Marine Weather and Tide Forecast for Hanamaulu, HI

DISCUSSION
Latest observations and model analyses indicate a deep closed low positioned around 800 miles northwest of Lihue this evening.
Erosion of mid-level stability evident on this afternoon's sounding out of Lihue has allowed low development to extend downward to the surface where surface pressure falls have bottomed out around 1011mb. At press time, the islands remain under the influence of moist prefrontal S/SE flow which has led to a steady increase in humidity at the surface and PWAT values climbing to around 1.5" over the western end of the state and closer to 1" in the vicinity of the Big Island and and Maui. Unstable cumulus is streaming from south to north over Oahu and Kauai supporting pockets of showers, a few of which have been heavy (around 1"/hr rates), but they are also transient in nature and have not posed a flooding threat. Some island interaction between Lanai and Molokai is also noted in which a convergent plume emanating from the former brought a period of heavy rainfall to Leeward Molokai earlier this evening.

Although no further strengthening of the low is anticipated, it will lose a couple more degrees of latitude during the next 24 hours or so which will allow the eastern periphery of the mid-level cold pool to impinge on Kauai during Wednesday and Thursday leading to a relative maximum in instability during that time. However, the coldest air and greatest instability aloft will remain confined well northwest of the area. Even with marginal mid-level temperatures, afternoon 50mb MLCAPE has climbed to around 1500 j/kg with equilibrium levels around 40kft over Kauai, more than sufficient for deep convection capable of producing torrential rainfall.

The lack of a low-level forcing mechanism has limited shower coverage thus far, but some shower development within convergent southerly low-level flow has been observed during the last few hours. Combined with increasingly cyclonic flow aloft and cooling aloft courtesy of the favorable nocturnal period, periods of training convection over Kauai and possibly even Oahu remain possible through the night. The latest high res guidance offers support for this reasoning and actually simulates heavier rainfall focusing over Kauai during the day Wednesday as the low makes its closest approach.

Light to moderate low-level flow retains a SE component through the second half of the week which may allow showers to acquire a land/sea breeze character. This would favor a transition away from an organized heavy rain threat to one in which afternoon convective flare ups could produce pockets of heavy rain over island interiors from Thursday onward as the resident low peels off to the north and east. The Flood Watch for Kauai remains in good shape and is still in effect through Wednesday afternoon. Long range guidance suggests moderate trades return for the first half of next week.

AVIATION

Moderate winds holding out of the southerly quadrant will steer developing showers over leeward areas, particularly those of Oahu and Kauai, the latter of which stands the greatest chance of observing periods of heavy rain and isolated thunderstorms. Showers remain scattered and disorganized in Maui County. MVFR is expected to become increasingly common over Leeward Kauai as the night wears on and, to a lesser extent, leeward Oahu. The heaviest showers will produce localized IFR.

No AIRMETs in effect. AIRMET Sierra for mountain obscuration may be needed for portions of Kauai and Oahu tonight.



MARINE

Gentle to moderate southerlies continues over the western end of the state with locally fresh SE prevailing over the eastern half. A low settling several hundred miles northwest of the islands results in a chance of thunderstorms mainly in the vicinity of Kauai through Thursday.

The existing small, medium-period NW swell peaked this afternoon and will subside through Wednesday with little in the way of surf for N and W facing shores expected thereafter. A slightly larger long- period NW swell is then expected this weekend, though surf is forecast to remain well below the advisory threshold for N and W facing shores.

A small, long period SSW swell filled in today. This swell will elevate surf along S facing shores above the summertime average, but still firmly below the advisory threshold as it peaks tonight and holds through Wednesday. This swell then slowly declines during the rest of the week.

Lack of upstream trades favors near-to-below average surf along E facing shores, though some chop resulting from current SE winds may reach the waters off of E facing shores during the next several days.

HFO WATCHES/WARNINGS/ADVISORIES
Flood Watch from noon HST today through Wednesday afternoon for Niihau-Kauai-