West Kaua’i

Today: Sunny with isolated showers in the morning, then partly sunny with scattered showers in the afternoon. Highs 86 to 91 near the shore to around 77 above 3000 feet. East winds up to 15 mph. Chance of rain 50 percent.

Tonight: Mostly cloudy with scattered showers. Lows around 73 near the shore to around 62 above 3000 feet. East winds up to 15 mph increasing to 10 to 15 mph after midnight. Chance of rain 50 percent.

Thursday: Partly sunny. Scattered showers in the morning, then numerous showers and isolated thunderstorms in the afternoon. Highs 85 to 90 near the shore to around 76 above 3000 feet. East winds up to 15 mph. Chance of rain 70 percent.

South Kaua’i

Today: Breezy. Partly sunny with scattered showers. Highs 82 to 90. Northeast winds 10 to 20 mph. Chance of rain 50 percent.

Tonight: Mostly cloudy with numerous showers. Lows around 73. Northeast winds 10 to 15 mph. Chance of rain 70 percent.

Thursday: Mostly cloudy. Numerous showers in the morning, then numerous showers and isolated thunderstorms in the afternoon. Highs 81 to 89. East winds 10 to 15 mph. Chance of rain 70 percent.

Kaua’i Mountains

Today: Breezy. Mostly cloudy with numerous showers. Highs 76 to 85 in the valleys to around 67 above 4000 feet. East winds up to 15 mph increasing to 10 to 20 mph in the afternoon. Chance of rain 70 percent.

Tonight: Frequent showers. Lows 64 to 71 in the valleys to around 59 above 4000 feet. East winds 10 to 15 mph decreasing to up to 15 mph after midnight. Chance of rain near 100 percent.

Thursday: Frequent showers in the morning, then frequent showers and isolated thunderstorms in the afternoon. Highs 75 to 83 in the valleys to around 66 above 4000 feet. East winds up to 15 mph increasing to 10 to 15 mph in the afternoon. Chance of rain near 100 percent.

North Kaua’i

Today: Partly sunny with scattered showers. Highs 78 to 88. East winds up to 15 mph. Chance of rain 50 percent.

Tonight: Mostly cloudy with numerous showers. Lows 65 to 73. Southeast winds 10 to 15 mph. Chance of rain 70 percent.

Thursday: Mostly cloudy. Numerous showers in the morning, then numerous showers and isolated thunderstorms in the afternoon. Highs 77 to 88. East winds up to 15 mph. Chance of rain 70 percent.

East Kaua’i

Today: Mostly cloudy with numerous showers. Highs 75 to 87. East winds 10 to 15 mph. Chance of rain 70 percent.

Tonight: Frequent showers. Lows 65 to 75. Northeast winds 10 to 15 mph decreasing to up to 10 mph after midnight. Chance of rain near 100 percent.

Thursday: Mostly cloudy. Frequent showers in the morning, then numerous showers and isolated thunderstorms in the afternoon. Highs 74 to 86. East winds up to 15 mph increasing to 10 to 15 mph in the afternoon. Chance of rain 90 percent.

Detailed Forecast

Synopsis

A wet trade wind weather pattern developing over the Hawaiian Islands will produce increasing rainfall, heavy at times, along with a few thunderstorms into Saturday. All island areas will see enhanced shower activity with the highest rainfall amounts favoring the windward mountain areas. Showers will decrease a bit this weekend as the upper low drifts westward away from the state. However, another weak upper low moves in near Kauai and Oahu by Monday triggering another round of wet trade wind weather potentially lasting into next week Wednesday.

Discussion

This mornings satellite imagery, water vapor channel, clearly shows two upper lows approaching the islands from the north. Divergent winds are shown along the southern and eastern flank of this low pressure system with thunderstorms forming around the cold core center of the low. In the lower levels, using the infrared satellite channel, we see long bands of unstable clouds likely associated with remnants of an old East Pacific cold front riding into the islands on the trade winds. These cloud bands produce enhanced showers caused by the instability surrounding the upper low and the convergent lifting of the trade winds forcing these unstable clouds orographically up the windward mountain slopes.
For today we expect to see easterly trade winds into the moderate to locally breezy range with increasing clouds and showers as the upper low moves in from the north and low level cloud bands riding in from the east begin to converge over the island chain. This mornings subsidence temperature inversion heights, as measured by upper air balloon soundings at Lihue and Hilo at 2 AM HST (12Z), have risen to the 7,000 to 8,000 foot level. This means scattered to numerous showers are likely near island mountains.
An extended period of wet weather will continue to affect all islands this week with periods of numerous showers falling across the state into Saturday. Some of these showers will be locally heavy at times, and thunderstorms are likely over the western islands of Niihau, Kauai and Oahu. The Big Island may also see a few thunderstorms on Thursday. Storm total rainfall estimates for this entire two to three day event may exceed 2 inches in terrain favored windward locations. Our confidence in which islands will see the heaviest showers is currently not high enough to trigger a flood watch at this time. However, a flood watch may be needed once we see how this pattern evolves in order to narrow down potential flooding threats to specific islands in the chain.
Showers will linger over the islands on Saturday, with a brief decrease in shower activity on Sunday as the upper low drifts westward moving away from the island chain. However, another weak upper low moves right in from the north on Monday, setting up in an almost identical location near Kauai and Oahu. Cold air surrounding this low and divergence aloft will bring yet another round of enhanced shower activity to the islands next week, from Monday into next Wednesday. Stay Tuned.

Aviation

Moderate to locally breezy easterly trade winds are expected with increasing clouds and showers over the next 12 to 24 hours. An upper level low will move in from the north and set up over the western half of the state as a bands of unstable low clouds drift into the region from the east. The convergence of the upper low with these unstable convergent low cloud bands will increase shower activity into Saturday.
Brief periods of MVFR conditions are possible in passing showers mainly over windward and mountain areas favoring the overnight and early morning hours. Isolated IFR conditions are possible in heavier showers.
No AIRMETs currently in effect. AIRMET Sierra will likely be issued later today for mountain obscurations. AIRMET Tango may be issued later today for upper level turbulence associated with stronger winds surrounding the approaching upper low.

Marine

Moderate to fresh trade winds are expected today as a weak surface troughing over the western end of the state advances west and dissipates. A Small Craft Advisory (SCA) for winds around Maui County and the Big Island remains in effect until 6 AM HST Thursday. An approaching upper low brings a chance for thunderstorms to area waters from later tonight into the weekend.
A moderate to long period NW (310-320) swell that peaked yesterday will slowly decline through the rest of the week into the weekend.
Small, medium period S swell continues through the week with minor longer period pulses arriving this morning and Saturday. E shores remain small through the near term, but will see increasing short period action as trades strengthen mid-week.

Fire weather

Wind speeds and afternoon minimum humidity levels will likely remain below critical fire weather thresholds this week. Unstable conditions will produce increasing wet weather trends across the state into Saturday. Locally heavy rain and thunderstorms are possible during this time period. Temperature inversion heights near Maui and the Big Island will range from the 7,000 to 8,000 feet elevation range today.

HFO Watches/Warnings/Advisories

Small Craft Advisory until 6 AM HST Thursday for Maalaea Bay, Pailolo Channel, Alenuihaha Channel, Big Island Leeward Waters, Big Island Southeast Waters.

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