 mph after midnight. Winds could gust as high as 25 mph. Chance of precipitation is 90%. New rainfall amounts between a quarter and half of an inch possible.

M.L.King Day: Frequent showers and possibly a thunderstorm. Some of the storms could produce heavy rainfall. High near 74. Windy, with a north northeast wind 18 to 24 mph, with gusts as high as 34 mph. Chance of precipitation is 90%. New rainfall amounts between three quarters and one inch possible.

Central Maui

Today: Scattered showers, mainly after 3pm. The rain could be heavy at times. Partly sunny, with a high near 81. Southwest wind around 5 mph becoming calm in the morning. Chance of precipitation is 30%.

Tonight: Scattered showers, then frequent showers and possibly a thunderstorm after midnight. Some of the storms could produce heavy rainfall. Low around 68. Breezy, with a light and variable wind becoming north northeast 13 to 18 mph after midnight. Winds could gust as high as 25 mph. Chance of precipitation is 80%. New rainfall amounts between a quarter and half of an inch possible.

M.L.King Day: Frequent showers and possibly a thunderstorm. Some of the storms could produce heavy rainfall. High near 74. Windy, with a north northeast wind 18 to 25 mph, with gusts as high as 36 mph. Chance of precipitation is 90%. New rainfall amounts between three quarters and one inch possible.

Upcountry

Today: Isolated showers. The rain could be heavy at times. Partly sunny, with a high near 69. West southwest wind 6 to 8 mph. Chance of precipitation is 20%.

Tonight: Showers likely, with thunderstorms also possible after 1am. Some of the storms could produce heavy rainfall. Mostly cloudy, with a low around 57. West southwest wind 6 to 13 mph becoming north after midnight. Chance of precipitation is 70%. New rainfall amounts between 2 and 3 inches possible.

M.L.King Day: Frequent showers and possibly a thunderstorm. Some of the storms could produce heavy rainfall. High near 61. Breezy, with a north northeast wind 17 to 20 mph, with gusts as high as 28 mph. Chance of precipitation is 80%. New rainfall amounts in excess of 4 inches possible.

East Maui

Today: Scattered showers, mainly after 4pm. The rain could be heavy at times. Partly sunny, with a high near 79. Calm wind becoming south around 5 mph. Chance of precipitation is 30%. New precipitation amounts of less than a tenth of an inch possible.

Tonight: Scattered showers, then showers likely and possibly a thunderstorm after 1am. Some of the storms could produce heavy rainfall. Mostly cloudy, with a low around 66. Light and variable wind becoming north 9 to 14 mph after midnight. Winds could gust as high as 18 mph. Chance of precipitation is 60%. New rainfall amounts between a half and three quarters of an inch possible.

M.L.King Day: Frequent showers and possibly a thunderstorm. Some of the storms could produce heavy rainfall. High near 72. Breezy, with a north wind 16 to 18 mph, with gusts as high as 25 mph. Chance of precipitation is 90%. New rainfall amounts between 1 and 2 inches possible.

Lanai City

Today: Showers likely, mainly between noon and 4pm. The rain could be heavy at times. Partly sunny, with a high near 74. Light southwest wind becoming south southwest 5 to 10 mph in the morning. Chance of precipitation is 60%. New precipitation amounts between a tenth and quarter of an inch possible.

Tonight: Scattered showers, then showers likely and possibly a thunderstorm after 1am. Some of the storms could produce heavy rainfall. Mostly cloudy, with a low around 65. Breezy, with a south southwest wind 10 to 15 mph becoming north 16 to 21 mph in the evening. Winds could gust as high as 29 mph. Chance of precipitation is 70%. New rainfall amounts between 2 and 3 inches possible.

M.L.King Day: Frequent showers and possibly a thunderstorm. Some of the storms could produce heavy rainfall. High near 71. Windy, with a north northeast wind 26 to 30 mph, with gusts as high as 44 mph. Chance of precipitation is 100%. New rainfall amounts in excess of 4 inches possible.

