Photo Credit: Brendan Stephen

West Kaua’i

Today: Mostly sunny with isolated showers. Highs around 88 near the shore to around 77 above 3000 feet. East winds up to 15 mph shifting to the west in the afternoon. Chance of rain 20 percent.

Tonight: Partly cloudy with isolated showers. Lows around 73 near the shore to around 63 above 3000 feet. Light winds. Chance of rain 20 percent.

Tuesday: Partly sunny. Scattered showers in the morning, then numerous showers in the afternoon. Highs 85 to 91 near the shore to around 76 above 3000 feet. Light winds becoming south up to 10 mph in the afternoon. Chance of rain 70 percent.

South Kaua’i

Today: Breezy. Partly sunny with scattered showers. Highs 81 to 89. Northeast winds 10 to 20 mph. Chance of rain 50 percent.

Tonight: Mostly cloudy with scattered showers. Lows around 74. Northeast winds 10 to 15 mph. Chance of rain 50 percent.

Tuesday: Mostly cloudy with numerous showers. Highs 82 to 90. Northeast winds 10 to 15 mph. Chance of rain 70 percent.

Kaua’i Mountains

Today: Mostly cloudy. Numerous showers in the morning, then scattered showers in the afternoon. Highs 74 to 84 in the valleys to around 66 above 4000 feet. East winds up to 15 mph. Chance of rain 70 percent.

Tonight: Mostly cloudy. Scattered showers in the evening, then numerous showers after midnight. Lows 65 to 72 in the valleys to around 61 above 4000 feet. Light winds. Chance of rain 70 percent.

Tuesday: Occasional showers. Highs 74 to 84 in the valleys to around 67 above 4000 feet. Northeast winds up to 15 mph. Chance of rain near 100 percent.

North Kaua’i

Today: Partly sunny with scattered showers. Highs 77 to 88. East winds up to 15 mph. Chance of rain 50 percent.

Tonight: Mostly cloudy. Scattered showers in the evening, then numerous showers after midnight. Lows 67 to 74. Southeast winds up to 10 mph in the evening becoming light. Chance of rain 70 percent.

Tuesday: Cloudy with occasional showers. Highs 76 to 88. East winds up to 15 mph shifting to the northeast in the afternoon. Chance of rain 90 percent.

East Kaua’i

Today: Mostly cloudy. Numerous showers in the morning, then scattered showers in the afternoon. Highs 74 to 86. Northeast winds 10 to 15 mph decreasing to up to 15 mph in the afternoon. Chance of rain 70 percent.

Tonight: Mostly cloudy with numerous showers. Lows 67 to 77. Northeast winds up to 10 mph in the evening becoming light. Chance of rain 70 percent.

Tuesday: Occasional showers. Highs 74 to 87. Northeast winds up to 15 mph increasing to 10 to 15 mph in the afternoon. Chance of rain near 100 percent.

Detailed Forecast

Synopsis

A plume of tropical moisture will bring the potential for locally heavy rain and a few thunderstorms through Tuesday, particularly over Maui and the Big Island. A cold front will approach the islands on Wednesday then stall and dissipate as a new high builds to its north. This will bring a period of wet trades and breezy to locally windy conditions to the islands later this week.

Discussion

Early morning satellite imagery shows a weak surface trough approximately 200 miles southeast of the Big Island. As this trough moves west over the next couple of days it will be the primary forecast concern due to the potential for heavy rain and localized flooding, particularly over windward Big Island and Maui. CIMSS MIMIC precipitable water imagery continues to show a broad area of 2+ inches of precipitable water around this trough. The morning sounding from Hilo measured around 1.77 inches of precipitable water which is close to the 90th percentile for this time of the year.
Guidance continues to show the trough and its plume of moisture moving west-northwestward across the islands through Tuesday. This surface trough looks to be supported by a passing upper level trough from the west which will help to increase instability across the state. The combination of the troughs at the surface and aloft along with enhanced tropical moisture will bring the potential for heavy rain and a few thunderstorms over mainly windward Big Island and Maui. A Flood Watch remains in effect for these islands through Tuesday. Otherwise, expect trade winds to strengthen a bit today as the surface trough increases the local pressure gradient, with clouds and passing showers affecting mainly windward and mauka areas of the smaller islands west of Maui.
Trade winds look to weaken slightly on Tuesday as the surface trough dissipates and a cold front approaches from the northwest. Oahu and Kauai may see a brief period of heavy rain on Tuesday before the trough dissipates, but confidence is not high enough to expand the Flood Watch to these islands.
Guidance continues to diverge beyond Wednesday and generally falls into two camps. Some model guidance (mainly the ECMWF) shows the cold front moving across the state sometime Wednesday through Thursday. However, other guidance suggests that the front will briefly stall over or just north of the islands during this time period before dissipating. Will continue to favor the non-ECMWF solution of the front reaching the state by Wednesday then stalling and dissipating over or just north of the islands by Thursday as a new high builds in behind the front. Based on the current forecast expect breezy to locally windy conditions to return later this week and into the weekend, but will likely need to make some adjustments as model guidance hopefully starts to converge on a single solution.

Aviation

Moderate to locally breezy trades for today. Low cigs, SHRA and some isol TSTMs are possible across the eastern side of the state through tomorrow. With an increase in SHRA coverage and intensity expect MVFR and isol IFR conds. On the western side of the state expect the typical windward and mauka SHRA with brief MVFR conds possible with those SHRA.
AIRMET Sierra is in effect for mtn obsc for windward Big Island. This may be expanded to include other islands when/if conds warrant.

Marine

A weak surface trough passing across the southern offshore waters today will tighten the regional pressure gradient enough to subtly strengthen trades for those waters surrounding Big Island and Maui County. A Small Craft Advisory is in effect for these waters through early Tuesday. The passage of this trough this afternoon through early Tuesday will introduce an eastern island nearshore and southeast-south offshore waters isolated thunderstorm threat. Fresh to locally strong trades will transition west-to- east across the local waters Wednesday as a Central Pacific cold front and its associated backing surface high north of the state move east. The pressure gradient over and upstream of the state may be tight enough in the front's wake or along the southern periphery of the surface high to support fresh trades into the weekend.
The ongoing medium period, moderate size northwest (330 degree) swell that peaked yesterday afternoon will gradually subside through the day. Near double head high surf will persist along better focused reefs (in relation to this 330 degree swell) to maintain advisory surf heights past noon. The swell and subsequent surf is expected to make a significant fall from this evening through Tuesday. To account for this slower decline, the High Surf Advisory for the north and west- facing shores of the smaller islands, except for only the north on Maui, will remain in effect through this afternoon. A mid to late week frontal passage north of the islands will be the source of the next medium to long period, moderate size northwest swell scheduled to arrive sometime Wednesday night into Thursday morning. Surf along east-facing shores will remain relatively small and choppy with select exposures still experiencing some of this northwest swell wrap. As trades strengthen during the second half of the week, east-facing shore chop will likely be trending upward.

HFO Watches/Warnings/Advisories

High Surf Advisory until 6 PM HST this evening 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 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.
Small Craft Advisory until 6 AM HST Tuesday for Maui County Windward Waters, Maalaea Bay, Pailolo Channel, Alenuihaha Channel, Big Island Windward Waters, Big Island Leeward Waters, Big Island Southeast Waters.

Data Courtesy of NOAA.gov