Latest Forecast for Mauna Kea Observatories
5 PM HST Wednesday 08 August (0300 UTC Thursday 9 August) 2018

Warnings
Fog, high humidity and rain
Moderate/strong (diminishing) winds

Cloud Cover and Precipitation Forecast
Fog, high humidity and light rain will continue to plague the summit throughout the night. Banding thick clouds are expected to stream in from the SE and contribute to extensive cloud cover through the night.

Summary of Key Meteorological Variables
Summit temperatures will be 1.5 C this evening and 2 C tomorrow morning. Winds will be from the ESE at 25-40 mph for the first half of the night, diminishing to 15-30 mph by the end of the night. Seeing and precipitable water are expected to exceed 1 arcsecond and 4 mm, respectively through the night.

Discussion
The atmosphere below 18 thousand feet will remain fairly saturated through early tomorrow evening, then will begin to slowly dry out as the inversion restrengthens near 10 thousand feet by sunrise Friday. Nonetheless, extensive fog, high humidity and light rain will continue to plague the summit for tonight and probably most of tomorrow night. There is also an outside chance for the development of isolated convection in the area, particularly along the southern half of the Big Island through tomorrow afternoon. Drier more stable conditions are set to return to the summit, once the inversion reaches full strength near 6-8 thousand feet for Friday, Saturday and Sunday night. Extensive daytime clouds are expected through tomorrow, then will taper for Friday, become minimal and short-lived for the weekend, only to pick up again for the early part of next week.

Banding thick clouds are expected to stream in from the SE, contributing to extensive cloud cover, if not overcast skies for tonight and probably most of tomorrow night. These clouds will eventually shift off toward the west and SW on Friday, leaving predominately clear skies for that night and Saturday night. However, another band of high clouds may move in from the SE during the second half of Sunday night.

Precipitable water is expected to exceed 4 mm for the next 2 nights, slip toward 2.5-3 mm for Friday, Saturday and at least the early part of Sunday night (it may trend toward 4 mm through the latter night).

While the boundary layer turbulence is set to diminish over the next 24 hours, a steady supply of moisture will contribute to poor/bad seeing through tomorrow night. A drier air mass, combined with relatively light laminar easterly flow aloft should allow seeing to improve early Friday evening and eventually settle in near 0.5-0.6 arcseconds during the second half of that night and Saturday night. There is a possibility that an increase in free atmospheric turbulence may degrade seeing toward more average-like values through Sunday night.

Little change since the morning forecast...While TC Hector will continue to march westward, helping to diminish summit-level winds over the course of the night, deep tropical moisture along its northern flank will keep the atmosphere below 18 thousand feet rather saturated during that time. A secondary influx of moisture drawn in from the SE along the passing storm's eastern flank is also expected to arrive during the second half of the night, then will shift westward through tomorrow night. Nonetheless, this abundant supply of tropical moisture will allow extensive fog, high humidity, overcast skies and light rain to the plague the summit for tonight and probably most of tomorrow night. There is also an outside chance for the development in convection in the area, but that should be limited to the SE and SW flanks of the Big Island and perhaps within the plume to the west. Dry/stable and clear/calm conditions/skies are set to return to the summit as Hector shifts far west of the state and the ridge to the NE regains control of the air mass, rebuilding the inversion near 6-8 thousand feet for Friday, Saturday and Sunday night. An upper-level low is expected to develop to the east of the Big Island and transverse westward late in the weekend. Initially the low will have little to no affect on summit conditions and may also allow relatively deep NE flow to prevail in the free atmosphere, which should help seeing to settle in near 0.5-0.6 arcseconds for much of the weekend. However, the low could begin to destabilize the air mass and bring high clouds in from the tropics as it progresses westward and over the state for the early part of next week.