Kaunakakai

Today: Isolated showers. The rain could be heavy at times. Mostly sunny, with a high near 79. Light west southwest wind becoming south southwest 5 to 10 mph in the morning. Chance of precipitation is 20%.

Tonight: Isolated showers, then frequent showers and possibly a thunderstorm after midnight. Some of the storms could produce heavy rainfall. Low around 70. Windy, with a west northwest wind 6 to 11 mph becoming north 18 to 23 mph in the evening. Winds could gust as high as 32 mph. Chance of precipitation is 90%. New rainfall amounts between a quarter and half of an inch possible.

M.L.King Day: Frequent showers and possibly a thunderstorm. Some of the storms could produce heavy rainfall. High near 74. Windy, with a north northeast wind 22 to 29 mph, with gusts as high as 41 mph. Chance of precipitation is 90%. New rainfall amounts between three quarters and one inch possible.

Detailed Forecast

Synopsis

A wet pattern is on tap for the islands during the first half of the coming week. This comes about as a front moves down the island chain this afternoon and tonight, then stalling around Maui and the Big Island between Monday and Tuesday. The front, or trough, is then slated to drift back to Oahu and Kauai between Tuesday afternoon and Wednesday. The front will have good support aloft from an upper level disturbance, resulting in heavy showers and isolated thunderstorms. On top of the rain, strong northeast trade winds will follow in behind the front. Moderate to strong easterly trade winds return to the region during the second half of the week with a bit of a wet flavor.

Discussion

A change in the weather is forthcoming. A cold front with strong trade winds and somewhat cooler temperatures behind it will move down the main Hawaiian Islands, starting with Kauai this afternoon, Oahu early this evening, Maui County during the rest of tonight/early Monday morning. The front is expected to come to a rest around Maui and the Alenuihaha Channel early Monday morning. Both the EC and GFS does not have the front crossing the Big Island. A couple of facts, the new ECMWF is about 3 hours slower than the GFS for the first 12 hours. The GFS is also drier once the front clears Kauai this evening. The current forecast package has the threat of thunderstorms ending over Kauai by midnight tonight in line with the GFS. Although rather windy, Kauai should be fine Monday through the first half of Tuesday.
Between Monday and Tuesday, Maui County and the Big Island will most likely get the brunt of the weather as upper level trough approaches Kauai slowly from the northwest, adding some extra energy to an already moist and unstable air mass. As noted in the evening discussion, there is a pool of cold air of minus 15 degrees centigrade at 18k feet. This temperature is a departure of about 8 degrees of the norm. Through Monday night, Oahu will be at the fringe of this weather. In the wake of the front, strong and gusty northeast trade winds will settle in, possibly strong enough to warrant a Wind Advisory for most, if not all of the smaller islands.
The upper trough stalls over Kauai, then evolves to a low southwest of Kauai on Tuesday. At the surface, the front weakens into a trough that moves west, back across Oahu and Kauai Tuesday afternoon and Tuesday night, giving these two islands another round of wet and unstable weather. By Wednesday afternoon, the surface trough will be west of Kauai. But the air mass over the islands remains unstable with plenty of low level moisture, leading to a showery trade wind regime that may persist through the second half of the work week. Thunderstorms are not part of the forecast during this stretch, but a second look will be taken in the coming days.
A Flash Flood Watch has been issued for Oahu and Maui County starting at 6 pm HST tonight. We left Kauai out of this watch since the front is expected to move through the island at about 15 to 20 mph before slowing down after reaching Oahu. This is a pretty fast speed, so a Flood Advisory will suffice to this situation. We also did not include the Big Island in the watch as well. Models are hinting at Monday night at the earliest for weather to turn sour there.
I have backed off on the cooler temps. Although it will be cooler, the strong trade winds, the clouds and showers will keep the overnight low from reaching the minimum dew point values, that is low 60’s on Oahu, and upper 50’s on Kauai. We are looking at overnight lows through Tuesday night in the 64 to 70 range.
Radar shows some hit and miss showers present on all but Oahu. An isolated shower or two is still present at Kawaihae Big Island. These showers can quick form, and quickly dissipate. This is the pattern for today before the front arrives.

