textproduct: Honolulu

SYNOPSIS

Locally breezy trades deliver limited showers windward and mauka into Thursday. An increase in showers is then expected Thursday night into the weekend followed by a return to drier and more stable trade wind pattern thereafter.

DISCUSSION

A closed upper low positioned north of Kauai supports a 55kt subtropical jet draped over the eastern end of the state and associated southwest flow aloft is advecting high clouds over and south of the Big Island. As the low drifts westward during the next few days, so too will the axis of SW flow aloft resulting in patchy high clouds areawide for the rest of the week. The low otherwise remains inconsequential through the near term as it resides above strongly stable mid-levels. At the surface, a stable trade wind pattern prevails with a few modest patches of moisture poised to bring periodic showers to windward and mauka areas the next couple of nights.

Upstream, an orphaned cold front aligned along & south of 30N presents as a narrow ribbon of enhanced moisture averaging about 5kft in depth except where deep convection exists along its convergent lead edge. Pockets of thunderstorms have been common along this boundary. As this moisture is ushered southwestward within prevailing trade winds, it will increasingly encounter a hostile mid-level environment that will tend to erode the deepest moisture and result in a trend toward decreasing deep convection as it approaches the islands. Low-level convergence itself will also tend to diminish with time given the dearth of mid-level support for the front. Ongoing breezy trades will further limit prospects for meaningful low-level convergence along which deep convection could organize. With that said, the resident upper low will offer mid/upper level lapse rates around 6.5C/km within a low density CAPE profile that will provide very marginal support for isolated thunderstorms within any pockets of deeper moisture, most likely over the western end of the state and at night when lapse rates are maximized. Low confidence at this juncture precludes inclusion of thunderstorms in the forecast over/around Oahu & Kauai, but PoPs have been bumped up a bit given high confidence in overall shower coverage.

Of note, guidance indicates ENE to NE flow through 700mb Friday night which is favorable for directing moisture toward the Hamakua Coast of the Big Island, an area in need of rainfall. Guidance also indicates moisture arrival prior to full development of the mountain breeze that would otherwise limit rainfall potential there.

A typical breezy trade wind pattern then prevails into next week.

AVIATION

Locally breezy trade winds will continue for the next several days. Although some clouds and showers embedded within the trades will be possible for windward and mountain areas, VFR flight conditions are expected to prevail for most locations.

AIRMET Tango has been issued for moderate low level turbulence downwind of island terrain. This AIRMET will likely be needed for the next several days as breezy trades persist.

MARINE

Surface high pressure will remain anchored north of the state for the next several days and bring moderate to locally strong trade winds across the waters. A brief uptick of wind speeds are expected tonight into Thursday with fresh to strong trades expected across many coastal waters. However, winds are expected to decrease back to moderate to locally strong speeds by Friday as a front drops south and weakens the high. With the latest high-res models showing a slight increase of wind speeds, the Small Craft Advisory has been adjusted to include Maui Windward waters tonight and may need to be expanded to the Kaiwi Channel for Thursday. These advisories will likely be scaled back to the just the typical windy areas around Maui and the Big Island from Friday onward.

Surf along south facing shores will be small over the next few days with mainly background south and southeast energy. A small to moderate, long-period, south southwest swell will build Saturday into Sunday and help boost south shore surf heights back up to near or above the September average.

Surf along north facing shores should see a small increase tomorrow as a short period northerly swell fills in. This small bump is expected to peak on Friday out of the north to north- northeast direction, followed by a short period north northwest swell this weekend. Moderate to locally strong trades near and upstream of the area will lead to choppy conditions along east facing shores over the next several days. A slight increase of windswell is expected over the weekend due to a fetch of strong winds northeast of the state.

FIRE WEATHER

Decreasing fire weather risk as humidity gradually increases through the week and into the weekend. Showers increase in coverage Thursday night into the weekend as deep pockets of moisture filter in on the trades helping to alleviate fire weather concerns. Sustained winds briefly hover near to below the critical fire weather threshold through Thursday afternoon then trend slightly lighter for the weekend.

HFO WATCHES/WARNINGS/ADVISORIES

Small Craft Advisory until 6 AM HST Thursday for Kauai Leeward Waters-Kauai Channel.

Small Craft Advisory until 6 PM HST Thursday for Maui County Windward Waters-Maalaea Bay-Pailolo Channel-Alenuihaha Channel- Big Island Leeward Waters-Big Island Southeast Waters.