FORECAST DISCUSSION

Synopsis

High pressure building north of the state will keep breezy trade winds in place through the week. Relatively dry and stable trade wind weather will prevail through Friday, with a more showery trade wind pattern expected this weekend into early next week.

Discussion

Water vapor imagery shows an upper level ridge just south of the Big Island, with southwesterly upper level flow across the state. At the surface, high pressure is located far northeast of the state, with weakening troughs to the northwest. 12z soundings from Lihue and Hilo show inversions less than 4kft and 6kft respectively. Precipitable waters were just under 1.5 inches, which is somewhat above normal for this time of year. Early morning MIMIC total precipitable water imagery shows similar moisture values extending 300 miles upstream to the northeast of the state, before dropping off steadily toward the subtropical high. Conditions are stable across the islands, which is reflected in both the cloud field over and upstream of us and in the overnight rainfall amounts. Only a couple gages saw any measurable rainfall, and those that did just received a hundredth or two. This stable pattern with lower than normal inversion heights will continue at least through Thursday night. Models show precipitable water dropping below normal, but this may be attributed to mid-level drying above the inversion; forecast soundings/cross sections show low-levels remaining nearly saturated. The current forecast has lower than normal rainfall chances, especially during the afternoon, which looks good given the shallow cloud layer. Otherwise our currently breezy trade winds will pick up a bit as the high shifts westward and strengthens. These breezy winds will continue into Friday before the next shift in the weather pattern. The 12z GFS/ECMWF continue to be in good agreement with respect to the subtropical high, which remains entrenched far north- northeast of the state into next week. The remnant low that was Georgette will dissipate well east of the area, though moisture it leaves behind will get picked up in the trades. By later Friday/Friday night, we see an increase in moisture depth spreading from east to west across the state. MIMIC precipitable water imagery shows higher TPW values extending a few hundred miles west of former Georgette (in addition to the clouds in this area having a more showery/unstable appearance in visible imagery), which accounts for the initial increase in moisture depth by Friday night. This surge in moisture combined with the locally breezy trade winds will lead to a wet trade pattern, with most rainfall across windward sections but more showers affecting leeward sides of the smaller islands as well.

Aviation

High pressure far NE of the state will support moderate to breezy NE to E trade winds. AIRMET TANGO for low level turbulence over and immediately S through W of the mountains is posted for all islands. The 12Z soundings from PHTO and PHLI show a strong low level inversion sloping from 6 kft at PHTO to 3.5 kft at PHLI. Morning AMDAR reports in and out of PHNL suggest the inversion over Oahu is between 3 and 3.6 kft, with winds near the inversion level reaching upwards of 30 kt. Conditions will remain fairly dry and stable, any passing showers will be brief and light. An overcast stratocumulus deck extending from the California coasts is pushing towards the state. These low stable clouds could introduce periods of MVFR cigs, mainly along the windward coasts, as they pass. Otherwise, expect VFR conditions.

Marine

High pressure far northeast of the state will strengthen and shift westward, keeping trade winds breezy across the area. A small craft advisory is currently in effect for a somewhat larger than typical area, a sign of the stronger gradient across the islands. The advisory may need to be expanded to additional areas tonight as the high strengthens, though we will wait to see how the current forecast verifies against the morning ASCAT pass to make that determination. Breezy trade winds will result in choppy, short-period surf along east facing shores, but surf heights are expected to remain below advisory levels. There will also be a series of small southerly swells through the weekend and into next week.

HFO Watches/Warnings/Advisories

Small Craft Advisory until 6 PM HST Friday for Oahu Leeward Waters, Kaiwi Channel, Maui County Windward Waters, Maui County Leeward Waters, Maalaea Bay, Pailolo Channel, Alenuihaha Channel, Big Island Windward Waters, Big Island Southeast Waters.

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Discussion/marine...Bravender Aviation...Eaton

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