Latest Forecast for Mauna Kea Observatories
5 PM HST Thursday 13 September (0300 UTC Friday 14 September) 2018

Warnings
High humidity
Chance for fog and afternoon convection/rain

Cloud Cover and Precipitation Forecast
There is a moderate/high risk for fog, high humidity and rain mainly during the first half of the night. Residual thick clouds may also linger in the area, while high clouds may fill out of the west as the night progresses.

Summary of Key Meteorological Variables
Summit temperatures will be near 1 C, with winds from the SE at 5-15 mph for the night. Seeing will likely exceed 1 arcsecond for this evening, perhaps improving toward 0.7-0.8 arcseconds as the night progresses. Precipitable water is expected to exceed 4 mm through the night.

Discussion
The atmosphere near the Big Island will remain fairly saturated and unstable, which may contribute to periods of fog and high humidity through at least this evening. While there is a good chance that the atmosphere will begin a drying trend as the inversion rebuilds near 8 thousand feet as the night progresses and especially through tomorrow, humidity may linger in the 50-70% range which could allow for short-lived fog through Saturday night (the odds are slim after tonight). Precipitation is unlikely beyond this evening and the summit will remain dry and stable for Sunday and Monday night. Daytime clouds will begin to taper over the next 2 days, becoming minimal and short-lived for Sunday and early part of next week.

Residual patches of thick clouds may also linger in the area and isolated cirrus from any convection exhaust may persist into this evening, while high clouds may fill out of the west as the night progresses. The former are expected to push off toward the west, but more widespread/organzied high clouds may fill out of the west and SW through tomorrow, contributing to periods of extensive cloud cover for much of the weekend. These clouds may push off toward the southern skies late Monday morning, opening up skies for that night.

Precipitable water is expected to linger near or exceed 4 mm throughout the forecast period.

Residual moisture and instability may contribute to poor/bad seeing through most of tonight (a modest improvement is possible toward the end of the night). However, a drier and especially more stable air mass, combined with relatively uniform westerly winds in the free atmosphere should allow seeing to settle in neaer 0.55-0.6 arcseconds for the following 4 nights. Some variability is possible mainly for tomorrow evening (due to lingering moisture) and again as pockets of upper-level turbulence pass through mainly for Sunday night.

No change since the morning forecast...Remnant moisture and low-level convergence left behind by Olivia will slowly move through the Island, while an upper-level low near Kauai slips off toward the west over the next 6-12 hours. Unfortunately, this moisture moisture/convergence combined with instability from the low allowed for the development of afternoon convection along the Big Island slopes (as expected), and may allow for more fog/high humidity at the summit mainly for this evening. There is good chance that departure of the low could help stabilize the atmosphere, which may permit humidity to slip toward 60%, and together with downslope flow may perhaps clear out any fog during the second half of the night. While the inversion is set to become more firmly established near 7-8 thousand feet as the low dissipates and slips further westward, residual moisture may still keep humidity near 50-70% which could allow for short-lived fog at the summit through Saturday night (though the odds are slim). In addition, cirrus exhaust associated with development of thunderstorms (between the interation of the upper-level low and remnants of Olivia) may fill in from the West via the redevelopment of the tropical upper-tropospheric trough and its embedded sub-tropical jet over the state over much of the weekend. The TUTT is not expected to affect the stability of the atmosphere and the re-establishment of the ridge to the north combined with the absence of any tropical cyclones in the area should ensure the summit steers free of moisture for at least Sunday and Monday night. Flow aloft will also take on a more uniform westerly flow, minimizing turbulence in the free atmosphere which should seeing settle in near 0.5-0.6 arcseconds after tonight.