FORECAST DISCUSSION

Synopsis

Expect mainly dry trade wind weather through early Friday, with brief trade showers mainly along windward facing slopes, especially over the eastern end of the island chain. Trade wind speeds are forecast to strengthen again starting late Friday, with a slight increase in trade showers expected mainly over windward sections this weekend.

Discussion

The western end of a surface ridge appears to be about 260 miles north of Lihue based on an ASCAT pass from late Wednesday evening. A surface front is also located about 525 miles north of Lihue. The close proximity of the front has weakened the surface ridge, and this has caused the pressure gradient to become relatively slack across the islands early this morning. As a result, trade wind speeds have continued to diminish overnight, especially over the western end of the state, where background surface winds are from an east-southeast direction. This weaker flow also allowed local land breeze circulations to develop over the leeward and interior sections of many of the islands. Aloft, water vapor imagery continues to show a middle tropospheric ridge near the Big Island, which is maintaining relatively stable atmospheric conditions across the islands. Elsewhere, the axis of an upper tropospheric short-wave trough is evident about 415 miles west of Lihue. This feature is moving rapidly toward the east- northeast. The main influence this trough aloft appears to be having on island weather are thin high clouds, mainly cirrus, which are streaming rapidly toward the east-northeast across the western islands this morning. These translucent clouds will partially block sunshine over some areas in a few hours after sunrise. The cold front north of the region is forecast to continue pushing eastward. This will keep the western end of the weak surface ridge close to the islands, with east to east-southeast surface winds. Note that the weaker background flow will likely allow local afternoon sea breezes and nighttime land breezes to develop over leeward and interior sections of the individual islands, especially on Kauai and Oahu, through tonight. The early morning sounding from Hilo shows moistening overnight compared with Wednesday afternoon, since the most recent precipitable water (PW) value increased to 1.27 inches. Elsewhere, the Lihue sounding still showed relatively dry conditions, with a PW value of 1.05 inches. In addition, the low- level trade wind inversion has increased to around 8 thousand feet at Hilo, while it remains below 6 thousand feet at Lihue. The latest forecast guidance continues to indicate additional low-level moisture will be advected over the eastern islands today. Satellite derived PW values of 1.3 inches extend a few hundred miles upstream from the Big Island. Therefore, expect the low-level flow to bring additional low clouds and showers from the east-southeast toward the Big Island, and possibly Maui, from this morning into Friday. The middle tropospheric ridge will likely erode near the eastern end of the state during the next couple of days. This is due in part to the short-wave trough moving by just to the north of the islands during the next 18 to 30 hours. Once this feature is northeast of the Big Island, the models suggest a sharp upper tropospheric trough will develop east and northeast of the Big Island starting later tonight or Friday, and persist into this weekend. This may allow some additional increase in inversion heights, especially near the windward Big Island this weekend. High pressure will gradually build north of the area after the front dissipates, resulting in an increase in trade wind speeds. The trades will also likely be from a more typical east-northeast direction starting late Friday, and continuing through this weekend. In addition, as the inversion heights gain altitude this weekend, expect an increase in trade wind showers, particularly along windward facing slopes of the eastern islands. The models also show a new high moving to a position far north of the islands late Saturday. The pressure gradient south of this feature will maintain locally breezy trade winds at least into Monday as it continues moving toward the east-northeast. There may be some fluctuations in trade speeds starting around Monday night or Tuesday. In addition, trade showers may also increase over some windward sections from Tuesday into the middle of next week.

Aviation

A cold front passing well north of the state will weaken the high pressure ridge, decreasing the strength of the trade winds and keeping the wind direction more east to southeasterly. Cloud remnants from an old frontal boundary to our east will drift into eastern sections of the Hawaiian Islands with isolated to scattered MVFR ceilings and a few showers in the forecast. Expect mostly VFR conditions elsewhere. No AIRMETs are in effect at this time.

Marine

A surface high is currently located far northeast of the area with a surface ridge trailing west to about 200NM north of Kauai. The high will move east and the ridge will weaken over the next couple of days as two fronts pass to the north of the state. Winds are expected to remain below Small Craft Advisory (SCA) speeds through Friday. The surface ridge will strengthen again north of the area starting Friday. Winds are expected to reach SCA strength by Saturday and may reach near-gale strength around the Big Island and Maui over the weekend as a new high builds north of the area. Surf along east facing shores will increase a bit again as well, with the strengthening trade winds. A storm east of Japan will produce a very large west-northwest swell reaching the islands Saturday. Surf from this swell will be well above the advisory threshold through early next week and may reach warning levels, most likely on Kauai and Niihau, where surf heights will be largest. Those islands will block some of the swell, so surf heights will be somewhat lower on the rest of the smaller islands. As this swell subsides next Tuesday, a new moderate northwest swell (from around 320 deg)is expected to arrive.

HFO Watches/Warnings/Advisories

None.

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Discussion...Houston Aviation...Bohlin Marine...TS

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