textproduct: Honolulu

SYNOPSIS

Moderate trade winds will focus showers over windward and mauka areas through today. A band of enhanced moisture (the remnants of an old dissipated front) will move across the state tonight into Friday, bringing an increase in rainfall and stronger trade winds. Drier trades return Saturday. Winds will turn southeast Sunday, followed by a potential increase in showers Sunday through Monday night. After that, a return to drier trades is likely.

DISCUSSION

Current radar as of 4 AM HST shows isolated showers moving east across mainly Kauai County and windward Big Island. Satellite shows mostly cloudy skies across Kauai and Niihau, with partly cloudy skies in most other areas. Winds at most reporting stations were out of the east to northeast at 5 to 15 mph, but a few leeward sites showed light west to northwest winds.

An upper level ridge will slowly move over the islands through Friday night, allowing a surface high to spread southward over us and to our east. This will allow a fresh push of trade winds to bring a moisture boundary (remnants of an old cold front) into the area today through tonight. This will enhance showers windward and mauka, and could lead to a few moderate showers. The atmosphere is relatively stable Friday into Saturday, so heavy rain and flooding are *not* considered serious concerns with this system.

Late Saturday into Sunday, a strong upper level low will move southeast toward the state. This will likely result in the formation of a surface low several hundred miles to the northwest, but models are very inconsistent (even more than usual, especially at this time range) with their solutions on strength and location. Therefore, there is a tremendous amount of uncertainty in the forecast for Sunday through Monday. The potential exists for quite a rainy period, with upper levels cold enough for thunderstorms. If this solution verifies, surface winds would come out of the southeast, and heavy rain is likely. The latest data suggests this is the most likely solution, but uncertainty is high enough that there is still a chance we could end up experiencing a more typical transition back to east or northeast trades, with showers continuing mainly windward and mauka. We will have to wait for some model convergence before we take a firm stand on how wet we will get.

AVIATION

A moderate to locally breezy easterly trade wind regime will continue today. Embedded trade wind showers will primarily affect windward and mauka areas. a moisture band will move down the island chain from the north tonight, potentially enhancing showers. Expect VFR outside of showers, where MVFR is likely.

No AIRMETs in effect. AIRMET Sierra for mountain obscurations is possible later today if showers/clouds increase as expected.

MARINE

Moderate to locally strong easterly trade winds will continue through Friday as high pressure builds north of the islands. The Small Craft Advisory continues through this afternoon for the typically windy waters around Maui County and the Big Island. Model trends indicate winds weakening and veering from the east to southeast direction this weekend, as an area of low pressure develops northwest of the state. The latest forecast guidance shows breezy to strong easterly trade winds building swiftly into Hawaiian Waters from Sunday into early next week.

A medium-period north-northwest (340-350 degree) swell continues to lower and shift from a more northerly (350-360 degree) direction. Another moderate, medium-period north-northeast (360-020 degree) swell will build into Hawaiian waters by early Friday morning, producing moderate surges into north facing harbors, as well as boosting surf heights along north facing shores to near advisory levels. The latest north swell model guidance may be running 2 to 3 feet too low as compared to the long upstream generating fetch area. In any case, we boosted the swell heights 2 feet above model guidance for this next long duration Friday through Monday north-northeast swell that will peak from late Friday into Saturday, then diminish into Monday. The next long period, small northwest (310-320 degree) swell will build into Hawaiian waters by next Tuesday and then slowly decrease through the end of next week.

Choppy east shore surf will continue into Friday, before decreasing this weekend as wind speeds begin to weaken shift out of the east-southeasterly direction. Stronger easterly trade winds starting on Sunday will build rough surf along east facing shores into early next week. Surf along south facing shores will remain tiny to small through the forecast period.

Peak astronomical monthly tides may produce minor coastal flooding through early next week, and may be enhanced along north facing shores later this week due to the next north swell. A Coastal Flood Statement has been issued to highlight this flooding potential through Friday night.

A building north-northeast swell over the next several days will also produce moderate surges for north facing harbors, especially for Kahului and Hilo. A Marine Weather Statement was issued this morning for maritime interests to watch for these impending harbor surges.

HFO WATCHES/WARNINGS/ADVISORIES

Small Craft Advisory until 6 PM HST this evening for Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big Island Southeast Waters.