Widely scattered light showers dot the Hawaiian coastal waters on this Tuesday evening. Rain gauge data shows just isolated light amounts during the past several hours, with most land locations remaining dry. Surface analysis depicts low pressure centered far northeast of the state, with a dissipating front/trough trailing SW to east and south of the Big Island. A 1022 mb high is centered about 1000 miles northwest of Kauai and is building slowly eastward, providing gentle trade wind flow across the state. Aloft, a weak upper level low is centered near Kauai and moving slowly northward, with a large mid/upper level ridge sprawling northwest of our region from west of the dateline to near 160°W. Mimic-tpw satellite imagery shows a stripe of higher moisture along the dissipating surface trough extending across the southern tip of the Big Island and south of the smaller islands. The airmass over the state as of 00Z contained near normal moisture, with the lihue/hilo soundings measuring precipitable water near 1.4 inches and inversions based around 7000 feet. Another area of somewhat higher moisture is approaching from the north, and appears poised to move across the western half of the state late tonight into Wednesday.
Overnight, expect shower activity to increase across windward waters and slopes over the western sections, due to the typical nocturnal rainfall maximum plus the aforementioned area of higher moisture moving in. A considerably drier airmass lies upstream from the Big Island, and would expect less shower activity there even during the late night period. May perform some minor updates to the overnight gridded/text forecasts to reflect the above thinking and short term trends, but no major changes are planned.
Surface high pressure will build slowly eastward to the north of the state through Wednesday night, maintaining a gentle background trade wind flow. Trades will remain light enough to allow localized sea breezes to develop once again Wednesday afternoon. Expect a hybrid pattern similar to that observed today, with showers and clouds focusing on windward slopes during the night and early morning, and across some of the leeward and interior areas during the afternoon. The area of increased moisture crossing the state on Wednesday may allow for a bit more leeward shower activity than was observed today, with a few locally heavier downpours possible.
From Thursday into the weekend, another cold front is forecast to drop southward across the north central Pacific. This will push the subtropical ridge south and east, disrupting the pressure gradient across the state. Local trade wind flow should weaken further or disappear entirely during this time, allowing for a period of more widespread daytime sea breezes and nighttime land breezes. This pattern will bring mostly clear nights and mornings with spotty showers over interior areas during the afternoons and evenings. The light winds will allow volcanic haze to build up around the Big Island, but it does not appear as if there will be enough southerly component in the flow to carry the haze over the smaller islands.
Forecast uncertainty increases during the weekend and into next week, when the models generally bring a tropical wave and associated surge of deep moisture northwestward toward the state. The 00Z guidance suite continues to show some variations in the timing and extent of this moisture. It appears that the increased pressure gradient between lower pressure to the south, and a narrow high pressure ridge between the islands and the stalled frontal boundary to our north should induce some increase in easterly flow over the state for Sunday and Monday. Rainfall amounts may increase with locally heavy showers becoming possible as well. Thereafter, a drier airmass should spread back into the state from the east, with the low level flow possibly veering toward the east-southeast as another cold front drops into the north central Pacific. This would bring the potential for volcanic haze to spread west northwestward over some of the smaller islands at that time.