Aviation

Latest observations indicate some expansion of shower coverage is underway within modestly convergent southwest flow that has emerged well ahead of an approaching cold front. The bulk of this activity is confined to Kauai and adjacent waters but will gradually shift east to include Oahu through the daytime hours when diurnal trends will act as a damper on both coverage and intensity. Prevailing VFR is therefore forecast for Oahu through Big Island for the balance of the daylight hours.
For Kauai, the probability of MVFR will begin increasing over Kauai by late this morning as low-level moisture increases. Issuance of AIRMET Sierra for tempo mountain obscuration is likely by this evening. The arrival of the cold front is expected for Kauai around 02z/4pm HST and around 06z/8pm HST for Oahu and will be marked by a greater coverage of showers, some locally heavy, followed by abrupt shift to gusty northerly winds.

Marine

A High Surf Warning for exposed N and W facing shores has been extended through Monday night, despite surf having already diminished below warning levels on Kauai, while also likely dropping below warning levels by tonight elsewhere. This is because another powerful NW swell building tonight into Monday will once again bring warning-level surf, with peak surf heights expected to be slightly lower than they were this most recent swell. Very strong N to NE winds will destroy any chances of quality surf along N facing shores with this new swell however.
A High Surf Advisory for exposed N and W facing shores of the Big Island was also extended through Monday night. Noteworthy that the PacIOOS nearshore Hilo buoy has recently reported combined seas near 14 feet with a dominant period near 16 seconds. If the full brunt of these swells were actually reaching the shoreline, we would expect surf heights to be well above warning-levels, but to the best of our knowledge, most of this swell energy is not realized at the coast with this particular swell direction. However, due to the potential for significant surges within Hilo and Kahului harbors today, a Marine Weather Statement is posted.
A Small Craft Advisory (SCA) remains in effect for all waters except Maalaea Bay through today, as the NW swell continues to produce seas greater than 10 feet. Seas will diminish slowly today, but will be ramping up again tonight and Monday, normally requiring an extension of the SCA. However, in this case a Gale Watch is posted for waters from the Alenuihaha Channel westward starting tonight as very strong N to NE winds are expected to develop after the passage of a cold front that is rapidly approaching from the NW, producing frequent gusts greater than gale force. We will be monitoring wind observations from NDBC buoys 51001/51101 NW of Kauai to determine the need to upgrade to a Gale Warning later today.
As the front approaches today, weakly-convergent, light to moderate S to SW winds will lead to the development of some pre- frontal showers through the day. The front will enter Kauai waters this afternoon, Oahu waters this evening, before stalling over the central portion of the island chain early Monday, bringing some heavy showers and the potential for thunderstorms. A developing sharp trough aloft on Monday and Tuesday will likely invigorate the lingering moisture through Wednesday as it drifts westward, keeping the potential for heavy showers and thunderstorms.

HFO Watches/Warnings/Advisories

Flash Flood Watch from this evening through Tuesday afternoon for Oahu, Molokai, Lanai, Kahoolawe, Maui.
High Surf Warning until 6 AM HST Tuesday for Niihau, Kauai Windward, Kauai Leeward, Waianae Coast, Oahu North Shore, Oahu Koolau, Olomana, Molokai, Maui Windward West, Maui Central Valley, Windward Haleakala.
High Surf Advisory until 6 AM HST Tuesday for Kona, Big Island North and East, Kohala.
Small Craft Advisory until 6 PM HST this evening for all Hawaiian waters except Maalaea Bay,
Gale Watch from this evening through late Monday night for Kauai Northwest Waters, Kauai Windward Waters, Kauai Leeward Waters, Kauai Channel, Oahu Windward Waters, Oahu Leeward Waters, Kaiwi Channel, Maui County Windward Waters, Maui County Leeward Waters, Maalaea Bay, Pailolo Channel, Alenuihaha Channel.

Data Courtesy of NOAA.gov