Marine Weather and Tide Forecast for Koloa, HI

DISCUSSION

No major changes since previous forecast. Satellite and radar imagery shows a band of light, isolated clouds and showers moving in from the east embedded in the breezy trades with showers mainly focused along windward and mauka areas. The afternoon sounding show relatively low inversion heights around 4 to 5 kft with precipitable water vapor slightly below normal. This setup will maintain rather dry, stable conditions tonight into Tuesday. A front roughly 600 miles northwest of Kauai will approach the state Tuesday. Upper level shortwave energy will also dig south during this time, supporting cyclogenesis along the attendant cold front north of the islands late Tuesday. The nascent low will quickly strengthen and occlude as it lifts northward away from the area.
Associated pressure falls will eradicate the local surface pressure gradient on Tuesday leading to expanding sea breezes during the afternoon followed by stable land breezes overnight.
Nothing more than a few interior clouds and isolated showers expected Tuesday afternoon.

As the low lifts north, shortwave energy aloft will pivot through the forecast area driving the cold front eastward through Kauai and Oahu on Wednesday and into the eastern end of the state on Wednesday night. This scenario won't be dissimilar to the most recent frontal passage which brought a brief round of rain followed by dewpoints falling into the upper 50s. Forcing aloft will be stronger this time, so slightly deeper moisture along the front may allow for a better coverage of showers. Mid-level lapse rates are adequate for an isolated thunderstorm or two along the front, but hardly impressive. Combined with low density CAPE profiles, it appears the chance for thunder will be fleeting over the islands themselves, the greater probabilities residing over the waters north and east of the area.

Frontal passage on Wednesday will therefore be accompanied by a transient period of light to moderate showers and just a slight chance of thunder. Followed by a transition to moderate to locally windy NW winds based on the latest Hi-resolution model guidance. Dewpoints will fall further into the 50s. Wednesday night through Friday night will feature rather cool weather characterized by large diurnal swings in temperature with typical daytime highs followed by overnight lows falling into the 50s in some locales, particularly sheltered and interior zones.

There remains enough uncertainty in the position of the aforementioned low that post-frontal low-level moist cyclonic flow could impinge on Kauai on Thursday bringing shallow showers to north and west facing facing slopes on Thursday. Primary uncertainty this forecast package is extent of eastward progress of the front and whether the incoming cool/low dewpoint airmass will penetrate as far east as the Big Island. At this time, model consensus suggests forward progress of the front stalls over or immediately east of the Big Island suggesting they are still in the game for a brief period of cooler weather. Dewpoints then rebound into the 60s as trades return for the weekend.

AVIATION
The trades will steadily ease tonight and Tuesday, with land breezes and sea breezes becoming increasingly dominant across the state. A few very light showers may affect windward slopes overnight into early Tuesday, otherwise dry weather and VFR conditions will prevail.

No AIRMETs are currently in effect. AIRMET Tango may be needed for moderate upper level turbulence over the western half of the state later tonight and expanding to the remainder of the island chain on Tuesday.



MARINE
Trade winds will decline through Tuesday, followed by the passage of a front passage Wednesday and Thursday. Trades will diminish as a surface ridge to the north is weakened and pushed southward, and winds will become light and variable when the ridge moves over the islands Tuesday afternoon. A front will move down the island chain Wednesday and Thursday, bringing a period of rainfall and possible thunderstorms. A brief period of fresh prefrontal southwest winds is excepted on Wednesday, mainly over the eastern half of the island chain. Following the frontal passage, moderate to strong northwest to north winds will result in localized accelerations through channels and leeward areas prompting the need for a Small Craft Advisory (SCA) for select zones Wednesday and Thursday. Winds will diminish late Thursday and Friday as the front stalls then dissipates near the Big Island. A brief return of trade winds is due on Saturday.

The current northwest swell will gradually decline over the next 24 hours and forerunners of another northwest swell will arrive late Tuesday. This swell will slowly build through Wednesday and peak Wednesday night and Thursday, leading to High Surf Advisory (HSA) conditions across most north and west facing shores from Kauai to Maui and SCA level seas over exposed waters. As this swell declines late Thursday and Friday, a storm low centered 600 to 700 nm north of Hawaii will send a medium period north swell that could produce HSA surf for north shores. The north swell will lower through the weekend and may be followed by a moderate to large northwest swell on next Monday.

East shore surf surf will decline overnight as trades diminish, though a small pulse of short period northeast swell will be possible Tuesday and Wednesday. Aside from areas exposed to wrapping north swell Thursday into the weekend, east shore surf will be small. The recent out of season south swell will fade by tomorrow, bringing back typical wintertime conditions for south shores. A tiny pulse of south-southwest swell is possible for the weekend.

FIRE WEATHER

Winds are forecast to remain below the threshold value while a reinforcing shot of dry air causes afternoon RH values to fall to near or slightly below 40 percent Thursday and Friday. Critical fire weather concerns are not anticipated during the forecast period.

HFO WATCHES/WARNINGS/ADVISORIES
None.