West Kaua’i

Tonight: Mostly cloudy with isolated showers. Lows 68 to 73 near the shore to around 61 above 3000 feet. East winds up to 10 mph. Chance of rain 20 percent.

Monday: Partly sunny. Scattered showers in the morning, then isolated showers in the afternoon. Highs 82 to 88 near the shore to around 74 above 3000 feet. East winds up to 10 mph. Chance of rain 40 percent.

Monday Night: Mostly cloudy. Isolated showers after midnight. Lows 68 to 73 near the shore to around 60 above 3000 feet. Light winds. Chance of rain 20 percent.

South Kaua’i

Tonight: Mostly cloudy with scattered showers. Lows around 72. Northeast winds around 10 mph. Chance of rain 50 percent.

Monday: Partly sunny with scattered showers. Highs 79 to 86. East winds around 10 mph. Chance of rain 50 percent.

Monday Night: Mostly cloudy. Isolated showers in the evening, then scattered showers after midnight. Lows around 71. Northeast winds around 10 mph in the evening becoming light. Chance of rain 40 percent.

Kaua’i Mountains

Tonight: Mostly cloudy. Numerous showers in the evening, then occasional showers after midnight. Lows 63 to 69 in the valleys to around 59 above 4000 feet. East winds 10 to 15 mph. Chance of rain 90 percent.

Monday: Mostly cloudy. Numerous showers in the morning, then scattered showers in the afternoon. Highs 73 to 80 in the valleys to around 65 above 4000 feet. East winds 10 to 15 mph. Chance of rain 70 percent.

Monday Night: Mostly cloudy with scattered showers. Lows 62 to 68 in the valleys to around 57 above 4000 feet. East winds around 10 mph in the evening becoming light. Chance of rain 50 percent.

North Kaua’i

Tonight: Mostly cloudy with scattered showers. Lows 64 to 72. East winds around 10 mph. Chance of rain 50 percent.

Monday: Partly sunny with scattered showers. Highs 75 to 84. East winds up to 10 mph. Chance of rain 50 percent.

Monday Night: Mostly cloudy with scattered showers. Lows 64 to 72. East winds up to 10 mph in the evening becoming light. Chance of rain 50 percent.

East Kaua’i

Tonight: Mostly cloudy with numerous showers. Lows 64 to 75. East winds around 10 mph. Chance of rain 70 percent.

Monday: Partly sunny. Numerous showers in the morning, then scattered showers in the afternoon. Highs 72 to 84. East winds around 10 mph. Chance of rain 70 percent.

Monday Night: Mostly cloudy with scattered showers. Lows 63 to 75. East winds up to 10 mph in the evening becoming light. Chance of rain 50 percent.

Detailed Forecast

Synopsis

Moderate easterly trade winds will focus showers across windward slopes through Monday, while a developing upper level low just southwest of the islands gradually pulls a swath of high clouds overhead. The upper level low will strengthen mid to late Tuesday into Wednesday and bring an increased threat of heavy showers and isolated thunderstorms on increasing trade winds, with flood potential increasing over the eastern end of the island chain. Chances of heavy rainfall will diminish late Wednesday and Thursday, though breezy and somewhat wet trade wind weather is expected. Another round of unsettled weather is possible next weekend.

Discussion

Moderate easterly trade winds will continue to focus showers across windward slopes through Monday. A sharp upper level trough bisecting the state this afternoon will develop into an upper level low centered just southwest of the island chain tonight. The upper low will pull a shield of high clouds over the area from the south and also erode the mid level ridge overhead, leading to the elimination of the inversion and a slight destabilization of the atmosphere. This could produce some enhancement of the showers in the trade wind flow, mainly within a diffuse band of moisture moving across the western half of the island chain tonight.
Trade winds will briefly diminish Monday night and early Tuesday as the atmosphere further destabilizes. Trades will ease as a cold front advances to within 200 miles northwest of Kauai and pushes the weakened subtropical ridge generating the trades to far northeast of Hawaii. The upper level trough driving the front will begin to absorb the upper level low just southwest of the state. The resulting negatively titled upper level trough will produce difluence aloft within a narrow jet stream over the islands. The greatest forcing associated with this jet stream will likely be just south of the Big Island, where an area of deep tropical moisture will be drawn northward. At this time, GFS and ECMWF ensemble guidance suggest that the deepest moisture and greatest threat of heavy rainfall will remain south of the islands through at least midday, with chances for rainfall along southeast and windward Big Island increasing during the afternoon. Meanwhile, closer to the colder temperatures aloft along the upper level trough axis, an isolated thunderstorm or heavy shower cannot be ruled out across other islands. In addition, a shield of thick high clouds will cover the entire island chain.
The highest chances for heavy rainfall favors late Tuesday afternoon or early evening into Wednesday, though differences in model guidance are producing some uncertainty. The wetter GFS is maintaining a more persistent upper level trough with somewhat strong difluence aloft, while the latest run of the ECMWF depicts a weaker upper level trough being absorbed more quickly into the large upper trough to the west. Both models suggest that the front will dissipate just north of the islands and that trade winds will ramp up as strong surface high pressure passes far to the north. Under this breezy, moist, and unstable trade wind flow, windward areas will be the focus for heavy rainfall, while leeward communities experience briefly heavy and potentially frequent passing showers. The greatest threat for flooding continues to point toward the eastern end of the island chain, mainly on Big Island, where difluence aloft may produce precipitable water values in excess of 2 inches. With trade winds blowing and thick high clouds limiting convection, the greatest flood potential appears to be over windward and southeast Big Island, where the 12Z GFS Ensemble 90th percentile solution shows 24 hour rainfall totals of around 5 inches. Given the model differences regarding potential rainfall totals, confidence is not high enough to issue a Flood Watch at this time. Along with the flood threat, the high summits of the Big Island could experience periods of heavy snow and strong winds, but the big question will be the snow level, which will be hovering close to summit elevation. We will hold off on any winter weather headlines for now due to uncertainty.
The threat for heavy rainfall will decrease during the day on Wednesday as high clouds diminish, but breezy and wet conditions are expected into Thursday. The heavy rainfall threat will drop as the upper level trough near Hawaii gets absorbed by the deeper upper level trough to the west, greatly diminishing difluence aloft. The strong surface high passing to the north will maintain breezy trade winds, and even though precipitable water will be decreasing, the GFS and ECMWF keep abundant moisture around the islands, suggesting a rather wet pattern.
A brief period of stable and drier trade wind conditions is expected on Friday, followed by another round of potentially active weather next weekend.

