Photo Credit: Braden Jarvis

West Kaua’i

Today: Sunny in the morning, then mostly cloudy with isolated showers in the afternoon. Highs around 84 near the shore to around 72 above 3000 feet. Northeast winds up to 15 mph. Chance of rain 20 percent.

Tonight: Cloudy with scattered showers. Lows around 68 near the shore to around 62 above 3000 feet. Northeast winds up to 15 mph. Chance of rain 50 percent.

Monday: Cloudy with scattered showers. Highs around 82 near the shore to around 70 above 3000 feet. East winds up to 15 mph. Chance of rain 50 percent.

South Kaua’i

Today: Mostly cloudy. Scattered showers in the morning, then isolated showers in the afternoon. Highs around 81. Northeast winds 10 to 15 mph. Chance of rain 40 percent.

Tonight: Cloudy with numerous showers. Lows around 68. Northeast winds 10 to 15 mph. Chance of rain 60 percent.

Monday: Cloudy with numerous showers. Highs around 79. Northeast winds 10 to 15 mph shifting to the east in the afternoon. Chance of rain 70 percent.

Kaua’i Mountains

Today: Mostly cloudy. Scattered showers in the morning, then isolated showers in the afternoon. Highs 71 to 78 in the valleys to around 62 above 4000 feet. East winds up to 15 mph. Chance of rain 50 percent.

Tonight: Cloudy with occasional showers. Lows 61 to 66 in the valleys to around 57 above 4000 feet. East winds up to 15 mph. Chance of rain 90 percent.

Monday: Cloudy with occasional showers. Highs 69 to 76 in the valleys to around 60 above 4000 feet. East winds up to 15 mph. Chance of rain 90 percent.

North Kaua’i

Today: Partly sunny. Scattered showers in the morning, then isolated showers in the afternoon. Highs 74 to 83. East winds up to 15 mph. Chance of rain 50 percent.

Tonight: Cloudy with numerous showers. Lows 63 to 69. East winds up to 10 mph. Chance of rain 70 percent.

Monday: Cloudy with numerous showers. Highs 72 to 81. Southeast winds up to 10 mph. Chance of rain 70 percent.

East Kaua’i

Today: Mostly cloudy. Scattered showers in the morning, then isolated showers in the afternoon. Highs 74 to 82. Northeast winds 10 to 15 mph. Chance of rain 50 percent.

Tonight: Cloudy with occasional showers. Lows 59 to 70. Northeast winds around 10 mph. Chance of rain 90 percent.

Monday: Cloudy. Occasional showers early in the morning, then numerous showers in the late morning and afternoon. Highs 72 to 80. East winds up to 10 mph. Chance of rain 90 percent.

Detailed Forecast

Synopsis

Moderate to locally breezy trade wind weather will continue through Monday with brief periods of clouds and showers favoring windward and mountain areas. A slight increase in shower trends will develop from Sunday afternoon to Monday. A developing kona low will then bring potential for heavy rain and thunderstorms to islands next week beginning with Kauai and Oahu on Tuesday and then spread statewide from late Tuesday into next weekend. Significant snowfall remains in the forecast for the highest summits of the Big Island.

Discussion

Water vapor band satellite imagery shows a kona low developing west of Kauai with a band of colder cloud tops and thunderstorms forming just east of the low center. This system will slowly drift east towards Kauai over the next few days. High level cirrus clouds, made up of ice crystals, are moving out ahead of this low pressure system and over the Hawaiian Islands today and tomorrow. Hawaiian radar imagery shows isolated to scattered showers drifting into the eastern slopes of each island on the trade winds. In the low levels, a wave in the easterlies shows up just north to northeast of the Big Island. This low level trough will drift into the state enhancing windward and mountain showers later tonight through Monday morning.
Medium range forecast guidance continues to show this kona low drifting slowly eastward towards Kauai through Tuesday. Shower activity increases over the western half of the state for Kauai by late Monday through Tuesday and for Oahu by Tuesday morning. Heavy rain and thunderstorms remain in the forecast for this first round with rainfall impacts mainly over Kauai County and Oahu.
Another kona low drops in from the northwest on Tuesday and sets up just west of Kauai by Wednesday. This second low is stronger than the first one with deeper southerly winds that will draw up deep tropical moisture into the eastern half of the state. This unstable tropical moisture will spread periods of showers,heavy at times, and thunderstorms across all islands from Wednesday through Saturday.
These cold core lows can produce significant amounts of rainfall in a very short time period. Flooding threats are elevated statewide as these two kona lows take aim on the islands. The heaviest rain areas typical occur under the strongest wind divergence aloft and we will have to wait until the forecast time period gets closer to identify where these areas might set up. Periods of wet weather with locally heavy rain and thunderstorms should be expected for all islands next week. Pay close attention the the weather forecast updates over the next few days as island by island impacts will evolve as the model guidance improves over time.
A Hydrologic Outlook remains in effect for heavy rain and possible flash flooding from Tuesday into the weekend. Over the next 24 to 36 hours we will consider the possibility of issuing a Flash Flood Watch, at least for the western islands for the Monday to Tuesday time period.

Aviation

Moderate trade winds will persist during the next 24 hours across the islands. Relatively dry conditions are expected today, with trade winds showers increasing tonight across the smaller islands as a band of deeper moisture moves through. Showers will favor windward slopes and coasts, with a few showers reaching leeward areas tonight. Some showers will likely develop over the leeward Big Island slopes this afternoon as well, then gradually fizzle out tonight. Predominantly VFR conditions are expected today, with windward areas seeing an increase in MVFR cigs/vsbys in showers tonight.
No AIRMETs are currently in effect and none are expected today. AIRMET Sierra may be needed for mountain obscuration in windward areas tonight.

Marine

High pressure rebuilding north of the state will produce moderate to locally strong trade winds through Monday. Thereafter, a developing kona low just to the west of the state will lead to moderate southerly winds and increasing widespread rainfall with the potential for thunderstorms through the week.
Recent NDBC buoy observations (51101) far northwest of the state showed the large northwest swell declining through the overnight hours. As a result, surf will slowly decline through the day as a High Surf Advisory (HSA) continues for effected north and west facing shores through the day. Surf heights should dip below advisory levels by Monday. A moderate medium period northwest swell is expected sometime in the middle of the week. The Small Craft Advisory (SCA) remains in place for most waters through today for a combination of the large northwest swell and increasing easterly trade winds. The SCA will scale back to just the typically windier waters around Maui County and the Big Island by this evening as the swell declines.
Surf along south facing shores will remain small with mainly background long-period south-southwest swell expected through the weekend.
Surf along east facing shores will receive a small boost through Monday with the brief return of locally fresh trades, but generally remain small through the upcoming week.

HFO Watches/Warnings/Advisories

High Surf Advisory until 6 PM HST this evening for north and west facing shores of Niihau, Kauai, Oahu, Molokai, and north facing shores of Maui.
Small Craft Advisory until 6 PM HST this evening for most Hawaiian Coastal Waters.
Small Craft Advisory from 6 PM this evening to 6 AM HST Monday for windier waters around Maui and the Big Island.
Small Craft Advisory until 6 AM HST Monday for Alenuihaha Channel.

Data Courtesy of NOAA.gov