textproduct: Honolulu

SYNOPSIS

Moderate trade winds will persist through this weekend and bring scattered showers to mainly windward and mauka locations. By early next week, a trough deepening west of the islands will act to weaken trade wind flow. By mid-week, this trough will approach the islands and bring gusty southerly winds and rounds of moderate to heavy rain.

DISCUSSION

IR satellite from this morning shows a layer of high clouds associated with a sub-tropical jet streaming west to east over most islands, thickest between Molokai and the Big Island. Scattered low clouds and embedded light showers, caught in the trade flow, have managed to squeeze out a few hundredths of an inch of accumulation overnight at several windward sites on each island. The 12Z RAOB sounding at Hilo clearly indicated a boundary layer temperature inversion at around 6000 feet, a dry and stable layer in the mid levels, and sharp increase of moisture in the upper levels (base of high clouds) at around 25000 feet. Latest guidance has these high clouds exiting the region by late tonight.

A transient surface high well northeast of the state is progged move eastward by this weekend, but will be reinforced by yet another surface high building in from the northwest. All in all, this should keep moderate trade winds in place across the region for the next several days. Low cloud cover with embedded light showers will continue to favor windward and mauka sections of the islands, especially during the nights and early mornings.

Monday into Tuesday, a large broad trough begins to sag southward, west of the state. In response, background flow weakens and shifts easterly, then southeasterly. A slight increase in moisture, along afternoon sea breeze development, could spark a few showers over island interiors and on mountain slopes.

Tuesday night through late week, a very unsettled weather pattern appears to be taking shape. Model solutions have been consistent in deepening the aforementioned trough west of the islands and eventually inducing a surface low and associated front. Ahead of these features, deep layer south or southwesterly flow will draw tropical moisture northward over the state. Southerly winds may become gusty at times, especially on the lee sides of any terrain. Various lobes of energy rotating around the trough will act to destabilize the atmosphere and provide lift. Rounds of heavier rain, unfortunately, will be possible and may lead to more flooding concerns. However, it is still too early to accurately determine amounts and which islands would be most affected. Please keep yourself updated through the weekend for the latest forecasts.

AVIATION

Moderate trade winds will persist through the 24 h period, with brief showers favoring windward areas overnight through the early morning periods. These showers could bring periods of MVFR CIGS/VSBYS.

AIRMET TANGO remains in effect across the state for moderate turbulence between FL280 and FL380, which will persist through the day today.

MARINE

Moderate to locally fresh trade winds will remain in place through the weekend. Winds will then begin to veer east to southeast by Monday ahead of a front approaching from the northwest and trough moving up from the southwest. This surface front and trough will be driven by an upper level trough, which could allow for isolated thunderstorms over the offshore and coastal waters through much of next week. A Small Craft Advisory remains in effect for the typically windier waters around Maui County and the Big Island through tonight.

Surf over north and east facing shores will remain choppy today as northeast swell decreases. By tonight through tomorrow, small, medium-period west-northwest (310 degree) energy will fill in and bump up surf for north and west facing shores. Then by Monday night through Wednesday, a moderate medium-period west- northwest swell will fill in and increase surf to just below advisory levels.

Along south facing shores, expect small reinforcements to provide periods of small surf through the weekend until a larger long period south swell fills in Monday through mid week. Strengthening southerly winds may lead to increasingly choppy conditions by mid week.

HFO WATCHES/WARNINGS/ADVISORIES

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