Aviation

Gentle to breezy easterly trade winds will hold through tonight and veer slightly east southeast Monday while weakening over the western half of the state. VFR conditions prevail across the state with a few spots briefly reaching MVFR conditions due to passing CLD and SHRA as an area of moisture continues to move through Oahu and Kauai this afternoon into evening hours. High CLD will build in from south around Big Island and Maui tonight into Monday. MVFR conditions are then possible across all windward and mauka areas overnight and during the early morning hours along windward and mauka areas as another band of moisture trapped with the trade flow moves east to west. Weaker winds Monday suggest that local land/sea breezes will become more dominant with possible CLD and SHRA building along the more sheltered leeward coasts and interior during the afternoon hours before clearing overnight with the land breeze, especially for Kauai and Oahu. Overcast conditions will be possible for the eastern half of the state Monday as high clouds continue to build overhead.
No AIRMETs are currently in effect but could be necessary overnight and early morning Monday as the next band of moisture moves through.

Marine

Trade wind speeds will continue to decrease and veer from a more E to SE direction from Monday to Tuesday as the high pressure ridge north of the Hawaiian Islands drifts E in response to an approaching cold front far NW of the state and a surface trough forming over the islands. Additionally, the cold front will advance into the offshore waters by Tuesday evening, bringing near-gale-force winds in its wake before the front diminishes over the NW coastal waters on Wednesday. Expect Small Craft Advisories to be issued for most of the Hawaiian coastal waters from at least Wednesday into Friday due to a combination of strengthening winds and rising seas. Isolated thunderstorms are possible over the Hawaiian waters through much of this week.
Expect multiple overlapping NW swells this week. This will keep surf heights boosted along exposed N and W facing shores. The challenge with this pattern will be in tracking the multiple swell energies from similar directions and similar periods. Wave height model guidance has trended towards combining these different swells for the Monday through Friday time period. Continued to decrease the NW swell heights from model output by a few feet with this forecast package to account for this discrepancy.
A small medium period NW (310-320 degree) swell peaked this afternoon and is expected to slowly decline into Monday. The next moderate, medium to long period NNW (320-340 degree) swell will then arrive late Tuesday, peaking Wednesday near High Surf Advisory thresholds along exposed N and W facing shores before gradually declining through the end of the week. Another overlapping small, medium period NNE (010-020 degree) swell arrives from Thursday into Friday. Then another small, long period NW (320-330 degree) swell arrives in Hawaiian waters by Friday, then holds through the weekend.
Surf along E facing shores will decline into Monday due to the weakening of the local and upstream trade winds, though there may be a brief uptick late tonight into Monday morning as a small medium period NE (040 degree) swell arrives. As the high pressure ridge moves east later this week, the combination of a long easterly fetch of 15 to 20 kt winds upstream of the islands from Tuesday through Thursday, along with strengthening local winds along the cold front, will produce moderate and choppy surf along E shores. For S shores, periods of tiny background south swell energy will linger.

Fire weather

Moderate trade winds and seasonably higher humidity will maintain conditions well short of critical fire weather thresholds through Monday as the inversion gradually weakens. Wet conditions are expected Tuesday into Thursday, with heavy rainfall possible over recent burn areas on the Big Island.

HFO Watches/Warnings/Advisories

None.

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